Claude Code Unveiled

The first architecture deep-dive, hidden feature catalog, and cost optimization guide for Claude Code — based on v2.1.88 source analysis (1,884 files, 132K lines of TypeScript).

中文版 | English

On March 31, 2026, Claude Code's TypeScript source (1,884 files, ~132K lines) was exposed through an unstripped .map file in the npm package. This repository includes the complete restored source code in src/ plus original analysis. This project extracts actionable insights — architecture patterns, hidden features, cost optimization, and best practices — all with source file references.

Table of Contents

• System Prompt (Complete Reconstruction)
• 87 Hidden Feature Flags
• 15 Hidden Slash Commands
• 25 Internal-Only Commands
• Undercover Mode (How Anthropic Hides AI Attribution)
• Cost Optimization (10 Tips from Source)
• Architecture Diagrams
• Claude Code vs Cursor vs Cline
• Telemetry & Privacy
• Remote Control & Killswitches
• Future Roadmap (Unreleased Features)
• CLAUDE.md Best Practices

System Prompt (Complete Reconstruction)

Full document | Source: src/constants/prompts.ts, src/utils/systemPrompt.ts

5-Layer Priority System

Priority 0: Override (loop mode, testing)        ← highest
Priority 1: Coordinator (multi-worker orchestration)
Priority 2: Agent (subagent definitions)
Priority 3: Custom (--system-prompt flag)
Priority 4: Default (standard Claude Code)       ← lowest
         + appendSystemPrompt (always appended unless override)

What the Default Prompt Actually Says

The system prompt is 9 major sections:

1. Identity — "You are an interactive agent that helps users with software engineering tasks"
2. System Rules — Tool execution, permission modes, prompt injection detection
3. Task Execution — The most detailed section:
   • Read code before modifying (mandatory)
   • Don't add unrequested features/refactoring
   • Don't create helpers for one-time operations
   • "Three similar lines of code is better than a premature abstraction"
   • Avoid OWASP Top 10 vulnerabilities
4. Careful Actions — Destructive operations need user confirmation (rm -rf, force push, etc.)
5. Tool Usage — Use dedicated tools (Read/Edit/Glob/Grep) instead of Bash
6. Tone — No emojis, concise, file_path:line_number format
7. Output Efficiency — "If you can say it in one sentence, don't use three"
8. Cache Boundary — __SYSTEM_PROMPT_DYNAMIC_BOUNDARY__ splits static/dynamic parts
9. Environment — CWD, platform, model name, knowledge cutoff

Cache Optimization

The prompt is split by __SYSTEM_PROMPT_DYNAMIC_BOUNDARY__:

• Before boundary (static): Cached globally, saves tokens on repeat calls
• After boundary (dynamic): User/session-specific, recalculated each time (memory, MCP, language, etc.)

Model Knowledge Cutoffs

87 Hidden Feature Flags

Full document | Source: src/commands.ts, various

These are compile-time switches via bun:bundle feature(). Most are dead-code-eliminated in the public npm build.

Core Codenames

Context & Compression

Obfuscated Runtime Config (tengu_* gates)

Runtime flags use tengu_ + random word pairs to deliberately hide their purpose:

tengu_frond_boric       → Analytics killswitch (Datadog/FirstParty)
tengu_passport_quail    → Memory extraction gate
tengu_moth_copse        → Memory extraction enable
tengu_bramble_lintel    → Memory extraction frequency
tengu_cicada_nap_ms     → Background refresh throttle
tengu_slate_prism       → Connector text summarization
tengu_amber_json_tools  → JSON tool format (token-efficient)
tengu_tool_pear         → Structured output (strict tools)

15 Hidden Slash Commands

Source: src/commands.ts

These require specific feature flags to appear:

25 Internal-Only Commands

Condition: USER_TYPE === 'ant' && !IS_DEMO

Undercover Mode

Full document | Source: src/utils/undercover.ts

When Anthropic employees (USER_TYPE=ant) work in public repositories, Claude Code automatically hides all internal information.

How It Works

isUndercover() check:
  USER_TYPE !== 'ant' → always OFF (external users unaffected)
  USER_TYPE === 'ant' →
    CLAUDE_CODE_UNDERCOVER=1 → forced ON
    Repo in internal allowlist → OFF
    Repo is public/unknown → ON (safe default)

There is NO force-OFF option. If Claude Code can't confirm it's an internal repo, undercover stays ON.

What Gets Injected

## UNDERCOVER MODE — CRITICAL

You are operating UNDERCOVER in a PUBLIC/OPEN-SOURCE repository.
NEVER include in commit messages or PR descriptions:
- Internal model codenames (Capybara, Tengu, etc.)
- Unreleased model version numbers
- Internal repo or project names
- The phrase "Claude Code" or any mention that you are an AI
- Co-Authored-By lines or any other attribution

Internal Repo Allowlist (22 private repos)

anthropics/claude-cli-internal, anthropics/anthropic, anthropics/apps,
anthropics/casino, anthropics/dbt, anthropics/dotfiles,
anthropics/terraform-config, anthropics/hex-export, anthropics/feedback-v2,
anthropics/labs, anthropics/argo-rollouts, anthropics/starling-configs,
anthropics/ts-tools, anthropics/ts-capsules, anthropics/feldspar-testing,
anthropics/trellis, anthropics/claude-for-hiring, anthropics/forge-web,
anthropics/infra-manifests, anthropics/mycro_manifests,
anthropics/mycro_configs, anthropics/mobile-apps

Source: src/utils/commitAttribution.ts lines 30-75

Cost Optimization

Full document | Sources: src/utils/modelCost.ts, src/utils/context.ts, src/services/compact/

Model Pricing (per million tokens)

Fast mode is 6x the price. Use it only for short, time-sensitive tasks.

Source: src/utils/modelCost.ts lines 36-69

10 Money-Saving Tips

| # | Tip | Source Evide