Skip to content

Streamable HTTP - SSE Failure Logging / Handling #877

Closed
Bug
Closed
Streamable HTTP - SSE Failure Logging / Handling#877
Bug
Assignees
Kehrlann
Labels
P3Nice to haves, rare edge casesbugSomething isn't working

Description

@jamesward
jamesward
opened on Mar 20, 2026

Version 1.1.0

I'm connecting to the https://www.javadocs.dev/mcp server - streamable http, but no SSE support. I get this on Spring app startup:

2026-03-19T21:33:47.310-06:00  INFO 114327 --- [ient-1-Worker-0] i.m.client.LifecycleInitializer          : Server response with Protocol: 2025-03-26, Capabilities: ServerCapabilities[completions=null, experimental=null, logging=null, prompts=null, resources=null, tools=ToolCapabilities[listChanged=false]], Info: Implementation[name=javadocs.dev, title=null, version=0.0.2] and Instructions null
2026-03-19T21:33:47.318-06:00  WARN 114327 --- [ient-1-Worker-0] i.m.client.LifecycleInitializer          : Handling exception

io.modelcontextprotocol.spec.McpTransportException: Unrecognized server error when connecting to SSE stream, status code: 404
	at io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport.lambda$reconnect$11(HttpClientStreamableHttpTransport.java:303) ~[mcp-core-1.1.0.jar:1.1.0]
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:389) ~[reactor-core-3.8.4.jar:3.8.4]
	at reactor.core.publisher.FluxCreate$BufferAsyncSink.drain(FluxCreate.java:887) ~[reactor-core-3.8.4.jar:3.8.4]
	at reactor.core.publisher.FluxCreate$BufferAsyncSink.next(FluxCreate.java:812) ~[reactor-core-3.8.4.jar:3.8.4]
	at reactor.core.publisher.FluxCreate$SerializedFluxSink.next(FluxCreate.java:164) ~[reactor-core-3.8.4.jar:3.8.4]
	at io.modelcontextprotocol.client.transport.ResponseSubscribers$BodilessResponseLineSubscriber.hookOnComplete(ResponseSubscribers.java:315) ~[mcp-core-1.1.0.jar:1.1.0]
	at reactor.core.publisher.BaseSubscriber.onComplete(BaseSubscriber.java:200) ~[reactor-core-3.8.4.jar:3.8.4]
	at org.reactivestreams.FlowAdapters$FlowToReactiveSubscriber.onComplete(FlowAdapters.java:221) ~[reactive-streams-1.0.4.jar:na]
	at java.net.http/jdk.internal.net.http.LineSubscriberAdapter$LineSubscription.loop(LineSubscriberAdapter.java:430) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$LockingRestartableTask.run(SequentialScheduler.java:182) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:149) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:207) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:280) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:233) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.LineSubscriberAdapter$LineSubscription.signalComplete(LineSubscriberAdapter.java:193) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.LineSubscriberAdapter.onComplete(LineSubscriberAdapter.java:114) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.HttpBodySubscriberWrapper.complete(HttpBodySubscriberWrapper.java:293) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.HttpBodySubscriberWrapper.onComplete(HttpBodySubscriberWrapper.java:401) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Stream.schedule(Stream.java:230) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$LockingRestartableTask.run(SequentialScheduler.java:182) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:149) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:207) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:280) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:233) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Stream.receiveDataFrame(Stream.java:427) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Stream.incoming(Stream.java:566) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Http2Connection.processFrame(Http2Connection.java:1070) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.frame.FramesDecoder.decode(FramesDecoder.java:155) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Http2Connection$FramesController.processReceivedData(Http2Connection.java:318) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Http2Connection.asyncReceive(Http2Connection.java:871) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Http2Connection$Http2TubeSubscriber.processQueue(Http2Connection.java:1840) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$LockingRestartableTask.run(SequentialScheduler.java:182) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:149) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:207) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:280) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:233) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Http2Connection$Http2TubeSubscriber.runOrSchedule(Http2Connection.java:1857) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Http2Connection$Http2TubeSubscriber.onNext(Http2Connection.java:1884) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.Http2Connection$Http2TubeSubscriber.onNext(Http2Connection.java:1818) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SSLTube$DelegateWrapper.onNext(SSLTube.java:219) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SSLTube$SSLSubscriberWrapper.onNext(SSLTube.java:499) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SSLTube$SSLSubscriberWrapper.onNext(SSLTube.java:304) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SubscriberWrapper$DownstreamPusher.run1(SubscriberWrapper.java:316) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SubscriberWrapper$DownstreamPusher.run(SubscriberWrapper.java:259) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$LockingRestartableTask.run(SequentialScheduler.java:182) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:149) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:207) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:280) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:233) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SubscriberWrapper.outgoing(SubscriberWrapper.java:232) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SubscriberWrapper.outgoing(SubscriberWrapper.java:198) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SSLFlowDelegate$Reader.processData(SSLFlowDelegate.java:465) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SSLFlowDelegate$Reader$ReaderDownstreamPusher.run(SSLFlowDelegate.java:283) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$LockingRestartableTask.run(SequentialScheduler.java:182) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:149) ~[java.net.http:na]
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:207) ~[java.net.http:na]
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090) ~[na:na]
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614) ~[na:na]
	at java.base/java.lang.Thread.run(Thread.java:1474) ~[na:na]

Everything works correctly. The MCP spec does say that the GET to /mcp can return a 405 response, which I tried but still got a similar logged error.

Activity

  1. Kehrlann
    self-assigned this
    on Mar 21, 2026

  2. Kehrlann commented on Apr 1, 2026

    @Kehrlann
    Kehrlann
    on Apr 1, 2026
    Contributor

    Issue introduced in #861

  3. Kehrlann
    added the
    Bug
    issue type on Apr 1, 2026
  4. Kehrlann
    added
    bugSomething isn't working
    P3Nice to haves, rare edge cases
    on Apr 1, 2026

  5. Kehrlann commented on Apr 2, 2026

    @Kehrlann
    Kehrlann
    on Apr 2, 2026
    Contributor

    @jamesward I'd be inclined to leave the 404 error as-is, but respect HTTP 405. Would that be acceptable?

  6. Kehrlann
    added a commit that references this issue on Apr 2, 2026

    HttpClientStreamableHttpTransport: handle HTTP 405

    4867755

  7. jamesward commented on Apr 2, 2026

    @jamesward
    jamesward
    on Apr 2, 2026
    ContributorAuthor

    I think that sounds like a spec-compliant approach. So that'd be good! Thank you.

  8. Kehrlann
    added 2 commits that reference this issue on Apr 2, 2026

    HttpClientStreamableHttpTransport: handle HTTP 405

    b325ace

    HttpClientStreamableHttpTransport: handle HTTP 405

    8e1c5d0

  9. Kehrlann commented on Apr 2, 2026

    @Kehrlann
    Kehrlann
    on Apr 2, 2026
    Contributor

    Fixed in #900 , forward ported to 5e77762

  10. Kehrlann
    closed this as completedon Apr 2, 2026
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Metadata

Metadata

Assignees

Labels

P3Nice to haves, rare edge casesbugSomething isn't working

Type

Bug

Projects

No projects

    Milestone

    No milestone

    Relationships

    None yet

    Development

    No branches or pull requests

    Issue actions