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02-hidden-features-and-codenames.md

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Hidden Features & Model Codenames

Based on Claude Code v2.1.88 decompiled source code analysis.

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)

Source code 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