Assertion MCP Server

Assertion — a second brain for your codebase

Gives Claude Code, OpenAI Codex, and Cursor memory that remembers the why behind your decisions and the strategy across your project — and keeps it current, superseding calls you've reversed instead of resurfacing them. It recalls that context automatically as you work, into a shared, project-scoped tree. Reads run over a hosted MCP endpoint (no local server); small hooks handle capture and context injection. One codebase serves all three agents (single copy of the logic). By Assertion AI.

Install, then sign in once in your browser. There is no key to copy. Requires a system python3 (for the stdlib hooks); no other deps.

Install — Claude Code

Already installed? Keep the copy you have. If you installed from this repo (assertion@assertion-ai), don't also install it from the Claude Code community catalog (assertion@claude-community). Two copies means two sets of hooks, so every turn is captured twice. /assertion:upgrade updates whichever copy you have.

# 1) in Claude Code:
/plugin marketplace add Assertion-AI/assertion
/plugin install assertion@assertion-ai
/reload-plugins

/reload-plugins turns the plugin on in the session you're in, with no restart. Or install from your terminal before you start Claude Code, and skip it: claude plugin marketplace add Assertion-AI/assertion && claude plugin install assertion@assertion-ai

2) Sign in:

/assertion:login

Your browser opens at Assertion Studio; click Connect (new here? sign up on that page first). Memory turns on in the same session: the memory tools and capture both pick up the sign-in, with no restart and no key to copy. Then try recall <topic>. Until you sign in, each session starts with a one-line notice that memory is off.

On a machine with no browser (SSH, a server), /assertion:login code shows a short code to confirm from any device instead. /assertion:logout signs this computer out and turns memory off.

Requirements: a system python3 (for the two stdlib hooks). No other deps — the memory tools connect over HTTP.

Using a key instead (CI, or a shared machine): ASSERTION_API_KEY still works and takes precedence over the sign-in. Put it in ~/.claude/settings.json as {"env": {"ASSERTION_API_KEY": "<key>"}} rather than a shell export, so it reaches the plugin however you launch Claude Code.

Upgrading: run /assertion:upgrade, then /reload-plugins. No restart needed.

Spaces: /assertion:space lists your memory spaces and shows which one this session uses; /assertion:space <name> switches this session only (/assertion:space personal to go back).

Install — OpenAI Codex

# 1) install the plugin
codex plugin marketplace add Assertion-AI/assertion
codex plugin add assertion@assertion-ai

# 2) sign in in your browser (no key to copy)
python3 "$(ls -d ~/.codex/plugins/cache/*/assertion/*/scripts | sort -V | tail -1)/assertion.py" login --client codex

Or, once Codex is open, ask it to "sign in to Assertion" (the plugin's assertion-login skill runs the same sign-in; "sign out of Assertion" runs assertion-logout). Either way the key is saved for the hooks and written into ~/.codex/config.toml for the memory tools.

1. Use Codex interactively (codex in a terminal, or the VS Code Codex panel — not codex exec). On first run, toggle the three hooks Trust on when prompted (once).

Capture + injection then work against the same shared tree, in CLI and VS Code. Full Codex details (recall/expand setup, dev override): plugin/codex/README.md.

Install — Cursor

Cursor has no plugin marketplace, so it installs via a one-time script that wires its hooks.json + mcp.json for you:

git clone https://github.com/Assertion-AI/assertion
cd assertion/plugin/cursor && python3 install_cursor.py   # signs you in in the browser

Then fully quit and reopen Cursor. The installer merges into any existing Cursor config (it won't touch your other hooks or MCP servers), backs up what it changes, and defaults to prod + the shared default workspace. In Cursor, /assertion-login signs in again, /assertion-space lists or switches memory spaces, and /upgrade updates the clone. Options (--key, --workspace, --server, --uninstall) and a manual fallback: plugin/cursor/README.md.

What's included

• MCP tools — query and curate your memory from inside the agent, over https://memory.assertion-ai.com:
  • recall — find past work relevant to what you're doing now
  • expand — open up a point to see the detail beneath it
  • evidence — see the supporting detail behind a claim
  • conflicts — surface where the record disagrees with itself
  • resolve — settle a conflict and keep the memory coherent
  • superseded — see what's been replaced as decisions changed
  • unsupersede — bring back something that was replaced
• Stop hook — captures each finished turn into the tree.
• SessionStart hook — injects the project's working-set into context.

Both hooks fail open: if the backend is unreachable, your session is never blocked.

Optional: show your memory space in the Claude Code status line

A permanent 📁 <space> indicator at the bottom of your terminal — instant, local, per-session-accurate. In ~/.claude/settings.json:

"statusLine": {
  "type": "command",
  "command": "python3 ~/.claude/plugins/cache/assertion-ai/assertion/scripts/statusline_space.py"
}

(If you already have a statusline command, chain this after it.)

License

The plugin in this repository is licensed under the Apache License 2.0. See NOTICE.

The license covers this client code only. The hosted Assertion service it connects to is governed by the Terms of Service, and your data by the privacy notice. "Assertion" is a trademark of Assertion AI, Inc.