Streamablehttp throws exception 'Client failed to initialize by explicit API call'

My MCP Server Code with python:

mcp = FastMCP(name="mcp_datetime")

@mcp.tool(name="get_datetime",description="""
This tool supports get current datetime.
""")
async def get_datetime()->TextContent:
    now = datetime.now()
    formatted = now.strftime("%Y-%m-%d %H:%M:%S")
    return TextContent(type="text",text=formatted)


if __name__ == '__main__':
    mcp.run(transport="streamable-http",host="0.0.0.0",port=8000,path="/mcp")

My client code is:

HttpClientStreamableHttpTransport transport = HttpClientStreamableHttpTransport.builder(some url )
                .build();

        // Create a synchronous McpClient


        try(McpSyncClient mcpClient = McpClient.sync(transport).requestTimeout(Duration.ofSeconds(10)).capabilities(McpSchema.ClientCapabilities.builder().sampling().build()).clientInfo(new McpSchema.Implementation("test","v1.0.0")).build()) {
            // Initialize the connection
            mcpClient.initialize();
            System.out.println("MCP client initialized.");

            // List available tools
            McpSchema.ListToolsResult tools = mcpClient.listTools();
            System.out.println("Available tools: " + tools.tools());

            // Call the "echo" tool
            Map<String, Object> params = Map.of("message", "Hello from Java over HTTP!");
            McpSchema.CallToolRequest callToolRequest = new McpSchema.CallToolRequest("get_datetime", params);
            McpSchema.CallToolResult result = mcpClient.callTool(callToolRequest);
            System.out.println("Tool call result: " + result.content());

        } catch (Exception e) {
            throw new RuntimeException(e);
        }

The exception is:

