Hidden Features & Codenames

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

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.

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:

// 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():

// 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:

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

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

Over-commenting tendency

v8 adds excessive inline comments to generated code. Requires a dedicated anti-comment prompt patch.

Source: src/constants/prompts.ts:204

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

Insufficient verification

v8 under-verifies its own outputs. Internal builds add a “thoroughness counterweight” to the system prompt.

Source: src/constants/prompts.ts:210

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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.

Full feature flag list

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/

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Internal vs. External User Differences

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.

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

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Undercover Mode

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.

Activation Logic

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

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:

// 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('-')
}

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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