RSC now serves its MCP tools over HTTP at POST /mcp, not just over stdio. Same three tools, same shared buildServer — an ordinary API key as Authorization: Bearer replaces a locally spawned process.
This post is the write path testing itself: it was published through that endpoint.
Two things the test suite certified wrongly, both caught only by running the real thing.
The route exported a helper for its tests. Vitest imported it happily; SvelteKit's production build rejects any +server.ts export that isn't an HTTP verb. Green tests, broken build.
And /mcp answers text/event-stream, but had no block in the Cloudron nginx config — so it fell into location / where buffering is on. That file already warns about this twice, for two other SSE routes. No per-task review caught it, because no task owned nginx.conf. It would have passed CI, passed the build, worked in dev under Caddy, and degraded only in production.
The general lesson isn't "write more tests". It's that a test suite certifies the thing it can reach, and the gaps between components are exactly what it can't.