Exception in thread "main" java.lang.RuntimeException: io.modelcontextprotocol.spec.McpError: Client failed to initialize by explicit API call
	at io.modelcontextprotocol.Demo.main(Demo.java:39)
Caused by: io.modelcontextprotocol.spec.McpError: Client failed to initialize by explicit API call
	at io.modelcontextprotocol.client.LifecycleInitializer.lambda$withIntitialization$2(LifecycleInitializer.java:289)
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:94)
	at reactor.core.publisher.SerializedSubscriber.onError(SerializedSubscriber.java:124)
	at reactor.core.publisher.SerializedSubscriber.onError(SerializedSubscriber.java:124)
	at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onError(FluxTimeout.java:219)
	at reactor.core.publisher.FluxMap$MapSubscriber.onError(FluxMap.java:134)
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:106)
	at reactor.core.publisher.Operators.error(Operators.java:198)
	at reactor.core.publisher.MonoError.subscribe(MonoError.java:53)
	at reactor.core.publisher.Mono.subscribe(Mono.java:4444)
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:103)
	at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onError(FluxPeekFuseable.java:234)
	at reactor.core.publisher.MonoFlatMap$FlatMapMain.onError(MonoFlatMap.java:180)
	at reactor.core.publisher.FluxHandle$HandleSubscriber.onError(FluxHandle.java:212)
	at reactor.core.publisher.SerializedSubscriber.onError(SerializedSubscriber.java:124)
	at reactor.core.publisher.SerializedSubscriber.onError(SerializedSubscriber.java:124)
	at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onError(FluxTimeout.java:219)
	at reactor.core.publisher.MonoCreate$DefaultMonoSink.error(MonoCreate.java:201)
	at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$9(McpClientSession.java:260)
	at reactor.core.publisher.LambdaMonoSubscriber.doError(LambdaMonoSubscriber.java:155)
	at reactor.core.publisher.LambdaMonoSubscriber.onError(LambdaMonoSubscriber.java:150)
	at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onError(FluxContextWrite.java:121)
	at reactor.core.publisher.MonoCreate$DefaultMonoSink.error(MonoCreate.java:201)
	at io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport.lambda$sendMessage$30(HttpClientStreamableHttpTransport.java:521)
	at reactor.core.publisher.FluxOnErrorReturn$ReturnSubscriber.onError(FluxOnErrorReturn.java:175)
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:106)
	at reactor.core.publisher.Operators.error(Operators.java:198)
	at reactor.core.publisher.FluxError.subscribe(FluxError.java:43)
	at reactor.core.publisher.Flux.subscribe(Flux.java:8660)
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:103)
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.checkTerminated(FluxFlatMap.java:842)
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.drainLoop(FluxFlatMap.java:608)
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.drain(FluxFlatMap.java:588)
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.onError(FluxFlatMap.java:451)
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.checkTerminated(FluxFlatMap.java:842)
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.drainLoop(FluxFlatMap.java:608)
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.drain(FluxFlatMap.java:588)
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.onError(FluxFlatMap.java:451)
	at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:416)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onNext(MonoFlatMapMany.java:250)
	at reactor.core.publisher.FluxCreate$BufferAsyncSink.drain(FluxCreate.java:814)
	at reactor.core.publisher.FluxCreate$BufferAsyncSink.next(FluxCreate.java:739)
	at reactor.core.publisher.FluxCreate$SerializedFluxSink.next(FluxCreate.java:161)
	at io.modelcontextprotocol.client.transport.ResponseSubscribers$BodilessResponseLineSubscriber.hookOnComplete(ResponseSubscribers.java:287)
	at reactor.core.publisher.BaseSubscriber.onComplete(BaseSubscriber.java:197)
	at org.reactivestreams.FlowAdapters$FlowToReactiveSubscriber.onComplete(FlowAdapters.java:221)
	at java.net.http/jdk.internal.net.http.LineSubscriberAdapter$LineSubscription.loop(LineSubscriberAdapter.java:430)
	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$SchedulableTask.run(SequentialScheduler.java:230)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:303)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:256)
	at java.net.http/jdk.internal.net.http.LineSubscriberAdapter$LineSubscription.signalComplete(LineSubscriberAdapter.java:193)
	at java.net.http/jdk.internal.net.http.LineSubscriberAdapter.onComplete(LineSubscriberAdapter.java:114)
	at java.net.http/jdk.internal.net.http.Http1Response$Http1BodySubscriber.complete(Http1Response.java:339)
	at java.net.http/jdk.internal.net.http.Http1Response$Http1BodySubscriber.onComplete(Http1Response.java:390)
	at java.net.http/jdk.internal.net.http.ResponseContent$FixedLengthBodyParser.onSubscribe(ResponseContent.java:555)
	at java.net.http/jdk.internal.net.http.Http1Response$BodyReader.onSubscribe(Http1Response.java:773)
	at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.handlePendingDelegate(Http1AsyncReceiver.java:395)
	at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.flush(Http1AsyncReceiver.java:214)
	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$SchedulableTask.run(SequentialScheduler.java:230)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:303)
	at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:256)
	at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.subscribe(Http1AsyncReceiver.java:436)
	at java.net.http/jdk.internal.net.http.Http1Response.lambda$readBody$3(Http1Response.java:456)
	at java.net.http/jdk.internal.net.http.HttpClientImpl$DelegatingExecutor.execute(HttpClientImpl.java:158)
	at java.net.http/jdk.internal.net.http.Http1Response.readBody(Http1Response.java:422)
	at java.net.http/jdk.internal.net.http.Http1Exchange.readBodyAsync(Http1Exchange.java:376)
	at java.net.http/jdk.internal.net.http.Exchange.readBodyAsync(Exchange.java:191)
	at java.net.http/jdk.internal.net.http.MultiExchange.lambda$responseAsync0$4(MultiExchange.java:353)
	at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1150)
	at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510)
	at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1773)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:840)
	Suppressed: java.lang.Exception: #block terminated with an error
		at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:99)
		at reactor.core.publisher.Mono.block(Mono.java:1710)
		at io.modelcontextprotocol.client.McpSyncClient.initialize(McpSyncClient.java:180)
		at io.modelcontextprotocol.Demo.main(Demo.java:25)

May I know what MCP server did you try this with?

I'm encountering the same error. After deploying this open-source project, I get an error when trying to access it using the streamablehttp protocol.

