> ## 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.
# Special systems
> In-depth reference for Claurst's advanced internal systems: BUDDY, KAIROS, ULTRAPLAN, Undercover Mode, Penguin Mode, system prompt architecture, and beta API headers.
Several subsystems in Claurst go beyond the core query loop. Some are gated behind compile-time feature flags and absent from standard external builds; others are always present but not exposed through the public interface.
BUDDY, KAIROS, and ULTRAPLAN are controlled by **compile-time feature flags**. They are dead-code-eliminated from builds that do not set the corresponding flag. Do not expect these subsystems to be available unless you build Claurst with the appropriate feature enabled.
***
**Feature flag:** `BUDDY`\
**Crate:** `cc-buddy` (`crates/buddy`)\
**Status:** Teaser window April 1–7 2026; full launch planned May 2026
BUDDY is a Tamagotchi-style companion pet that lives in your terminal alongside the input prompt. Each user gets a deterministic companion — same user always hatches the same species.
### Gacha and PRNG
Species is determined by a **Mulberry32 PRNG** seeded from your `userId` hash with the salt `'friend-2026-401'`:
```rust theme={null}
// Mulberry32 — deterministic, reproducible per user
fn mulberry32(seed: u32) -> impl FnMut() -> f64 {
move || {
// seed |= 0; seed = seed + 0x6D2B79F5 | 0;
// ... fast 32-bit mixing
}
}
```
The seed is derived from your user ID, so the same user always hatches the same buddy.
### Species and rarity
18 species across 5 rarity tiers. Rarity determines drop probability; shiny is a separate 1% roll independent of rarity.
| Rarity | Drop chance | Species |
| --------- | ----------- | -------------------------------------------------------------- |
| Common | 60% | Pebblecrab, Dustbunny, Mossfrog, Twigling, Dewdrop, Puddlefish |
| Uncommon | 25% | Cloudferret, Gustowl, Bramblebear, Thornfox |
| Rare | 10% | Crystaldrake, Deepstag, Lavapup |
| Epic | 4% | Stormwyrm, Voidcat, Aetherling |
| Legendary | 1% | Cosmoshale, Nebulynx |
A shiny Legendary Nebulynx has a **0.01%** chance (1% rarity × 1% shiny).
### Stats
Each buddy has 5 procedurally generated stats in the range 0–100:
| Stat | Description |
| ----------- | ------------------------------------ |
| `DEBUGGING` | Aptitude for finding bugs |
| `PATIENCE` | Tolerance for long compilation times |
| `CHAOS` | Tendency toward unexpected behavior |
| `WISDOM` | Accumulated experience |
| `SNARK` | Commentary sharpness |
### Sprites
Sprites are rendered as **5 lines × 12 characters** ASCII art with multiple animation frames (idle and reaction). They sit next to the user input prompt.
### Soul description
On first hatch, Claude generates a short personality description for the buddy — its "soul". The system prompt tells Claude:
```
A small {species} named {name} sits beside the user's input box and
occasionally comments in a speech bubble. You're not {name} — it's a
separate watcher.
```
The buddy has its own personality and responds when addressed by name.
### Key types (`cc-buddy`)
| Type | Description |
| --------- | -------------------------------------------------------------------------------- |
| `Buddy` | Species, name, rarity, `is_shiny`, stats, soul description, current sprite frame |
| `Species` | 18-variant enum with associated rarity tier |
| `Stats` | Five `u8` fields, 0–100 each |
**Feature flags:** `PROACTIVE` / `KAIROS`\
**Status:** Internal only; absent from all external builds
KAIROS is a persistent, always-running Claude assistant that does not wait for user input. It watches, logs, and proactively acts on things it notices.
### How it works
KAIROS maintains **append-only daily log files** tracking observations, decisions, and actions throughout the day. On a regular interval it receives `` prompts that let it decide whether to act proactively or stay quiet.
```
```
The system has a **15-second blocking budget**: any proactive action that would block the user's workflow for more than 15 seconds is deferred.
### Brief mode
When KAIROS is active, the `Brief` output mode produces extremely concise responses — suitable for a persistent assistant that should not flood the terminal.
