tc39-mcp MCP Server

by xyzzylabs Your server? Claim it

answering

tc39-mcp is answering right now. Last checked 5 min ago. 203 installs a week from npm. It exposes 17 tools. Last commit 1 Nov 2026.

Independent MCP server for the TC39 specs (ECMA-262 + ECMA-402): clauses, search, diffs, history.

Installs per day peak 44 · avg 11 · +35% w/w

a month agotoday

Uptime history 91 days of history · worst day 93%

91 days agonow

100.0%

Uptime 24h

91 of 91 checks

Tools

read from the server

138 ms

Response time

average over 24h

203

Installs / week

npm and PyPI

What changed 3

Every tool that appeared, vanished or quietly changed what it asks for. Recorded since 8 September 2026. No other catalogue keeps this.

8 Oct a tool changed version

7 Oct a tool changed version

8 Sep a tool changed version

Nothing serious here today

Today is the operative word: we check tc39-mcp every 15 minutes and re-read its code on every release. Watch it and you find out the day that stops being true.

Connect this server

Endpoint below is the one we actually reach during checks — not the one copied from a README. Last verified 5 min ago.

Claude Code Claude Desktop Codex Cursor VS Code

run in your terminal

claude mcp add tc39-mcp --transport http https://mcp.xyzzylabs.ai/tc39/mcp

Settings → Connectors → Add custom connector, then paste this address

https://mcp.xyzzylabs.ai/tc39/mcp

~/.codex/config.toml

[mcp_servers.tc39-mcp]
url = "https://mcp.xyzzylabs.ai/tc39/mcp"

.cursor/mcp.json

{
  "mcpServers": {
    "tc39-mcp": {
      "url": "https://mcp.xyzzylabs.ai/tc39/mcp"
    }
  }
}

.vscode/mcp.json

{
  "servers": {
    "tc39-mcp": {
      "type": "http",
      "url": "https://mcp.xyzzylabs.ai/tc39/mcp"
    }
  }
}

Available tools 17

Read directly from the server with tools/list, grouped by what they act on. If a tool disappears, we record the date.

spec

spec.about

Self-description of this MCP server: package name + version, per-snapshot pin metadata (sha, fetched_at, biblio_commit, clause_count) for every supported (spec, edition), plus test262 + proposals index headers when present.

spec.crossrefs

For a clause id, return its outgoing references (clauses it cites) and/or incoming references (clauses that cite it — the back-reference index the parse alone doesn't expose). The reverse index is AOID-densified from step text. `direction`: 'in' | 'out' | 'both' (default). Outgoing also carries an `external` category: the clause's citations to external specs (Unicode, IETF, WHATWG) as resolvable URLs. Set `include_cross_spec: true` to also resolve outgoing references from ECMA-262 → ECMA-402 (or vice versa).

spec.diff

Clause-level diff of one clause across two editions of a spec. Reports identical / modified / added / removed plus a field-level breakdown (title, signature, step count, reworded step indices, notes, crossrefs). `from` defaults to the latest stable release, `to` to main.

spec.global_search

Run spec.search across both ECMA-262 and ECMA-402 in one call and interleave hits by score. Each hit is tagged with the spec it came from. Use it when you don't know which spec defines a symbol. `search_steps` also matches algorithm step text.

spec.grammar

Query grammar productions captured from the spec's `<emu-grammar>` blocks. `{ nonterminal }` returns every production for that non-terminal (exact match); `{ contains }` filters by RHS / name substring; neither lists all non-terminals + their production counts. `include_sdo` folds in SDO-attached productions (off by default).

spec.sdo_index

Index Syntax-Directed Operations by the grammar production they're defined on. `by: 'production'` (default) groups SDOs under each production; `by: 'sdo'` groups productions under each SDO title. `filter` narrows to keys containing a substring (case-insensitive).

spec.search

Search the parsed spec by clause id / aoid / title (+ step text with `search_steps`). Aoid-exact ranks first. `at: '<sha>'` searches a historical main snapshot.

spec.snapshots

List the live (spec, edition, sha, fetched_at) snapshots the hosted Worker is serving from R2. Filter by `spec` ('262'|'402') or `edition` (e.g. 'main', 'es2026'). Historical SHA-pinned copies are reachable via `at:` on clause.get / spec.search but aren't enumerated here.