Project: https://github.com/wenb1n-dev/mysql_mcp_server_pro

Here is my code:

    HttpClientStreamableHttpTransport build = HttpClientStreamableHttpTransport
            .builder("http://127.0.0.1:3000/mcp")
            .build();

    var client = McpClient.async(build)
            .initializationTimeout(Duration.ofSeconds(20))
            .build();
    client.initialize().block();

    client.listTools().subscribe(a->{
        a.tools().stream().forEach(t->{
            System.out.println(t.name());
            System.out.println(t.description());
            System.out.println(t.inputSchema());
        });
    });

exception is

`Java HotSpot(TM) 64-Bit Server VM warning: Sharing is only supported for boot loader classes because bootstrap classpath has been appended
2025-07-31T19:11:11.770+08:00 WARN 7086 --- [ai-mcp] [ient-2-Worker-0] i.m.client.LifecycleInitializer : Handling exception

java.lang.RuntimeException: Failed to send message: DummyEvent[responseInfo=jdk.internal.net.http.ResponseInfoImpl@72c829ef]
at io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport.lambda$sendMessage$27(HttpClientStreamableHttpTransport.java:512) ~[mcp-0.11.0.jar:0.11.0]
at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:388) ~[reactor-core-3.6.15.jar:3.6.15]
at reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onNext(MonoFlatMapMany.java:251) ~[reactor-core-3.6.15.jar:3.6.15]
at reactor.core.publisher.FluxCreate$BufferAsyncSink.drain(FluxCreate.java:880) ~[reactor-core-3.6.15.jar:3.6.15]
at reactor.core.publisher.FluxCreate$BufferAsyncSink.next(FluxCreate.java:805) ~[reactor-core-3.6.15.jar:3.6.15]
at reactor.core.publisher.FluxCreate$SerializedFluxSink.next(FluxCreate.java:163) ~[reactor-core-3.6.15.jar:3.6.15]
at io.modelcontextprotocol.client.transport.ResponseSubscribers$BodilessResponseLineSubscriber.hookOnComplete(ResponseSubscribers.java:287) ~[mcp-0.11.0.jar:0.11.0]
at reactor.core.publisher.BaseSubscriber.onComplete(BaseSubscriber.java:197) ~[reactor-core-3.6.15.jar:3.6.15]
at org.reactivestreams.FlowAdapters$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:205) ~[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:230) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:303) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:256) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.LineSubscriberAdapter$LineSubscription.signalComplete(LineSubscriberAdapter.java:193) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.LineSubscriberAdapter.onComplete(LineSubscriberAdapter.java:114) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response$Http1BodySubscriber.complete(Http1Response.java:339) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response$Http1BodySubscriber.onComplete(Http1Response.java:390) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.ResponseContent$FixedLengthBodyParser.onSubscribe(ResponseContent.java:555) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response$BodyReader.onSubscribe(Http1Response.java:773) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.handlePendingDelegate(Http1AsyncReceiver.java:395) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.flush(Http1AsyncReceiver.java:214) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler$LockingRestartableTask.run(SequentialScheduler.java:205) ~[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:230) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:303) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:256) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.subscribe(Http1AsyncReceiver.java:436) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response.lambda$readBody$3(Http1Response.java:456) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.HttpClientImpl$DelegatingExecutor.execute(HttpClientImpl.java:158) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response.readBody(Http1Response.java:422) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Exchange.readBodyAsync(Http1Exchange.java:376) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Exchange.readBodyAsync(Exchange.java:191) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.MultiExchange.lambda$responseAsync0$4(MultiExchange.java:353) ~[java.net.http:na]
at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1150) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run$$$capture(CompletableFuture.java:1773) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[na:na]
at java.base/java.lang.Thread.run(Thread.java:842) ~[na:na]

