MCP endpoint corrupts Next.js 16 proxy Response realm after StreamableHTTPServerTransport

[ouchanip]

MCP endpoint corrupts Next.js 16 proxy Response realm after StreamableHTTPServerTransport

Summary

Self-hosted prompts.chat on Docker/Next.js 16.1.0 becomes globally broken after /api/mcp handles Streamable HTTP requests. Once triggered, unrelated routes start returning 500 with:

TypeError: Expected an instance of Response to be returned
    at .../next/dist/server/web/adapter.js

PR #1087 (singleton McpServer + AsyncLocalStorage) does not fix this. The failure is not caused by per-request McpServer construction.

Environment

• ghcr.io/f/prompts.chat:latest
• Next.js 16.1.0
• @modelcontextprotocol/sdk 1.25.1
• MCP endpoint: src/pages/api/mcp.ts
• Docker self-host with external PostgreSQL

Reproduction

1. Start prompts.chat with PCHAT_FEATURE_MCP=true.
2. Send one or more Streamable HTTP initialize requests:

curl -sS -X POST http://localhost:14444/api/mcp \
  -H 'content-type: application/json' \
  -H 'accept: application/json, text/event-stream' \
  --data '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"repro","version":"0.1.0"}}}'

3. Hit / or any route that goes through Next proxy/middleware.
4. The app may start returning 500 with Expected an instance of Response to be returned.

Root Cause

@modelcontextprotocol/sdk/server/streamableHttp.js uses @hono/node-server's getRequestListener() without options. Hono defaults overrideGlobalObjects to true and replaces global.Request and global.Response.

Minimal proof inside the container:

node --input-type=module -e '
const nativeResponse = globalThis.Response;
const nativeRequest = globalThis.Request;
const { getRequestListener } = await import("@hono/node-server");
console.log("before", globalThis.Response === nativeResponse, globalThis.Request === nativeRequest, globalThis.Response.name);
getRequestListener(() => new Response("ok"));
console.log("after", globalThis.Response === nativeResponse, globalThis.Request === nativeRequest, globalThis.Response.name);
const r = new nativeResponse("ok");
console.log("native instance of current Response", r instanceof Response);
'

Observed:

before true true Response
after false false _Response
native instance of current Response false

Next.js proxy/middleware later checks that the returned value is instanceof Response. After Hono changes global.Response, NextResponse.next() is no longer an instance of the current global Response, so Next throws.

Workaround/Fix Used

Avoid the MCP SDK Node transport wrapper and use the SDK Web Standard transport directly:

• Replace StreamableHTTPServerTransport with WebStandardStreamableHTTPServerTransport.
• Convert NextApiRequest headers to Web Headers.
• Pass the already-parsed JSON-RPC body through transport.handleRequest(webRequest, { parsedBody }).
• Pipe the returned Web Response back to NextApiResponse with Readable.fromWeb.

This avoids @hono/node-server entirely, so global.Request/global.Response stay untouched.

Validation

On the patched image:

• single initialize: 200
• 100 initialize requests: 100/100 200
• 100 tools/call search_prompts requests: 100/100 200
• /: 200 after stress tests
• /api/health: 200 after stress tests
• logs containing Expected an instance of Response: 0
• logs containing MCP error: 0

Suggested Upstream Options

1. Patch prompts.chat to use WebStandardStreamableHTTPServerTransport in Next.js.
2. Or ask @modelcontextprotocol/sdk to call Hono getRequestListener(..., { overrideGlobalObjects: false }) in StreamableHTTPServerTransport.
3. Re-open/revisit refactor: singleton McpServer with AsyncLocalStorage context #1087 only if keeping a singleton McpServer is still desired, but it is not the root cause of this self-hosted Docker crash.

[coderabbitai]

🔗 Related PRs

#1035 - Fix: Docker login configuration error [merged]
#1055 - Fix(docker): remove premature nextjs start [merged]
#1086 - perf: reduce Vercel costs with 12 targeted optimizations ($6.4K → $800-1.5K/mo) [merged]
#1117 - fix: return 405 on MCP GET per Streamable HTTP spec [merged]
#1118 - perf: optimize MCP handler DB queries and response sizes [merged]

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