feat: cache-tracking progress

rust/crates/api/src/client.rs

@@ -1,4 +1,5 @@
 use std::collections::VecDeque;
+use std::sync::{Arc, Mutex};
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
 use runtime::{
@@ -8,8 +9,9 @@ use runtime::{
 use serde::Deserialize;
 
 use crate::error::ApiError;
+use crate::prompt_cache::{PromptCache, PromptCacheRecord, PromptCacheStats};
 use crate::sse::SseParser;
-use crate::types::{MessageRequest, MessageResponse, StreamEvent};
+use crate::types::{MessageRequest, MessageResponse, StreamEvent, Usage};
 
 const DEFAULT_BASE_URL: &str = "https://api.anthropic.com";
 const ANTHROPIC_VERSION: &str = "2023-06-01";
@@ -108,6 +110,8 @@ pub struct AnthropicClient {
     max_retries: u32,
     initial_backoff: Duration,
     max_backoff: Duration,
+    prompt_cache: Option<PromptCache>,
+    last_prompt_cache_record: Arc<Mutex<Option<PromptCacheRecord>>>,
 }
 
 impl AnthropicClient {
@@ -120,6 +124,8 @@ impl AnthropicClient {
             max_retries: DEFAULT_MAX_RETRIES,
             initial_backoff: DEFAULT_INITIAL_BACKOFF,
             max_backoff: DEFAULT_MAX_BACKOFF,
+            prompt_cache: None,
+            last_prompt_cache_record: Arc::new(Mutex::new(None)),
         }
     }
 
@@ -132,6 +138,8 @@ impl AnthropicClient {
             max_retries: DEFAULT_MAX_RETRIES,
             initial_backoff: DEFAULT_INITIAL_BACKOFF,
             max_backoff: DEFAULT_MAX_BACKOFF,
+            prompt_cache: None,
+            last_prompt_cache_record: Arc::new(Mutex::new(None)),
         }
     }
 
@@ -189,6 +197,30 @@ impl AnthropicClient {
         self
     }
 
+    #[must_use]
+    pub fn with_prompt_cache(mut self, prompt_cache: PromptCache) -> Self {
+        self.prompt_cache = Some(prompt_cache);
+        self
+    }
+
+    #[must_use]
+    pub fn prompt_cache(&self) -> Option<&PromptCache> {
+        self.prompt_cache.as_ref()
+    }
+
+    #[must_use]
+    pub fn prompt_cache_stats(&self) -> Option<PromptCacheStats> {
+        self.prompt_cache.as_ref().map(PromptCache::stats)
+    }
+
+    #[must_use]
+    pub fn take_last_prompt_cache_record(&self) -> Option<PromptCacheRecord> {
+        self.last_prompt_cache_record()
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner)
+            .take()
+    }
+
     #[must_use]
     pub fn auth_source(&self) -> &AuthSource {
         &self.auth
@@ -198,10 +230,19 @@ impl AnthropicClient {
         &self,
         request: &MessageRequest,
     ) -> Result<MessageResponse, ApiError> {
+        self.store_last_prompt_cache_record(None);
         let request = MessageRequest {
             stream: false,
             ..request.clone()
         };
+        if let Some(prompt_cache) = &self.prompt_cache {
+            if let Some(response) = prompt_cache.lookup_completion(&request) {
+                self.store_last_prompt_cache_record(Some(prompt_cache_record_from_stats(
+                    prompt_cache.stats(),
+                )));
+                return Ok(response);
+            }
+        }
         let response = self.send_with_retry(&request).await?;
         let request_id = request_id_from_headers(response.headers());
         let mut response = response
@@ -211,6 +252,10 @@ impl AnthropicClient {
         if response.request_id.is_none() {
             response.request_id = request_id;
         }
+        if let Some(prompt_cache) = &self.prompt_cache {
+            let record = prompt_cache.record_response(&request, &response);
+            self.store_last_prompt_cache_record(Some(record));
+        }
         Ok(response)
     }
 
@@ -218,6 +263,7 @@ impl AnthropicClient {
         &self,
         request: &MessageRequest,
     ) -> Result<MessageStream, ApiError> {
+        self.store_last_prompt_cache_record(None);
         let response = self
             .send_with_retry(&request.clone().with_streaming())
             .await?;
@@ -227,9 +273,30 @@ impl AnthropicClient {
             parser: SseParser::new(),
             pending: VecDeque::new(),
             done: false,
+            cache_tracking: self
+                .prompt_cache
+                .as_ref()
+                .map(|prompt_cache| StreamCacheTracking {
+                    prompt_cache: prompt_cache.clone(),
+                    request: request.clone().with_streaming(),
+                    last_usage: None,
+                    finalized: false,
+                    last_record: self.last_prompt_cache_record.clone(),
+                }),
         })
     }
 
+    fn store_last_prompt_cache_record(&self, record: Option<PromptCacheRecord>) {
+        *self
+            .last_prompt_cache_record()
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner) = record;
+    }
+
+    fn last_prompt_cache_record(&self) -> &Arc<Mutex<Option<PromptCacheRecord>>> {
+        &self.last_prompt_cache_record
+    }
+
     pub async fn exchange_oauth_code(
         &self,
         config: &OAuthConfig,
@@ -527,6 +594,7 @@ pub struct MessageStream {
     parser: SseParser,
     pending: VecDeque<StreamEvent>,
     done: bool,
+    cache_tracking: Option<StreamCacheTracking>,
 }
 
 impl MessageStream {
@@ -538,6 +606,9 @@ impl MessageStream {
     pub async fn next_event(&mut self) -> Result<Option<StreamEvent>, ApiError> {
         loop {
             if let Some(event) = self.pending.pop_front() {
+                if let Some(cache_tracking) = &mut self.cache_tracking {
+                    cache_tracking.observe(&event);
+                }
                 return Ok(Some(event));
             }
 
@@ -545,8 +616,14 @@ impl MessageStream {
                 let remaining = self.parser.finish()?;
                 self.pending.extend(remaining);
                 if let Some(event) = self.pending.pop_front() {
+                    if let Some(cache_tracking) = &mut self.cache_tracking {
+                        cache_tracking.observe(&event);
+                    }
                     return Ok(Some(event));
                 }
+                if let Some(cache_tracking) = &mut self.cache_tracking {
+                    cache_tracking.finalize();
+                }
                 return Ok(None);
             }
 
@@ -562,6 +639,53 @@ impl MessageStream {
     }
 }
 
+#[derive(Debug, Clone)]
+struct StreamCacheTracking {
+    prompt_cache: PromptCache,
+    request: MessageRequest,
+    last_usage: Option<Usage>,
+    finalized: bool,
+    last_record: Arc<Mutex<Option<PromptCacheRecord>>>,
+}
+
+impl StreamCacheTracking {
+    fn observe(&mut self, event: &StreamEvent) {
+        match event {
+            StreamEvent::MessageStart(event) => {
+                self.last_usage = Some(event.message.usage.clone());
+            }
+            StreamEvent::MessageDelta(event) => {
+                self.last_usage = Some(event.usage.clone());
+            }
+            StreamEvent::ContentBlockStart(_)
+            | StreamEvent::ContentBlockDelta(_)
+            | StreamEvent::ContentBlockStop(_)
+            | StreamEvent::MessageStop(_) => {}
+        }
+    }
+
+    fn finalize(&mut self) {
+        if self.finalized {
+            return;
+        }
+        if let Some(usage) = &self.last_usage {
+            let record = self.prompt_cache.record_usage(&self.request, usage);
+            *self
+                .last_record
+                .lock()
+                .unwrap_or_else(std::sync::PoisonError::into_inner) = Some(record);
+        }
+        self.finalized = true;
+    }
+}
+
+fn prompt_cache_record_from_stats(stats: PromptCacheStats) -> PromptCacheRecord {
+    PromptCacheRecord {
+        cache_break: None,
+        stats,
+    }
+}
+
 async fn expect_success(response: reqwest::Response) -> Result<reqwest::Response, ApiError> {
     let status = response.status();
     if status.is_success() {
@@ -606,7 +730,7 @@ mod tests {
     use super::{ALT_REQUEST_ID_HEADER, REQUEST_ID_HEADER};
     use std::io::{Read, Write};
     use std::net::TcpListener;
-    use std::sync::{Mutex, OnceLock};
+    use std::sync::atomic::{AtomicU64, Ordering};
     use std::thread;
     use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
@@ -616,19 +740,15 @@ mod tests {
         now_unix_timestamp, oauth_token_is_expired, resolve_saved_oauth_token,
         resolve_startup_auth_source, AnthropicClient, AuthSource, OAuthTokenSet,
     };
+    use crate::test_env_lock;
     use crate::types::{ContentBlockDelta, MessageRequest};
 
-    fn env_lock() -> std::sync::MutexGuard<'static, ()> {
-        static LOCK: OnceLock<Mutex<()>> = OnceLock::new();
-        LOCK.get_or_init(|| Mutex::new(()))
-            .lock()
-            .expect("env lock")
-    }
-
     fn temp_config_home() -> std::path::PathBuf {
+        static NEXT_ID: AtomicU64 = AtomicU64::new(0);
         std::env::temp_dir().join(format!(
-            "api-oauth-test-{}-{}",
+            "api-oauth-test-{}-{}-{}",
             std::process::id(),
+            NEXT_ID.fetch_add(1, Ordering::Relaxed),
             SystemTime::now()
                 .duration_since(UNIX_EPOCH)
                 .expect("time")
@@ -668,7 +788,7 @@ mod tests {
 
     #[test]
     fn read_api_key_requires_presence() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         std::env::remove_var("ANTHROPIC_AUTH_TOKEN");
         std::env::remove_var("ANTHROPIC_API_KEY");
         std::env::remove_var("CLAUDE_CONFIG_HOME");
@@ -678,7 +798,7 @@ mod tests {
 
     #[test]
     fn read_api_key_requires_non_empty_value() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         std::env::set_var("ANTHROPIC_AUTH_TOKEN", "");
         std::env::remove_var("ANTHROPIC_API_KEY");
         let error = super::read_api_key().expect_err("empty key should error");
@@ -688,7 +808,7 @@ mod tests {
 
     #[test]
     fn read_api_key_prefers_api_key_env() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         std::env::set_var("ANTHROPIC_AUTH_TOKEN", "auth-token");
         std::env::set_var("ANTHROPIC_API_KEY", "legacy-key");
         assert_eq!(
@@ -701,7 +821,7 @@ mod tests {
 
     #[test]
     fn read_auth_token_reads_auth_token_env() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         std::env::set_var("ANTHROPIC_AUTH_TOKEN", "auth-token");
         assert_eq!(super::read_auth_token().as_deref(), Some("auth-token"));
         std::env::remove_var("ANTHROPIC_AUTH_TOKEN");
@@ -721,7 +841,7 @@ mod tests {
 
     #[test]
     fn auth_source_from_env_combines_api_key_and_bearer_token() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         std::env::set_var("ANTHROPIC_AUTH_TOKEN", "auth-token");
         std::env::set_var("ANTHROPIC_API_KEY", "legacy-key");
         let auth = AuthSource::from_env().expect("env auth");
@@ -733,7 +853,7 @@ mod tests {
 
     #[test]
     fn auth_source_from_saved_oauth_when_env_absent() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         let config_home = temp_config_home();
         std::env::set_var("CLAUDE_CONFIG_HOME", &config_home);
         std::env::remove_var("ANTHROPIC_AUTH_TOKEN");
@@ -772,7 +892,7 @@ mod tests {
 
     #[test]
     fn resolve_saved_oauth_token_refreshes_expired_credentials() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         let config_home = temp_config_home();
         std::env::set_var("CLAUDE_CONFIG_HOME", &config_home);
         std::env::remove_var("ANTHROPIC_AUTH_TOKEN");
@@ -804,7 +924,7 @@ mod tests {
 
     #[test]
     fn resolve_startup_auth_source_uses_saved_oauth_without_loading_config() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         let config_home = temp_config_home();
         std::env::set_var("CLAUDE_CONFIG_HOME", &config_home);
         std::env::remove_var("ANTHROPIC_AUTH_TOKEN");
@@ -828,7 +948,7 @@ mod tests {
 
     #[test]
     fn resolve_startup_auth_source_errors_when_refreshable_token_lacks_config() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         let config_home = temp_config_home();
         std::env::set_var("CLAUDE_CONFIG_HOME", &config_home);
         std::env::remove_var("ANTHROPIC_AUTH_TOKEN");
@@ -860,7 +980,7 @@ mod tests {
 
     #[test]
     fn resolve_saved_oauth_token_preserves_refresh_token_when_refresh_response_omits_it() {
-        let _guard = env_lock();
+        let _guard = test_env_lock();
         let config_home = temp_config_home();
         std::env::set_var("CLAUDE_CONFIG_HOME", &config_home);
         std::env::remove_var("ANTHROPIC_AUTH_TOKEN");

rust/crates/api/src/lib.rs

@@ -1,5 +1,6 @@
 mod client;
 mod error;
+mod prompt_cache;
 mod sse;
 mod types;
 
@@ -8,6 +9,10 @@ pub use client::{
     AnthropicClient, AuthSource, MessageStream, OAuthTokenSet,
 };
 pub use error::ApiError;
+pub use prompt_cache::{
+    CacheBreakEvent, PromptCache, PromptCacheConfig, PromptCachePaths, PromptCacheRecord,
+    PromptCacheStats,
+};
 pub use sse::{parse_frame, SseParser};
 pub use types::{
     ContentBlockDelta, ContentBlockDeltaEvent, ContentBlockStartEvent, ContentBlockStopEvent,
@@ -15,3 +20,11 @@ pub use types::{
     MessageResponse, MessageStartEvent, MessageStopEvent, OutputContentBlock, StreamEvent,
     ToolChoice, ToolDefinition, ToolResultContentBlock, Usage,
 };
+
+#[cfg(test)]
+pub(crate) fn test_env_lock() -> std::sync::MutexGuard<'static, ()> {
+    static LOCK: std::sync::OnceLock<std::sync::Mutex<()>> = std::sync::OnceLock::new();
+    LOCK.get_or_init(|| std::sync::Mutex::new(()))
+        .lock()
+        .unwrap_or_else(std::sync::PoisonError::into_inner)
+}

