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fix(server): notifications not delivered over Streamable HTTP (#587) (#599)

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## Changes

- fix(server): keep SSE connection open until explicitly cancelled
- conformance: add logging to test_tool_with_logging and update
troubleshooting documentation
- chore(conformance): remove 5 passing server tests from conformance
baselines:
    - tools-call-with-logging
    - tools-call-with-progress
    - tools-call-sampling
    - tools-call-elicitation
    - elicitation-sep1034-defaults

## Motivation and Context

Two bugs in StreamableHttpServerTransport caused server-to-client
notifications (e.g. notifications/message during tool calls) to be
silently dropped.
   
**Bug 1 — GET SSE stream closed immediately**
`handleGetRequest` rejected GET requests with 405 when
`enableJsonResponse = true` (which `mcpStreamableHttp` always sets).
Since the Ktor `sse {}` handler commits response headers before the body
runs, the reject failed silently and the function returned, causing Ktor
to close the SSE stream. There was also no `awaitCancellation()` to keep
the connection alive.
**Bug 2 — Notifications discarded in JSON mode**

In `send()`, notifications with a relatedRequestId entered the
POST-stream path but hit `if (!isTerminated) return` without being
forwarded anywhere.
   
**Fix**
- Remove the enableJsonResponse guard from handleGetRequest. The GET SSE
stream is an always-required notification channel, orthogonal to how
POST responses are delivered.
- Add `awaitCancellation()` to keep the GET SSE connection open until
the client disconnects or the transport closes.
- In `send()`, route notifications with a relatedRequestId to the
standalone GET SSE stream when `enableJsonResponse = true`.

## How Has This Been Tested?

Verified against the MCP conformance test suite — the
ToolsCallWithLogging scenario now passes. ngrep confirms the `GET /mcp`
stream stays open and `notifications/message` events arrive before the
`tools/call` response.


## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context

Fixes #587
Requires #585 to be merged first.

---------

Co-authored-by: devcrocod <devcrocod@gmail.com>
This commit is contained in:
Konstantin Pavlov 2026-03-12 16:52:27 +02:00 • committed by GitHub
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15
conformance-test/README.md
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@ -114,16 +114,11 @@ Tests the conformance server against all server scenarios:
8 scenarios are expected to fail due to current SDK limitations (tracked in [
`conformance-baseline.yml`](conformance-baseline.yml).
| Scenario | Suite | Root Cause |
|---------------------------------------|--------|--------------------------------------------------------------------------------------------------------------------------------------------------------|
| `tools-call-with-logging` | server | Notifications from tool handlers have no `relatedRequestId`; transport routes them to the standalone SSE stream instead of the request-specific stream |
| `tools-call-with-progress` | server | *(same as above)* |
| `tools-call-sampling` | server | *(same as above)* |
| `tools-call-elicitation` | server | *(same as above)* |
| `elicitation-sep1034-defaults` | server | *(same as above)* |
| `elicitation-sep1330-enums` | server | *(same as above)* |
| `resources-templates-read` | server | SDK does not implement `addResourceTemplate()` with URI pattern matching; resources are looked up by exact URI |
| `elicitation-sep1034-client-defaults` | client | SDK does not fill in `default` values from the elicitation request schema before sending the response |
| Scenario | Suite | Root Cause |
|---------------------------------------|--------|----------------------------------------------------------------------------------------------------------------|
| `elicitation-sep1330-enums` | server | *(same as above)* |
| `resources-templates-read` | server | SDK does not implement `addResourceTemplate()` with URI pattern matching; resources are looked up by exact URI |
| `elicitation-sep1034-client-defaults` | client | SDK does not fill in `default` values from the elicitation request schema before sending the response |
These failures reveal SDK gaps and are intentionally not fixed in this module.