diff --git a/rust/crates/api/src/providers/openai_compat.rs b/rust/crates/api/src/providers/openai_compat.rs
index 3344765..140d93e 100644
--- a/rust/crates/api/src/providers/openai_compat.rs
+++ b/rust/crates/api/src/providers/openai_compat.rs
@@ -801,6 +801,10 @@ fn build_chat_completion_request(request: &MessageRequest, config: OpenAiCompatC
             payload["stop"] = json!(stop);
         }
     }
+    // reasoning_effort for OpenAI-compatible reasoning models (o4-mini, o3, etc.)
+    if let Some(effort) = &request.reasoning_effort {
+        payload["reasoning_effort"] = json!(effort);
+    }
 
     payload
 }
@@ -1216,6 +1220,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn reasoning_effort_is_included_when_set() {
+        let payload = build_chat_completion_request(
+            &MessageRequest {
+                model: "o4-mini".to_string(),
+                max_tokens: 1024,
+                messages: vec![InputMessage::user_text("think hard")],
+                reasoning_effort: Some("high".to_string()),
+                ..Default::default()
+            },
+            OpenAiCompatConfig::openai(),
+        );
+        assert_eq!(payload["reasoning_effort"], json!("high"));
+    }
+
+    #[test]
+    fn reasoning_effort_omitted_when_not_set() {
+        let payload = build_chat_completion_request(
+            &MessageRequest {
+                model: "gpt-4o".to_string(),
+                max_tokens: 64,
+                messages: vec![InputMessage::user_text("hello")],
+                ..Default::default()
+            },
+            OpenAiCompatConfig::openai(),
+        );
+        assert!(payload.get("reasoning_effort").is_none());
+    }
+
     #[test]
     fn openai_streaming_requests_include_usage_opt_in() {
         let payload = build_chat_completion_request(
@@ -1333,6 +1366,7 @@ mod tests {
             frequency_penalty: Some(0.5),
             presence_penalty: Some(0.3),
             stop: Some(vec!["\n".to_string()]),
+            reasoning_effort: None,
         };
         let payload = build_chat_completion_request(&request, OpenAiCompatConfig::openai());
         assert_eq!(payload["temperature"], 0.7);
diff --git a/rust/crates/api/src/types.rs b/rust/crates/api/src/types.rs
index 830b3de..e136a76 100644
--- a/rust/crates/api/src/types.rs
+++ b/rust/crates/api/src/types.rs
@@ -26,6 +26,11 @@ pub struct MessageRequest {
     pub presence_penalty: Option<f64>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub stop: Option<Vec<String>>,
+    /// Reasoning effort level for OpenAI-compatible reasoning models (e.g. `o4-mini`).
+    /// Accepted values: `"low"`, `"medium"`, `"high"`. Omitted when `None`.
+    /// Silently ignored by backends that do not support it.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub reasoning_effort: Option<String>,
 }
 
 impl MessageRequest {