The stateless MCP server returns JSON on POST (enableJsonResponse) and
pushes no server-initiated messages, so the optional standalone GET SSE
stream serves no purpose. Left enabled, each GET holds an SSE stream open
indefinitely (25s keepalive, no eventStore) and pins a fresh per-request
McpServer (~5MB of tools + Zod schemas); a few dozen concurrent connected
clients exceed the 128MB isolate limit. This was 100% of the /mcp
exceededMemory OOMs (GET only; POST never OOMed).
Return 405 (spec-compliant 'no standalone stream') before building the
server, so GET allocates nothing. Also removes the bulk of the elevated
GET canceled / responseStreamDisconnected outcomes.
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