Stabilize flaky test: McpCompletionTests.testCompletionErrorOnMissingContext

McpCompletionTests.testCompletionErrorOnMissingContext と、リンクされた GitHub Actions の失敗ログから始め、その後、接続のクローズと保留中のリクエストに関連する SSE クライアント/サーバーセッションのライフサイクルを調査します。完了の条件は、ビルド間でテストの信頼性が維持され、対象の race の間に 404 Session not found エラーで失敗しなくなることです。

McpCompletionTests.testCompletionErrorOnMissingContext is sometimes failing and recovers after simple build restart.
**Error:** Session not found: <sessionId>

Likely causes:
The server removes the session when SSE connection closes, but the client still has pending requests in flight. This is a race condition between:

• SSE connection closing → server removes session
• Client sending next request → "Session not found"

2026-02-09 23:31:09 [HttpClient-1-Worker-0] WARN  i.m.c.t.HttpClientSseClientTransport - SSE stream observed an error
java.io.IOException: chunked transfer encoding, state: READING_LENGTH
	at java.net.http/jdk.internal.net.http.common.Utils.wrapWithExtraDetail(Utils.java:375)
	at java.net.http/jdk.internal.net.http.Http1Response$BodyReader.onReadError(Http1Response.java:660)
	at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.checkForErrors(Http1AsyncReceiver.java:302)
	at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.flush(Http1AsyncReceiver.java:268)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$LockingRestartableTask.run(SequentialScheduler.java:205)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:149)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:230)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: java.io.EOFException: EOF reached while reading
	at java.net.http/jdk.internal.net.http.Http1AsyncReceiver$Http1TubeSubscriber.onComplete(Http1AsyncReceiver.java:597)
	at java.net.http/jdk.internal.net.http.SocketTube$InternalReadPublisher$ReadSubscription.signalCompletion(SocketTube.java:648)
	at java.net.http/jdk.internal.net.http.SocketTube$InternalReadPublisher$InternalReadSubscription.read(SocketTube.java:853)
	at java.net.http/jdk.internal.net.http.SocketTube$SocketFlowTask.run(SocketTube.java:181)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:230)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:303)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:256)
	at java.net.http/jdk.internal.net.http.SocketTube$InternalReadPublisher$InternalReadSubscription.signalReadable(SocketTube.java:782)
	at java.net.http/jdk.internal.net.http.SocketTube$InternalReadPublisher$ReadEvent.signalEvent(SocketTube.java:965)
	at java.net.http/jdk.internal.net.http.SocketTube$SocketFlowEvent.handle(SocketTube.java:253)
	at java.net.http/jdk.internal.net.http.HttpClientImpl$SelectorManager.handleEvent(HttpClientImpl.java:1402)
	at java.net.http/jdk.internal.net.http.HttpClientImpl$SelectorManager.lambda$run$3(HttpClientImpl.java:1347)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
	at java.net.http/jdk.internal.net.http.HttpClientImpl$SelectorManager.run(HttpClientImpl.java:1347)
2026-02-09 23:31:09 [ForkJoinPool.commonPool-worker-1] ERROR i.m.c.t.HttpClientSseClientTransport - Error sending message: Sending message failed with a non-OK HTTP code: 404 - {"cause":null,"jsonRpcError":null,"localizedMessage":"Session not found: 7a9438d6-5965-456a-99af-26523f108d68","message":"Session not found: 7a9438d6-5965-456a-99af-26523f108d68","stackTrace":

https://github.com/modelcontextprotocol/java-sdk/actions/runs/21845041347/job/63038831474#step:4:15277