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docs/en/02-hidden-features-and-codenames.md

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+# Hidden Features & Model Codenames
+
+> Based on publicly available online references and community discussions on Claude Code v2.1.88.
+
+## Model Codename System
+
+Anthropic uses **animal names** as internal model codenames. These are aggressively protected from leaking into external builds.
+
+### Known Codenames
+
+| Codename | Role | Evidence |
+|----------|------|----------|
+| **Tengu** (天狗) | Product/telemetry prefix, possibly a model | Used as `tengu_*` prefix for all 250+ analytics events and feature flags |
+| **Capybara** | Sonnet-series model, currently at v8 | `capybara-v2-fast[1m]`, prompt patches for v8 behavior issues |
+| **Fennec** (耳廓狐) | Predecessor to Opus 4.6 | Migration: `fennec-latest` → `opus` |
+| **Numbat** (袋食蚁兽) | Next model launch | Comment: "Remove this section when we launch numbat" |
+
+### Codename Protection
+
+The `undercover` mode explicitly lists protected codenames:
+
+```typescript
+// src/utils/undercover.ts:48-49
+NEVER include in commit messages or PR descriptions:
+- Internal model codenames (animal names like Capybara, Tengu, etc.)
+- Unreleased model version numbers (e.g., opus-4-7, sonnet-4-8)
+```
+
+The build system uses `scripts/excluded-strings.txt` to scan for leaked codenames. Buddy system species are encoded via `String.fromCharCode()` to avoid triggering the canary:
+
+```typescript
+// src/buddy/types.ts:10-13
+// One species name collides with a model-codename canary in excluded-strings.txt.
+// The check greps build output (not source), so runtime-constructing the value keeps
+// the literal out of the bundle while the check stays armed for the actual codename.
+```
+
+That colliding species is **capybara** — both a pet species and a model codename.
+
+### Capybara Behavior Issues (v8)
+
+The architecture reveals specific behavioral problems with Capybara v8:
+
+1. **Stop sequence false trigger** (~10% rate when `<functions>` at prompt tail)
+   - Source: `src/utils/messages.ts:2141`
+
+2. **Empty tool_result causes zero output**
+   - Source: `src/utils/toolResultStorage.ts:281`
+
+3. **Over-commenting** — requires dedicated anti-comment prompt patches
+   - Source: `src/constants/prompts.ts:204`
+
+4. **High false-claims rate**: v8 has 29-30% FC rate vs v4's 16.7%
+   - Source: `src/constants/prompts.ts:237`
+
+5. **Insufficient verification** — requires "thoroughness counterweight"
+   - Source: `src/constants/prompts.ts:210`
+
+## Feature Flag Naming Convention
+
+All feature flags use the `tengu_` prefix with **random word pairs** to obscure their purpose:
+
+| Flag | Purpose |
+|------|---------|
+| `tengu_onyx_plover` | Auto Dream (background memory consolidation) |
+| `tengu_coral_fern` | memdir feature |
+| `tengu_moth_copse` | Another memdir switch |
+| `tengu_herring_clock` | Team memory |
+| `tengu_passport_quail` | Path feature |
+| `tengu_slate_thimble` | Another memdir switch |
+| `tengu_sedge_lantern` | Away Summary |
+| `tengu_frond_boric` | Analytics kill switch |
+| `tengu_amber_quartz_disabled` | Voice mode kill switch |
+| `tengu_amber_flint` | Agent teams |
+| `tengu_hive_evidence` | Verification agent |
+
+The random word pattern (adjective/material + nature/object) prevents external observers from inferring feature purpose from flag names alone.
+
+## Internal vs External User Difference
+
+Anthropic employees (`USER_TYPE === 'ant'`) receive significantly better treatment:
+
+### Prompt Differences (`src/constants/prompts.ts`)
+
+| Dimension | External Users | Internal (ant) |
+|-----------|---------------|----------------|
+| Output style | "Be extra concise" | "Err on the side of more explanation" |
+| False-claims mitigation | None | Dedicated Capybara v8 patches |
+| Numeric length anchors | None | "≤25 words between tools, ≤100 words final" |
+| Verification agent | None | Required for non-trivial changes |
+| Comment guidance | Generic | Dedicated anti-over-commenting prompt |
+| Proactive correction | None | "If user has misconception, say so" |
+
+### Tool Access
+
+Internal users have access to tools not available externally:
+- `REPLTool` — REPL mode
+- `SuggestBackgroundPRTool` — background PR suggestions
+- `TungstenTool` — performance monitoring panel
+- `VerifyPlanExecutionTool` — plan verification
+- Agent nesting (agents spawning agents)
+
+## Hidden Commands
+
+| Command | Status | Description |
+|---------|--------|-------------|
+| `/btw` | Active | Ask side questions without interrupting |
+| `/stickers` | Active | Order Claude Code stickers (opens browser) |
+| `/thinkback` | Active | 2025 Year in Review |
+| `/effort` | Active | Set model effort level |
+| `/good-claude` | Stub | Hidden placeholder |
+| `/bughunter` | Stub | Hidden placeholder |