Tc39 MCP Server

tc39-mcp

📖 Docs: tc39-mcp.chicoxyzzy.workers.dev — Get started · Tools · Cookbook · Editions · Architecture · Hosting · Sponsor

Give MCP-speaking AI agents structural access to the JS spec. Claude Code, Claude Desktop, Cursor, MCP Inspector, and anything else that speaks the Model Context Protocol can now call clause.get sec-tonumber and get back parsed JSON (algorithm steps as discrete arrays, cross-references as ids, signatures as typed values) instead of being handed a 4 MB spec.html to grep through. Tools cover ECMA-262 (the core language) and ECMA-402 (the Intl API): clauses, algorithm steps, cross-references both ways, edition diffs, upstream git history, test262 search, proposal lookup. Every response is SHA-pinned to a specific upstream commit so anything an agent cites stays reproducible.

Offline-first by default: the stdio transport (npx tc39-mcp) ships every parsed snapshot in the tarball, so once installed it runs entirely offline — no network call per agent query, no leakage of which clause the agent is reading. The hosted Cloudflare Worker is the HTTP alternative when you want a shared network endpoint; it auto-refreshes from upstream every ~4 hours.

Install + first call

Wire into Claude Code, Claude Desktop, Cursor, or any MCP-speaking client via .mcp.json:

{
  "mcpServers": {
    "tc39": { "command": "npx", "args": ["tc39-mcp"] }
  }
}

The first run downloads ~50 MB (the parsed snapshots ship in the tarball — no separate fetch step needed, and every subsequent call is served from local disk with no network round-trip). Then in your client:

use clause.get to read sec-tonumber and show me the steps

You should see structured JSON back:

{
  "meta": {
    "id": "sec-tonumber",
    "aoid": "ToNumber",
    "title": "ToNumber ( argument )",
    "number": "7.1.4",
    "kind": "op"
  },
  "signatureRaw": "ToNumber ( _argument_: an ECMAScript language value, ): either a normal completion containing a Number or a throw completion",
  "algorithms": [
    { "steps": [
        { "text": "If _argument_ is a Number, return _argument_." },
        { "text": "If _argument_ is either *undefined* or a Symbol, throw a *TypeError* exception." },
        { "text": "If _argument_ is *null*, return *+0*<sub>𝔽</sub>." },
        "..."
    ]}
  ],
  "crossrefs": ["sec-tonumber-applied-to-the-string-type", "..."]
}

Five-minute walkthrough: docs/getting-started.md.

Other transports

Hosted Cloudflare Worker (HTTP)

{
  "mcpServers": {
    "tc39": {
      "type": "http",
      "url": "https://tc39-mcp.chicoxyzzy.workers.dev/mcp"
    }
  }
}

Anonymous traffic is rate-limited to 30 req/min per IP. Sponsors get 300 req/min per key by adding an Authorization: Bearer tcms_… header.

Global CLI

npm i -g tc39-mcp
tc39-mcp                     # reads stdio

What it's good at

• Letting an agent reason about the spec without hallucinating. Structured JSON answers ground the model on real spec text: step numbering, cross-reference targets, signature shapes, edition deltas, conformance tests. Anything cited resolves to a specific clause id at a specific SHA — easy to verify, easy to reproduce.
• Finding the clause you want from a hint. spec.search ranks AOID-exact matches first; spec.symbol_resolve decodes [[Prototype]] / %Object.prototype% / ~enumerate~.
• Following references both ways. spec.crossrefs returns what a clause cites AND who cites it. AOID-densified so bare mentions in step text count, not just <emu-xref> hrefs. include_cross_spec resolves 262 ↔ 402 hops. (Cookbook recipe 1.)
• Comparing editions and tracking prose drift. spec.diff between any two editions back to ES2016; spec.history walks the upstream git log via pickaxe search. (Cookbook recipe 2.)
• Finding test262 coverage for a clause. test262.search with prefix-matched esid: catches sec-tonumber AND sec-tonumber-applied-to-the-string-type in one call.
• Mapping proposals to the spec. proposal.list / proposal.get from a structured index of tc39/proposals, refreshed on the same 4-hour cadence as the specs.
• Running entirely offline (stdio). Once npx tc39-mcp has installed, every tool call is served from on-disk snapshots — no network round-trip per query, no leakage of which clause the agent is reading, no upstream rate limit. The hosted Worker is the HTTP alternative for shared / multi-tenant use.

Tools (19 across 5 namespaces)

Full reference (input schemas, output types, example calls per tool): docs/tools.md — auto-generated from the schemas so it never drifts.

Specs + editions

Every spec-reading tool accepts spec ("262" or "402", default "262") and edition (default "latest").

• ECMA-262: es2016 – es2025, main. (ES5 / ES5.1 / ES6 have no upstream tags and aren't supported.)
• ECMA-402: es2025-candidate, main. (No annual final-release tagging upstream.)
• Aliases: latest is spec-aware (262 → current stable release; 402 → main). draft / next → main on both.

Full table + how to add new releases: docs/editions.md.

Sponsorship

The free anonymous tier of the hosted Worker stays free — 30 req/min per IP, no signup, no key. Sponsorship is optional and goes toward keeping the hosted Worker running comfortably as usage grows.

Sponsors at any tier ≥ $5/mo get an API key (tcms_…) that gives them 300 req/min bucketed per-key instead of per-IP, so the same key can drive an interactive chat, a CI pipeline, and a batch agent without those processes competing for budget.

To sponsor: https://github.com/sponsors/xyzzylabs. The maintainer DMs you the key via GitHub Sponsors. Full tier setup + client-side wiring instructions: docs/sponsor.md.

Build from source (contributors)

End users don't need this — the npm package and the hosted Worker are the supported surfaces above. This is for working on the server itself.

git clone https://github.com/xyzzylabs/tc39-mcp
cd tc39-mcp
npm install
npm run fetch-spec               # ~2 min, ~150 MB — both specs at every supported edition
npm run parse                    # spec.html → build/spec-<spec>-<edition>.json
npm run fetch-test262            # optional, enables test262.* (~300 MB)
npm run build-test262-index
npm run fetch-proposals          # optional, enables proposal.* (~50 MB)
npm run build-proposals-index
npm run mcp                      # start the stdio MCP server against your source

Point Claude Code at your local source instead of the published bin:

{
  "mcpServers": {
    "tc39": {
      "type": "stdio",
      "command": "npm",
      "args": ["run", "mcp"],
      "cwd": "/abs/path/to/tc39-mcp"
    }
  }
}

Docs

Hosted at tc39-mcp.chicoxyzzy.workers.dev — searchable, dark-mode-friendly, auto-rebuilt on every refresh so /snapshots always reflects the live SHAs.

In-repo (also browseable on GitHub):

• docs/getting-started.md — install → wire → first call → verify. Five minutes.
• docs/tools.md — every tool, every field, every example. Auto-generated from source.
• docs/cookbook.md — multi-tool recipes: cross-spec lookups, prose-drift tracking, grammar/SDO cross-references, test262 coverage, proposal-to-clause mapping.
• docs/editions.md — supported editions + alias resolution.
• docs/architecture.md — data pipeline, parser, cache, memory model.
• docs/deployment.md — local stdio, npm CLI, hosted Cloudflare Worker, refresh model, observability, sponsor lifecycle.
• docs/sponsor.md — sponsorship tiers, key setup, security model.
• CONTRIBUTING.md — what kinds of changes land easily, what won't.
• SECURITY.md — threat model + responsible disclosure.
• CHANGELOG.md — version history + auto-refresh convention.

License

MIT