tc39-mcp

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Independent project — not an official Ecma International or TC39 publication. Reads the publicly published ECMAScript specs (ECMA-262 + ECMA-402).

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.

Snapshots resolve through a local cache → hosted Worker → bundled fallback chain. The stdio transport (npx tc39-mcp) fetches each snapshot from the hosted Cloudflare Worker on a cold cache, writes it under ~/.cache/tc39-mcp/, and serves it from disk thereafter — revalidating only when the local copy is older than ~4 hours (a conditional If-None-Match request). The npm package also bundles the latest stable + main editions of both specs plus the test262 and proposals indexes; when the Worker is unreachable, those are served straight from the package (the offline fallback — not written to the cache). The hosted Worker is also the HTTP alternative when you want a shared network endpoint; its R2 data 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 the npm package (latest stable + main editions plus the proposals and test262 indexes are bundled). The first call for a given snapshot fetches it from the hosted Worker and caches it locally; subsequent calls are served from disk, revalidated against the Worker only after the ~4-hour freshness window. If the Worker is unreachable, the bundled editions still answer offline. 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"
    }
  }
}

Traffic is rate-limited to 30 req/min per IP.

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, covering both ECMA-262 and ECMA-402 (Intl) proposals — filter by spec. Refreshed on the same 4-hour cadence as the specs.

• Local cache, bundled fallback (stdio). Once a snapshot is

cached under ~/.cache/tc39-mcp/, tool calls are served from disk and only revalidated against the hosted Worker after the ~4-hour freshness window (a conditional If-None-Match request that carries the R2 object key, never a clause-id). Bundled editions answer offline when the Worker is unreachable. 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`](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 – es2026, main. (ES5 / ES5.1 / ES6

have no upstream tags and aren't supported.)

• ECMA-402: es2016 – es2026, main. (402 publishes each

annual edition as an esYYYY branch rather than a tag; the fetch step resolves a branch or a tag the same way.)

• Aliases: latest is spec-aware (each spec → its current

stable release, es2026 today). draft / next → main on both.

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

Self-hosting snapshots

The stdio server fetches snapshots from the public hosted Worker at https://tc39-mcp.chicoxyzzy.workers.dev/r2/<key> (cache → Worker → bundled fallback), so on a strict-egress network it falls back to the bundled editions and can't reach the others. Override the base URL via TC39_MCP_BASE_URL to point at a private mirror — useful for strict-egress networks, air-gapped environments, or running against a self-hosted Worker:

TC39_MCP_BASE_URL=https://my-mirror.example.com npx tc39-mcp

The endpoint just needs to serve the same key structure (spec-<spec>-<edition>.json, test262-index.json, proposals-index.json) — a plain static file server works. If it returns ETags, the server revalidates with If-None-Match (cheap 304s); without them it just refetches the full object when a cached copy goes stale. To populate a mirror, run npm run parse against a local checkout (see below) and upload build/*.json to your bucket of choice.

The cache lives at $XDG_CACHE_HOME/tc39-mcp (or ~/.cache/tc39-mcp when XDG_CACHE_HOME is unset).

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", "--silent", "mcp"],
      "cwd": "/abs/path/to/tc39-mcp"
    }
  }
}

--silent keeps npm's lifecycle banner off stdout, so the MCP client receives a clean JSON-RPC stream.

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.

• CONTRIBUTING.md — what kinds of changes

land easily, what won't.

• SECURITY.md — threat model + responsible

disclosure.

• CHANGELOG.md — version history + auto-refresh

convention.

Privacy Policy

tc39-mcp is a read-only spec lookup service. The stdio transport (npx tc39-mcp) sends no telemetry and never transmits your queries — snapshots are fetched from the hosted Worker on a cold or stale cache and served from local disk otherwise; those fetches carry R2 object keys, never clause-ids or tool arguments. The hosted Cloudflare Worker collects only standard request metadata (IP for rate limiting, timestamps, request headers); it does not log request bodies, set cookies, or share data with third parties.

Full policy: tc39-mcp.chicoxyzzy.workers.dev/privacy

For privacy questions, open an issue with the privacy label on GitHub.

License

MIT