rust/crates/api/src/prompt_cache.rs

@@ -0,0 +1,727 @@
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::sync::{Arc, Mutex};
+use std::time::{Duration, SystemTime, UNIX_EPOCH};
+
+use serde::{Deserialize, Serialize};
+
+use crate::types::{MessageRequest, MessageResponse, Usage};
+
+const DEFAULT_COMPLETION_TTL_SECS: u64 = 30;
+const DEFAULT_PROMPT_TTL_SECS: u64 = 5 * 60;
+const DEFAULT_BREAK_MIN_DROP: u32 = 2_000;
+const MAX_SANITIZED_LENGTH: usize = 80;
+const REQUEST_FINGERPRINT_VERSION: u32 = 1;
+const REQUEST_FINGERPRINT_PREFIX: &str = "v1";
+const FNV_OFFSET_BASIS: u64 = 0xcbf2_9ce4_8422_2325;
+const FNV_PRIME: u64 = 0x0000_0100_0000_01b3;
+
+#[derive(Debug, Clone)]
+pub struct PromptCacheConfig {
+    pub session_id: String,
+    pub completion_ttl: Duration,
+    pub prompt_ttl: Duration,
+    pub cache_break_min_drop: u32,
+}
+
+impl PromptCacheConfig {
+    #[must_use]
+    pub fn new(session_id: impl Into<String>) -> Self {
+        Self {
+            session_id: session_id.into(),
+            completion_ttl: Duration::from_secs(DEFAULT_COMPLETION_TTL_SECS),
+            prompt_ttl: Duration::from_secs(DEFAULT_PROMPT_TTL_SECS),
+            cache_break_min_drop: DEFAULT_BREAK_MIN_DROP,
+        }
+    }
+}
+
+impl Default for PromptCacheConfig {
+    fn default() -> Self {
+        Self::new("default")
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct PromptCachePaths {
+    pub root: PathBuf,
+    pub session_dir: PathBuf,
+    pub completion_dir: PathBuf,
+    pub session_state_path: PathBuf,
+    pub stats_path: PathBuf,
+}
+
+impl PromptCachePaths {
+    #[must_use]
+    pub fn for_session(session_id: &str) -> Self {
+        let root = base_cache_root();
+        let session_dir = root.join(sanitize_path_segment(session_id));
+        let completion_dir = session_dir.join("completions");
+        Self {
+            root,
+            session_state_path: session_dir.join("session-state.json"),
+            stats_path: session_dir.join("stats.json"),
+            session_dir,
+            completion_dir,
+        }
+    }
+
+    #[must_use]
+    pub fn completion_entry_path(&self, request_hash: &str) -> PathBuf {
+        self.completion_dir.join(format!("{request_hash}.json"))
+    }
+}
+
+#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
+pub struct PromptCacheStats {
+    pub tracked_requests: u64,
+    pub completion_cache_hits: u64,
+    pub completion_cache_misses: u64,
+    pub completion_cache_writes: u64,
+    pub expected_invalidations: u64,
+    pub unexpected_cache_breaks: u64,
+    pub total_cache_creation_input_tokens: u64,
+    pub total_cache_read_input_tokens: u64,
+    pub last_cache_creation_input_tokens: Option<u32>,
+    pub last_cache_read_input_tokens: Option<u32>,
+    pub last_request_hash: Option<String>,
+    pub last_completion_cache_key: Option<String>,
+    pub last_break_reason: Option<String>,
+    pub last_cache_source: Option<String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct CacheBreakEvent {
+    pub unexpected: bool,
+    pub reason: String,
+    pub previous_cache_read_input_tokens: u32,
+    pub current_cache_read_input_tokens: u32,
+    pub token_drop: u32,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PromptCacheRecord {
+    pub cache_break: Option<CacheBreakEvent>,
+    pub stats: PromptCacheStats,
+}
+
+#[derive(Debug, Clone)]
+pub struct PromptCache {
+    inner: Arc<Mutex<PromptCacheInner>>,
+}
+
+impl PromptCache {
+    #[must_use]
+    pub fn new(session_id: impl Into<String>) -> Self {
+        Self::with_config(PromptCacheConfig::new(session_id))
+    }
+
+    #[must_use]
+    pub fn with_config(config: PromptCacheConfig) -> Self {
+        let paths = PromptCachePaths::for_session(&config.session_id);
+        let stats = read_json::<PromptCacheStats>(&paths.stats_path).unwrap_or_default();
+        let previous = read_json::<TrackedPromptState>(&paths.session_state_path);
+        Self {
+            inner: Arc::new(Mutex::new(PromptCacheInner {
+                config,
+                paths,
+                stats,
+                previous,
+            })),
+        }
+    }
+
+    #[must_use]
+    pub fn paths(&self) -> PromptCachePaths {
+        self.lock().paths.clone()
+    }
+
+    #[must_use]
+    pub fn stats(&self) -> PromptCacheStats {
+        self.lock().stats.clone()
+    }
+
+    #[must_use]
+    pub fn lookup_completion(&self, request: &MessageRequest) -> Option<MessageResponse> {
+        let request_hash = request_hash_hex(request);
+        let (paths, ttl) = {
+            let inner = self.lock();
+            (inner.paths.clone(), inner.config.completion_ttl)
+        };
+        let entry_path = paths.completion_entry_path(&request_hash);
+        let entry = read_json::<CompletionCacheEntry>(&entry_path);
+        let Some(entry) = entry else {
+            let mut inner = self.lock();
+            inner.stats.completion_cache_misses += 1;
+            inner.stats.last_completion_cache_key = Some(request_hash);
+            persist_state(&inner);
+            return None;
+        };
+
+        if entry.fingerprint_version != current_fingerprint_version() {
+            let mut inner = self.lock();
+            inner.stats.completion_cache_misses += 1;
+            inner.stats.last_completion_cache_key = Some(request_hash.clone());
+            let _ = fs::remove_file(entry_path);
+            persist_state(&inner);
+            return None;
+        }
+
+        let expired = now_unix_secs().saturating_sub(entry.cached_at_unix_secs) >= ttl.as_secs();
+        let mut inner = self.lock();
+        inner.stats.last_completion_cache_key = Some(request_hash.clone());
+        if expired {
+            inner.stats.completion_cache_misses += 1;
+            let _ = fs::remove_file(entry_path);
+            persist_state(&inner);
+            return None;
+        }
+
+        inner.stats.completion_cache_hits += 1;
+        apply_usage_to_stats(
+            &mut inner.stats,
+            &entry.response.usage,
+            &request_hash,
+            "completion-cache",
+        );
+        inner.previous = Some(TrackedPromptState::from_usage(
+            request,
+            &entry.response.usage,
+        ));
+        persist_state(&inner);
+        Some(entry.response)
+    }
+
+    #[must_use]
+    pub fn record_response(
+        &self,
+        request: &MessageRequest,
+        response: &MessageResponse,
+    ) -> PromptCacheRecord {
+        self.record_usage_internal(request, &response.usage, Some(response))
+    }
+
+    #[must_use]
+    pub fn record_usage(&self, request: &MessageRequest, usage: &Usage) -> PromptCacheRecord {
+        self.record_usage_internal(request, usage, None)
+    }
+
+    fn record_usage_internal(
+        &self,
+        request: &MessageRequest,
+        usage: &Usage,
+        response: Option<&MessageResponse>,
+    ) -> PromptCacheRecord {
+        let request_hash = request_hash_hex(request);
+        let mut inner = self.lock();
+        let previous = inner.previous.clone();
+        let current = TrackedPromptState::from_usage(request, usage);
+        let cache_break = detect_cache_break(&inner.config, previous.as_ref(), &current);
+
+        inner.stats.tracked_requests += 1;
+        apply_usage_to_stats(&mut inner.stats, usage, &request_hash, "api-response");
+        if let Some(event) = &cache_break {
+            if event.unexpected {
+                inner.stats.unexpected_cache_breaks += 1;
+            } else {
+                inner.stats.expected_invalidations += 1;
+            }
+            inner.stats.last_break_reason = Some(event.reason.clone());
+        }
+
+        inner.previous = Some(current);
+        if let Some(response) = response {
+            write_completion_entry(&inner.paths, &request_hash, response);
+            inner.stats.completion_cache_writes += 1;
+        }
+        persist_state(&inner);
+
+        PromptCacheRecord {
+            cache_break,
+            stats: inner.stats.clone(),
+        }
+    }
+
+    fn lock(&self) -> std::sync::MutexGuard<'_, PromptCacheInner> {
+        self.inner
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner)
+    }
+}
+
+#[derive(Debug)]
+struct PromptCacheInner {
+    config: PromptCacheConfig,
+    paths: PromptCachePaths,
+    stats: PromptCacheStats,
+    previous: Option<TrackedPromptState>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+struct CompletionCacheEntry {
+    cached_at_unix_secs: u64,
+    #[serde(default = "current_fingerprint_version")]
+    fingerprint_version: u32,
+    response: MessageResponse,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+struct TrackedPromptState {
+    observed_at_unix_secs: u64,
+    #[serde(default = "current_fingerprint_version")]
+    fingerprint_version: u32,
+    model_hash: u64,
+    system_hash: u64,
+    tools_hash: u64,
+    messages_hash: u64,
+    cache_read_input_tokens: u32,
+}
+
+impl TrackedPromptState {
+    fn from_usage(request: &MessageRequest, usage: &Usage) -> Self {
+        let hashes = RequestFingerprints::from_request(request);
+        Self {
+            observed_at_unix_secs: now_unix_secs(),
+            fingerprint_version: current_fingerprint_version(),
+            model_hash: hashes.model,
+            system_hash: hashes.system,
+            tools_hash: hashes.tools,
+            messages_hash: hashes.messages,
+            cache_read_input_tokens: usage.cache_read_input_tokens,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+struct RequestFingerprints {
+    model: u64,
+    system: u64,
+    tools: u64,
+    messages: u64,
+}
+
+impl RequestFingerprints {
+    fn from_request(request: &MessageRequest) -> Self {
+        Self {
+            model: hash_serializable(&request.model),
+            system: hash_serializable(&request.system),
+            tools: hash_serializable(&request.tools),
+            messages: hash_serializable(&request.messages),
+        }
+    }
+}
+
+fn detect_cache_break(
+    config: &PromptCacheConfig,
+    previous: Option<&TrackedPromptState>,
+    current: &TrackedPromptState,
+) -> Option<CacheBreakEvent> {
+    let previous = previous?;
+    if previous.fingerprint_version != current.fingerprint_version {
+        return Some(CacheBreakEvent {
+            unexpected: false,
+            reason: format!(
+                "fingerprint version changed (v{} -> v{})",
+                previous.fingerprint_version, current.fingerprint_version
+            ),
+            previous_cache_read_input_tokens: previous.cache_read_input_tokens,
+            current_cache_read_input_tokens: current.cache_read_input_tokens,
+            token_drop: previous
+                .cache_read_input_tokens
+                .saturating_sub(current.cache_read_input_tokens),
+        });
+    }
+    let token_drop = previous
+        .cache_read_input_tokens
+        .saturating_sub(current.cache_read_input_tokens);
+    if token_drop < config.cache_break_min_drop {
+        return None;
+    }
+
+    let mut reasons = Vec::new();
+    if previous.model_hash != current.model_hash {
+        reasons.push("model changed");
+    }
+    if previous.system_hash != current.system_hash {
+        reasons.push("system prompt changed");
+    }
+    if previous.tools_hash != current.tools_hash {
+        reasons.push("tool definitions changed");
+    }
+    if previous.messages_hash != current.messages_hash {
+        reasons.push("message payload changed");
+    }
+
+    let elapsed = current
+        .observed_at_unix_secs
+        .saturating_sub(previous.observed_at_unix_secs);
+
+    let (unexpected, reason) = if reasons.is_empty() {
+        if elapsed > config.prompt_ttl.as_secs() {
+            (
+                false,
+                format!("possible prompt cache TTL expiry after {elapsed}s"),
+            )
+        } else {
+            (
+                true,
+                "cache read tokens dropped while prompt fingerprint remained stable".to_string(),
+            )
+        }
+    } else {
+        (false, reasons.join(", "))
+    };
+
+    Some(CacheBreakEvent {
+        unexpected,
+        reason,
+        previous_cache_read_input_tokens: previous.cache_read_input_tokens,
+        current_cache_read_input_tokens: current.cache_read_input_tokens,
+        token_drop,
+    })
+}
+
+fn apply_usage_to_stats(
+    stats: &mut PromptCacheStats,
+    usage: &Usage,
+    request_hash: &str,
+    source: &str,
+) {
+    stats.total_cache_creation_input_tokens += u64::from(usage.cache_creation_input_tokens);
+    stats.total_cache_read_input_tokens += u64::from(usage.cache_read_input_tokens);
+    stats.last_cache_creation_input_tokens = Some(usage.cache_creation_input_tokens);
+    stats.last_cache_read_input_tokens = Some(usage.cache_read_input_tokens);
+    stats.last_request_hash = Some(request_hash.to_string());
+    stats.last_cache_source = Some(source.to_string());
+}
+
+fn persist_state(inner: &PromptCacheInner) {
+    let _ = ensure_cache_dirs(&inner.paths);
+    let _ = write_json(&inner.paths.stats_path, &inner.stats);
+    if let Some(previous) = &inner.previous {
+        let _ = write_json(&inner.paths.session_state_path, previous);
+    }
+}
+
+fn write_completion_entry(
+    paths: &PromptCachePaths,
+    request_hash: &str,
+    response: &MessageResponse,
+) {
+    let _ = ensure_cache_dirs(paths);
+    let entry = CompletionCacheEntry {
+        cached_at_unix_secs: now_unix_secs(),
+        fingerprint_version: current_fingerprint_version(),
+        response: response.clone(),
+    };
+    let _ = write_json(&paths.completion_entry_path(request_hash), &entry);
+}
+
+fn ensure_cache_dirs(paths: &PromptCachePaths) -> std::io::Result<()> {
+    fs::create_dir_all(&paths.completion_dir)
+}
+
+fn write_json<T: Serialize>(path: &Path, value: &T) -> std::io::Result<()> {
+    let json = serde_json::to_vec_pretty(value)
+        .map_err(|error| std::io::Error::new(std::io::ErrorKind::InvalidData, error))?;
+    fs::write(path, json)
+}
+
+fn read_json<T: for<'de> Deserialize<'de>>(path: &Path) -> Option<T> {
+    let bytes = fs::read(path).ok()?;
+    serde_json::from_slice(&bytes).ok()
+}
+
+fn request_hash_hex(request: &MessageRequest) -> String {
+    format!(
+        "{REQUEST_FINGERPRINT_PREFIX}-{:016x}",
+        hash_serializable(request)
+    )
+}
+
+fn hash_serializable<T: Serialize>(value: &T) -> u64 {
+    let json = serde_json::to_vec(value).unwrap_or_default();
+    stable_hash_bytes(&json)
+}
+
+fn sanitize_path_segment(value: &str) -> String {
+    let sanitized: String = value
+        .chars()
+        .map(|ch| if ch.is_ascii_alphanumeric() { ch } else { '-' })
+        .collect();
+    if sanitized.len() <= MAX_SANITIZED_LENGTH {
+        return sanitized;
+    }
+    let suffix = format!("-{:x}", hash_string(value));
+    format!(
+        "{}{}",
+        &sanitized[..MAX_SANITIZED_LENGTH.saturating_sub(suffix.len())],
+        suffix
+    )
+}
+
+fn hash_string(value: &str) -> u64 {
+    stable_hash_bytes(value.as_bytes())
+}
+
+fn base_cache_root() -> PathBuf {
+    if let Some(config_home) = std::env::var_os("CLAUDE_CONFIG_HOME") {
+        return PathBuf::from(config_home)
+            .join("cache")
+            .join("prompt-cache");
+    }
+    if let Some(home) = std::env::var_os("HOME") {
+        return PathBuf::from(home)
+            .join(".claude")
+            .join("cache")
+            .join("prompt-cache");
+    }
+    std::env::temp_dir().join("claude-prompt-cache")
+}
+
+fn now_unix_secs() -> u64 {
+    SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .map_or(0, |duration| duration.as_secs())
+}
+
+const fn current_fingerprint_version() -> u32 {
+    REQUEST_FINGERPRINT_VERSION
+}
+
+fn stable_hash_bytes(bytes: &[u8]) -> u64 {
+    let mut hash = FNV_OFFSET_BASIS;
+    for byte in bytes {
+        hash ^= u64::from(*byte);
+        hash = hash.wrapping_mul(FNV_PRIME);
+    }
+    hash
+}
+
+#[cfg(test)]
+mod tests {
+    use std::time::{Duration, SystemTime, UNIX_EPOCH};
+
+    use super::{
+        detect_cache_break, read_json, request_hash_hex, sanitize_path_segment, PromptCache,
+        PromptCacheConfig, PromptCachePaths, TrackedPromptState, REQUEST_FINGERPRINT_PREFIX,
+    };
+    use crate::test_env_lock;
+    use crate::types::{InputMessage, MessageRequest, MessageResponse, OutputContentBlock, Usage};
+
+    #[test]
+    fn path_builder_sanitizes_session_identifier() {
+        let paths = PromptCachePaths::for_session("session:/with spaces");
+        let session_dir = paths
+            .session_dir
+            .file_name()
+            .and_then(|value| value.to_str())
+            .expect("session dir name");
+        assert_eq!(session_dir, "session--with-spaces");
+        assert!(paths.completion_dir.ends_with("completions"));
+        assert!(paths.stats_path.ends_with("stats.json"));
+        assert!(paths.session_state_path.ends_with("session-state.json"));
+    }
+
+    #[test]
+    fn request_fingerprint_drives_unexpected_break_detection() {
+        let request = sample_request("same");
+        let previous = TrackedPromptState::from_usage(
+            &request,
+            &Usage {
+                input_tokens: 0,
+                cache_creation_input_tokens: 0,
+                cache_read_input_tokens: 6_000,
+                output_tokens: 0,
+            },
+        );
+        let current = TrackedPromptState::from_usage(
+            &request,
+            &Usage {
+                input_tokens: 0,
+                cache_creation_input_tokens: 0,
+                cache_read_input_tokens: 1_000,
+                output_tokens: 0,
+            },
+        );
+        let event = detect_cache_break(&PromptCacheConfig::default(), Some(&previous), &current)
+            .expect("break should be detected");
+        assert!(event.unexpected);
+        assert!(event.reason.contains("stable"));
+    }
+
+    #[test]
+    fn changed_prompt_marks_break_as_expected() {
+        let previous_request = sample_request("first");
+        let current_request = sample_request("second");
+        let previous = TrackedPromptState::from_usage(
+            &previous_request,
+            &Usage {
+                input_tokens: 0,
+                cache_creation_input_tokens: 0,
+                cache_read_input_tokens: 6_000,
+                output_tokens: 0,
+            },
+        );
+        let current = TrackedPromptState::from_usage(
+            &current_request,
+            &Usage {
+                input_tokens: 0,
+                cache_creation_input_tokens: 0,
+                cache_read_input_tokens: 1_000,
+                output_tokens: 0,
+            },
+        );
+        let event = detect_cache_break(&PromptCacheConfig::default(), Some(&previous), &current)
+            .expect("break should be detected");
+        assert!(!event.unexpected);
+        assert!(event.reason.contains("message payload changed"));
+    }
+
+    #[test]
+    fn completion_cache_round_trip_persists_recent_response() {
+        let _guard = test_env_lock();
+        let temp_root = std::env::temp_dir().join(format!(
+            "prompt-cache-test-{}-{}",
+            std::process::id(),
+            SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .expect("time")
+                .as_nanos()
+        ));
+        std::env::set_var("CLAUDE_CONFIG_HOME", &temp_root);
+        let cache = PromptCache::new("unit-test-session");
+        let request = sample_request("cache me");
+        let response = sample_response(42, 12, "cached");
+
+        assert!(cache.lookup_completion(&request).is_none());
+        let record = cache.record_response(&request, &response);
+        assert!(record.cache_break.is_none());
+
+        let cached = cache
+            .lookup_completion(&request)
+            .expect("cached response should load");
+        assert_eq!(cached.content, response.content);
+
+        let stats = cache.stats();
+        assert_eq!(stats.completion_cache_hits, 1);
+        assert_eq!(stats.completion_cache_misses, 1);
+        assert_eq!(stats.completion_cache_writes, 1);
+
+        let persisted = read_json::<super::PromptCacheStats>(&cache.paths().stats_path)
+            .expect("stats should persist");
+        assert_eq!(persisted.completion_cache_hits, 1);
+
+        std::fs::remove_dir_all(temp_root).expect("cleanup temp root");
+        std::env::remove_var("CLAUDE_CONFIG_HOME");
+    }
+
+    #[test]
+    fn distinct_requests_do_not_collide_in_completion_cache() {
+        let _guard = test_env_lock();
+        let temp_root = std::env::temp_dir().join(format!(
+            "prompt-cache-distinct-{}-{}",
+            std::process::id(),
+            SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .expect("time")
+                .as_nanos()
+        ));
+        std::env::set_var("CLAUDE_CONFIG_HOME", &temp_root);
+        let cache = PromptCache::new("distinct-request-session");
+        let first_request = sample_request("first");
+        let second_request = sample_request("second");
+
+        let response = sample_response(42, 12, "cached");
+        let _ = cache.record_response(&first_request, &response);
+
+        assert!(cache.lookup_completion(&second_request).is_none());
+
+        std::fs::remove_dir_all(temp_root).expect("cleanup temp root");
+        std::env::remove_var("CLAUDE_CONFIG_HOME");
+    }
+
+    #[test]
+    fn expired_completion_entries_are_not_reused() {
+        let _guard = test_env_lock();
+        let temp_root = std::env::temp_dir().join(format!(
+            "prompt-cache-expired-{}-{}",
+            std::process::id(),
+            SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .expect("time")
+                .as_nanos()
+        ));
+        std::env::set_var("CLAUDE_CONFIG_HOME", &temp_root);
+        let cache = PromptCache::with_config(PromptCacheConfig {
+            session_id: "expired-session".to_string(),
+            completion_ttl: Duration::ZERO,
+            ..PromptCacheConfig::default()
+        });
+        let request = sample_request("expire me");
+        let response = sample_response(7, 3, "stale");
+
+        let _ = cache.record_response(&request, &response);
+
+        assert!(cache.lookup_completion(&request).is_none());
+        let stats = cache.stats();
+        assert_eq!(stats.completion_cache_hits, 0);
+        assert_eq!(stats.completion_cache_misses, 1);
+
+        std::fs::remove_dir_all(temp_root).expect("cleanup temp root");
+        std::env::remove_var("CLAUDE_CONFIG_HOME");
+    }
+
+    #[test]
+    fn sanitize_path_caps_long_values() {
+        let long_value = "x".repeat(200);
+        let sanitized = sanitize_path_segment(&long_value);
+        assert!(sanitized.len() <= 80);
+    }
+
+    #[test]
+    fn request_hashes_are_versioned_and_stable() {
+        let request = sample_request("stable");
+        let first = request_hash_hex(&request);
+        let second = request_hash_hex(&request);
+        assert_eq!(first, second);
+        assert!(first.starts_with(REQUEST_FINGERPRINT_PREFIX));
+    }
+
+    fn sample_request(text: &str) -> MessageRequest {
+        MessageRequest {
+            model: "claude-3-7-sonnet-latest".to_string(),
+            max_tokens: 64,
+            messages: vec![InputMessage::user_text(text)],
+            system: Some("system".to_string()),
+            tools: None,
+            tool_choice: None,
+            stream: false,
+        }
+    }
+
+    fn sample_response(
+        cache_read_input_tokens: u32,
+        output_tokens: u32,
+        text: &str,
+    ) -> MessageResponse {
+        MessageResponse {
+            id: "msg_test".to_string(),
+            kind: "message".to_string(),
+            role: "assistant".to_string(),
+            content: vec![OutputContentBlock::Text {
+                text: text.to_string(),
+            }],
+            model: "claude-3-7-sonnet-latest".to_string(),
+            stop_reason: Some("end_turn".to_string()),
+            stop_sequence: None,
+            usage: Usage {
+                input_tokens: 10,
+                cache_creation_input_tokens: 5,
+                cache_read_input_tokens,
+                output_tokens,
+            },
+            request_id: Some("req_test".to_string()),
+        }
+    }
+}

