> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/yocxy2/claurst/llms.txt
> Use this file to discover all available pages before exploring further.

# 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                                                 |

<Warning>
  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.
</Warning>

### Building with features enabled (Rust)

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

```bash theme={null}
# 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`:

```toml Cargo.toml theme={null}
[features]
default = []
coordinator-mode = []
bridge-mode = []
```

<Note>
  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.
</Note>

## Runtime feature flags (GrowthBook)

Claurst uses [GrowthBook](https://www.growthbook.io/) 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                                         |

```bash theme={null}
# 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 theme={null}
// 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 theme={null}
// 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.