Cairn MCP Server

中文 | English

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Cairn structures your project's historical decisions, rejected paths, and accepted trade-offs into a three-layer format that AI coding assistants read automatically — so they work within your project's real constraints instead of suggesting in a vacuum.

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The Problem

Your project is 18 months old. In month 7, you migrated from Redux to Zustand — the boilerplate cost wasn't worth it for a two-person team. In month 11, you tried tRPC, hit integration issues with your existing REST clients, and rolled back. Neither decision is documented anywhere except your memory.

Today, you open a new AI session to refactor a module.

AI suggestion #1: Use Redux Toolkit.
AI suggestion #2: Migrate your API layer to tRPC.

Both are reasonable — if you don't know the history. You spend 10 minutes explaining what's already been ruled out. Next week, same session, same loop.

This isn't a model capability problem. Claude, GPT, Gemini — they're all smart enough. They just don't know what this specific project has already tried and why it didn't work.

Every new session, AI is a brilliant architect who just walked in. Cairn gives it the project memory of a colleague who's been here from the start.

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What is Cairn

Cairn is an AI path-dependency constraint system — a structured format for recording what your project has decided, what it has rejected, and what trade-offs it has accepted.

It is not a documentation system. Every entry must change what AI suggests. If a piece of information doesn't alter AI behavior, it doesn't belong in Cairn.

Cairn fills the gap between what AI tools already handle well:

What tools handle today	What Cairn adds
Coding style, naming conventions	Rejected directions and why
Current tech stack and architecture	Stage-aware constraints (MVP vs growth)
What's being built now	Accepted debt that should not be touched
How to write code	What paths have already been walked

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How It Works

Cairn uses a three-layer directory at your repository root:

.cairn/
├── output.md          # Layer 1: global constraints, read every session
├── SKILL.md           # Operating protocol; AI reads this to know how to maintain .cairn/
├── domains/           # Layer 2: domain design context, read during planning
│   ├── api-layer.md
│   └── auth.md
└── history/           # Layer 3: structured source events, queried on demand
    ├── 2023-09_trpc-experiment-rejection.md
    └── 2024-01_auth-debt-accepted.md

Layer	File	When AI Reads	Token Budget
Global Constraints	.cairn/output.md	Every session, always	500 target / 800 max
Domain Context	.cairn/domains/*.md	During planning & design	200–400 per file
Decision History	.cairn/history/*.md	On-demand precise queries	Unlimited

Runtime flow:

1. AI reads output.md at session start — establishes what's off-limits and the current project stage
2. When planning a feature, AI reads the relevant domain file — understands that area's evolution and pitfalls
3. When full historical detail is needed, AI queries history/ — structured source events with rejected alternatives

Three Constraint Types

Three concepts produce distinct AI behavior changes:

Concept	What It Means	AI Behavior
no-go	A direction evaluated and excluded	Never suggest it
accepted debt	A known defect intentionally left in place	Never attempt to fix it
known pitfalls	An operational trap in a specific domain	Actively avoid trigger conditions

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Implementation

Cairn is a pure file format — no runtime, no background process, no external service required. The .cairn/ directory is plain Markdown with a defined structure, versioned alongside your code.

Layer 1 (output.md): Six required YAML-headlined sections (stage, no-go, hooks, stack, debt, open questions). Hard-limited to 800 tokens. AI reads this before every response.

Layer 2 (domain files): YAML frontmatter with hooks keyword lists for intent detection, followed by Markdown sections for no-go rules, known pitfalls, and evolution history. Written to be replaced wholesale, not appended — the source events stay in history/.

Layer 3 (history files): Bare key: value format (no YAML fences). Each file is a structured source event with scope, status, behavior_effect, and confidence, plus decision fields such as summary, rejected, chosen, reason, and revisit_when. output.md and domain files are runtime projections of this source layer. The rejected field remains the most critical — it's what AI is most likely to re-propose.

Protocol layer (.cairn/SKILL.md): The full operating protocol — when to read each layer, how to interpret constraints, when and how to write back to history/domains/output. Copied from skills/claude-code/SKILL.md by cairn init. AI reads this once per session.