rust/crates/api/tests/client_integration.rs

@@ -1,17 +1,25 @@
 use std::collections::HashMap;
 use std::sync::Arc;
+use std::sync::{Mutex as StdMutex, OnceLock};
 use std::time::Duration;
 
 use api::{
     AnthropicClient, ApiError, ContentBlockDelta, ContentBlockDeltaEvent, ContentBlockStartEvent,
     InputContentBlock, InputMessage, MessageDeltaEvent, MessageRequest, OutputContentBlock,
-    StreamEvent, ToolChoice, ToolDefinition,
+    PromptCache, StreamEvent, ToolChoice, ToolDefinition,
 };
 use serde_json::json;
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 use tokio::net::TcpListener;
 use tokio::sync::Mutex;
 
+fn env_lock() -> std::sync::MutexGuard<'static, ()> {
+    static LOCK: OnceLock<StdMutex<()>> = OnceLock::new();
+    LOCK.get_or_init(|| StdMutex::new(()))
+        .lock()
+        .unwrap_or_else(std::sync::PoisonError::into_inner)
+}
+
 #[tokio::test]
 async fn send_message_posts_json_and_parses_response() {
     let state = Arc::new(Mutex::new(Vec::<CapturedRequest>::new()));
@@ -45,6 +53,8 @@ async fn send_message_posts_json_and_parses_response() {
     assert_eq!(response.id, "msg_test");
     assert_eq!(response.total_tokens(), 16);
     assert_eq!(response.request_id.as_deref(), Some("req_body_123"));
+    assert_eq!(response.usage.cache_creation_input_tokens, 0);
+    assert_eq!(response.usage.cache_read_input_tokens, 0);
     assert_eq!(
         response.content,
         vec![OutputContentBlock::Text {
@@ -76,11 +86,55 @@ async fn send_message_posts_json_and_parses_response() {
 }
 
 #[tokio::test]
+async fn send_message_parses_prompt_cache_token_usage_from_response() {
+    let state = Arc::new(Mutex::new(Vec::<CapturedRequest>::new()));
+    let body = concat!(
+        "{",
+        "\"id\":\"msg_cache_tokens\",",
+        "\"type\":\"message\",",
+        "\"role\":\"assistant\",",
+        "\"content\":[{\"type\":\"text\",\"text\":\"Cache tokens\"}],",
+        "\"model\":\"claude-3-7-sonnet-latest\",",
+        "\"stop_reason\":\"end_turn\",",
+        "\"stop_sequence\":null,",
+        "\"usage\":{\"input_tokens\":12,\"cache_creation_input_tokens\":321,\"cache_read_input_tokens\":654,\"output_tokens\":4}",
+        "}"
+    );
+    let server = spawn_server(
+        state,
+        vec![http_response("200 OK", "application/json", body)],
+    )
+    .await;
+
+    let client = AnthropicClient::new("test-key").with_base_url(server.base_url());
+    let response = client
+        .send_message(&sample_request(false))
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(response.usage.input_tokens, 12);
+    assert_eq!(response.usage.cache_creation_input_tokens, 321);
+    assert_eq!(response.usage.cache_read_input_tokens, 654);
+    assert_eq!(response.usage.output_tokens, 4);
+}
+
+#[tokio::test]
+#[allow(clippy::await_holding_lock)]
 async fn stream_message_parses_sse_events_with_tool_use() {
+    let _guard = env_lock();
+    let temp_root = std::env::temp_dir().join(format!(
+        "api-stream-cache-{}-{}",
+        std::process::id(),
+        std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .expect("time")
+            .as_nanos()
+    ));
+    std::env::set_var("CLAUDE_CONFIG_HOME", &temp_root);
     let state = Arc::new(Mutex::new(Vec::<CapturedRequest>::new()));
     let sse = concat!(
         "event: message_start\n",
-        "data: {\"type\":\"message_start\",\"message\":{\"id\":\"msg_stream\",\"type\":\"message\",\"role\":\"assistant\",\"content\":[],\"model\":\"claude-3-7-sonnet-latest\",\"stop_reason\":null,\"stop_sequence\":null,\"usage\":{\"input_tokens\":8,\"output_tokens\":0}}}\n\n",
+        "data: {\"type\":\"message_start\",\"message\":{\"id\":\"msg_stream\",\"type\":\"message\",\"role\":\"assistant\",\"content\":[],\"model\":\"claude-3-7-sonnet-latest\",\"stop_reason\":null,\"stop_sequence\":null,\"usage\":{\"input_tokens\":8,\"cache_creation_input_tokens\":13,\"cache_read_input_tokens\":21,\"output_tokens\":0}}}\n\n",
         "event: content_block_start\n",
         "data: {\"type\":\"content_block_start\",\"index\":0,\"content_block\":{\"type\":\"tool_use\",\"id\":\"toolu_123\",\"name\":\"get_weather\",\"input\":{}}}\n\n",
         "event: content_block_delta\n",
@@ -88,7 +142,7 @@ async fn stream_message_parses_sse_events_with_tool_use() {
         "event: content_block_stop\n",
         "data: {\"type\":\"content_block_stop\",\"index\":0}\n\n",
         "event: message_delta\n",
-        "data: {\"type\":\"message_delta\",\"delta\":{\"stop_reason\":\"tool_use\",\"stop_sequence\":null},\"usage\":{\"input_tokens\":8,\"output_tokens\":1}}\n\n",
+        "data: {\"type\":\"message_delta\",\"delta\":{\"stop_reason\":\"tool_use\",\"stop_sequence\":null},\"usage\":{\"input_tokens\":8,\"cache_creation_input_tokens\":34,\"cache_read_input_tokens\":55,\"output_tokens\":1}}\n\n",
         "event: message_stop\n",
         "data: {\"type\":\"message_stop\"}\n\n",
         "data: [DONE]\n\n"
@@ -106,7 +160,8 @@ async fn stream_message_parses_sse_events_with_tool_use() {
 
     let client = AnthropicClient::new("test-key")
         .with_auth_token(Some("proxy-token".to_string()))
-        .with_base_url(server.base_url());
+        .with_base_url(server.base_url())
+        .with_prompt_cache(PromptCache::new("stream-session"));
     let mut stream = client
         .stream_message(&sample_request(false))
         .await
@@ -160,6 +215,20 @@ async fn stream_message_parses_sse_events_with_tool_use() {
     let captured = state.lock().await;
     let request = captured.first().expect("server should capture request");
     assert!(request.body.contains("\"stream\":true"));
+
+    let cache_stats = client
+        .prompt_cache_stats()
+        .expect("prompt cache stats should exist");
+    assert_eq!(cache_stats.tracked_requests, 1);
+    assert_eq!(cache_stats.last_cache_creation_input_tokens, Some(34));
+    assert_eq!(cache_stats.last_cache_read_input_tokens, Some(55));
+    assert_eq!(
+        cache_stats.last_cache_source.as_deref(),
+        Some("api-response")
+    );
+
+    std::fs::remove_dir_all(temp_root).expect("cleanup temp root");
+    std::env::remove_var("CLAUDE_CONFIG_HOME");
 }
 
 #[tokio::test]
@@ -243,6 +312,121 @@ async fn surfaces_retry_exhaustion_for_persistent_retryable_errors() {
     }
 }
 
+#[tokio::test]
+#[allow(clippy::await_holding_lock)]
+async fn send_message_reuses_recent_completion_cache_entries() {
+    let _guard = env_lock();
+    let temp_root = std::env::temp_dir().join(format!(
+        "api-prompt-cache-{}-{}",
+        std::process::id(),
+        std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .expect("time")
+            .as_nanos()
+    ));
+    std::env::set_var("CLAUDE_CONFIG_HOME", &temp_root);
+
+    let state = Arc::new(Mutex::new(Vec::<CapturedRequest>::new()));
+    let server = spawn_server(
+        state.clone(),
+        vec![http_response(
+            "200 OK",
+            "application/json",
+            "{\"id\":\"msg_cached\",\"type\":\"message\",\"role\":\"assistant\",\"content\":[{\"type\":\"text\",\"text\":\"Cached once\"}],\"model\":\"claude-3-7-sonnet-latest\",\"stop_reason\":\"end_turn\",\"stop_sequence\":null,\"usage\":{\"input_tokens\":3,\"cache_creation_input_tokens\":5,\"cache_read_input_tokens\":4000,\"output_tokens\":2}}",
+        )],
+    )
+    .await;
+
+    let client = AnthropicClient::new("test-key")
+        .with_base_url(server.base_url())
+        .with_prompt_cache(PromptCache::new("integration-session"));
+
+    let first = client
+        .send_message(&sample_request(false))
+        .await
+        .expect("first request should succeed");
+    let second = client
+        .send_message(&sample_request(false))
+        .await
+        .expect("second request should reuse cache");
+
+    assert_eq!(first.content, second.content);
+    assert_eq!(state.lock().await.len(), 1);
+
+    let cache_stats = client
+        .prompt_cache_stats()
+        .expect("prompt cache stats should exist");
+    assert_eq!(cache_stats.completion_cache_hits, 1);
+    assert_eq!(cache_stats.completion_cache_misses, 1);
+    assert_eq!(cache_stats.completion_cache_writes, 1);
+
+    std::fs::remove_dir_all(temp_root).expect("cleanup temp root");
+    std::env::remove_var("CLAUDE_CONFIG_HOME");
+}
+
+#[tokio::test]
+#[allow(clippy::await_holding_lock)]
+async fn send_message_tracks_unexpected_prompt_cache_breaks() {
+    let _guard = env_lock();
+    let temp_root = std::env::temp_dir().join(format!(
+        "api-prompt-break-{}-{}",
+        std::process::id(),
+        std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .expect("time")
+            .as_nanos()
+    ));
+    std::env::set_var("CLAUDE_CONFIG_HOME", &temp_root);
+
+    let state = Arc::new(Mutex::new(Vec::<CapturedRequest>::new()));
+    let server = spawn_server(
+        state,
+        vec![
+            http_response(
+                "200 OK",
+                "application/json",
+                "{\"id\":\"msg_one\",\"type\":\"message\",\"role\":\"assistant\",\"content\":[{\"type\":\"text\",\"text\":\"One\"}],\"model\":\"claude-3-7-sonnet-latest\",\"stop_reason\":\"end_turn\",\"stop_sequence\":null,\"usage\":{\"input_tokens\":3,\"cache_creation_input_tokens\":5,\"cache_read_input_tokens\":6000,\"output_tokens\":2}}",
+            ),
+            http_response(
+                "200 OK",
+                "application/json",
+                "{\"id\":\"msg_two\",\"type\":\"message\",\"role\":\"assistant\",\"content\":[{\"type\":\"text\",\"text\":\"Two\"}],\"model\":\"claude-3-7-sonnet-latest\",\"stop_reason\":\"end_turn\",\"stop_sequence\":null,\"usage\":{\"input_tokens\":3,\"cache_creation_input_tokens\":0,\"cache_read_input_tokens\":1000,\"output_tokens\":2}}",
+            ),
+        ],
+    )
+    .await;
+
+    let request = sample_request(false);
+    let client = AnthropicClient::new("test-key")
+        .with_base_url(server.base_url())
+        .with_prompt_cache(PromptCache::with_config(api::PromptCacheConfig {
+            session_id: "break-session".to_string(),
+            completion_ttl: Duration::from_secs(0),
+            ..api::PromptCacheConfig::default()
+        }));
+
+    client
+        .send_message(&request)
+        .await
+        .expect("first response should succeed");
+    client
+        .send_message(&request)
+        .await
+        .expect("second response should succeed");
+
+    let cache_stats = client
+        .prompt_cache_stats()
+        .expect("prompt cache stats should exist");
+    assert_eq!(cache_stats.unexpected_cache_breaks, 1);
+    assert_eq!(
+        cache_stats.last_break_reason.as_deref(),
+        Some("cache read tokens dropped while prompt fingerprint remained stable")
+    );
+
+    std::fs::remove_dir_all(temp_root).expect("cleanup temp root");
+    std::env::remove_var("CLAUDE_CONFIG_HOME");
+}
+
 #[tokio::test]
 #[ignore = "requires ANTHROPIC_API_KEY and network access"]
 async fn live_stream_smoke_test() {

rust/crates/commands/src/lib.rs

@@ -125,8 +125,8 @@ const SLASH_COMMAND_SPECS: &[SlashCommandSpec] = &[
     },
     SlashCommandSpec {
         name: "session",
-        summary: "List, switch, or fork managed local sessions",
+        summary: "List or switch managed local sessions",
-        argument_hint: Some("[list|switch <session-id>|fork [branch-name]]"),
+        argument_hint: Some("[list|switch <session-id>]"),
         resume_supported: false,
     },
 ];
@@ -229,7 +229,7 @@ pub fn resume_supported_slash_commands() -> Vec<&'static SlashCommandSpec> {
 pub fn render_slash_command_help() -> String {
     let mut lines = vec![
         "Slash commands".to_string(),
-        "  [resume] means the command also works with --resume SESSION.jsonl".to_string(),
+        "  [resume] means the command also works with --resume SESSION.json".to_string(),
     ];
     for spec in slash_command_specs() {
         let name = match spec.argument_hint {
@@ -365,19 +365,12 @@ mod tests {
                 target: Some("abc123".to_string())
             })
         );
-        assert_eq!(
-            SlashCommand::parse("/session fork incident-review"),
-            Some(SlashCommand::Session {
-                action: Some("fork".to_string()),
-                target: Some("incident-review".to_string())
-            })
-        );
     }
 
