ContextLib

A project library that outlives every agent session.

Real screenshot of the sample library (an invented project).

Each project's knowledge — decisions, requirements, facts, lessons, glossary, sources and returned work — is stored as plain Markdown files with a small front-matter block, in one folder per project. The folders can sit on an external SSD. A person can understand a project from Finder alone. Agents read and write the same files through a CLI (ctxlib) and an MCP server (context_* tools). Any database is a search index rebuilt from the files, never the source of truth.

It is the open core of the context plugin of AgentBrain.

The Python package is agentbrain_contextlib. The CLI is ctxlib. Python 3.9+, standard library only. Git is optional.

Why

Agent sessions end. Chat logs drift. A store that only a model can parse is a dead end the first time you open Finder on an airplane.

ContextLib keeps the durable layer as files you can read, diff, copy, and take to another Mac. The search index is disposable. The Markdown is the product.

Related MCP server: mcp-rag

60-second quickstart

pip install "git+https://github.com/willykeenan/agentbrain-contextlib"

ctxlib init ~/ContextLib --project lighthouse-app \
  --name "Lighthouse App" \
  --purpose "Harbor dashboard for lamp status, fog signals, and traffic"

ctxlib add lighthouse-app decision "Use keeper logs as lamp status" \
  --body "**Decision:** Lamp status comes from keeper logs, not AIS." \
  --author mara \
  --tag lamp --tag keeper-log

ctxlib brief lighthouse-app
ctxlib doctor lighthouse-app --ten-minute

The library path is --library PATH, else env CONTEXTLIB_ROOT, else ~/ContextLib.

A finished sample you can open in Finder without installing anything lives at examples/sample-library/. Start at that README, then the project BRIEF, then decisions/INDEX.md.

Full walkthrough: docs/QUICKSTART.md.

CLI (ctxlib)

ctxlib [--library PATH] COMMAND

Types: decision, requirement, fact, lesson, glossary, source, return, brief-note.

MCP (context_* tools)

ctxlib mcp --identity ID is a stdio JSON-RPC server. Protocol versions: 2024-11-05, 2025-03-26, 2025-06-18.

Tools: context_brief, context_search, context_get, context_record, context_supersede, context_review_due, context_review, context_capture, context_export, context_import, context_status.

The identity is fixed at start. Tools cannot write as another identity. Errors come back as isError results.

Claude Code

{
  "mcpServers": {
    "contextlib": {
      "command": "ctxlib",
      "args": ["mcp", "--identity", "claude:project"],
      "env": { "CONTEXTLIB_ROOT": "/Volumes/SSD/ContextLib" }
    }
  }
}

Codex

{
  "mcpServers": {
    "contextlib": {
      "command": "ctxlib",
      "args": ["mcp", "--identity", "codex:project"],
      "env": { "CONTEXTLIB_ROOT": "/Volumes/SSD/ContextLib" }
    }
  }
}

Cursor

In .cursor/mcp.json (project) or ~/.cursor/mcp.json (user):

{
  "mcpServers": {
    "contextlib": {
      "command": "ctxlib",
      "args": ["mcp", "--identity", "cursor:project"],
      "env": { "CONTEXTLIB_ROOT": "/Volumes/SSD/ContextLib" }
    }
  }
}

Details: docs/MCP.md.

Record format

A record is UTF-8 Markdown. It starts with a YAML-like front-matter block (scalars, quoted strings, lists of scalars, lists of flat maps) and a Markdown body. The body hash is stored in the front-matter; bodies are not rewritten.

---
id: dec-20260926-7f3a
type: decision
title: Use plain files as the source of truth
status: current
project: lighthouse-app
scope: {lane: workstream, team: owner}
created: 2026-09-26T14:03:00Z
author: mara
approved_by: mara — "files first, database is only an index"
evidence:
  - {ref: "project:BRIEF.md"}
  - {ref: "url:https://example.com/x"}
supersedes: []
superseded_by: null
review_by: 2027-03-25
tags: [storage, library]
sensitivity: normal
body_sha256: "…"
---
**Decision:** …
**Why:** …
**Consequences:** …
**How to verify:** …

Evidence refs are project:<relative path>, ssd:<relative path>, room:<name>#<seq>, url:<https url>, or note:<text>. Absolute home paths and secrets are refused at write time.

Ids: <prefix>-<YYYYMMDD>-<4 hex> with prefixes dec, req, fact, les, glo, src, ret, note.

Files: <YYYY-MM-DD>_<title-slug>_<id-suffix>.md. Superseded records move to superseded/ with the body hash still valid.

Library layout

<library root>/
├── README.md              how to read this in 10 minutes
├── INDEX.md               one line per project
├── library.json           {"format": "contextlib/1", "library_id", "created"}
└── projects/<slug>/
    ├── README.md BRIEF.md GLOSSARY.md TIMELINE.md project.json
    ├── decisions/ requirements/ facts/ lessons/ glossary/ sources/ returns/
    │   ├── INDEX.md
    │   ├── <YYYY-MM-DD>_<title-slug>_<id-suffix>.md
    │   └── superseded/
    ├── inbox/             proposed records
    ├── exports/
    └── .contextlib/       ledger + rebuildable index.sqlite

Keeping the library on an SSD, using it unplugged, and moving it between Macs: docs/SSD.md.

Develop

PYTHONPATH=src python3 -m unittest discover -s tests
python3 tools/check.py packaging

Python 3.9+, standard library only. See CONTRIBUTING.md and CHANGELOG.md.

Apache-2.0, copyright KE Studios. SECURITY.md for reports.

ContextLib is the open library layer of AgentBrain (agentrooms.io).

Run with Docker

The image is published at ghcr.io/willykeenan/agentbrain-contextlib for Apple silicon and Intel. Mount your library at /library:

claude mcp add contextlib -- docker run -i --rm --user "$(id -u):$(id -g)" -v ~/ContextLib:/library ghcr.io/willykeenan/agentbrain-contextlib mcp --identity you