MCP servers deployed on AWS Lambda Function URLs stopped working after upgrading from v1.24.2 to v1.25.0+

[severi]

1.25.0: Connection fails
1.25.3: Connection works but tool calls return null

(didn't test 1.25.1-1.25.2)

Suspect PR #1209's fetch transport doesn't play well with Lambda + serverless-express which expects Node.js-style streaming.

To Reproduce

1. Deploy MCP server to Lambda Function URL with StreamableHTTPServerTransport + @codegenie/serverless-express
2. Connect with Claude Code
3. Call any tool

Expected behavior

Tool calls return results (works fine on 1.24.2).

Logs

1.25.0:
Failed to reconnect to test-mcp-server.

1.25.3:
{"code":"invalid_union","errors":[[{"expected":"object","code":"invalid_type","path":[],"message":"Invalid input: expected object, received null"}]],"path":[],"message":"Invalid input"}

[felixweinberger]

this looks related to #1414 — both seem to stem from the fetch transport refactor in #1209.

could you share a minimal reproduction? ideally a small Lambda handler + serverless-express setup we can deploy or test locally. the version bisection is helpful but we need something runnable to dig into what's different between the Node.js stream path and the fetch transport path in a Lambda context.

[severi]

sure, here you go

https://github.com/severi/mcp-sdk-lambda-repro