     #[test]
     fn renders_help_from_shared_specs() {
         let help = render_slash_command_help();
-        assert!(help.contains("works with --resume SESSION.jsonl"));
+        assert!(help.contains("works with --resume SESSION.json"));
         assert!(help.contains("/help"));
         assert!(help.contains("/status"));
         assert!(help.contains("/compact"));
@@ -392,24 +385,26 @@ mod tests {
         assert!(help.contains("/diff"));
         assert!(help.contains("/version"));
         assert!(help.contains("/export [file]"));
-        assert!(help.contains("/session [list|switch <session-id>|fork [branch-name]]"));
+        assert!(help.contains("/session [list|switch <session-id>]"));
         assert_eq!(slash_command_specs().len(), 15);
         assert_eq!(resume_supported_slash_commands().len(), 11);
     }
 
     #[test]
     fn compacts_sessions_via_slash_command() {
-        let mut session = Session::new();
+        let session = Session {
-        session.messages = vec![
+            version: 1,
-            ConversationMessage::user_text("a ".repeat(200)),
+            messages: vec![
-            ConversationMessage::assistant(vec![ContentBlock::Text {
+                ConversationMessage::user_text("a ".repeat(200)),
-                text: "b ".repeat(200),
+                ConversationMessage::assistant(vec![ContentBlock::Text {
-            }]),
+                    text: "b ".repeat(200),
-            ConversationMessage::tool_result("1", "bash", "ok ".repeat(200), false),
+                }]),
-            ConversationMessage::assistant(vec![ContentBlock::Text {
+                ConversationMessage::tool_result("1", "bash", "ok ".repeat(200), false),
-                text: "recent".to_string(),
+                ConversationMessage::assistant(vec![ContentBlock::Text {
-            }]),
+                    text: "recent".to_string(),
-        ];
+                }]),
+            ],
+        };
 
         let result = handle_slash_command(
             "/compact",
@@ -460,12 +455,6 @@ mod tests {
             CompactionConfig::default()
         )
         .is_none());
-        assert!(handle_slash_command(
-            "/resume session.jsonl",
-            &session,
-            CompactionConfig::default()
-        )
-        .is_none());
         assert!(handle_slash_command("/config", &session, CompactionConfig::default()).is_none());
         assert!(
             handle_slash_command("/config env", &session, CompactionConfig::default()).is_none()

rust/crates/runtime/src/compact.rs

@@ -99,14 +99,13 @@ pub fn compact_session(session: &Session, config: CompactionConfig) -> Compactio
     }];
     compacted_messages.extend(preserved);
 
-    let mut compacted_session = session.clone();
-    compacted_session.messages = compacted_messages;
-    compacted_session.record_compaction(summary.clone(), removed.len());
-
     CompactionResult {
         summary,
         formatted_summary,
-        compacted_session,
+        compacted_session: Session {
+            version: session.version,
+            messages: compacted_messages,
+        },
         removed_message_count: removed.len(),
     }
 }
@@ -391,8 +390,10 @@ mod tests {
 
     #[test]
     fn leaves_small_sessions_unchanged() {
-        let mut session = Session::new();
+        let session = Session {
-        session.messages = vec![ConversationMessage::user_text("hello")];
+            version: 1,
+            messages: vec![ConversationMessage::user_text("hello")],
+        };
 
         let result = compact_session(&session, CompactionConfig::default());
         assert_eq!(result.removed_message_count, 0);
@@ -403,21 +404,23 @@ mod tests {
 
     #[test]
     fn compacts_older_messages_into_a_system_summary() {
-        let mut session = Session::new();
+        let session = Session {
-        session.messages = vec![
+            version: 1,
-            ConversationMessage::user_text("one ".repeat(200)),
+            messages: vec![
-            ConversationMessage::assistant(vec![ContentBlock::Text {
+                ConversationMessage::user_text("one ".repeat(200)),
-                text: "two ".repeat(200),
+                ConversationMessage::assistant(vec![ContentBlock::Text {
-            }]),
+                    text: "two ".repeat(200),
-            ConversationMessage::tool_result("1", "bash", "ok ".repeat(200), false),
+                }]),
-            ConversationMessage {
+                ConversationMessage::tool_result("1", "bash", "ok ".repeat(200), false),
-                role: MessageRole::Assistant,
+                ConversationMessage {
-                blocks: vec![ContentBlock::Text {
+                    role: MessageRole::Assistant,
-                    text: "recent".to_string(),
+                    blocks: vec![ContentBlock::Text {
-                }],
+                        text: "recent".to_string(),
-                usage: None,
+                    }],
-            },
+                    usage: None,
-        ];
+                },
+            ],
+        };
 
         let result = compact_session(
             &session,

rust/crates/runtime/src/conversation.rs

@@ -25,9 +25,19 @@ pub enum AssistantEvent {
         input: String,
     },
     Usage(TokenUsage),
+    PromptCache(PromptCacheEvent),
     MessageStop,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PromptCacheEvent {
+    pub unexpected: bool,
+    pub reason: String,
+    pub previous_cache_read_input_tokens: u32,
+    pub current_cache_read_input_tokens: u32,
+    pub token_drop: u32,
+}
+
 pub trait ApiClient {
     fn stream(&mut self, request: ApiRequest) -> Result<Vec<AssistantEvent>, RuntimeError>;
 }
@@ -84,6 +94,7 @@ impl std::error::Error for RuntimeError {}
 pub struct TurnSummary {
     pub assistant_messages: Vec<ConversationMessage>,
     pub tool_results: Vec<ConversationMessage>,
+    pub prompt_cache_events: Vec<PromptCacheEvent>,
     pub iterations: usize,
     pub usage: TokenUsage,
 }
@@ -118,7 +129,7 @@ where
             tool_executor,
             permission_policy,
             system_prompt,
-            RuntimeFeatureConfig::default(),
+            &RuntimeFeatureConfig::default(),
         )
     }
 
@@ -129,7 +140,7 @@ where
         tool_executor: T,
         permission_policy: PermissionPolicy,
         system_prompt: Vec<String>,
-        feature_config: RuntimeFeatureConfig,
+        feature_config: &RuntimeFeatureConfig,
     ) -> Self {
         let usage_tracker = UsageTracker::from_session(&session);
         Self {
@@ -140,7 +151,7 @@ where
             system_prompt,
             max_iterations: usize::MAX,
             usage_tracker,
-            hook_runner: HookRunner::from_feature_config(&feature_config),
+            hook_runner: HookRunner::from_feature_config(feature_config),
         }
     }
 
@@ -156,11 +167,12 @@ where
         mut prompter: Option<&mut dyn PermissionPrompter>,
     ) -> Result<TurnSummary, RuntimeError> {
         self.session
-            .push_user_text(user_input.into())
+            .messages
-            .map_err(|error| RuntimeError::new(error.to_string()))?;
+            .push(ConversationMessage::user_text(user_input.into()));
 
         let mut assistant_messages = Vec::new();
         let mut tool_results = Vec::new();
+        let mut prompt_cache_events = Vec::new();
         let mut iterations = 0;
 
         loop {
@@ -176,10 +188,12 @@ where
                 messages: self.session.messages.clone(),
             };
             let events = self.api_client.stream(request)?;
-            let (assistant_message, usage) = build_assistant_message(events)?;
+            let (assistant_message, usage, turn_prompt_cache_events) =
+                build_assistant_message(events)?;
             if let Some(usage) = usage {
                 self.usage_tracker.record(usage);
             }
+            prompt_cache_events.extend(turn_prompt_cache_events);
             let pending_tool_uses = assistant_message
                 .blocks
                 .iter()
@@ -191,9 +205,7 @@ where
                 })
                 .collect::<Vec<_>>();
 
-            self.session
+            self.session.messages.push(assistant_message.clone());
-                .push_message(assistant_message.clone())
-                .map_err(|error| RuntimeError::new(error.to_string()))?;
             assistant_messages.push(assistant_message);
 
             if pending_tool_uses.is_empty() {
@@ -251,9 +263,7 @@ where
                         ConversationMessage::tool_result(tool_use_id, tool_name, reason, true)
                     }
                 };
-                self.session
+                self.session.messages.push(result_message.clone());
-                    .push_message(result_message.clone())
-                    .map_err(|error| RuntimeError::new(error.to_string()))?;
                 tool_results.push(result_message);
             }
         }
@@ -261,6 +271,7 @@ where
         Ok(TurnSummary {
             assistant_messages,
             tool_results,
+            prompt_cache_events,
             iterations,
             usage: self.usage_tracker.cumulative_usage(),
         })
@@ -286,11 +297,6 @@ where
         &self.session
     }
 
-    #[must_use]
-    pub fn fork_session(&self, branch_name: Option<String>) -> Session {
-        self.session.fork(branch_name)
-    }
-
     #[must_use]
     pub fn into_session(self) -> Session {
         self.session
@@ -299,9 +305,17 @@ where
 
 fn build_assistant_message(
     events: Vec<AssistantEvent>,
-) -> Result<(ConversationMessage, Option<TokenUsage>), RuntimeError> {
+) -> Result<
+    (
+        ConversationMessage,
+        Option<TokenUsage>,
+        Vec<PromptCacheEvent>,
+    ),
+    RuntimeError,
+> {
     let mut text = String::new();
     let mut blocks = Vec::new();
+    let mut prompt_cache_events = Vec::new();
     let mut finished = false;
     let mut usage = None;
 
@@ -313,6 +327,7 @@ fn build_assistant_message(
                 blocks.push(ContentBlock::ToolUse { id, name, input });
             }
             AssistantEvent::Usage(value) => usage = Some(value),
+            AssistantEvent::PromptCache(event) => prompt_cache_events.push(event),
             AssistantEvent::MessageStop => {
                 finished = true;
             }
@@ -333,6 +348,7 @@ fn build_assistant_message(
     Ok((
         ConversationMessage::assistant_with_usage(blocks, usage),
         usage,
+        prompt_cache_events,
     ))
 }
 
@@ -405,7 +421,7 @@ impl ToolExecutor for StaticToolExecutor {
 #[cfg(test)]
 mod tests {
     use super::{
-        ApiClient, ApiRequest, AssistantEvent, ConversationRuntime, RuntimeError,
+        ApiClient, ApiRequest, AssistantEvent, ConversationRuntime, PromptCacheEvent, RuntimeError,
         StaticToolExecutor,
     };
     use crate::compact::CompactionConfig;
@@ -417,9 +433,7 @@ mod tests {
     use crate::prompt::{ProjectContext, SystemPromptBuilder};
     use crate::session::{ContentBlock, MessageRole, Session};
     use crate::usage::TokenUsage;
-    use std::fs;
     use std::path::PathBuf;
-    use std::time::{SystemTime, UNIX_EPOCH};
 
     struct ScriptedApiClient {
         call_count: usize,
@@ -464,6 +478,15 @@ mod tests {
                             cache_creation_input_tokens: 1,
                             cache_read_input_tokens: 3,
                         }),
+                        AssistantEvent::PromptCache(PromptCacheEvent {
+                            unexpected: true,
+                            reason:
+                                "cache read tokens dropped while prompt fingerprint remained stable"
+                                    .to_string(),
+                            previous_cache_read_input_tokens: 6_000,
+                            current_cache_read_input_tokens: 1_000,
+                            token_drop: 5_000,
+                        }),
                         AssistantEvent::MessageStop,
                     ])
                 }
@@ -517,8 +540,10 @@ mod tests {
         assert_eq!(summary.iterations, 2);
         assert_eq!(summary.assistant_messages.len(), 2);
         assert_eq!(summary.tool_results.len(), 1);
+        assert_eq!(summary.prompt_cache_events.len(), 1);
         assert_eq!(runtime.session().messages.len(), 4);
         assert_eq!(summary.usage.output_tokens, 10);
+        assert!(summary.prompt_cache_events[0].unexpected);
         assert!(matches!(
             runtime.session().messages[1].blocks[1],
             ContentBlock::ToolUse { .. }
@@ -620,7 +645,7 @@ mod tests {
             }),
             PermissionPolicy::new(PermissionMode::DangerFullAccess),
             vec!["system".to_string()],
-            RuntimeFeatureConfig::default().with_hooks(RuntimeHookConfig::new(
+            &RuntimeFeatureConfig::default().with_hooks(RuntimeHookConfig::new(
                 vec![shell_snippet("printf 'blocked by hook'; exit 2")],
                 Vec::new(),
             )),
@@ -686,7 +711,7 @@ mod tests {
             StaticToolExecutor::new().register("add", |_input| Ok("4".to_string())),
             PermissionPolicy::new(PermissionMode::DangerFullAccess),
             vec!["system".to_string()],
-            RuntimeFeatureConfig::default().with_hooks(RuntimeHookConfig::new(
+            &RuntimeFeatureConfig::default().with_hooks(RuntimeHookConfig::new(
                 vec![shell_snippet("printf 'pre hook ran'")],
                 vec![shell_snippet("printf 'post hook ran'")],
             )),
@@ -708,7 +733,7 @@ mod tests {
             "post hook should preserve non-error result: {output:?}"
         );
         assert!(
-            output.contains("4"),
+            output.contains('4'),
             "tool output missing value: {output:?}"
         );
         assert!(
@@ -798,86 +823,6 @@ mod tests {
             result.compacted_session.messages[0].role,
             MessageRole::System
         );
-        assert_eq!(
-            result.compacted_session.session_id,
-            runtime.session().session_id
-        );
-        assert!(result.compacted_session.compaction.is_some());
-    }
-
-    #[test]
-    fn persists_conversation_turn_messages_to_jsonl_session() {
-        struct SimpleApi;
-        impl ApiClient for SimpleApi {
-            fn stream(
-                &mut self,
-                _request: ApiRequest,
-            ) -> Result<Vec<AssistantEvent>, RuntimeError> {
-                Ok(vec![
-                    AssistantEvent::TextDelta("done".to_string()),
-                    AssistantEvent::MessageStop,
-                ])
-            }
-        }
-
-        let path = temp_session_path("persisted-turn");
-        let session = Session::new().with_persistence_path(path.clone());
-        let mut runtime = ConversationRuntime::new(
-            session,
-            SimpleApi,
-            StaticToolExecutor::new(),
-            PermissionPolicy::new(PermissionMode::DangerFullAccess),
-            vec!["system".to_string()],
-        );
-
-        runtime
-            .run_turn("persist this turn", None)
-            .expect("turn should succeed");
-
-        let restored = Session::load_from_path(&path).expect("persisted session should reload");
-        fs::remove_file(&path).expect("temp session file should be removable");
-
-        assert_eq!(restored.messages.len(), 2);
-        assert_eq!(restored.messages[0].role, MessageRole::User);
-        assert_eq!(restored.messages[1].role, MessageRole::Assistant);
-        assert_eq!(restored.session_id, runtime.session().session_id);
-    }
-
-    #[test]
-    fn forks_runtime_session_without_mutating_original() {
-        let mut session = Session::new();
-        session
-            .push_user_text("branch me")
-            .expect("message should append");
-
-        let runtime = ConversationRuntime::new(
-            session.clone(),
-            ScriptedApiClient { call_count: 0 },
-            StaticToolExecutor::new(),
-            PermissionPolicy::new(PermissionMode::DangerFullAccess),
-            vec!["system".to_string()],
-        );
-
-        let forked = runtime.fork_session(Some("alt-path".to_string()));
-
-        assert_eq!(forked.messages, session.messages);
-        assert_ne!(forked.session_id, session.session_id);
-        assert_eq!(
-            forked
-                .fork
-                .as_ref()
-                .map(|fork| (fork.parent_session_id.as_str(), fork.branch_name.as_deref())),
-            Some((session.session_id.as_str(), Some("alt-path")))
-        );
-        assert!(runtime.session().fork.is_none());
-    }
-
-    fn temp_session_path(label: &str) -> PathBuf {
-        let nanos = SystemTime::now()
-            .duration_since(UNIX_EPOCH)
-            .expect("system time should be after epoch")
-            .as_nanos();
-        std::env::temp_dir().join(format!("runtime-conversation-{label}-{nanos}.json"))
     }
 