### Exclusive tools
KAIROS has access to three tools not available to regular Claude Code sessions:
| Tool | Description |
| ------------------ | ---------------------------------------------------------- |
| `SendUserFile` | Push files directly to the user (notifications, summaries) |
| `PushNotification` | Send push notifications to the user's device |
| `SubscribePR` | Subscribe to and monitor pull request activity |
These tools are only compiled when the `KAIROS` feature flag is enabled. Attempting to invoke them in a standard build will result in an unknown-tool error.
**Status:** Internal; gated behind feature configuration\
**Model:** Opus 4.6 (configured via `tengu_ultraplan_model`)
ULTRAPLAN offloads a complex planning task to a remote **Cloud Container Runtime (CCR) session**, gives it up to 30 minutes to think, and lets the user approve the result from a browser.
### Flow
1. Claude Code identifies a task that needs deep planning.
2. It spins up a remote CCR session running Opus 4.6.
3. The terminal enters a polling state — checking every **3 seconds** for the result.
4. A browser-based UI lets you watch the planning happen and approve or reject it.
5. On approval, the sentinel value `__ULTRAPLAN_TELEPORT_LOCAL__` is used to return the result to the local terminal.
```
sentinel: __ULTRAPLAN_TELEPORT_LOCAL__
poll interval: 3 seconds
max planning time: 30 minutes
remote model: opus-4-6 (tengu_ultraplan_model)
```
The sentinel string `__ULTRAPLAN_TELEPORT_LOCAL__` is how the remote planning result is "teleported" back into the local session. It is never meant to be typed by the user.
**Activation:** `CLAUDE_CODE_UNDERCOVER=1` (explicit) or automatic in public/open-source repos\
**Status:** Present in all builds; injected into system prompt when active
Undercover Mode prevents Claude from accidentally revealing Anthropic-internal information in commits and pull requests. It is activated automatically unless the repo remote matches an internal allowlist.
### System prompt injection
When active, the following block is injected into the system prompt:
```
## UNDERCOVER MODE - CRITICAL
You are operating UNDERCOVER in a PUBLIC/OPEN-SOURCE repository. Your commit
messages, PR titles, and PR bodies MUST NOT contain ANY Anthropic-internal
information. Do not blow your cover.
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)
- Internal repo or project names
- Internal tooling, Slack channels, or short links (e.g., go/cc, #claude-code-…)
- The phrase "Claude Code" or any mention that you are an AI
- Co-Authored-By lines or any other attribution
```
### Activation logic
| Condition | Result |
| -------------------------------------- | ------------------------------------- |
| `CLAUDE_CODE_UNDERCOVER=1` | Forces Undercover Mode ON in any repo |
| Repo remote matches internal allowlist | Undercover Mode OFF |
| Repo remote does not match allowlist | Undercover Mode ON (automatic) |
There is **no force-OFF**. If the system is not confident it is in an internal repo, it stays undercover. `CLAUDE_CODE_UNDERCOVER=1` can only force it on, not off.
### Internal model codenames referenced
The instruction explicitly lists animal names as internal codenames to suppress: **Capybara**, **Tengu**, **Fennec**.
**Internal name:** Penguin Mode\
**User-facing name:** Fast mode\
**Slash command:** `/fast`
Fast mode is internally called "Penguin Mode" throughout the codebase.
### Configuration keys
| Key | Value |
| ---------------- | ------------------------------------- |
| API endpoint | `/api/claude_code_penguin_mode` |
| Config key | `penguinModeOrgEnabled` |
| Kill-switch flag | `tengu_penguins_off` |
| Analytics event | `tengu_org_penguin_mode_fetch_failed` |
| Beta header | `fast-mode-2026-02-01` |
### Enabling
```
/fast
```
Penguin Mode is enabled per-organization. The org-level setting is fetched at startup and cached. If the fetch fails, the analytics event `tengu_org_penguin_mode_fetch_failed` is recorded and the mode falls back to standard.
The system prompt is not a single static string. It is assembled at runtime from **modular, cached sections**.
