`simple-streamablehttp-stateless` example will not return data if `id=0`

[howardjohn]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

I ran the simple-streamablehttp-stateless example, then connected via curl.

If I send:

curl localhost:3000/mcp/ --json '{"jsonrpc":"2.0","id":0,"method":"tools/call","params":{"_meta":{"progressToken":0},"name":"start-notification-stream","arguments":{"interval":0.1,"count":5,"caller":""}}}' -H "Accept: application/json, text/event-stream"

it does NOT work; there are no notifications sent.

If I send (changing only the id):

curl localhost:3000/mcp/ --json '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"_meta":{"progressToken":0},"name":"start-notification-stream","arguments":{"interval":0.1,"count":5,"caller":""}}}' -H "Accept: application/json, text/event-stream"

I do get notifications.

If I run the same example in the typescript SDK, it works with both id.

Log:

Sending notification 1/5 from caller:
2025-07-30 13:14:04,335 - root - DEBUG - Request stream _GET_stream not found
                                for message. Still processing message as the client
                                might reconnect and replay.
Sending notification 2/5 from caller:
2025-07-30 13:14:04,435 - root - DEBUG - Request stream _GET_stream not found
                                for message. Still processing message as the client
                                might reconnect and replay.
Sending notification 3/5 from caller:
2025-07-30 13:14:04,536 - root - DEBUG - Request stream _GET_stream not found
                                for message. Still processing message as the client
                                might reconnect and replay.
Sending notification 4/5 from caller:
2025-07-30 13:14:04,637 - root - DEBUG - Request stream _GET_stream not found
                                for message. Still processing message as the client
                                might reconnect and replay.
Sending notification 5/5 from caller:
2025-07-30 13:14:04,737 - root - DEBUG - Request stream _GET_stream not found
                                for message. Still processing message as the client
                                might reconnect and replay.

Example Code

Python & MCP Python SDK

latest

[Br1an67]

I'd like to work on this. The root cause is a falsy check (if related_request_id) in BaseSession.send_notification that treats integer 0 as False, dropping the metadata and misrouting notifications to the GET stream.

PR: #2187