2025-07-31T19:11:11.775+08:00 WARN 7086 --- [ai-mcp] [ient-2-Worker-0] i.m.client.LifecycleInitializer : Failed to initialize

java.lang.RuntimeException: Failed to send message: DummyEvent[responseInfo=jdk.internal.net.http.ResponseInfoImpl@72c829ef]
at io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport.lambda$sendMessage$27(HttpClientStreamableHttpTransport.java:512) ~[mcp-0.11.0.jar:0.11.0]
at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:388) ~[reactor-core-3.6.15.jar:3.6.15]
at reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onNext(MonoFlatMapMany.java:251) ~[reactor-core-3.6.15.jar:3.6.15]
at reactor.core.publisher.FluxCreate$BufferAsyncSink.drain(FluxCreate.java:880) ~[reactor-core-3.6.15.jar:3.6.15]
at reactor.core.publisher.FluxCreate$BufferAsyncSink.next(FluxCreate.java:805) ~[reactor-core-3.6.15.jar:3.6.15]
at reactor.core.publisher.FluxCreate$SerializedFluxSink.next(FluxCreate.java:163) ~[reactor-core-3.6.15.jar:3.6.15]
at io.modelcontextprotocol.client.transport.ResponseSubscribers$BodilessResponseLineSubscriber.hookOnComplete(ResponseSubscribers.java:287) ~[mcp-0.11.0.jar:0.11.0]
at reactor.core.publisher.BaseSubscriber.onComplete(BaseSubscriber.java:197) ~[reactor-core-3.6.15.jar:3.6.15]
at org.reactivestreams.FlowAdapters$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:205) ~[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:230) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:303) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:256) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.LineSubscriberAdapter$LineSubscription.signalComplete(LineSubscriberAdapter.java:193) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.LineSubscriberAdapter.onComplete(LineSubscriberAdapter.java:114) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response$Http1BodySubscriber.complete(Http1Response.java:339) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response$Http1BodySubscriber.onComplete(Http1Response.java:390) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.ResponseContent$FixedLengthBodyParser.onSubscribe(ResponseContent.java:555) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response$BodyReader.onSubscribe(Http1Response.java:773) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.handlePendingDelegate(Http1AsyncReceiver.java:395) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.flush(Http1AsyncReceiver.java:214) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler$LockingRestartableTask.run(SequentialScheduler.java:205) ~[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:230) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:303) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:256) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.subscribe(Http1AsyncReceiver.java:436) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response.lambda$readBody$3(Http1Response.java:456) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.HttpClientImpl$DelegatingExecutor.execute(HttpClientImpl.java:158) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Response.readBody(Http1Response.java:422) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Http1Exchange.readBodyAsync(Http1Exchange.java:376) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.Exchange.readBodyAsync(Exchange.java:191) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.MultiExchange.lambda$responseAsync0$4(MultiExchange.java:353) ~[java.net.http:na]
at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1150) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run$$$capture(CompletableFuture.java:1773) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[na:na]
at java.base/java.lang.Thread.run(Thread.java:842) ~[na:na]

io.modelcontextprotocol.spec.McpError: Client failed to initialize by explicit API call

