claude-code-source-code/docs/en/02-hidden-features-and-codenames.md

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:

// 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:

// 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