spec.symbol_resolve

Resolve spec notation like `[[Prototype]]` (internal slot), `%Object.prototype%` (well-known intrinsic), or `~number~` (sigil enum): return clauses that mention or define it, ranked by occurrence with a bump for definition-y sections.

spec.tables

List or fetch parsed `<emu-table>` content. `{ id }` returns exactly that table (full columns + rows); otherwise list table summaries, optionally narrowed by a `filter` substring over the caption or id.

spec.well_known_intrinsics

Enumerate the well-known intrinsics in a spec with their probable defining clauses. Driven from the canonical §6.1.7.4 WKI table when present (ECMA-262), else a `%X%` prose scan (e.g. ECMA-402). `filter` narrows by bare-name substring.

clause

clause.get

Fetch a parsed TC39 clause as structured JSON. `spec` selects '262' (default) or '402'. `edition` defaults to 'latest'. `at: '<sha>'` pins to a historical main snapshot (only valid for edition='main'); omit to query the live snapshot.

clause.list

List parsed spec clauses with optional filters (kind, section prefix, has_algorithm). `spec` selects '262' or '402'. `at: '<sha>'` queries a historical main snapshot.

clause.outline

Return the section tree (table of contents) for a parsed (spec, edition). `depth` caps how deep the tree descends (1 = top-level only); `under` limits the tree to descendants of one clause id.

proposal

proposal.get

Fetch one TC39 proposal by slug (exact) or name (case-insensitive).

proposal.list

List TC39 proposals from the static index (tc39/proposals). Filter by spec ('262'|'402'), stage ('0'|'1'|'2'|'2.7'|'3'|'finished'|'inactive'|'active'), champion (substring), or contains (name/slug substring).

test262

test262.search

Search the tc39/test262 conformance suite from its indexed front-matter. `query` AND-matches whitespace tokens (case-insensitive) across each test's description + path; `esid` prefix-matches the front-matter esid. Returns ranked hits (path, GitHub url at the indexed SHA, esid, description, features, flags), capped at `limit` (default 20). Supply at least one of `query` / `esid`.

Endpoints

URL	Transport	State	Latency	Checked
https://mcp.xyzzylabs.ai/tc39/mcp	streamable-http	answering	95 ms	5 min ago

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tc39-mcp — questions

Answers built from our own checks of this server.

What can tc39-mcp do?

It exposes 17 tools, read directly from the server on our last check. Among them: clause.get, clause.list, clause.outline, proposal.get, proposal.list, spec.about and 11 more. The full list with descriptions is on this page — we take it from the server itself via tools/list, not from a README. How MCP servers expose tools in the first place →

What is tc39-mcp mostly used for?

Its tools cluster around spec, clause and proposal. That is what this server is built to work with — the grouping comes from the actual tool names, not from a category we assigned.

Is tc39-mcp working right now?

We send a real MCP handshake every 15 minutes. Over the last 24 hours 91 of 91 checks got a reply (100.0%), average response time 138 ms. The bar chart above shows every period we have measured.

How do I connect tc39-mcp?

Copy the ready config from this page. We generate it for Claude Code, Claude Desktop, Codex, Cursor and VS Code, each in the form that client actually reads. It is a remote server, so there is nothing to install — the client connects to the address.

Does tc39-mcp need an API key?

No. tc39-mcp completed a full MCP handshake with us as an anonymous client and listed its tools without asking for anything. All 17 of them are readable on this page. This is what we observed, not what the docs claim.

How fast is tc39-mcp?

It answers our handshake in 138 ms on average, which is faster than 85% of all working MCP servers we measure. That puts it in the quick quarter of the ecosystem. The comparison comes from our own checks across the whole registry, every 15 minutes.

How many people use tc39-mcp?

The npm package tc39-mcp was installed 203 times in the last week. Week over week that is +35%. We show installs rather than GitHub stars on purpose: a star is a bookmark, an install is someone actually running it.

Is tc39-mcp open source?

The code is open source: it is published under the MIT licence, written in TypeScript and 2 stars on GitHub. The licence covers the code in the repository, not the hosted service at mcp.xyzzylabs.ai, which runs on its operator's own terms.