at io.modelcontextprotocol.client.LifecycleInitializer.lambda$withIntitialization$2(LifecycleInitializer.java:289)
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:94)
at reactor.core.publisher.SerializedSubscriber.onError(SerializedSubscriber.java:124)
at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onError(FluxTimeout.java:220)
at reactor.core.publisher.FluxMap$MapSubscriber.onError(FluxMap.java:134)
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:106)
at reactor.core.publisher.Operators.error(Operators.java:198)
at reactor.core.publisher.MonoError.subscribe(MonoError.java:53)
at reactor.core.publisher.Mono.subscribe(Mono.java:4576)
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:103)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onError(FluxPeekFuseable.java:234)
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onError(MonoFlatMap.java:180)
at reactor.core.publisher.FluxHandle$HandleSubscriber.onError(FluxHandle.java:213)
at reactor.core.publisher.SerializedSubscriber.onError(SerializedSubscriber.java:124)
at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onError(FluxTimeout.java:220)
at reactor.core.publisher.MonoCreate$DefaultMonoSink.error(MonoCreate.java:205)
at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$9(McpClientSession.java:260)
at reactor.core.publisher.LambdaMonoSubscriber.doError(LambdaMonoSubscriber.java:155)
at reactor.core.publisher.LambdaMonoSubscriber.onError(LambdaMonoSubscriber.java:150)
at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onError(FluxContextWrite.java:121)
at reactor.core.publisher.MonoCreate$DefaultMonoSink.error(MonoCreate.java:205)
at io.modelcontextprotocol.client.transport.HttpClientStreamableHttpTransport.lambda$sendMessage$30(HttpClientStreamableHttpTransport.java:521)
at reactor.core.publisher.FluxOnErrorReturn$ReturnSubscriber.onError(FluxOnErrorReturn.java:175)
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:106)
at reactor.core.publisher.Operators.error(Operators.java:198)
at reactor.core.publisher.FluxError.subscribe(FluxError.java:43)
at reactor.core.publisher.Flux.subscribe(Flux.java:8848)
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:103)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.checkTerminated(FluxFlatMap.java:846)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.drainLoop(FluxFlatMap.java:612)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.drain(FluxFlatMap.java:592)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.onError(FluxFlatMap.java:455)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.checkTerminated(FluxFlatMap.java:846)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.drainLoop(FluxFlatMap.java:612)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.drain(FluxFlatMap.java:592)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.onError(FluxFlatMap.java:455)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:418)
at reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onNext(MonoFlatMapMany.java:251)
at reactor.core.publisher.FluxCreate$BufferAsyncSink.drain(FluxCreate.java:880)
at reactor.core.publisher.FluxCreate$BufferAsyncSink.next(FluxCreate.java:805)
at reactor.core.publisher.FluxCreate$SerializedFluxSink.next(FluxCreate.java:163)
at io.modelcontextprotocol.client.transport.ResponseSubscribers$BodilessResponseLineSubscriber.hookOnComplete(ResponseSubscribers.java:287)
at reactor.core.publisher.BaseSubscriber.onComplete(BaseSubscriber.java:197)
at org.reactivestreams.FlowAdapters$FlowToReactiveSubscriber.onComplete(FlowAdapters.java:221)
at java.net.http/jdk.internal.net.http.LineSubscriberAdapter$LineSubscription.loop(LineSubscriberAdapter.java:430)
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$SchedulableTask.run(SequentialScheduler.java:230)
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:303)
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:256)
at java.net.http/jdk.internal.net.http.LineSubscriberAdapter$LineSubscription.signalComplete(LineSubscriberAdapter.java:193)
at java.net.http/jdk.internal.net.http.LineSubscriberAdapter.onComplete(LineSubscriberAdapter.java:114)
at java.net.http/jdk.internal.net.http.Http1Response$Http1BodySubscriber.complete(Http1Response.java:339)
at java.net.http/jdk.internal.net.http.Http1Response$Http1BodySubscriber.onComplete(Http1Response.java:390)
at java.net.http/jdk.internal.net.http.ResponseContent$FixedLengthBodyParser.onSubscribe(ResponseContent.java:555)
at java.net.http/jdk.internal.net.http.Http1Response$BodyReader.onSubscribe(Http1Response.java:773)
at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.handlePendingDelegate(Http1AsyncReceiver.java:395)
at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.flush(Http1AsyncReceiver.java:214)
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$SchedulableTask.run(SequentialScheduler.java:230)
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:303)
at java.net.http/jdk.internal.net.http.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:256)
at java.net.http/jdk.internal.net.http.Http1AsyncReceiver.subscribe(Http1AsyncReceiver.java:436)
at java.net.http/jdk.internal.net.http.Http1Response.lambda$readBody$3(Http1Response.java:456)
at java.net.http/jdk.internal.net.http.HttpClientImpl$DelegatingExecutor.execute(HttpClientImpl.java:158)
at java.net.http/jdk.internal.net.http.Http1Response.readBody(Http1Response.java:422)
at java.net.http/jdk.internal.net.http.Http1Exchange.readBodyAsync(Http1Exchange.java:376)
at java.net.http/jdk.internal.net.http.Exchange.readBodyAsync(Exchange.java:191)
at java.net.http/jdk.internal.net.http.MultiExchange.lambda$responseAsync0$4(MultiExchange.java:353)
at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1150)
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run$$$capture(CompletableFuture.java:1773)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java)
at --- Async.Stack.Trace --- (captured by IntelliJ IDEA debugger)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.<init>(CompletableFuture.java:1754)
at java.base/java.util.concurrent.CompletableFuture.completeAsync(CompletableFuture.java:2673)
at java.net.http/jdk.internal.net.http.PlainHttpConnection$ConnectEvent.handle(PlainHttpConnection.java:144)
at java.net.http/jdk.internal.net.http.HttpClientImpl$SelectorManager.handleEvent(HttpClientImpl.java:984)
at java.net.http/jdk.internal.net.http.HttpClientImpl$SelectorManager.lambda$run$3(HttpClientImpl.java:939)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
at java.net.http/jdk.internal.net.http.HttpClientImpl$SelectorManager.run(HttpClientImpl.java:939)
Suppressed: java.lang.Exception: #block terminated with an error
	at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:104)
	at reactor.core.publisher.Mono.block(Mono.java:1779)
	at com.star.ai.test.aimcp.StreamMcpTest.simpleConnectionTest(StreamMcpTest.java:68)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

