> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/sanbuphy/claude-code-source-code/llms.txt
> Use this file to discover all available pages before exploring further.

# Hidden Features & Codenames

> Internal codenames, feature flags, and hidden functionality discovered in Claude Code v2.1.88 source code.

<Warning>
  This page contains research findings derived from decompiled source code of `@anthropic-ai/claude-code` v2.1.88. It is intended for educational and research purposes only. All source code is the intellectual property of Anthropic.
</Warning>

## Model Codename System

Anthropic uses animal names as internal model codenames. The build system actively prevents these names from appearing in published artifacts.

### Known Codenames

| Codename     | Animal              | Role                                       | Evidence                                                                 |
| ------------ | ------------------- | ------------------------------------------ | ------------------------------------------------------------------------ |
| **Tengu**    | 天狗 (Japanese demon) | Product/telemetry prefix; possibly a model | Used as `tengu_*` prefix for all 250+ analytics events and feature flags |
| **Capybara** | Capybara            | Sonnet-series model, currently at v8       | `capybara-v2-fast[1m]`, dedicated prompt patches for v8 behavior issues  |
| **Fennec**   | 耳廓狐 (Fennec fox)    | Predecessor to Opus 4.6                    | Migration: `fennec-latest` → `opus`                                      |
| **Numbat**   | 袋食蚁兽 (Numbat)       | Next upcoming model                        | Comment: `// @[MODEL LAUNCH]: Remove this section when we launch numbat` |

### Codename Protection

The `undercover` mode lists explicitly protected terms:

```typescript theme={null}
// 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 pipeline scans compiled output for leaked codenames via `scripts/excluded-strings.txt`. To avoid triggering the canary, the buddy system pet species "capybara" is constructed at runtime using `String.fromCharCode()`:

```typescript theme={null}
// 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.
```

### Capybara v8 Known Behavioral Issues

The source code documents specific bugs in the current Capybara (Sonnet) v8 model that required workarounds:

<AccordionGroup>
  <Accordion title="Stop sequence false trigger (~10% rate)">
    Occurs when `<functions>` appears at the prompt tail. Dedicated mitigation exists in the agent loop.

    Source: `src/utils/messages.ts:2141`
  </Accordion>

  <Accordion title="Empty tool_result causes zero output">
    An empty `tool_result` message causes the model to produce no response. Handled in storage layer.

    Source: `src/utils/toolResultStorage.ts:281`
  </Accordion>

  <Accordion title="Over-commenting tendency">
    v8 adds excessive inline comments to generated code. Requires a dedicated anti-comment prompt patch.

    Source: `src/constants/prompts.ts:204`
  </Accordion>

  <Accordion title="Elevated false-claims rate">
    v8 has a 29–30% false-claims rate compared to v4's 16.7%. Internal users receive a dedicated mitigation prompt.

    Source: `src/constants/prompts.ts:237`
  </Accordion>

  <Accordion title="Insufficient verification">
    v8 under-verifies its own outputs. Internal builds add a "thoroughness counterweight" to the system prompt.

    Source: `src/constants/prompts.ts:210`
  </Accordion>
</AccordionGroup>

***

## Feature Flag Naming Convention

All feature flags use the `tengu_` prefix followed by **random word pairs** (adjective/material + nature/object). This pattern is intentional — it prevents external observers from inferring the feature's purpose from the flag name alone.

<AccordionGroup>
  <Accordion title="Full feature flag list" icon="flag">
    | Flag                          | Purpose                                      |
    | ----------------------------- | -------------------------------------------- |
    | `tengu_onyx_plover`           | Auto Dream (background memory consolidation) |
    | `tengu_coral_fern`            | memdir feature                               |
    | `tengu_moth_copse`            | memdir switch (secondary)                    |
    | `tengu_herring_clock`         | Team memory                                  |
    | `tengu_passport_quail`        | Path feature                                 |
    | `tengu_slate_thimble`         | memdir switch (tertiary)                     |
    | `tengu_sedge_lantern`         | Away Summary                                 |
    | `tengu_frond_boric`           | Analytics kill switch                        |
    | `tengu_amber_quartz_disabled` | Voice mode emergency off                     |
    | `tengu_amber_flint`           | Agent teams                                  |
    | `tengu_hive_evidence`         | Verification agent                           |
    | `tengu_penguins_off`          | Fast mode kill switch                        |
    | `tengu_marble_sandcastle`     | Fast mode availability                       |
    | `tengu_ant_model_override`    | Internal employee model override             |

    Source: `src/services/analytics/`, `src/utils/`, `src/voice/`
  </Accordion>
</AccordionGroup>

***

## Internal vs. External User Differences

<Warning>
  Anthropic employees (`USER_TYPE === 'ant'`) receive materially different model behavior, prompt quality, and tool access compared to external users. This disparity is hardcoded in the source.
</Warning>

### Prompt Differences

Source: `src/constants/prompts.ts`

| Dimension               | External Users     | Internal (`ant`) Users                      |
| ----------------------- | ------------------ | ------------------------------------------- |
| 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 unavailable externally:

* **`REPLTool`** — Interactive REPL mode (VM sandbox)
* **`SuggestBackgroundPRTool`** — Background PR suggestions
* **`TungstenTool`** — Performance monitoring panel
* **`VerifyPlanExecutionTool`** — Plan execution verification
* **Agent nesting** — Agents spawning sub-agents

***

## Undercover Mode

<Warning>
  Undercover mode instructs the AI to conceal its identity and present contributions as human-authored when Anthropic employees use Claude Code in public or open-source repositories. **There is no force-off switch.**
</Warning>

### Activation Logic

Source: `src/utils/undercover.ts:28-37`

