[mcollina]

Problem

The legacy GET /mcp SSE handler calls reply.hijack() and then raw.writeHead(200). Headers that hooks put on the reply (CORS, security headers) are dropped. A cross-origin browser client can neither open the stream nor read Mcp-Session-Id.

While writing the test I found a second bug on the same path: the error branch calls reply.send() after the hijack. Fastify ignores that, so a failure while opening the stream (the session store being down, for example) never got a response, and the client hung until it timed out.

Fix

• Read reply.getHeaders() before hijack() and set them on the raw response. Then set the SSE headers on top, so Content-Type and Cache-Control still win. This follows what the 2026-07-28 streams do on feat: implement MCP 2026-07-28 alongside the handshake revisions #170.
• On the hijacked error path, write a 500 JSON response on the raw response. It keeps the hook headers and switches Content-Type to application/json. If the head has already been sent, the stream is destroyed instead. Errors raised before the hijack still go through reply.send() as before.

onSend hooks still do not run for this response, as with any hijacked reply. Headers have to be set in onRequest/preHandler, which is where CORS plugins set them.

Tests

test/sse-hook-headers.test.ts:

• the 200 stream response carries the headers set by an onRequest hook, next to text/event-stream and Mcp-Session-Id
• with sessionStore.create failing, a real listening server answers 500 with JSON and the hook headers. Before this fix it timed out.

Fixes #335

🤖 Generated with Claude Code

https://claude.ai/code/session_01LmhsFjk8W96X2PvmShmDm9