Adapter guide blocks: Each AI tool's config file gets a 12-line block pointing to .cairn/SKILL.md. Installed by cairn init. Regenerated by cairn init --refresh-skills.

Language support (v0.0.2): .cairn/ content can be maintained in any language. The CLI and init script follow CAIRN_LANG (auto-detected from $LANG). AI skill adapters include language-continuity rules so new entries match existing file language. Format contracts (section headers, field names) remain English ASCII regardless of content language.

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Quality Guarantees

Specification-driven: spec/FORMAT.md is the authoritative reference for all three layers. Every tool, script, and example must conform to it. When in doubt, the spec wins.

Test coverage:

• Shell test suite: 600+ assertions across 8 test files (CLI, init script, format validation, protocol conformance)
• MCP Server test suite: 270+ assertions across 15 Vitest test files (parsers, all 6 tools)

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Quick Start

Install

Option A: Interactive init script (recommended)

curl -sL https://raw.githubusercontent.com/zzf2333/Cairn/main/scripts/cairn-init.sh -o cairn-init.sh
chmod +x cairn-init.sh
./cairn-init.sh

The script guides you through 5 steps (~30 minutes):

1. Choose your project's domains (11 standard options)
2. Fill in output.md (stage, no-go, stack, accepted debt)
3. Initialize history/ with an entry template
4. Initialize domains/ (empty to start — this is normal)
5. Install the guide block for your AI tool(s) (points to .cairn/SKILL.md for the full protocol)

Option B: CLI

git clone https://github.com/zzf2333/Cairn ~/.cairn
echo 'export PATH="$HOME/.cairn/cli:$PATH"' >> ~/.zshrc   # or ~/.bashrc
source ~/.zshrc
cairn init    # interactive, delegates to cairn-init.sh

Requires Bash 3.2+ (macOS system bash is sufficient). Symlinks are also supported — the script resolves its real path via readlink.

Option C: MCP Server only

# From npm
npm install -g cairn-mcp-server

# Or from source
git clone https://github.com/zzf2333/Cairn
cd Cairn/mcp && npm install && npm run build

Requires Node.js 18+.

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Daily Usage

After cairn init, the AI handles ongoing .cairn/ maintenance automatically. No daily CLI commands needed — the AI reads output.md and SKILL.md at session start, loads domain files when planning, and writes history entries directly after task completion.

# Check health (after onboarding a new dev, or when something seems off)
cairn doctor

# Refresh guide blocks and .cairn/SKILL.md after updating Cairn
cairn init --refresh-skills

# Install guide blocks globally (covers projects without per-project init)
cairn init --global

Task completion protocol

After every non-trivial task, the AI produces a structured Cairn reflection block:

Task completion summary
- Completed work: switched internal services from REST to tRPC
- Changed files / domains: src/api/, api-layer
- Risk level: medium

Cairn reflection
- Result: memory-updated
- Impacted domains: api-layer
- History recorded: history/2026-04_trpc-adoption.md
- Output updated: no
- Domains updated: api-layer
- Audit required: no
- Next action: none

cairn: recorded 1 event: history/2026-04_trpc-adoption.md

Result is one of no-op (nothing written), memory-updated (write-back occurred), or audit-required (migration risk — follow-up needed). See spec/TASK-COMPLETION-PROTOCOL.md for the normative definition.

Review with git diff .cairn/. Edit the file directly if anything needs adjusting.

Stale detection: cairn doctor compares each domain file's updated: frontmatter against the recorded_date of its history entries:

$ cairn doctor

⚠  api-layer  stale: 2 new history entries since 2024-03
⚠  .cairn/SKILL.md not found — run: cairn init
✓  auth        up to date (2024-06)

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Update

Init script / CLI:

cd ~/.cairn && git pull
# PATH stays valid — no re-install needed

MCP Server (npm):

npm install -g cairn-mcp-server@latest

MCP Server (from source):

cd /path/to/Cairn && git pull
cd mcp && npm install && npm run build
# Running server picks up the new build automatically on next restart

Guide blocks / SKILL.md: Run cairn init --refresh-skills to refresh guide blocks and .cairn/SKILL.md without touching output.md, domains/, or history/. See the upgrade guide for details.

