Feature flags

Claurst uses compile-time feature gates to ship a single codebase that produces different binaries for different deployment contexts. Gated features compile away completely in standard builds.

Claurst inherits the feature-gating architecture of the original Claude Code TypeScript codebase. Features are toggled at compile time using Bun’s feature() bundler function (TypeScript) or Cargo features (Rust port), and at runtime via GrowthBook and environment variable overrides.

Compile-time feature flags

The bundler constant-folds feature checks and eliminates dead branches. This means a feature that is not enabled produces zero code in the output binary — not a disabled path, but no code at all.

Known feature flags

Flag	What it gates
PROACTIVE / KAIROS	Always-on assistant mode — Claude receives <tick> heartbeat prompts and acts proactively between user turns
KAIROS_BRIEF	Brief command — ultra-concise output mode designed for the persistent KAIROS assistant
BRIDGE_MODE	Remote control via claude.ai — JWT-authenticated WebSocket/SSE bridge for syncing sessions to the cloud
DAEMON	Background daemon mode — runs a persistent Claurst process in the background
VOICE_MODE	Voice input — speech-to-text integration for prompt entry
WORKFLOW_SCRIPTS	Workflow automation — enables the WorkflowTool and WorkflowScripts command
COORDINATOR_MODE	Multi-agent orchestration — transforms Claurst into a coordinator that spawns and directs parallel worker agents
TRANSCRIPT_CLASSIFIER	AFK mode — ML-based automatic permission approval using a transcript classifier (afk-mode-2026-01-31 beta header)
BUDDY	Companion pet system — Tamagotchi-style ASCII companion with species, stats, and Claude-generated personality
NATIVE_CLIENT_ATTESTATION	Client attestation — Bun’s HTTP stack overwrites the cch=00000 placeholder in the billing header with a computed hash
HISTORY_SNIP	History snipping — enables the SnipTool for extracting snippets from conversation history
EXPERIMENTAL_SKILL_SEARCH	Skill discovery — experimental search over the skills/commands registry

These flags control features that are internal to Anthropic’s production builds. Standard Claurst builds do not enable them. Attempting to enable internal-only flags in a community build will produce compilation errors or missing dependencies.

Building with features enabled (Rust)

The Claurst Rust workspace uses Cargo features. To enable a feature for a local build:

# Build with coordinator mode enabled
cargo build --features coordinator-mode

# Build with multiple features
cargo build --features "coordinator-mode,bridge-mode"

# Build the release binary with voice mode
cargo build --release --features voice-mode

To permanently enable features during development, add them to Cargo.toml:

Cargo.toml

[features]
default = []
coordinator-mode = []
bridge-mode = []

Feature names in Cargo.toml use kebab-case (coordinator-mode), while the original TypeScript source uses UPPER_SNAKE_CASE (COORDINATOR_MODE). They refer to the same concepts.

Runtime feature flags (GrowthBook)

Claurst uses GrowthBook for runtime feature gating and A/B testing. Runtime flags are fetched at startup and cached aggressively — many checks use stale values intentionally to avoid blocking the main loop.

All GrowthBook feature flags in Claude Code use the tengu_ prefix (Tengu is the internal project codename).

Examples of tengu_ prefixed runtime flags:

Flag	Effect
tengu_attribution_header	Kill-switch for the x-anthropic-billing-header
tengu_hawthorn_window	Overrides MAX_TOOL_RESULTS_PER_MESSAGE_CHARS
tengu_ultraplan_model	Configures the remote model used for ULTRAPLAN sessions
tengu_penguins_off	Kill-switch for Fast Mode (internally “Penguin Mode”)
tengu_scratch	Enables the shared scratchpad directory for coordinator workers

GrowthBook client keys are environment-dependent:

Environment	Key
External (prod)	sdk-zAZezfDKGoZuXXKe
Anthropic internal (prod)	sdk-xRVcrliHIlrg4og4
Anthropic internal (dev)	sdk-yZQvlplybuXjYh6L

Environment variable overrides

Several features can be toggled at runtime via environment variables, without recompiling:

Variable	Effect
CLAUDE_CODE_UNDERCOVER=1	Forces Undercover Mode on — prevents internal information from appearing in commits and PRs
CLAUDE_CODE_COORDINATOR_MODE=1	Activates multi-agent coordinator mode
CLAUDE_CODE_VERIFY_PLAN	Enables the VerifyPlanExecutionTool
ANTHROPIC_API_KEY	API key (preferred over storing in settings)
ANTHROPIC_BASE_URL	Override the API base URL for custom endpoints or proxies
BRAVE_SEARCH_API_KEY	Enables Brave Search as the WebSearchTool backend

# Run in coordinator mode without a feature-flag build
CLAUDE_CODE_COORDINATOR_MODE=1 claude

# Force undercover mode for a public repo session
CLAUDE_CODE_UNDERCOVER=1 claude

# Use a custom API endpoint
ANTHROPIC_BASE_URL=https://my-proxy.example.com claude

Dead-code elimination

Features gated behind a compile-time flag produce zero footprint in builds where they are disabled. There is no runtime check, no disabled code path, and no binary size overhead from unused features.

This is done via Bun’s constant-folding for TypeScript:

// TypeScript (original)
if (feature('COORDINATOR_MODE')) {
  // This entire block is eliminated from external builds
  registerCoordinatorTools()
}

And via Cargo #[cfg] attributes in the Rust port:

// Rust
#[cfg(feature = "coordinator-mode")]
fn register_coordinator_tools(registry: &mut ToolRegistry) {
    // Compiled in only when the feature is enabled
}

USER_TYPE gating

Beyond compile-time feature flags, some behavior is gated on USER_TYPE === 'ant' — meaning the user is an Anthropic employee. This gates:

• Access to staging API endpoints (api-staging.anthropic.com)
• Internal beta headers (cli-internal-2026-02-09)
• The ConfigTool and TungstenTool (internal-only tools)
• Debug prompt dumping to ~/.config/claude/dump-prompts/
• The /security-review slash command

These features are present in the source but are not accessible in standard builds regardless of environment variables.