### Structure
```
┌─────────────────────────────────────────────────┐
│ Static sections (cacheable across orgs) │
│ - Core instructions │
│ - Tool descriptions │
│ - CYBER_RISK_INSTRUCTION │
├─────────────────────────────────────────────────┤
│ SYSTEM_PROMPT_DYNAMIC_BOUNDARY │
├─────────────────────────────────────────────────┤
│ Dynamic sections (per user / per session) │
│ - User context │
│ - CLAUDE.md content │
│ - Git status │
│ - Undercover Mode injection (if active) │
└─────────────────────────────────────────────────┘
```
### Key markers and functions
| Symbol | Description |
| ----------------------------------------- | -------------------------------------------------------------------------------------------------------------------------- |
| `SYSTEM_PROMPT_DYNAMIC_BOUNDARY` | Marker string that splits cacheable static sections from volatile dynamic sections |
| `DANGEROUS_uncachedSystemPromptSection()` | Explicitly marks a section as non-cacheable (volatile) |
| `ContextBuilder::build_system_context()` | Builds OS, working directory, and git context |
| `ContextBuilder::build_user_context()` | Discovers and concatenates CLAUDE.md files from the current directory up to filesystem root and from `~/.claude/CLAUDE.md` |
The naming `DANGEROUS_uncachedSystemPromptSection` reflects a hard lesson: adding volatile content to the cached portion of the prompt silently breaks prompt caching for all users. The `DANGEROUS_` prefix is a deliberate warning to future engineers.
### CYBER\_RISK\_INSTRUCTION
One section of the static prompt carries a mandatory review header:
```
IMPORTANT: DO NOT MODIFY THIS INSTRUCTION WITHOUT SAFEGUARDS TEAM REVIEW
This instruction is owned by the Safeguards team (David Forsythe, Kyla Guru)
```
This instruction draws clear lines around authorized security research — permitted actions versus destructive techniques and supply chain compromise.
### Caching
Static sections are wrapped with `CacheControl::ephemeral()` via the `SystemPrompt::Blocks` variant in `cc-api`. The dynamic boundary ensures only truly volatile content bypasses the cache.
Claude Code negotiates a set of beta feature headers with the Anthropic API on every request. These are set via the `anthropic-beta` header in `cc-api`.
The base header constant in `cc-core::constants`:
```
ANTHROPIC_BETA_HEADER =
"interleaved-thinking-2025-05-14,token-efficient-tools-2025-02-19,files-api-2025-04-14"
```
The full list of beta features negotiated (from `constants/betas.ts` in the original TypeScript source):
| Beta header | Feature |
| ------------------------------------- | ---------------------------------------------- |
| `interleaved-thinking-2025-05-14` | Extended thinking interleaved with text |
| `context-1m-2025-08-07` | 1 million token context window |
| `structured-outputs-2025-12-15` | Structured output format |
| `web-search-2025-03-05` | Web search tool |
| `advanced-tool-use-2025-11-20` | Advanced tool use |
| `effort-2025-11-24` | Effort level control |
| `task-budgets-2026-03-13` | Task budget management |
| `prompt-caching-scope-2026-01-05` | Prompt cache scoping |
| `fast-mode-2026-02-01` | Fast mode (Penguin Mode) |
| `redact-thinking-2026-02-12` | Redacted thinking blocks |
| `token-efficient-tools-2026-03-28` | Token-efficient tool schemas |
| `afk-mode-2026-01-31` | AFK mode |
| `cli-internal-2026-02-09` | Internal-only features (Anthropic employees) |
| `advisor-tool-2026-03-01` | Advisor tool |
| `summarize-connector-text-2026-03-13` | Connector text summarization |
| `files-api-2025-04-14` | Files API |
| `token-efficient-tools-2025-02-19` | Token-efficient tool schemas (earlier version) |
`redact-thinking`, `afk-mode`, and `advisor-tool` are not yet publicly released at the time of writing.
Not all headers are sent on every request. The `cc-api` `AnthropicClient` selects the appropriate subset based on the request configuration (e.g., `thinking` enabled, `KAIROS` mode, etc.).