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Uninstall

CLI:

# Remove the PATH export line from ~/.zshrc (or ~/.bashrc), then:
rm -rf ~/.cairn

MCP Server:

npm uninstall -g cairn-mcp-server
# Remove the "cairn" block from your MCP settings file
# (Claude Code: ~/.claude/settings.json or .claude/settings.json)

Skill adapters:
Delete the adapter file you copied during setup (see table in Supported AI Tools below).

Project data (.cairn/):
The .cairn/ directory belongs to your project repository. Removing Cairn tooling does not delete it — remove it manually if desired, or leave it as documentation.

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MCP Server

The MCP Server (Phase 3) upgrades Cairn from file-injection to typed tool calls. Instead of relying on AI tools to infer when to load context, it exposes six precise tools that match AI intent against domain frontmatter hooks fields.

Tools

Tool	Description
cairn_output	Read .cairn/output.md — Layer 1 global constraints
cairn_domain	Read .cairn/domains/<name>.md — Layer 2 domain context
cairn_query	Search .cairn/history/ — Layer 3 events, with domain/type filters
cairn_match	Match keywords (and optional files paths) against domain hooks — returns confidence levels (high/medium/low) and related domain advisory
cairn_write_history	Write a history entry directly to .cairn/history/ (for pure-MCP clients)
cairn_doctor	Run cairn doctor --json and return structured health check results

Configuration

Claude Code — add to ~/.claude/mcp.json (global) or .claude/mcp.json (project):

{
    "mcpServers": {
        "cairn": {
            "command": "cairn-mcp-server"
        }
    }
}

Cursor — add to .cursor/mcp.json in your project:

{
    "mcpServers": {
        "cairn": {
            "command": "cairn-mcp-server"
        }
    }
}

The server resolves .cairn/ by walking up from process.cwd(), or via the CAIRN_ROOT environment variable if you need to pin to a specific project.

See mcp/README.md for full configuration options and the recommended AI workflow.

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Supported AI Tools

Tool	Guide block file	Install Location
Claude Code	skills/claude-code/SKILL.md (canonical)	.cairn/SKILL.md + .claude/CLAUDE.md
Cursor	skills/cursor.mdc	.cairn/SKILL.md + .cursor/rules/cairn.mdc
Cline / Roo Code	skills/cline.md	.cairn/SKILL.md + .clinerules
Windsurf	skills/windsurf.md	.cairn/SKILL.md + .windsurfrules
GitHub Copilot	skills/copilot-instructions.md	.cairn/SKILL.md + .github/copilot-instructions.md
Codex CLI	skills/codex.md	.cairn/SKILL.md + AGENTS.md
Gemini CLI	skills/gemini-cli.md	.cairn/SKILL.md + GEMINI.md
OpenCode	skills/opencode.md	.cairn/SKILL.md + AGENTS.md

cairn init handles installation for all eight tools automatically. The full protocol lives in .cairn/SKILL.md (copied from skills/claude-code/SKILL.md); each tool's config file gets a 12-line guide block that points to it.

The data layer (.cairn/) is fully tool-agnostic — it travels with your repository.

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Example

examples/saas-18mo/ is a complete three-layer example from an 18-month SaaS project — the same project from the story at the top of this document:

• output.md — stage early-growth, no-go rules (tRPC, Redux, Kubernetes), active stack, accepted debts
• Three domain files: api-layer, auth, state-management
• Four history events: state management migration, tRPC rejection, auth debt acceptance, growth stage transition

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Documentation

Document	Contents
spec/FORMAT.md	Complete format reference for all three layers (authoritative)
spec/TASK-COMPLETION-PROTOCOL.md	Task completion protocol: reflection block format and doctor signals
spec/DESIGN.md	Why Cairn is designed the way it is
spec/vs-adr.md	How Cairn relates to Architecture Decision Records
spec/adoption-guide.md	Step-by-step Init and Reactive adoption guide
mcp/README.md	MCP Server configuration and tool reference
CHANGELOG.md	Version history

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License

MIT