```typescript theme={null}
export function isUndercover(): boolean {
  if (process.env.USER_TYPE === 'ant') {
    if (isEnvTruthy(process.env.CLAUDE_CODE_UNDERCOVER)) return true
    // Auto: active unless positively confirmed internal repo
    return getRepoClassCached() !== 'internal'
  }
  return false
}
```

Key properties:

* **Internal only**: Only activates for Anthropic employees (`USER_TYPE === 'ant'`)
* **Default ON**: Active in all repos that are not on the internal allowlist
* **No force-OFF**: The source explicitly states: *"There is NO force-OFF. This guards against model codename leaks"*
* **External builds**: Dead-code-eliminated by the bundler; never executes for non-Anthropic users

### The Prompt Given to the Model

Source: `src/utils/undercover.ts:39-69`

```
## UNDERCOVER MODE — CRITICAL

You are operating UNDERCOVER in a PUBLIC/OPEN-SOURCE repository. Your commit
messages, PR titles, and PR bodies MUST NOT contain ANY Anthropic-internal
information. Do not blow your cover.

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)
- Internal repo or project names (e.g., claude-cli-internal, anthropics/…)
- Internal tooling, Slack channels, or short links (e.g., go/cc, #claude-code-…)
- The phrase "Claude Code" or any mention that you are an AI
- Any hint of what model or version you are
- Co-Authored-By lines or any other attribution

Write commit messages as a human developer would — describe only what the code
change does.

GOOD:
- "Fix race condition in file watcher initialization"
- "Add support for custom key bindings"

BAD (never write these):
- "Fix bug found while testing with Claude Capybara"
- "1-shotted by claude-opus-4-6"
- "Generated with Claude Code"
- "Co-Authored-By: Claude Opus 4.6 <…>"
```

### Implications for Open-Source Communities

When Anthropic employees use Claude Code to contribute to open-source projects under undercover mode:

1. Code is written by AI, but commits appear human-authored
2. No `Co-Authored-By: Claude` attribution is added
3. No "Generated with Claude Code" markers are included
4. Project maintainers and contributors cannot identify AI-generated contributions
5. This may conflict with open-source transparency norms and individual project contribution policies

### Model Codename Masking

A separate masking function further protects codenames in any output that does reference a model name:

```typescript theme={null}
// src/utils/model/model.ts:386-392
function maskModelCodename(baseName: string): string {
  // e.g. capybara-v2-fast → cap*****-v2-fast
  const [codename = '', ...rest] = baseName.split('-')
  const masked = codename.slice(0, 3) + '*'.repeat(Math.max(0, codename.length - 3))
  return [masked, ...rest].join('-')
}
```

***

## Hidden Slash Commands

The following slash commands exist in source but are not listed in the standard help output:

| Command        | Status | Description                                               |
| -------------- | ------ | --------------------------------------------------------- |
| `/btw`         | Active | Ask a side question without interrupting the current task |
| `/stickers`    | Active | Opens browser to order Claude Code stickers               |
| `/thinkback`   | Active | 2025 Year in Review                                       |
| `/effort`      | Active | Set model effort level for the current session            |
| `/good-claude` | Stub   | Hidden placeholder with no implementation                 |
| `/bughunter`   | Stub   | Hidden placeholder with no implementation                 |