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cc @mattgodbolt — this is in the MCP endpoint from #8644. ## The problem `compile` in the MCP tool calls `baseCompiler.compile()` directly, so a compiler served by a sub-server gets attempted on whichever node handled the MCP request. There's no compiler binary there to launch, so every remotely-hosted compiler fails through `/mcp`: ``` runChild():486 Launching child process failed ``` Reproducible on production right now — `vcpp_v19_51_VS18_6_x64` through `compiler-explorer.com/mcp` fails, while the same id and options through `/api/compiler/<id>/compile` works. It affects any remote, not just the Windows ones. `CompileHandler.handle` has the branch that makes this work for REST: ```ts const remote = compiler.getRemote(); if (remote) { req.url = remote.path; this.proxy.web(req, res, {target: remote.target, changeOrigin: true}, ...); return; } ``` There's no equivalent anywhere in `lib/mcp` — `git log -S"getRemote" -- lib/mcp` is empty, so this has been the case since the endpoint landed. ## The change Check `getRemote()` and, when it's set, POST an equivalent request to `remote.target + remote.path` rather than compiling locally. A tool call has no `req`/`res` to hand to `http-proxy`, hence an outbound `fetch` instead of the proxy. The sub-server returns the same JSON shape `compile()` returns locally, so truncation, `buildResult` and `execResult` handling below are untouched. One wart: `result` is annotated `Awaited<ReturnType<typeof baseCompiler.compile>>` rather than `CompilationResult`, because the declared wire type has `asm?: string | ParsedAsmResultLine[]` and `truncateLines` wants the array form — typing it as `CompilationResult` doesn't compile. There's a cast on the remote branch with a comment explaining why. Happy to do this differently if you'd rather. ## Testing Unit tests: the existing fake compiler had no `getRemote`, so it's added there; two new cases cover the remote path posting to the sub-server (and never calling `compile()` locally) and a failing sub-server response surfacing as a tool error. Manually, against a local instance configured with the `gpu` and `winprod` remotes: | compiler | before | after | | --- | --- | --- | | `vcpp_v19_51_VS18_6_x64` `/O2` | `runChild():486` | `code 0`, `lea eax, DWORD PTR [rcx+1]` | | `vcpp_v19_51_VS18_6_x86` `/O2 /std:c++20` | — | `code 0`, `mov eax, DWORD PTR _x$[esp-4]` | | `clang-cl2216-v19_50_VS18_2` `/O2` | — | `code 0` | Compile errors propagate too: a deliberate typo returns `code 2` with `<source>(1): error C2065: 'nope': undeclared identifier`. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude Opus 5 <noreply@anthropic.com>