     #[cfg(windows)]

rust/crates/runtime/src/hooks.rs

@@ -64,7 +64,7 @@ impl HookRunner {
 
     #[must_use]
     pub fn run_pre_tool_use(&self, tool_name: &str, tool_input: &str) -> HookRunResult {
-        self.run_commands(
+        Self::run_commands(
             HookEvent::PreToolUse,
             self.config.pre_tool_use(),
             tool_name,
@@ -82,7 +82,7 @@ impl HookRunner {
         tool_output: &str,
         is_error: bool,
     ) -> HookRunResult {
-        self.run_commands(
+        Self::run_commands(
             HookEvent::PostToolUse,
             self.config.post_tool_use(),
             tool_name,
@@ -93,7 +93,6 @@ impl HookRunner {
     }
 
     fn run_commands(
-        &self,
         event: HookEvent,
         commands: &[String],
         tool_name: &str,
@@ -118,7 +117,7 @@ impl HookRunner {
         let mut messages = Vec::new();
 
         for command in commands {
-            match self.run_command(
+            match Self::run_command(
                 command,
                 event,
                 tool_name,
@@ -150,7 +149,6 @@ impl HookRunner {
     }
 
     fn run_command(
-        &self,
         command: &str,
         event: HookEvent,
         tool_name: &str,

rust/crates/runtime/src/lib.rs

@@ -31,8 +31,8 @@ pub use config::{
     ScopedMcpServerConfig, CLAUDE_CODE_SETTINGS_SCHEMA_NAME,
 };
 pub use conversation::{
-    ApiClient, ApiRequest, AssistantEvent, ConversationRuntime, RuntimeError, StaticToolExecutor,
+    ApiClient, ApiRequest, AssistantEvent, ConversationRuntime, PromptCacheEvent, RuntimeError,
-    ToolError, ToolExecutor, TurnSummary,
+    StaticToolExecutor, ToolError, ToolExecutor, TurnSummary,
 };
 pub use file_ops::{
     edit_file, glob_search, grep_search, read_file, write_file, EditFileOutput, GlobSearchOutput,
@@ -76,10 +76,7 @@ pub use remote::{
     RemoteSessionContext, UpstreamProxyBootstrap, UpstreamProxyState, DEFAULT_REMOTE_BASE_URL,
     DEFAULT_SESSION_TOKEN_PATH, DEFAULT_SYSTEM_CA_BUNDLE, NO_PROXY_HOSTS, UPSTREAM_PROXY_ENV_KEYS,
 };
-pub use session::{
+pub use session::{ContentBlock, ConversationMessage, MessageRole, Session, SessionError};
-    ContentBlock, ConversationMessage, MessageRole, Session, SessionCompaction, SessionError,
-    SessionFork,
-};
 pub use usage::{
     format_usd, pricing_for_model, ModelPricing, TokenUsage, UsageCostEstimate, UsageTracker,
 };

rust/crates/runtime/src/session.rs

@@ -1,19 +1,11 @@
 use std::collections::BTreeMap;
 use std::fmt::{Display, Formatter};
-use std::fs::{self, OpenOptions};
+use std::fs;
-use std::io::Write;
+use std::path::Path;
-use std::path::{Path, PathBuf};
-use std::sync::atomic::{AtomicU64, Ordering};
-use std::time::{SystemTime, UNIX_EPOCH};
 
 use crate::json::{JsonError, JsonValue};
 use crate::usage::TokenUsage;
 
-const SESSION_VERSION: u32 = 1;
-const ROTATE_AFTER_BYTES: u64 = 256 * 1024;
-const MAX_ROTATED_FILES: usize = 3;
-static SESSION_ID_COUNTER: AtomicU64 = AtomicU64::new(0);
-
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum MessageRole {
     System,
@@ -48,49 +40,11 @@ pub struct ConversationMessage {
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
-pub struct SessionCompaction {
-    pub count: u32,
-    pub removed_message_count: usize,
-    pub summary: String,
-}
-
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub struct SessionFork {
-    pub parent_session_id: String,
-    pub branch_name: Option<String>,
-}
-
-#[derive(Debug, Clone, PartialEq, Eq)]
-struct SessionPersistence {
-    path: PathBuf,
-}
-
-#[derive(Debug, Clone)]
 pub struct Session {
     pub version: u32,
-    pub session_id: String,
-    pub created_at_ms: u64,
-    pub updated_at_ms: u64,
     pub messages: Vec<ConversationMessage>,
-    pub compaction: Option<SessionCompaction>,
-    pub fork: Option<SessionFork>,
-    persistence: Option<SessionPersistence>,
 }
 
-impl PartialEq for Session {
-    fn eq(&self, other: &Self) -> bool {
-        self.version == other.version
-            && self.session_id == other.session_id
-            && self.created_at_ms == other.created_at_ms
-            && self.updated_at_ms == other.updated_at_ms
-            && self.messages == other.messages
-            && self.compaction == other.compaction
-            && self.fork == other.fork
-    }
-}
-
-impl Eq for Session {}
-
 #[derive(Debug)]
 pub enum SessionError {
     Io(std::io::Error),
@@ -125,84 +79,20 @@ impl From<JsonError> for SessionError {
 impl Session {
     #[must_use]
     pub fn new() -> Self {
-        let now = current_time_millis();
         Self {
-            version: SESSION_VERSION,
+            version: 1,
-            session_id: generate_session_id(),
-            created_at_ms: now,
-            updated_at_ms: now,
             messages: Vec::new(),
-            compaction: None,
-            fork: None,
-            persistence: None,
         }
     }
 
-    #[must_use]
-    pub fn with_persistence_path(mut self, path: impl Into<PathBuf>) -> Self {
-        self.persistence = Some(SessionPersistence { path: path.into() });
-        self
-    }
-
-    #[must_use]
-    pub fn persistence_path(&self) -> Option<&Path> {
-        self.persistence.as_ref().map(|value| value.path.as_path())
-    }
-
     pub fn save_to_path(&self, path: impl AsRef<Path>) -> Result<(), SessionError> {
-        let path = path.as_ref();
+        fs::write(path, self.to_json().render())?;
-        rotate_session_file_if_needed(path)?;
-        write_atomic(path, &self.render_jsonl_snapshot())?;
-        cleanup_rotated_logs(path)?;
         Ok(())
     }
 
     pub fn load_from_path(path: impl AsRef<Path>) -> Result<Self, SessionError> {
-        let path = path.as_ref();
         let contents = fs::read_to_string(path)?;
-        let session = match JsonValue::parse(&contents) {
+        Self::from_json(&JsonValue::parse(&contents)?)
-            Ok(value) => Self::from_json(&value)?,
-            Err(_) => Self::from_jsonl(&contents)?,
-        };
-        Ok(session.with_persistence_path(path.to_path_buf()))
-    }
-
-    pub fn push_message(&mut self, message: ConversationMessage) -> Result<(), SessionError> {
-        self.touch();
-        self.messages.push(message.clone());
-        self.append_persisted_message(&message)
-    }
-
-    pub fn push_user_text(&mut self, text: impl Into<String>) -> Result<(), SessionError> {
-        self.push_message(ConversationMessage::user_text(text))
-    }
-
-    pub fn record_compaction(&mut self, summary: impl Into<String>, removed_message_count: usize) {
-        self.touch();
-        let count = self.compaction.as_ref().map_or(1, |value| value.count + 1);
-        self.compaction = Some(SessionCompaction {
-            count,
-            removed_message_count,
-            summary: summary.into(),
-        });
-    }
-
-    #[must_use]
-    pub fn fork(&self, branch_name: Option<String>) -> Self {
-        let now = current_time_millis();
-        Self {
-            version: self.version,
-            session_id: generate_session_id(),
-            created_at_ms: now,
-            updated_at_ms: now,
-            messages: self.messages.clone(),
-            compaction: self.compaction.clone(),
-            fork: Some(SessionFork {
-                parent_session_id: self.session_id.clone(),
-                branch_name: normalize_optional_string(branch_name),
-            }),
-            persistence: None,
-        }
     }
 
     #[must_use]
@@ -212,18 +102,6 @@ impl Session {
             "version".to_string(),
             JsonValue::Number(i64::from(self.version)),
         );
-        object.insert(
-            "session_id".to_string(),
-            JsonValue::String(self.session_id.clone()),
-        );
-        object.insert(
-            "created_at_ms".to_string(),
-            JsonValue::Number(i64_from_u64(self.created_at_ms, "created_at_ms")),
-        );
-        object.insert(
-            "updated_at_ms".to_string(),
-            JsonValue::Number(i64_from_u64(self.updated_at_ms, "updated_at_ms")),
-        );
         object.insert(
             "messages".to_string(),
             JsonValue::Array(
@@ -233,12 +111,6 @@ impl Session {
                     .collect(),
             ),
         );
-        if let Some(compaction) = &self.compaction {
-            object.insert("compaction".to_string(), compaction.to_json());
-        }
-        if let Some(fork) = &self.fork {
-            object.insert("fork".to_string(), fork.to_json());
-        }
         JsonValue::Object(object)
     }
 
@@ -259,179 +131,7 @@ impl Session {
             .iter()
             .map(ConversationMessage::from_json)
             .collect::<Result<Vec<_>, _>>()?;
-        let now = current_time_millis();
+        Ok(Self { version, messages })
-        let session_id = object
-            .get("session_id")
-            .and_then(JsonValue::as_str)
-            .map(ToOwned::to_owned)
-            .unwrap_or_else(generate_session_id);
-        let created_at_ms = object
-            .get("created_at_ms")
-            .map(|value| required_u64_from_value(value, "created_at_ms"))
-            .transpose()?
-            .unwrap_or(now);
-        let updated_at_ms = object
-            .get("updated_at_ms")
-            .map(|value| required_u64_from_value(value, "updated_at_ms"))
-            .transpose()?
-            .unwrap_or(created_at_ms);
-        let compaction = object
-            .get("compaction")
-            .map(SessionCompaction::from_json)
-            .transpose()?;
-        let fork = object.get("fork").map(SessionFork::from_json).transpose()?;
-        Ok(Self {
-            version,
-            session_id,
-            created_at_ms,
-            updated_at_ms,
-            messages,
-            compaction,
-            fork,
-            persistence: None,
-        })
-    }
-
-    fn from_jsonl(contents: &str) -> Result<Self, SessionError> {
-        let mut version = SESSION_VERSION;
-        let mut session_id = None;
-        let mut created_at_ms = None;
-        let mut updated_at_ms = None;
-        let mut messages = Vec::new();
-        let mut compaction = None;
-        let mut fork = None;
-
-        for (line_number, raw_line) in contents.lines().enumerate() {
-            let line = raw_line.trim();
-            if line.is_empty() {
-                continue;
-            }
-            let value = JsonValue::parse(line).map_err(|error| {
-                SessionError::Format(format!(
-                    "invalid JSONL record at line {}: {}",
-                    line_number + 1,
-                    error
-                ))
-            })?;
-            let object = value.as_object().ok_or_else(|| {
-                SessionError::Format(format!(
-                    "JSONL record at line {} must be an object",
-                    line_number + 1
-                ))
-            })?;
-            match object
-                .get("type")
-                .and_then(JsonValue::as_str)
-                .ok_or_else(|| {
-                    SessionError::Format(format!(
-                        "JSONL record at line {} missing type",
-                        line_number + 1
-                    ))
-                })? {
-                "session_meta" => {
-                    version = required_u32(object, "version")?;
-                    session_id = Some(required_string(object, "session_id")?);
-                    created_at_ms = Some(required_u64(object, "created_at_ms")?);
-                    updated_at_ms = Some(required_u64(object, "updated_at_ms")?);
-                    fork = object.get("fork").map(SessionFork::from_json).transpose()?;
-                }
-                "message" => {
-                    let message_value = object.get("message").ok_or_else(|| {
-                        SessionError::Format(format!(
-                            "JSONL record at line {} missing message",
-                            line_number + 1
-                        ))
-                    })?;
-                    messages.push(ConversationMessage::from_json(message_value)?);
-                }
-                "compaction" => {
-                    compaction = Some(SessionCompaction::from_json(&JsonValue::Object(
-                        object.clone(),
-                    ))?);
-                }
-                other => {
-                    return Err(SessionError::Format(format!(
-                        "unsupported JSONL record type at line {}: {other}",
-                        line_number + 1
-                    )))
-                }
-            }
-        }
-
-        let now = current_time_millis();
-        Ok(Self {
-            version,
-            session_id: session_id.unwrap_or_else(generate_session_id),
-            created_at_ms: created_at_ms.unwrap_or(now),
-            updated_at_ms: updated_at_ms.unwrap_or(created_at_ms.unwrap_or(now)),
-            messages,
-            compaction,
-            fork,
-            persistence: None,
-        })
-    }
-
-    fn render_jsonl_snapshot(&self) -> String {
-        let mut lines = vec![self.meta_record().render()];
-        if let Some(compaction) = &self.compaction {
-            lines.push(compaction.to_jsonl_record().render());
-        }
-        lines.extend(
-            self.messages
-                .iter()
-                .map(|message| message_record(message).render()),
-        );
-        let mut rendered = lines.join("\n");
-        rendered.push('\n');
-        rendered
-    }
-
-    fn append_persisted_message(&self, message: &ConversationMessage) -> Result<(), SessionError> {
-        let Some(path) = self.persistence_path() else {
-            return Ok(());
-        };
-
-        let needs_bootstrap = !path.exists() || fs::metadata(path)?.len() == 0;
-        if needs_bootstrap {
-            self.save_to_path(path)?;
-            return Ok(());
-        }
-
-        let mut file = OpenOptions::new().append(true).open(path)?;
-        writeln!(file, "{}", message_record(message).render())?;
-        Ok(())
-    }
-
-    fn meta_record(&self) -> JsonValue {
-        let mut object = BTreeMap::new();
-        object.insert(
-            "type".to_string(),
-            JsonValue::String("session_meta".to_string()),
-        );
-        object.insert(
-            "version".to_string(),
-            JsonValue::Number(i64::from(self.version)),
-        );
-        object.insert(
-            "session_id".to_string(),
-            JsonValue::String(self.session_id.clone()),
-        );
-        object.insert(
-            "created_at_ms".to_string(),
-            JsonValue::Number(i64_from_u64(self.created_at_ms, "created_at_ms")),
-        );
-        object.insert(
-            "updated_at_ms".to_string(),
-            JsonValue::Number(i64_from_u64(self.updated_at_ms, "updated_at_ms")),
-        );
-        if let Some(fork) = &self.fork {
-            object.insert("fork".to_string(), fork.to_json());
-        }
-        JsonValue::Object(object)
-    }
-
-    fn touch(&mut self) {
-        self.updated_at_ms = current_time_millis();
     }
 }
 
@@ -624,92 +324,6 @@ impl ContentBlock {
     }
 }
 
-impl SessionCompaction {
-    #[must_use]
-    pub fn to_json(&self) -> JsonValue {
-        let mut object = BTreeMap::new();
-        object.insert(
-            "count".to_string(),
-            JsonValue::Number(i64::from(self.count)),
-        );
-        object.insert(
-            "removed_message_count".to_string(),
-            JsonValue::Number(i64_from_usize(
-                self.removed_message_count,
-                "removed_message_count",
-            )),
-        );
-        object.insert(
-            "summary".to_string(),
-            JsonValue::String(self.summary.clone()),
-        );
-        JsonValue::Object(object)
-    }
-
-    #[must_use]
-    pub fn to_jsonl_record(&self) -> JsonValue {
-        let mut object = self
-            .to_json()
-            .as_object()
-            .cloned()
-            .expect("compaction should render to object");
-        object.insert(
-            "type".to_string(),
-            JsonValue::String("compaction".to_string()),
-        );
-        JsonValue::Object(object)
-    }
-
-    fn from_json(value: &JsonValue) -> Result<Self, SessionError> {
-        let object = value
-            .as_object()
-            .ok_or_else(|| SessionError::Format("compaction must be an object".to_string()))?;
-        Ok(Self {
-            count: required_u32(object, "count")?,
-            removed_message_count: required_usize(object, "removed_message_count")?,
-            summary: required_string(object, "summary")?,
-        })
-    }
-}
-
-impl SessionFork {
-    #[must_use]
-    pub fn to_json(&self) -> JsonValue {
-        let mut object = BTreeMap::new();
-        object.insert(
-            "parent_session_id".to_string(),
-            JsonValue::String(self.parent_session_id.clone()),
-        );
-        if let Some(branch_name) = &self.branch_name {
-            object.insert(
-                "branch_name".to_string(),
-                JsonValue::String(branch_name.clone()),
-            );
-        }
-        JsonValue::Object(object)
-    }
-
-    fn from_json(value: &JsonValue) -> Result<Self, SessionError> {
-        let object = value
-            .as_object()
-            .ok_or_else(|| SessionError::Format("fork metadata must be an object".to_string()))?;
-        Ok(Self {
-            parent_session_id: required_string(object, "parent_session_id")?,
-            branch_name: object
-                .get("branch_name")
-                .and_then(JsonValue::as_str)
-                .map(ToOwned::to_owned),
-        })
-    }
-}
-
-fn message_record(message: &ConversationMessage) -> JsonValue {
-    let mut object = BTreeMap::new();
-    object.insert("type".to_string(), JsonValue::String("message".to_string()));
-    object.insert("message".to_string(), message.to_json());
-    JsonValue::Object(object)
-}
-
 fn usage_to_json(usage: TokenUsage) -> JsonValue {
     let mut object = BTreeMap::new();
     object.insert(
@@ -762,155 +376,22 @@ fn required_u32(object: &BTreeMap<String, JsonValue>, key: &str) -> Result<u32,
     u32::try_from(value).map_err(|_| SessionError::Format(format!("{key} out of range")))
 }
 
