Errata

@formachines/errata-mcp

An MCP server for Errata: places where AI models write outdated code for PyPI and npm libraries, each with a fix verified in a sandbox. Three tools; stdio; no API key.

Install

Claude Code:

claude mcp add errata -- npx -y @formachines/errata-mcp

Any client with a JSON config (Claude Desktop, Cursor, Windsurf, VS Code …):

{

"mcpServers": { "errata": { "command": "npx", "args": ["-y", "@formachines/errata-mcp"] } } }

Node 20 or newer. Registry name: ai.formachines/errata.

Tools

Each entry in an answer carries verdict (apply / lead / none), grade (verified / possible / not_affected / unknown), reason, wrong, right, detect, error, fails_on, verified_on, url.

Example, check_package {"ecosystem":"pypi","package":"click","version":"8.5.0"} against the live base:

{"status":"found","package":"pypi/click","installed":"8.5.0","entries":[{"id":"pypi/click/8.5.0/0001","verdict":"apply","grade":"verified","reason":"checked on click 8.5.0; installed 8.5.0; proof.auto_apply true","symbol":"click.Parameter","wrong":"parameter = click.Parameter([\"--x\"])","right":"parameter = click.Option([\"--x\"])","detect":["click.Parameter"],"error":"Can't instantiate abstract class Parameter with abstract methods _parse_decls, add_to_parser","fails_on":"8.5.0","verified_on":["8.5.0"],"url":"https://formachines.ai/errata/pypi/click/0001-click-parameter/"}]}

and the same package at a version nobody checked, "version":"8.6.0": status: "unconfirmed_version", verdict lead, reason *installed 8.6.0 is newer than the versions checked (8.5.0); one checked version is not a range*.

Rule

One rule for the server, the CLI, the skill and the site (printed in every page footer, llms.txt and the JSON agent_rule): apply a correction on your own only when the installed version is one it was checked on (versions.checked_on: fails_on and every recheck) and its proof.auto_apply is true. A newer version than the checked ones, one between works_on and fails_on, or an unknown one is lead; works_on or older is none.

Merged and withdrawn entries are never offered: answers list them under not_offered with the entry that carries a merged one now. report refuses them before sending. An unreachable base is technical_error with isError: true, never no_coverage.

Environment

Requests carry the User-Agent formachines-errata-mcp/ , so the site counts MCP traffic apart.

Search misses (since 0.1.2). When search_error finds nothing, the server sends POST https://formachines.ai/v1/miss with the source (mcp), the exception type and its message scrubbed of paths, numbers, IP addresses, hex, UUIDs, e-mails, token-like strings and quoted values other than plain names ('numpy' stays, 'my secret' becomes ), at most 200 characters. A dotted type keeps only its last part (acme.errors.BillingError → BillingError). No code, no project name, no other lines of the traceback; text with no SomeError: … line sends nothing. It is sent in the background (the answer never waits for it, a network error is ignored) and only on a miss. The receiver scrubs again, keeps no IP address and uses the list to decide which errors to cover next. ERRATA_NO_TELEMETRY=1 turns it off.

Overhead

Measured 2026-11-02 against formachines.ai: the first check_package costs 3 requests (index 5.9 kB, changelog 31 kB, the package JSON 5–12 kB), later calls in the next ten minutes only the package JSON (or nothing, when cached). search_error costs one 31 kB request for the error index. Sizes are uncompressed; since 2026-11-02 the site sends JSON gzipped, about a fifth of that on the wire. Answers are compact: 0.6 kB for click, 1.9 kB for typescript (two entries), 0.9 kB for a matched error.

What the server does not promise

• Not a full scan. The base covers a few dozen packages; no_coverage means no corrections, not that the code is current.
• No range proof. One checked version is not a range; newer versions are lead.
• Not an applier. The server never edits code; apply says the base allows applying the fix as is.
• Reports are not published. report sends to a quarantine that is rechecked in the sandbox before anything changes; a note is never shown to other agents.
• No offline mode. Without the network every tool answers technical_error.

Check it yourself

npx -y @modelcontextprotocol/inspector --cli npx -y @formachines/errata-mcp --method tools/list

Development

npm install

npm test npm run typecheck npm pack

src/core is shared with errata-cli and kept identical in both. Tests run against a snapshot of formachines.ai/errata in test/fixtures/site (2026-11-02); npm test starts the built server over stdio with the SDK client and runs the seven acceptance cases; scripts/inspect-live.sh repeats them with MCP Inspector against the live base. server.json follows the registry schema 2025-12-11.

MIT licence. The data is CC BY 4.0 (formachines.ai/errata).