compiler-explorer/docs/MCP.md

MCP endpoint

Compiler Explorer ships a built-in Model Context Protocol server that lets LLM clients drive CE the same way the web UI does: list languages and compilers, compile (and optionally execute) source, look up assembly instruction docs, and round-trip short URLs.

The endpoint is unauthenticated and lives alongside the public REST API on every CE deployment.

Connection details

• URL: https://godbolt.org/mcp (or <your-ce-host>/mcp for self-hosted).
• Transport: Streamable HTTP, stateless (one JSON-RPC POST per call, no session id).
• Methods accepted: POST for tool calls; OPTIONS for CORS preflight. GET and DELETE return 405 Method Not Allowed.
• Auth: none.

Using it from a Claude client

In Claude Desktop or Claude Code, add a custom HTTP MCP server pointing at https://godbolt.org/mcp. No credentials required.

For Claude Code on the command line:

claude mcp add --transport http compiler-explorer https://godbolt.org/mcp

Registry listing

The server is listed in the official MCP Registry as io.github.compiler-explorer/compiler-explorer. The registry stores metadata only — it records where the endpoint lives, not what is running behind it — so publishing is deliberately not part of a deploy. Nothing here changes when the site updates.

The published record is server.json in the repository root. Re-publish only when that file changes:

1. Edit server.json and bump its version. Published versions are immutable and the registry rejects a repeat of one that already exists. It versions the metadata record, and has nothing to do with CE's gh-NNNNN build numbers.

2. Install mcp-publisher and run:

   mcp-publisher login github   # requires Owner/admin on the compiler-explorer org
   mcp-publisher validate
   mcp-publisher publish
   

The io.github.compiler-explorer/* namespace is granted by GitHub org ownership, so no DNS records or domain proofs are involved.

Tools

Successful responses are JSON in a single text content block. Error responses set isError: true and put a plain-text human-readable message in the same block (not a JSON envelope) — e.g. Short link "abc" not found, Compilation error: ....

list_languages

Returns every supported language, with its default compiler id and the number of compilers that target it. Read-only.

list_compilers

List compilers, optionally filtered by language, instructionSet, or a free-text match. The latestPerMajor flag reduces the firehose to "newest stable per (language, arch, semver major), all nightly + prerelease, no experimentals" — the right default when an LLM is picking a compiler. lean: true drops everything but id and name. Read-only.

list_libraries

List libraries available for a given language, with the same match/maxResults/lean controls as list_compilers. Read-only.

compile

Compile a source string with a chosen compiler and options, optionally linking libraries and optionally executing the result. With execute: true the program runs in CE's ephemeral sandbox; runtime output appears at the top level and the compile diagnostics move to buildResult. Output is line-capped (defaults: 500 asm, 100 stdout, 100 stderr) — raise the maxAsmLines / maxStdoutLines / maxStderrLines caps to retrieve more. Read-only from the connector's point of view; sandbox effects do not escape the call.

lookup_asm_instruction

Retrieve documentation for an assembly mnemonic in a given instruction set (amd64, arm64, riscv, …). Read-only.

generate_short_url

Persist a Compiler Explorer short URL that captures source, language, compiler, options, and libraries, and return its https://godbolt.org/z/... form. Additive only — repeat calls with the same payload return the same URL (the storage layer dedupes by config hash). Marked as a write tool with idempotentHint: true.

get_shortlink_info

Resolve an https://godbolt.org/z/<id> URL (or just the id) back into source plus compiler config. The returned compiler entries use the same shape compile accepts ({compiler, options, libraries:[{id, version}]}), so a shortlink can be re-compiled without translation. Multi-pane shortlinks (executors, conformance views, CMake trees) are flattened to the basic compile inputs. Read-only.

Notes for tool authors

• All tools carry MCP annotations (title, readOnlyHint/destructiveHint, openWorldHint: false); see lib/mcp/tools/*.ts.
• The server uses StreamableHTTPServerTransport in stateless mode (sessionIdGenerator: undefined). Each POST is a complete request/response cycle.
• Tool implementations live in lib/mcp/tools/ and are wired up in lib/mcp/index.ts. Tests live in test/mcp/.

Reporting issues

File MCP-specific bugs at https://github.com/compiler-explorer/compiler-explorer/issues.