-fn required_u64(object: &BTreeMap<String, JsonValue>, key: &str) -> Result<u64, SessionError> {
-    let value = object
-        .get(key)
-        .ok_or_else(|| SessionError::Format(format!("missing {key}")))?;
-    required_u64_from_value(value, key)
-}
-
-fn required_u64_from_value(value: &JsonValue, key: &str) -> Result<u64, SessionError> {
-    let value = value
-        .as_i64()
-        .ok_or_else(|| SessionError::Format(format!("missing {key}")))?;
-    u64::try_from(value).map_err(|_| SessionError::Format(format!("{key} out of range")))
-}
-
-fn required_usize(object: &BTreeMap<String, JsonValue>, key: &str) -> Result<usize, SessionError> {
-    let value = object
-        .get(key)
-        .and_then(JsonValue::as_i64)
-        .ok_or_else(|| SessionError::Format(format!("missing {key}")))?;
-    usize::try_from(value).map_err(|_| SessionError::Format(format!("{key} out of range")))
-}
-
-fn i64_from_u64(value: u64, key: &str) -> i64 {
-    i64::try_from(value).unwrap_or_else(|_| panic!("{key} out of range for JSON number"))
-}
-
-fn i64_from_usize(value: usize, key: &str) -> i64 {
-    i64::try_from(value).unwrap_or_else(|_| panic!("{key} out of range for JSON number"))
-}
-
-fn normalize_optional_string(value: Option<String>) -> Option<String> {
-    value.and_then(|value| {
-        let trimmed = value.trim();
-        if trimmed.is_empty() {
-            None
-        } else {
-            Some(trimmed.to_string())
-        }
-    })
-}
-
-fn current_time_millis() -> u64 {
-    SystemTime::now()
-        .duration_since(UNIX_EPOCH)
-        .map(|duration| duration.as_millis() as u64)
-        .unwrap_or_default()
-}
-
-fn generate_session_id() -> String {
-    let millis = current_time_millis();
-    let counter = SESSION_ID_COUNTER.fetch_add(1, Ordering::Relaxed);
-    format!("session-{millis}-{counter}")
-}
-
-fn write_atomic(path: &Path, contents: &str) -> Result<(), SessionError> {
-    if let Some(parent) = path.parent() {
-        fs::create_dir_all(parent)?;
-    }
-    let temp_path = temporary_path_for(path);
-    fs::write(&temp_path, contents)?;
-    fs::rename(temp_path, path)?;
-    Ok(())
-}
-
-fn temporary_path_for(path: &Path) -> PathBuf {
-    let file_name = path
-        .file_name()
-        .and_then(|value| value.to_str())
-        .unwrap_or("session");
-    path.with_file_name(format!(
-        "{file_name}.tmp-{}-{}",
-        current_time_millis(),
-        SESSION_ID_COUNTER.fetch_add(1, Ordering::Relaxed)
-    ))
-}
-
-fn rotate_session_file_if_needed(path: &Path) -> Result<(), SessionError> {
-    let Ok(metadata) = fs::metadata(path) else {
-        return Ok(());
-    };
-    if metadata.len() < ROTATE_AFTER_BYTES {
-        return Ok(());
-    }
-    let rotated_path = rotated_log_path(path);
-    fs::rename(path, rotated_path)?;
-    Ok(())
-}
-
-fn rotated_log_path(path: &Path) -> PathBuf {
-    let stem = path
-        .file_stem()
-        .and_then(|value| value.to_str())
-        .unwrap_or("session");
-    path.with_file_name(format!("{stem}.rot-{}.jsonl", current_time_millis()))
-}
-
-fn cleanup_rotated_logs(path: &Path) -> Result<(), SessionError> {
-    let Some(parent) = path.parent() else {
-        return Ok(());
-    };
-    let stem = path
-        .file_stem()
-        .and_then(|value| value.to_str())
-        .unwrap_or("session");
-    let prefix = format!("{stem}.rot-");
-    let mut rotated_paths = fs::read_dir(parent)?
-        .filter_map(Result::ok)
-        .map(|entry| entry.path())
-        .filter(|entry_path| {
-            entry_path
-                .file_name()
-                .and_then(|value| value.to_str())
-                .is_some_and(|name| name.starts_with(&prefix) && name.ends_with(".jsonl"))
-        })
-        .collect::<Vec<_>>();
-
-    rotated_paths.sort_by_key(|entry_path| {
-        fs::metadata(entry_path)
-            .and_then(|metadata| metadata.modified())
-            .unwrap_or(UNIX_EPOCH)
-    });
-
-    let remove_count = rotated_paths.len().saturating_sub(MAX_ROTATED_FILES);
-    for stale_path in rotated_paths.into_iter().take(remove_count) {
-        fs::remove_file(stale_path)?;
-    }
-    Ok(())
-}
-
 #[cfg(test)]
 mod tests {
-    use super::{
+    use super::{ContentBlock, ConversationMessage, MessageRole, Session};
-        cleanup_rotated_logs, rotate_session_file_if_needed, ContentBlock, ConversationMessage,
-        MessageRole, Session, SessionFork,
-    };
-    use crate::json::JsonValue;
     use crate::usage::TokenUsage;
     use std::fs;
-    use std::path::PathBuf;
     use std::time::{SystemTime, UNIX_EPOCH};
 
     #[test]
-    fn persists_and_restores_session_jsonl() {
+    fn persists_and_restores_session_json() {
         let mut session = Session::new();
         session
-            .push_user_text("hello")
+            .messages
-            .expect("user message should append");
+            .push(ConversationMessage::user_text("hello"));
         session
-            .push_message(ConversationMessage::assistant_with_usage(
+            .messages
+            .push(ConversationMessage::assistant_with_usage(
                 vec![
                     ContentBlock::Text {
                         text: "thinking".to_string(),
@@ -927,15 +408,16 @@ mod tests {
                     cache_creation_input_tokens: 1,
                     cache_read_input_tokens: 2,
                 }),
-            ))
+            ));
-            .expect("assistant message should append");
+        session.messages.push(ConversationMessage::tool_result(
-        session
+            "tool-1", "bash", "hi", false,
-            .push_message(ConversationMessage::tool_result(
+        ));
-                "tool-1", "bash", "hi", false,
-            ))
-            .expect("tool result should append");
 
-        let path = temp_session_path("jsonl");
+        let nanos = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .expect("system time should be after epoch")
+            .as_nanos();
+        let path = std::env::temp_dir().join(format!("runtime-session-{nanos}.json"));
         session.save_to_path(&path).expect("session should save");
         let restored = Session::load_from_path(&path).expect("session should load");
         fs::remove_file(&path).expect("temp file should be removable");
@@ -946,157 +428,5 @@ mod tests {
             restored.messages[1].usage.expect("usage").total_tokens(),
             17
         );
-        assert_eq!(restored.session_id, session.session_id);
-    }
-
-    #[test]
-    fn loads_legacy_session_json_object() {
-        let path = temp_session_path("legacy");
-        let legacy = JsonValue::Object(
-            [
-                ("version".to_string(), JsonValue::Number(1)),
-                (
-                    "messages".to_string(),
-                    JsonValue::Array(vec![ConversationMessage::user_text("legacy").to_json()]),
-                ),
-            ]
-            .into_iter()
-            .collect(),
-        );
-        fs::write(&path, legacy.render()).expect("legacy file should write");
-
-        let restored = Session::load_from_path(&path).expect("legacy session should load");
-        fs::remove_file(&path).expect("temp file should be removable");
-
-        assert_eq!(restored.messages.len(), 1);
-        assert_eq!(
-            restored.messages[0],
-            ConversationMessage::user_text("legacy")
-        );
-        assert!(!restored.session_id.is_empty());
-    }
-
-    #[test]
-    fn appends_messages_to_persisted_jsonl_session() {
-        let path = temp_session_path("append");
-        let mut session = Session::new().with_persistence_path(path.clone());
-        session
-            .save_to_path(&path)
-            .expect("initial save should succeed");
-        session
-            .push_user_text("hi")
-            .expect("user append should succeed");
-        session
-            .push_message(ConversationMessage::assistant(vec![ContentBlock::Text {
-                text: "hello".to_string(),
-            }]))
-            .expect("assistant append should succeed");
-
-        let restored = Session::load_from_path(&path).expect("session should replay from jsonl");
-        fs::remove_file(&path).expect("temp file should be removable");
-
-        assert_eq!(restored.messages.len(), 2);
-        assert_eq!(restored.messages[0], ConversationMessage::user_text("hi"));
-    }
-
-    #[test]
-    fn persists_compaction_metadata() {
-        let path = temp_session_path("compaction");
-        let mut session = Session::new();
-        session
-            .push_user_text("before")
-            .expect("message should append");
-        session.record_compaction("summarized earlier work", 4);
-        session.save_to_path(&path).expect("session should save");
-
-        let restored = Session::load_from_path(&path).expect("session should load");
-        fs::remove_file(&path).expect("temp file should be removable");
-
-        let compaction = restored.compaction.expect("compaction metadata");
-        assert_eq!(compaction.count, 1);
-        assert_eq!(compaction.removed_message_count, 4);
-        assert!(compaction.summary.contains("summarized"));
-    }
-
-    #[test]
-    fn forks_sessions_with_branch_metadata_and_persists_it() {
-        let path = temp_session_path("fork");
-        let mut session = Session::new();
-        session
-            .push_user_text("before fork")
-            .expect("message should append");
-
-        let forked = session
-            .fork(Some("investigation".to_string()))
-            .with_persistence_path(path.clone());
-        forked
-            .save_to_path(&path)
-            .expect("forked session should save");
-
-        let restored = Session::load_from_path(&path).expect("forked session should load");
-        fs::remove_file(&path).expect("temp file should be removable");
-
-        assert_ne!(restored.session_id, session.session_id);
-        assert_eq!(
-            restored.fork,
-            Some(SessionFork {
-                parent_session_id: session.session_id,
-                branch_name: Some("investigation".to_string()),
-            })
-        );
-        assert_eq!(restored.messages, forked.messages);
-    }
-
-    #[test]
-    fn rotates_and_cleans_up_large_session_logs() {
-        let path = temp_session_path("rotation");
-        fs::write(&path, "x".repeat((super::ROTATE_AFTER_BYTES + 10) as usize))
-            .expect("oversized file should write");
-        rotate_session_file_if_needed(&path).expect("rotation should succeed");
-        assert!(
-            !path.exists(),
-            "original path should be rotated away before rewrite"
-        );
-
-        for _ in 0..5 {
-            let rotated = super::rotated_log_path(&path);
-            fs::write(&rotated, "old").expect("rotated file should write");
-        }
-        cleanup_rotated_logs(&path).expect("cleanup should succeed");
-
-        let rotated_count = rotation_files(&path).len();
-        assert!(rotated_count <= super::MAX_ROTATED_FILES);
-        for rotated in rotation_files(&path) {
-            fs::remove_file(rotated).expect("rotated file should be removable");
-        }
-    }
-
-    fn temp_session_path(label: &str) -> PathBuf {
-        let nanos = SystemTime::now()
-            .duration_since(UNIX_EPOCH)
-            .expect("system time should be after epoch")
-            .as_nanos();
-        std::env::temp_dir().join(format!("runtime-session-{label}-{nanos}.json"))
-    }
-
-    fn rotation_files(path: &PathBuf) -> Vec<PathBuf> {
-        let stem = path
-            .file_stem()
-            .and_then(|value| value.to_str())
-            .expect("temp path should have file stem")
-            .to_string();
-        fs::read_dir(path.parent().expect("temp path should have parent"))
-            .expect("temp dir should read")
-            .filter_map(Result::ok)
-            .map(|entry| entry.path())
-            .filter(|entry_path| {
-                entry_path
-                    .file_name()
-                    .and_then(|value| value.to_str())
-                    .is_some_and(|name| {
-                        name.starts_with(&format!("{stem}.rot-")) && name.ends_with(".jsonl")
-                    })
-            })
-            .collect()
     }
 }

rust/crates/runtime/src/usage.rs

@@ -286,19 +286,21 @@ mod tests {
 
     #[test]
     fn reconstructs_usage_from_session_messages() {
-        let mut session = Session::new();
+        let session = Session {
-        session.messages = vec![ConversationMessage {
+            version: 1,
-            role: MessageRole::Assistant,
+            messages: vec![ConversationMessage {
-            blocks: vec![ContentBlock::Text {
+                role: MessageRole::Assistant,
-                text: "done".to_string(),
+                blocks: vec![ContentBlock::Text {
+                    text: "done".to_string(),
+                }],
+                usage: Some(TokenUsage {
+                    input_tokens: 5,
+                    output_tokens: 2,
+                    cache_creation_input_tokens: 1,
+                    cache_read_input_tokens: 0,
+                }),
             }],
-            usage: Some(TokenUsage {
+        };
-                input_tokens: 5,
-                output_tokens: 2,
-                cache_creation_input_tokens: 1,
-                cache_read_input_tokens: 0,
-            }),
-        }];
 
         let tracker = UsageTracker::from_session(&session);
         assert_eq!(tracker.turns(), 1);

rust/crates/rusty-claude-cli/src/main.rs

@@ -9,12 +9,13 @@ use std::io::{self, Read, Write};
 use std::net::TcpListener;
 use std::path::{Path, PathBuf};
 use std::process::Command;
-use std::time::UNIX_EPOCH;
+use std::time::{SystemTime, UNIX_EPOCH};
 
 use api::{
     resolve_startup_auth_source, AnthropicClient, AuthSource, ContentBlockDelta, InputContentBlock,
-    InputMessage, MessageRequest, MessageResponse, OutputContentBlock,
+    InputMessage, MessageRequest, MessageResponse, OutputContentBlock, PromptCache,
-    StreamEvent as ApiStreamEvent, ToolChoice, ToolDefinition, ToolResultContentBlock,
+    PromptCacheRecord, StreamEvent as ApiStreamEvent, ToolChoice, ToolDefinition,
+    ToolResultContentBlock,
 };
 
 use commands::{
@@ -28,8 +29,8 @@ use runtime::{
     parse_oauth_callback_request_target, save_oauth_credentials, ApiClient, ApiRequest,
     AssistantEvent, CompactionConfig, ConfigLoader, ConfigSource, ContentBlock,
     ConversationMessage, ConversationRuntime, MessageRole, OAuthAuthorizationRequest, OAuthConfig,
-    OAuthTokenExchangeRequest, PermissionMode, PermissionPolicy, ProjectContext, RuntimeError,
+    OAuthTokenExchangeRequest, PermissionMode, PermissionPolicy, ProjectContext, PromptCacheEvent,
-    Session, TokenUsage, ToolError, ToolExecutor, UsageTracker,
+    RuntimeError, Session, TokenUsage, ToolError, ToolExecutor, UsageTracker,
 };
 use serde_json::json;
 use tools::{execute_tool, mvp_tool_specs, ToolSpec};
@@ -47,8 +48,6 @@ const DEFAULT_OAUTH_CALLBACK_PORT: u16 = 4545;
 const VERSION: &str = env!("CARGO_PKG_VERSION");
 const BUILD_TARGET: Option<&str> = option_env!("TARGET");
 const GIT_SHA: Option<&str> = option_env!("GIT_SHA");
-const PRIMARY_SESSION_EXTENSION: &str = "jsonl";
-const LEGACY_SESSION_EXTENSION: &str = "json";
 
 type AllowedToolSet = BTreeSet<String>;
 
@@ -591,19 +590,7 @@ fn print_version() {
 }
 
 fn resume_session(session_path: &Path, commands: &[String]) {
-    let resolved_path = if session_path.exists() {
+    let session = match Session::load_from_path(session_path) {
-        session_path.to_path_buf()
-    } else {
-        match resolve_session_reference(&session_path.display().to_string()) {
-            Ok(handle) => handle.path,
-            Err(error) => {
-                eprintln!("failed to restore session: {error}");
-                std::process::exit(1);
-            }
-        }
-    };
-
-    let session = match Session::load_from_path(&resolved_path) {
         Ok(session) => session,
         Err(error) => {
             eprintln!("failed to restore session: {error}");
@@ -614,7 +601,7 @@ fn resume_session(session_path: &Path, commands: &[String]) {
     if commands.is_empty() {
         println!(
             "Restored session from {} ({} messages).",
-            resolved_path.display(),
+            session_path.display(),
             session.messages.len()
         );
         return;
@@ -626,7 +613,7 @@ fn resume_session(session_path: &Path, commands: &[String]) {
             eprintln!("unsupported resumed command: {raw_command}");
             std::process::exit(2);
         };
-        match run_resume_command(&resolved_path, &session, &command) {
+        match run_resume_command(session_path, &session, &command) {
             Ok(ResumeCommandOutcome {
                 session: next_session,
                 message,
@@ -987,8 +974,6 @@ struct ManagedSessionSummary {
     path: PathBuf,
     modified_epoch_secs: u64,
     message_count: usize,
-    parent_session_id: Option<String>,
-    branch_name: Option<String>,
 }
 
 struct LiveCli {
@@ -1008,10 +993,10 @@ impl LiveCli {
         permission_mode: PermissionMode,
     ) -> Result<Self, Box<dyn std::error::Error>> {
         let system_prompt = build_system_prompt()?;
-        let session_state = Session::new();
+        let session = create_managed_session_handle()?;
-        let session = create_managed_session_handle(&session_state.session_id)?;
         let runtime = build_runtime(
-            session_state.with_persistence_path(session.path.clone()),
+            Session::new(),
+            session.id.clone(),
             model.clone(),
             system_prompt.clone(),
             enable_tools,
@@ -1067,13 +1052,14 @@ impl LiveCli {
         let mut permission_prompter = CliPermissionPrompter::new(self.permission_mode);
         let result = self.runtime.run_turn(input, Some(&mut permission_prompter));
         match result {
-            Ok(_) => {
+            Ok(summary) => {
                 spinner.finish(
                     "✨ Done",
                     TerminalRenderer::new().color_theme(),
                     &mut stdout,
                 )?;
                 println!();
+                print_prompt_cache_events(&summary);
                 self.persist_session()?;
                 Ok(())
             }
@@ -1103,6 +1089,7 @@ impl LiveCli {
         let session = self.runtime.session().clone();
         let mut runtime = build_runtime(
             session,
+            self.session.id.clone(),
             self.model.clone(),
             self.system_prompt.clone(),
             true,
@@ -1122,6 +1109,7 @@ impl LiveCli {
                 "iterations": summary.iterations,
                 "tool_uses": collect_tool_uses(&summary),
                 "tool_results": collect_tool_results(&summary),
+                "prompt_cache_events": collect_prompt_cache_events(&summary),
                 "usage": {
                     "input_tokens": summary.usage.input_tokens,
                     "output_tokens": summary.usage.output_tokens,
@@ -1249,6 +1237,7 @@ impl LiveCli {
         let message_count = session.messages.len();
         self.runtime = build_runtime(
             session,
+            self.session.id.clone(),
             model.clone(),
             self.system_prompt.clone(),
             true,
@@ -1292,6 +1281,7 @@ impl LiveCli {
         self.permission_mode = permission_mode_from_label(normalized);
         self.runtime = build_runtime(
             session,
+            self.session.id.clone(),
             self.model.clone(),
             self.system_prompt.clone(),
             true,
@@ -1314,10 +1304,10 @@ impl LiveCli {
             return Ok(false);
         }
 
