## Motivation and Context
When a GET SSE stream disconnects and the client reconnects on the same
session, the new GET is silently rejected because the previous stream's
`STANDALONE_SSE_STREAM_ID` entry in `streamsMapping` hasn't been cleaned
up yet. Cleanup via `invokeOnCompletion` is asynchronous — there is a
race window where the stale entry persists and blocks reconnection.
Additionally, Ktor's `sse {}` handler commits 200 OK headers before
`handleGetRequest` runs, so the intended
`call.reject(HttpStatusCode.Conflict, ...)` cannot change the
already-committed HTTP status. On all tested engines (test host, CIO,
Netty), clients receive a 200 OK with an immediately-closed empty SSE
stream rather than a 409 Conflict. Server-side `CallLogging` reports 409
(from the response object, not the wire status), which is misleading.
### What this PR does
1. **Explicitly closes the old SSE session** before establishing a new
stream. If the old session is already dead, `close()` is a no-op. This
is the synchronous cleanup that the async `invokeOnCompletion` couldn't
provide.
2. **Replaces `invokeOnCompletion` with `try/finally`** around
`awaitCancellation()` — cleanup runs during cancellation propagation
(earlier in the lifecycle) rather than after job completion. An identity
check (`=== newContext`) ensures only this stream's entry is removed,
not a replacement's.
3. **Adds identity-based removal in `emitOnStream`** — a failed `send()`
on an old stream can no longer accidentally evict the new stream's
mapping.
4. **Rethrows `CancellationException`** instead of swallowing it,
preserving structured concurrency.
The TypeScript reference implementation avoids this race via synchronous
`ReadableStream.cancel()` cleanup. The Kotlin coroutine equivalent
(`invokeOnCompletion`) is inherently async. The Go SDK uses `defer
stream.release()` under a mutex for the same reason. The Java SDK uses
unconditional `AtomicReference.set()` replacement with
`compareAndExchange` on close.
Fixes#715
## How Has This Been Tested?
All tests run against latest `main` (`bf8dc6d`). ktlint and detekt pass
clean.
- 2 new unit tests (Ktor test host): reconnect after disconnect,
concurrent stream takeover
- 2 new integration tests (real CIO embedded server): same scenarios
over real HTTP
- Manual verification against a live Ktor+Netty MCP server (curl):
confirmed 200 OK on the wire with immediately-closed stream on reconnect
(pre-fix), live stream on reconnect (post-fix)
- All existing server unit tests pass (76 total)
- All existing integration tests pass (306 total)
## Breaking Changes
The server no longer returns 409 Conflict for a second GET SSE stream on
the same session. Instead, the old stream is explicitly closed and the
new stream takes over. The 409 was already unreachable as an HTTP status
— Ktor's `sse {}` handler commits 200 OK before the transport handler
runs on all tested engines (test host, CIO, Netty), so clients never
received 409 on the wire.
## Types of changes
- [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
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed
## Additional context
This bug likely became reachable after #681 (April 10) fixed the Netty
`appendSseHeaders` crash that previously prevented GET SSE from working
at all on Netty.
Cross-SDK comparison of how each SDK handles a second GET SSE on the
same session:
| SDK | Behavior | Cleanup mechanism | Stale mapping bug? |
|-----|----------|-------------------|-------------------|
| TypeScript (reference) | 409 Conflict | Synchronous
`ReadableStream.cancel()` | No |
| Go | 409 Conflict | `defer stream.release()` under mutex | No |
| Java | Replaces (no 409) | `AtomicReference.set()` +
`compareAndExchange` on close | No |
| C# | 400 Bad Request | Boolean flag, never reset (no reconnect
support) | No |
| Kotlin (before fix) | 409 (unreachable on wire) | Async
`invokeOnCompletion` | **Yes** |
| Kotlin (this PR) | Closes old + replaces | `try/finally` +
identity-based removal | No |
---------
Co-authored-by: devcrocod <devcrocod@gmail.com>
a238fc0a2e