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.

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BUDDY — companion pet system

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':

// 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

KAIROS — always-on assistant

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 <tick> prompts that let it decide whether to act proactively or stay quiet.

<tick>

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.

ULTRAPLAN — 30-minute remote planning

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.

Undercover Mode — internal identity protection

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.

Penguin Mode — fast mode

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.

System prompt architecture

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.

Beta API headers

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.).