`

May I know what MCP server did you try this with?

the mcp server with python code

mcp = FastMCP(name="mcp_datetime")

@mcp.tool(name="get_datetime",description="""
This tool supports get current datetime.
""")
async def get_datetime()->TextContent:
    now = datetime.now()
    formatted = now.strftime("%Y-%m-%d %H:%M:%S")
    return TextContent(type="text",text=formatted)


if __name__ == '__main__':
    mcp.run(transport="streamable-http",host="0.0.0.0",port=8000,path="/mcp")

@tzolov

Check this issue: #396
From what I can understand, the issue is recurring from Python servers that uses FastMCP both from mcp package and fastmcp package. @tzolov

I was able to reproduce this with a python FastMCP server as well as with a Typescript MCP server by following the npm instructions in https://github.com/phuctm97/modelfetch/, which uses this lib: @modelcontextprotocol/sdk/server/mcp.js

By comparing the responses from the streamablehttp server implemented in Python and Node.js, I have a rough idea of the cause. In the Java client, during the call to {"jsonrpc":"2.0","method":"notifications/initialized"}, it parses the value of the content-type in the response header. The Python server returns content-type=application/json, while the Node.js implementation does not return the content-type header. Therefore, if you want to use the current 0.11.0 version of the Java client with an MCP service implemented in Python, you need to manually remove the content-type header from the response header if possible.

@star362

I also see the same client error as @star362 when trying to connect to our in-house servlet mcp server (jetty) implemented using the HttpServletStatelessServerTransport. It does not seem to happen with the prior server implementation that was based upon HttpServletSseServerTransportProvider (both release version 0.11.2)

The HttpClientStreamableHttpTransport and theHttpClientSseClientTransport both allow setting a customized HttpClient.Builder.

I believe this should be enough to resolve the issue:

HttpClient.Builder clientBuilder = HttpClient.newBuilder().version(HttpClient.Version.HTTP_1_1).followRedirects(HttpClient.Redirect.NORMAL);

HttpClientStreamableHttpTransport transport = HttpClientStreamableHttpTransport.builder(some url )
                .clientBuilder(clientBuilder)
                .build();
....

@tzolov

.clientBuilder(clientBuilder)

@tzolov This can easily resolve this issue. I will close the PR.

Resolved by #433 (comment)

.clientBuilder(clientBuilder)

@tzolov这样就能轻松解决这个问题。我会关闭这个 PR。

I am using version 0.15.0. However, the issue still cannot be resolved using the method mentioned in the comments. Just like the person who raised the question, my MCP service is implemented through FastMCP.

still getting this on 0.17.2, even after using

HttpClient.Builder clientBuilder = HttpClient.newBuilder().version(HttpClient.Version.HTTP_1_1).followRedirects(HttpClient.Redirect.NORMAL);

HttpClientStreamableHttpTransport transport = HttpClientStreamableHttpTransport.builder(some url )
                .clientBuilder(clientBuilder)
                .build();

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Streamablehttp throws exception 'Client failed to initialize by explicit API call' #433

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Streamablehttp throws exception 'Client failed to initialize by explicit API call' #433

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bharat-avn-14416 commented on Jul 31, 2025

star362 commented on Jul 31, 2025

twypx commented on Aug 4, 2025

bharat-avn-14416 commented on Aug 4, 2025

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