-        let session_state = Session::new();
+        self.session = create_managed_session_handle()?;
-        self.session = create_managed_session_handle(&session_state.session_id)?;
         self.runtime = build_runtime(
-            session_state.with_persistence_path(self.session.path.clone()),
+            Session::new(),
+            self.session.id.clone(),
             self.model.clone(),
             self.system_prompt.clone(),
             true,
@@ -1351,9 +1341,9 @@ impl LiveCli {
         let handle = resolve_session_reference(&session_ref)?;
         let session = Session::load_from_path(&handle.path)?;
         let message_count = session.messages.len();
-        let session_id = session.session_id.clone();
         self.runtime = build_runtime(
             session,
+            handle.id.clone(),
             self.model.clone(),
             self.system_prompt.clone(),
             true,
@@ -1361,10 +1351,7 @@ impl LiveCli {
             self.allowed_tools.clone(),
             self.permission_mode,
         )?;
-        self.session = SessionHandle {
+        self.session = handle;
-            id: session_id,
-            path: handle.path,
-        };
         println!(
             "{}",
             format_resume_report(
@@ -1427,41 +1414,9 @@ impl LiveCli {
                 let handle = resolve_session_reference(target)?;
                 let session = Session::load_from_path(&handle.path)?;
                 let message_count = session.messages.len();
-                let session_id = session.session_id.clone();
                 self.runtime = build_runtime(
                     session,
-                    self.model.clone(),
+                    handle.id.clone(),
-                    self.system_prompt.clone(),
-                    true,
-                    true,
-                    self.allowed_tools.clone(),
-                    self.permission_mode,
-                )?;
-                self.session = SessionHandle {
-                    id: session_id,
-                    path: handle.path,
-                };
-                println!(
-                    "Session switched\n  Active session   {}\n  File             {}\n  Messages         {}",
-                    self.session.id,
-                    self.session.path.display(),
-                    message_count,
-                );
-                Ok(true)
-            }
-            Some("fork") => {
-                let forked = self.runtime.fork_session(target.map(ToOwned::to_owned));
-                let parent_session_id = self.session.id.clone();
-                let handle = create_managed_session_handle(&forked.session_id)?;
-                let branch_name = forked
-                    .fork
-                    .as_ref()
-                    .and_then(|fork| fork.branch_name.clone());
-                let forked = forked.with_persistence_path(handle.path.clone());
-                let message_count = forked.messages.len();
-                forked.save_to_path(&handle.path)?;
-                self.runtime = build_runtime(
-                    forked,
                     self.model.clone(),
                     self.system_prompt.clone(),
                     true,
@@ -1471,19 +1426,15 @@ impl LiveCli {
                 )?;
                 self.session = handle;
                 println!(
-                    "Session forked\n  Parent session   {}\n  Active session   {}\n  Branch           {}\n  File             {}\n  Messages         {}",
+                    "Session switched\n  Active session   {}\n  File             {}\n  Messages         {}",
-                    parent_session_id,
                     self.session.id,
-                    branch_name.as_deref().unwrap_or("(unnamed)"),
                     self.session.path.display(),
                     message_count,
                 );
                 Ok(true)
             }
             Some(other) => {
-                println!(
+                println!("Unknown /session action '{other}'. Use /session list or /session switch <session-id>.");
-                    "Unknown /session action '{other}'. Use /session list, /session switch <session-id>, or /session fork [branch-name]."
-                );
                 Ok(false)
             }
         }
@@ -1496,6 +1447,7 @@ impl LiveCli {
         let skipped = removed == 0;
         self.runtime = build_runtime(
             result.compacted_session,
+            self.session.id.clone(),
             self.model.clone(),
             self.system_prompt.clone(),
             true,
@@ -1516,61 +1468,44 @@ fn sessions_dir() -> Result<PathBuf, Box<dyn std::error::Error>> {
     Ok(path)
 }
 
-fn create_managed_session_handle(
+fn create_managed_session_handle() -> Result<SessionHandle, Box<dyn std::error::Error>> {
-    session_id: &str,
+    let id = generate_session_id();
-) -> Result<SessionHandle, Box<dyn std::error::Error>> {
+    let path = sessions_dir()?.join(format!("{id}.json"));
-    let id = session_id.to_string();
-    let path = sessions_dir()?.join(format!("{id}.{PRIMARY_SESSION_EXTENSION}"));
     Ok(SessionHandle { id, path })
 }
 
+fn generate_session_id() -> String {
+    let millis = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .map(|duration| duration.as_millis())
+        .unwrap_or_default();
+    format!("session-{millis}")
+}
+
 fn resolve_session_reference(reference: &str) -> Result<SessionHandle, Box<dyn std::error::Error>> {
     let direct = PathBuf::from(reference);
-    let looks_like_path = direct.extension().is_some() || direct.components().count() > 1;
     let path = if direct.exists() {
         direct
-    } else if looks_like_path {
-        return Err(format!("session not found: {reference}").into());
     } else {
-        resolve_managed_session_path(reference)?
+        sessions_dir()?.join(format!("{reference}.json"))
     };
+    if !path.exists() {
+        return Err(format!("session not found: {reference}").into());
+    }
     let id = path
-        .file_name()
+        .file_stem()
         .and_then(|value| value.to_str())
-        .and_then(|name| {
-            name.strip_suffix(&format!(".{PRIMARY_SESSION_EXTENSION}"))
-                .or_else(|| name.strip_suffix(&format!(".{LEGACY_SESSION_EXTENSION}")))
-        })
         .unwrap_or(reference)
         .to_string();
     Ok(SessionHandle { id, path })
 }
 
-fn resolve_managed_session_path(session_id: &str) -> Result<PathBuf, Box<dyn std::error::Error>> {
-    let directory = sessions_dir()?;
-    for extension in [PRIMARY_SESSION_EXTENSION, LEGACY_SESSION_EXTENSION] {
-        let path = directory.join(format!("{session_id}.{extension}"));
-        if path.exists() {
-            return Ok(path);
-        }
-    }
-    Err(format!("session not found: {session_id}").into())
-}
-
-fn is_managed_session_file(path: &Path) -> bool {
-    path.extension()
-        .and_then(|ext| ext.to_str())
-        .is_some_and(|extension| {
-            extension == PRIMARY_SESSION_EXTENSION || extension == LEGACY_SESSION_EXTENSION
-        })
-}
-
 fn list_managed_sessions() -> Result<Vec<ManagedSessionSummary>, Box<dyn std::error::Error>> {
     let mut sessions = Vec::new();
     for entry in fs::read_dir(sessions_dir()?)? {
         let entry = entry?;
         let path = entry.path();
-        if !is_managed_session_file(&path) {
+        if path.extension().and_then(|ext| ext.to_str()) != Some("json") {
             continue;
         }
         let metadata = entry.metadata()?;
@@ -1580,41 +1515,19 @@ fn list_managed_sessions() -> Result<Vec<ManagedSessionSummary>, Box<dyn std::er
             .and_then(|time| time.duration_since(UNIX_EPOCH).ok())
             .map(|duration| duration.as_secs())
             .unwrap_or_default();
-        let (id, message_count, parent_session_id, branch_name) = Session::load_from_path(&path)
+        let message_count = Session::load_from_path(&path)
-            .map(|session| {
+            .map(|session| session.messages.len())
-                let parent_session_id = session
+            .unwrap_or_default();
-                    .fork
+        let id = path
-                    .as_ref()
+            .file_stem()
-                    .map(|fork| fork.parent_session_id.clone());
+            .and_then(|value| value.to_str())
-                let branch_name = session
+            .unwrap_or("unknown")
-                    .fork
+            .to_string();
-                    .as_ref()
-                    .and_then(|fork| fork.branch_name.clone());
-                (
-                    session.session_id,
-                    session.messages.len(),
-                    parent_session_id,
-                    branch_name,
-                )
-            })
-            .unwrap_or_else(|_| {
-                (
-                    path.file_stem()
-                        .and_then(|value| value.to_str())
-                        .unwrap_or("unknown")
-                        .to_string(),
-                    0,
-                    None,
-                    None,
-                )
-            });
         sessions.push(ManagedSessionSummary {
             id,
             path,
             modified_epoch_secs,
             message_count,
-            parent_session_id,
-            branch_name,
         });
     }
     sessions.sort_by(|left, right| right.modified_epoch_secs.cmp(&left.modified_epoch_secs));
@@ -1637,23 +1550,11 @@ fn render_session_list(active_session_id: &str) -> Result<String, Box<dyn std::e
         } else {
             "○ saved"
         };
-        let lineage = match (
-            session.branch_name.as_deref(),
-            session.parent_session_id.as_deref(),
-        ) {
-            (Some(branch_name), Some(parent_session_id)) => {
-                format!(" branch={branch_name} from={parent_session_id}")
-            }
-            (None, Some(parent_session_id)) => format!(" from={parent_session_id}"),
-            (Some(branch_name), None) => format!(" branch={branch_name}"),
-            (None, None) => String::new(),
-        };
         lines.push(format!(
-            "  {id:<20} {marker:<10} msgs={msgs:<4} modified={modified}{lineage} path={path}",
+            "  {id:<20} {marker:<10} msgs={msgs:<4} modified={modified} path={path}",
             id = session.id,
             msgs = session.message_count,
             modified = session.modified_epoch_secs,
-            lineage = lineage,
             path = session.path.display(),
         ));
     }
@@ -2022,8 +1923,10 @@ fn build_runtime_feature_config(
         .clone())
 }
 
+#[allow(clippy::too_many_arguments)]
 fn build_runtime(
     session: Session,
+    session_id: String,
     model: String,
     system_prompt: Vec<String>,
     enable_tools: bool,
@@ -2034,11 +1937,17 @@ fn build_runtime(
 {
     Ok(ConversationRuntime::new_with_features(
         session,
-        AnthropicRuntimeClient::new(model, enable_tools, emit_output, allowed_tools.clone())?,
+        AnthropicRuntimeClient::new(
+            model,
+            enable_tools,
+            emit_output,
+            allowed_tools.clone(),
+            session_id,
+        )?,
         CliToolExecutor::new(allowed_tools, emit_output),
         permission_policy(permission_mode),
         system_prompt,
-        build_runtime_feature_config()?,
+        &build_runtime_feature_config()?,
     ))
 }
 
@@ -2103,11 +2012,13 @@ impl AnthropicRuntimeClient {
         enable_tools: bool,
         emit_output: bool,
         allowed_tools: Option<AllowedToolSet>,
+        session_id: impl Into<String>,
     ) -> Result<Self, Box<dyn std::error::Error>> {
         Ok(Self {
             runtime: tokio::runtime::Runtime::new()?,
             client: AnthropicClient::from_auth(resolve_cli_auth_source()?)
-                .with_base_url(api::read_base_url()),
+                .with_base_url(api::read_base_url())
+                .with_prompt_cache(PromptCache::new(session_id)),
             model,
             enable_tools,
             emit_output,
@@ -2222,8 +2133,8 @@ impl ApiClient for AnthropicRuntimeClient {
                         events.push(AssistantEvent::Usage(TokenUsage {
                             input_tokens: delta.usage.input_tokens,
                             output_tokens: delta.usage.output_tokens,
-                            cache_creation_input_tokens: 0,
+                            cache_creation_input_tokens: delta.usage.cache_creation_input_tokens,
-                            cache_read_input_tokens: 0,
+                            cache_read_input_tokens: delta.usage.cache_read_input_tokens,
                         }));
                     }
                     ApiStreamEvent::MessageStop(_) => {
@@ -2238,6 +2149,8 @@ impl ApiClient for AnthropicRuntimeClient {
                 }
             }
 
+            push_prompt_cache_record(&self.client, &mut events);
+
             if !saw_stop
                 && events.iter().any(|event| {
                     matches!(event, AssistantEvent::TextDelta(text) if !text.is_empty())
@@ -2262,7 +2175,9 @@ impl ApiClient for AnthropicRuntimeClient {
                 })
                 .await
                 .map_err(|error| RuntimeError::new(error.to_string()))?;
-            response_to_events(response, out)
+            let mut events = response_to_events(response, out)?;
+            push_prompt_cache_record(&self.client, &mut events);
+            Ok(events)
         })
     }
 }
@@ -2323,6 +2238,39 @@ fn collect_tool_results(summary: &runtime::TurnSummary) -> Vec<serde_json::Value
         .collect()
 }
 
+fn collect_prompt_cache_events(summary: &runtime::TurnSummary) -> Vec<serde_json::Value> {
+    summary
+        .prompt_cache_events
+        .iter()
+        .map(|event| {
+            json!({
+                "unexpected": event.unexpected,
+                "reason": event.reason,
+                "previous_cache_read_input_tokens": event.previous_cache_read_input_tokens,
+                "current_cache_read_input_tokens": event.current_cache_read_input_tokens,
+                "token_drop": event.token_drop,
+            })
+        })
+        .collect()
+}
+
+fn print_prompt_cache_events(summary: &runtime::TurnSummary) {
+    for event in &summary.prompt_cache_events {
+        let label = if event.unexpected {
+            "Prompt cache break"
+        } else {
+            "Prompt cache invalidation"
+        };
+        println!(
+            "{label}: {} (cache read {} -> {}, drop {})",
+            event.reason,
+            event.previous_cache_read_input_tokens,
+            event.current_cache_read_input_tokens,
+            event.token_drop,
+        );
+    }
+}
+
 fn slash_command_completion_candidates() -> Vec<String> {
     slash_command_specs()
         .iter()
@@ -2469,18 +2417,20 @@ fn first_visible_line(text: &str) -> &str {
 }
 
 fn format_bash_result(icon: &str, parsed: &serde_json::Value) -> String {
+    use std::fmt::Write as _;
+
     let mut lines = vec![format!("{icon} \x1b[38;5;245mbash\x1b[0m")];
     if let Some(task_id) = parsed
         .get("backgroundTaskId")
         .and_then(|value| value.as_str())
     {
-        lines[0].push_str(&format!(" backgrounded ({task_id})"));
+        let _ = write!(lines[0], " backgrounded ({task_id})");
     } else if let Some(status) = parsed
         .get("returnCodeInterpretation")
         .and_then(|value| value.as_str())
         .filter(|status| !status.is_empty())
     {
-        lines[0].push_str(&format!(" {status}"));
+        let _ = write!(lines[0], " {status}");
     }
 
     if let Some(stdout) = parsed.get("stdout").and_then(|value| value.as_str()) {
@@ -2502,15 +2452,15 @@ fn format_read_result(icon: &str, parsed: &serde_json::Value) -> String {
     let path = extract_tool_path(file);
     let start_line = file
         .get("startLine")
-        .and_then(|value| value.as_u64())
+        .and_then(serde_json::Value::as_u64)
         .unwrap_or(1);
     let num_lines = file
         .get("numLines")
-        .and_then(|value| value.as_u64())
+        .and_then(serde_json::Value::as_u64)
         .unwrap_or(0);
     let total_lines = file
         .get("totalLines")
-        .and_then(|value| value.as_u64())
+        .and_then(serde_json::Value::as_u64)
         .unwrap_or(num_lines);
     let content = file
         .get("content")
@@ -2536,8 +2486,7 @@ fn format_write_result(icon: &str, parsed: &serde_json::Value) -> String {
     let line_count = parsed
         .get("content")
         .and_then(|value| value.as_str())
-        .map(|content| content.lines().count())
+        .map_or(0, |content| content.lines().count());
-        .unwrap_or(0);
     format!(
         "{icon} \x1b[1;32m✏️ {} {path}\x1b[0m \x1b[2m({line_count} lines)\x1b[0m",
         if kind == "create" { "Wrote" } else { "Updated" },
@@ -2568,7 +2517,7 @@ fn format_edit_result(icon: &str, parsed: &serde_json::Value) -> String {
     let path = extract_tool_path(parsed);
     let suffix = if parsed
         .get("replaceAll")
-        .and_then(|value| value.as_bool())
+        .and_then(serde_json::Value::as_bool)
         .unwrap_or(false)
     {
         " (replace all)"
@@ -2596,7 +2545,7 @@ fn format_edit_result(icon: &str, parsed: &serde_json::Value) -> String {
 fn format_glob_result(icon: &str, parsed: &serde_json::Value) -> String {
     let num_files = parsed
         .get("numFiles")
-        .and_then(|value| value.as_u64())
+        .and_then(serde_json::Value::as_u64)
         .unwrap_or(0);
     let filenames = parsed
         .get("filenames")
@@ -2620,11 +2569,11 @@ fn format_glob_result(icon: &str, parsed: &serde_json::Value) -> String {
 fn format_grep_result(icon: &str, parsed: &serde_json::Value) -> String {
     let num_matches = parsed
         .get("numMatches")
-        .and_then(|value| value.as_u64())
+        .and_then(serde_json::Value::as_u64)
         .unwrap_or(0);
     let num_files = parsed
         .get("numFiles")
-        .and_then(|value| value.as_u64())
+        .and_then(serde_json::Value::as_u64)
         .unwrap_or(0);
     let content = parsed
         .get("content")
@@ -2731,6 +2680,26 @@ fn response_to_events(
     Ok(events)
 }
 
+fn push_prompt_cache_record(client: &AnthropicClient, events: &mut Vec<AssistantEvent>) {
+    if let Some(event) = client
+        .take_last_prompt_cache_record()
+        .and_then(prompt_cache_record_to_runtime_event)
+    {
+        events.push(AssistantEvent::PromptCache(event));
+    }
+}
+
+fn prompt_cache_record_to_runtime_event(record: PromptCacheRecord) -> Option<PromptCacheEvent> {
+    let cache_break = record.cache_break?;
+    Some(PromptCacheEvent {
+        unexpected: cache_break.unexpected,
+        reason: cache_break.reason,
+        previous_cache_read_input_tokens: cache_break.previous_cache_read_input_tokens,
+        current_cache_read_input_tokens: cache_break.current_cache_read_input_tokens,
+        token_drop: cache_break.token_drop,
+    })
+}
+
 struct CliToolExecutor {
     renderer: TerminalRenderer,
     emit_output: bool,
@@ -2857,7 +2826,7 @@ fn print_help_to(out: &mut impl Write) -> io::Result<()> {
     writeln!(out, "      Shorthand non-interactive prompt mode")?;
     writeln!(
         out,
-        "  claw --resume SESSION.jsonl [/status] [/compact] [...]"
+        "  claw --resume SESSION.json [/status] [/compact] [...]"
     )?;
     writeln!(
         out,
@@ -2917,7 +2886,7 @@ fn print_help_to(out: &mut impl Write) -> io::Result<()> {
     )?;
     writeln!(
         out,
-        "  claw --resume session.jsonl /status /diff /export notes.txt"
+        "  claw --resume session.json /status /diff /export notes.txt"
     )?;
     writeln!(out, "  claw login")?;
     writeln!(out, "  claw init")?;
@@ -2931,23 +2900,18 @@ fn print_help() {
 #[cfg(test)]
 mod tests {
     use super::{
-        create_managed_session_handle, filter_tool_specs, format_compact_report,
+        filter_tool_specs, format_compact_report, format_cost_report, format_model_report,
-        format_cost_report, format_model_report, format_model_switch_report,
+        format_model_switch_report, format_permissions_report, format_permissions_switch_report,
-        format_permissions_report, format_permissions_switch_report, format_resume_report,
+        format_resume_report, format_status_report, format_tool_call_start, format_tool_result,
-        format_status_report, format_tool_call_start, format_tool_result,
         normalize_permission_mode, parse_args, parse_git_status_metadata, print_help_to,
         push_output_block, render_config_report, render_memory_report, render_repl_help,
-        resolve_model_alias, resolve_session_reference, response_to_events,
+        resolve_model_alias, response_to_events, resume_supported_slash_commands, status_context,
-        resume_supported_slash_commands, status_context, CliAction, CliOutputFormat, SlashCommand,
+        CliAction, CliOutputFormat, SlashCommand, StatusUsage, DEFAULT_MODEL,
-        StatusUsage, DEFAULT_MODEL,
     };
     use api::{MessageResponse, OutputContentBlock, Usage};
-    use runtime::{
+    use runtime::{AssistantEvent, ContentBlock, ConversationMessage, MessageRole, PermissionMode};
-        AssistantEvent, ContentBlock, ConversationMessage, MessageRole, PermissionMode, Session,
-    };
     use serde_json::json;
     use std::path::PathBuf;
-    use std::sync::{Mutex, OnceLock};
 
     #[test]
     fn defaults_to_repl_when_no_args() {
@@ -3121,13 +3085,13 @@ mod tests {
     fn parses_resume_flag_with_slash_command() {
         let args = vec![
             "--resume".to_string(),
-            "session.jsonl".to_string(),
+            "session.json".to_string(),
             "/compact".to_string(),
         ];
         assert_eq!(
             parse_args(&args).expect("args should parse"),
             CliAction::ResumeSession {
-                session_path: PathBuf::from("session.jsonl"),
+                session_path: PathBuf::from("session.json"),
                 commands: vec!["/compact".to_string()],
             }
         );
@@ -3137,7 +3101,7 @@ mod tests {
     fn parses_resume_flag_with_multiple_slash_commands() {
         let args = vec![
             "--resume".to_string(),
-            "session.jsonl".to_string(),
+            "session.json".to_string(),
             "/status".to_string(),
             "/compact".to_string(),
             "/cost".to_string(),
@@ -3145,7 +3109,7 @@ mod tests {
         assert_eq!(
             parse_args(&args).expect("args should parse"),
             CliAction::ResumeSession {
-                session_path: PathBuf::from("session.jsonl"),
+                session_path: PathBuf::from("session.json"),
                 commands: vec![
                     "/status".to_string(),
                     "/compact".to_string(),
@@ -3173,7 +3137,7 @@ mod tests {
     fn shared_help_uses_resume_annotation_copy() {
         let help = commands::render_slash_command_help();
         assert!(help.contains("Slash commands"));
-        assert!(help.contains("works with --resume SESSION.jsonl"));
+        assert!(help.contains("works with --resume SESSION.json"));
     }
 
     #[test]
@@ -3193,7 +3157,7 @@ mod tests {
         assert!(help.contains("/diff"));
         assert!(help.contains("/version"));
         assert!(help.contains("/export [file]"));
-        assert!(help.contains("/session [list|switch <session-id>|fork [branch-name]]"));
+        assert!(help.contains("/session [list|switch <session-id>]"));
         assert!(help.contains("/exit"));
     }
 
@@ -3214,9 +3178,9 @@ mod tests {
 
     #[test]
     fn resume_report_uses_sectioned_layout() {
-        let report = format_resume_report("session.jsonl", 14, 6);
+        let report = format_resume_report("session.json", 14, 6);
         assert!(report.contains("Session resumed"));
-        assert!(report.contains("Session file     session.jsonl"));
+        assert!(report.contains("Session file     session.json"));
         assert!(report.contains("Messages         14"));
         assert!(report.contains("Turns            6"));
     }
@@ -3318,7 +3282,7 @@ mod tests {
             "workspace-write",
             &super::StatusContext {
                 cwd: PathBuf::from("/tmp/project"),
-                session_path: Some(PathBuf::from("session.jsonl")),
+                session_path: Some(PathBuf::from("session.json")),
                 loaded_config_files: 2,
                 discovered_config_files: 3,
                 memory_file_count: 4,
@@ -3335,7 +3299,7 @@ mod tests {
         assert!(status.contains("Cwd              /tmp/project"));
         assert!(status.contains("Project root     /tmp"));
         assert!(status.contains("Git branch       main"));
-        assert!(status.contains("Session          session.jsonl"));
+        assert!(status.contains("Session          session.json"));
         assert!(status.contains("Config files     loaded 2/3"));
         assert!(status.contains("Memory files     4"));
     }
@@ -3410,9 +3374,9 @@ mod tests {
     #[test]
     fn parses_resume_and_config_slash_commands() {
         assert_eq!(
-            SlashCommand::parse("/resume saved-session.jsonl"),
+            SlashCommand::parse("/resume saved-session.json"),
             Some(SlashCommand::Resume {
-                session_path: Some("saved-session.jsonl".to_string())
+                session_path: Some("saved-session.json".to_string())
             })
         );
         assert_eq!(
@@ -3431,65 +3395,6 @@ mod tests {
         );
         assert_eq!(SlashCommand::parse("/memory"), Some(SlashCommand::Memory));
         assert_eq!(SlashCommand::parse("/init"), Some(SlashCommand::Init));
-        assert_eq!(
-            SlashCommand::parse("/session fork incident-review"),
-            Some(SlashCommand::Session {
-                action: Some("fork".to_string()),
-                target: Some("incident-review".to_string())
-            })
-        );
-    }
-
-    #[test]
-    fn help_mentions_jsonl_resume_examples() {
-        let mut help = Vec::new();
-        print_help_to(&mut help).expect("help should render");
-        let help = String::from_utf8(help).expect("help should be utf8");
-        assert!(help.contains("claw --resume SESSION.jsonl"));
-        assert!(help.contains("claw --resume session.jsonl /status /diff /export notes.txt"));
-    }
-
-    #[test]
-    fn managed_sessions_default_to_jsonl_and_resolve_legacy_json() {
-        let _guard = cwd_lock().lock().expect("cwd lock");
-        let workspace = temp_workspace("session-resolution");
-        std::fs::create_dir_all(&workspace).expect("workspace should create");
-        let previous = std::env::current_dir().expect("cwd");
-        std::env::set_current_dir(&workspace).expect("switch cwd");
-
-        let handle = create_managed_session_handle("session-alpha").expect("jsonl handle");
-        assert!(handle.path.ends_with("session-alpha.jsonl"));
-
-        let legacy_path = workspace.join(".claude/sessions/legacy.json");
-        std::fs::create_dir_all(
-            legacy_path
-                .parent()
-                .expect("legacy path should have parent directory"),
-        )
-        .expect("session dir should exist");
-        Session::new()
-            .with_persistence_path(legacy_path.clone())
-            .save_to_path(&legacy_path)
-            .expect("legacy session should save");
-
-        let resolved = resolve_session_reference("legacy").expect("legacy session should resolve");
-        assert_eq!(resolved.path, legacy_path);
-
-        std::env::set_current_dir(previous).expect("restore cwd");
-        std::fs::remove_dir_all(workspace).expect("workspace should clean up");
-    }
-
-    fn cwd_lock() -> &'static Mutex<()> {
-        static LOCK: OnceLock<Mutex<()>> = OnceLock::new();
-        LOCK.get_or_init(|| Mutex::new(()))
-    }
-
-    fn temp_workspace(label: &str) -> PathBuf {
-        let nanos = std::time::SystemTime::now()
-            .duration_since(std::time::UNIX_EPOCH)
-            .expect("system time should be after epoch")
-            .as_nanos();
-        std::env::temp_dir().join(format!("claw-cli-{label}-{nanos}"))
     }
 
     #[test]

rust/crates/rusty-claude-cli/src/render.rs

@@ -286,7 +286,7 @@ impl TerminalRenderer {
     ) {
         match event {
             Event::Start(Tag::Heading { level, .. }) => {
-                self.start_heading(state, level as u8, output)
+                Self::start_heading(state, level as u8, output);
             }
             Event::End(TagEnd::Paragraph) => output.push_str("\n\n"),
             Event::Start(Tag::BlockQuote(..)) => self.start_quote(state, output),
@@ -426,7 +426,7 @@ impl TerminalRenderer {
         }
     }
 
-    fn start_heading(&self, state: &mut RenderState, level: u8, output: &mut String) {
+    fn start_heading(state: &mut RenderState, level: u8, output: &mut String) {
         state.heading_level = Some(level);
         if !output.is_empty() {
             output.push('\n');

rust/crates/tools/src/lib.rs

@@ -5,15 +5,15 @@ use std::time::{Duration, Instant};
 
 use api::{
     read_base_url, AnthropicClient, ContentBlockDelta, InputContentBlock, InputMessage,
-    MessageRequest, MessageResponse, OutputContentBlock, StreamEvent as ApiStreamEvent, ToolChoice,
+    MessageRequest, MessageResponse, OutputContentBlock, PromptCache, PromptCacheRecord,
-    ToolDefinition, ToolResultContentBlock,
+    StreamEvent as ApiStreamEvent, ToolChoice, ToolDefinition, ToolResultContentBlock,
 };
 use reqwest::blocking::Client;
 use runtime::{
     edit_file, execute_bash, glob_search, grep_search, load_system_prompt, read_file, write_file,
     ApiClient, ApiRequest, AssistantEvent, BashCommandInput, ContentBlock, ConversationMessage,
     ConversationRuntime, GrepSearchInput, MessageRole, PermissionMode, PermissionPolicy,
-    RuntimeError, Session, TokenUsage, ToolError, ToolExecutor,
+    PromptCacheEvent, RuntimeError, Session, TokenUsage, ToolError, ToolExecutor,
 };
 use serde::{Deserialize, Serialize};
 use serde_json::{json, Value};
@@ -1466,7 +1466,8 @@ fn build_agent_runtime(
         .clone()
         .unwrap_or_else(|| DEFAULT_AGENT_MODEL.to_string());
     let allowed_tools = job.allowed_tools.clone();
-    let api_client = AnthropicRuntimeClient::new(model, allowed_tools.clone())?;
+    let api_client =
+        AnthropicRuntimeClient::new(model, allowed_tools.clone(), job.manifest.agent_id.clone())?;
     let tool_executor = SubagentToolExecutor::new(allowed_tools);
     Ok(ConversationRuntime::new(
         Session::new(),
@@ -1643,10 +1644,15 @@ struct AnthropicRuntimeClient {
 }
 
 impl AnthropicRuntimeClient {
-    fn new(model: String, allowed_tools: BTreeSet<String>) -> Result<Self, String> {
+    fn new(
+        model: String,
+        allowed_tools: BTreeSet<String>,
+        session_id: impl Into<String>,
+    ) -> Result<Self, String> {
         let client = AnthropicClient::from_env()
             .map_err(|error| error.to_string())?
-            .with_base_url(read_base_url());
+            .with_base_url(read_base_url())
+            .with_prompt_cache(PromptCache::new(session_id));
         Ok(Self {
             runtime: tokio::runtime::Runtime::new().map_err(|error| error.to_string())?,
             client,
@@ -1657,6 +1663,7 @@ impl AnthropicRuntimeClient {
 }
 
 impl ApiClient for AnthropicRuntimeClient {
+    #[allow(clippy::too_many_lines)]
     fn stream(&mut self, request: ApiRequest) -> Result<Vec<AssistantEvent>, RuntimeError> {
         let tools = tool_specs_for_allowed_tools(Some(&self.allowed_tools))
             .into_iter()
@@ -1726,8 +1733,8 @@ impl ApiClient for AnthropicRuntimeClient {
                         events.push(AssistantEvent::Usage(TokenUsage {
                             input_tokens: delta.usage.input_tokens,
                             output_tokens: delta.usage.output_tokens,
-                            cache_creation_input_tokens: 0,
+                            cache_creation_input_tokens: delta.usage.cache_creation_input_tokens,
-                            cache_read_input_tokens: 0,
+                            cache_read_input_tokens: delta.usage.cache_read_input_tokens,
                         }));
                     }
                     ApiStreamEvent::MessageStop(_) => {
@@ -1737,6 +1744,8 @@ impl ApiClient for AnthropicRuntimeClient {
                 }
             }
 
+            push_prompt_cache_record(&self.client, &mut events);
+
             if !saw_stop
                 && events.iter().any(|event| {
                     matches!(event, AssistantEvent::TextDelta(text) if !text.is_empty())
@@ -1761,7 +1770,9 @@ impl ApiClient for AnthropicRuntimeClient {
                 })
                 .await
                 .map_err(|error| RuntimeError::new(error.to_string()))?;
-            Ok(response_to_events(response))
+            let mut events = response_to_events(response);
+            push_prompt_cache_record(&self.client, &mut events);
+            Ok(events)
         })
     }
 }
@@ -1884,6 +1895,26 @@ fn response_to_events(response: MessageResponse) -> Vec<AssistantEvent> {
     events
 }
 
+fn push_prompt_cache_record(client: &AnthropicClient, events: &mut Vec<AssistantEvent>) {
+    if let Some(event) = client
+        .take_last_prompt_cache_record()
+        .and_then(prompt_cache_record_to_runtime_event)
+    {
+        events.push(AssistantEvent::PromptCache(event));
+    }
+}
+
+fn prompt_cache_record_to_runtime_event(record: PromptCacheRecord) -> Option<PromptCacheEvent> {
+    let cache_break = record.cache_break?;
+    Some(PromptCacheEvent {
+        unexpected: cache_break.unexpected,
+        reason: cache_break.reason,
+        previous_cache_read_input_tokens: cache_break.previous_cache_read_input_tokens,
+        current_cache_read_input_tokens: cache_break.current_cache_read_input_tokens,
+        token_drop: cache_break.token_drop,
+    })
+}
+
 fn final_assistant_text(summary: &runtime::TurnSummary) -> String {
     summary
         .assistant_messages