HttpClientSseClientTransport Connection Timeout

[gakkiyomi]

Bug description
For the same MCP SSE service endpoint, both the mcp-inspector and Python SDK work properly, but the Java SDK fails.

Environment
JDK17
Java SDK: 0.10.0
Python SDK: mcp[cli]>=1.9.0
MCP Inspector: v0.15.0

Steps to reproduce
MCP Inspector

Python Code

import asyncio

from mcp.client.sse import sse_client
from mcp import ClientSession
from objprint import op


async def main():
    async with sse_client(
        "https://xxxxxxxxx.com/mcp/proxy/testchairstream/init"
    ) as (
        stdio,
        write,
    ):
        async with ClientSession(stdio, write) as session:
            # init ClientSession
            await session.initialize()

            # list all tools
            response = await session.list_tools()
            print(response.tools)


if __name__ == "__main__":
    asyncio.run(main())

However, when I use the Java SDK to run this demo, it fails to work.
A timeout exception occurs during client initialization, specifically when executing client.initialize()

    @Override
    public List<? extends MCPTool> generateTools() {
        UrlParser.SseEndpointInfo info;
        try {
            //https://xxxxxxxxx.com/mcp/proxy/testchairstream/init
            //info = parseUrl("http://localhost:8000/sse");
            info = new UrlParser.SseEndpointInfo("https://xxxxxxxxx.com", "/mcp/proxy/testchairstream/init");
        } catch (Exception e) {
            throw new BizException(ErrorCode.SERVICE_ONLINE_FAIL, e.getMessage());
        }
        var transport = HttpClientSseClientTransport.builder(info.baseUri())
                .sseEndpoint(info.sseEndpoint())
                .objectMapper(new ObjectMapper())
                .requestBuilder(HttpRequest.newBuilder())
                .clientBuilder(HttpClient.newBuilder().followRedirects(HttpClient.Redirect.ALWAYS))
                .build();
        McpSyncClient client = McpClient.sync(transport)
                .clientInfo(new McpSchema.Implementation("gov-ai-service-platform-proxy", "1.0.0"))
                .build();
        try {
            client.initialize();  //Will throw an error on this line
            List<McpSchema.Tool> tools1 = client.listTools().tools();
            return tools1.stream().map(x -> new MCPBridgeTool(x, client)).toList();
        } catch (Exception e) {
            log.error(e.getMessage(), e);
        }
        return null;
    }

Below is the stack trace:

2025-07-01 15:57:15.437 [main][] [] ERROR c.a.g.d.mcp.metadata.MCPMetaData - java.util.concurrent.TimeoutException: Did not observe any item or terminal signal within 20000ms in 'Mono.deferContextual ⇢ at io.modelcontextprotocol.spec.McpClientSession.sendRequest(McpClientSession.java:233)' (and no fallback has been configured)
reactor.core.Exceptions$ReactiveException: java.util.concurrent.TimeoutException: Did not observe any item or terminal signal within 20000ms in 'Mono.deferContextual ⇢ at io.modelcontextprotocol.spec.McpClientSession.sendRequest(McpClientSession.java:233)' (and no fallback has been configured)
	at reactor.core.Exceptions.propagate(Exceptions.java:410)
	at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:102)
	at reactor.core.publisher.Mono.block(Mono.java:1779)
	at io.modelcontextprotocol.client.McpSyncClient.initialize(McpSyncClient.java:171)
	at com.alipay.govaiserviceplatform.domain.mcp.metadata.MCPMetaData.generateTools(MCPMetaData.java:75)
	at com.alipay.govaiserviceplatform.core.mcp.server.BuiltInMCPServer.getTools(BuiltInMCPServer.java:47)
	at com.alipay.govaiserviceplatform.core.mcp.server.BaseMCPServer.start(BaseMCPServer.java:49)
	at com.alipay.govaiserviceplatform.core.config.BuiltInMCPServerRegistry.createServerRegistration(BuiltInMCPServerRegistry.java:85)
	at com.alipay.govaiserviceplatform.core.config.BuiltInMCPServerRegistry.lambda$postProcessBeanFactory$1(BuiltInMCPServerRegistry.java:70)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.obtainInstanceFromSupplier(AbstractAutowireCapableBeanFactory.java:1297)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.obtainInstanceFromSupplier(DefaultListableBeanFactory.java:1033)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.obtainFromSupplier(AbstractAutowireCapableBeanFactory.java:1257)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1200)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:569)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:529)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:339)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:373)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:337)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:207)
	at org.springframework.boot.web.servlet.ServletContextInitializerBeans.getOrderedBeansOfType(ServletContextInitializerBeans.java:230)
	at org.springframework.boot.web.servlet.ServletContextInitializerBeans.getOrderedBeansOfType(ServletContextInitializerBeans.java:221)
	at org.springframework.boot.web.servlet.ServletContextInitializerBeans.addServletContextInitializerBeans(ServletContextInitializerBeans.java:106)
	at org.springframework.boot.web.servlet.ServletContextInitializerBeans.<init>(ServletContextInitializerBeans.java:95)
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.getServletContextInitializerBeans(ServletWebServerApplicationContext.java:271)
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.selfInitialize(ServletWebServerApplicationContext.java:245)
	at org.springframework.boot.web.embedded.tomcat.TomcatStarter.onStartup(TomcatStarter.java:52)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:4464)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:164)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1203)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1193)
	at java.base/java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:264)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java)
	at org.apache.tomcat.util.threads.InlineExecutorService.execute(InlineExecutorService.java:75)
	at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:145)
	at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:749)
	at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:772)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:164)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1203)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1193)
	at java.base/java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:264)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java)
	at org.apache.tomcat.util.threads.InlineExecutorService.execute(InlineExecutorService.java:75)
	at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:145)
	at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:749)
	at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:203)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:164)
	at org.apache.catalina.core.StandardService.startInternal(StandardService.java:412)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:164)
	at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:870)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:164)
	at org.apache.catalina.startup.Tomcat.start(Tomcat.java:438)
	at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.initialize(TomcatWebServer.java:128)
	at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.<init>(TomcatWebServer.java:107)
	at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getTomcatWebServer(TomcatServletWebServerFactory.java:517)
	at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getWebServer(TomcatServletWebServerFactory.java:219)
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.createWebServer(ServletWebServerApplicationContext.java:193)
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:167)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:621)
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:146)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:753)
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:439)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:318)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1362)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1351)
	at com.alipay.govaiserviceplatform.GovAIServicePlatformApplication.main(GovAIServicePlatformApplication.java:25)
	Suppressed: java.lang.Exception: #block terminated with an error
		at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:104)
		... 63 common frames omitted
Caused by: java.util.concurrent.TimeoutException: Did not observe any item or terminal signal within 20000ms in 'Mono.deferContextual ⇢ at io.modelcontextprotocol.spec.McpClientSession.sendRequest(McpClientSession.java:233)' (and no fallback has been configured)
	at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.handleTimeout(FluxTimeout.java:296)
	Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Assembly trace from producer [reactor.core.publisher.MonoTimeout] :
	reactor.core.publisher.Mono.timeout(Mono.java:5007)
	io.modelcontextprotocol.spec.McpClientSession.sendRequest(McpClientSession.java:245)
Error has been observed at the following site(s):
	*__Mono.timeout ⇢ at io.modelcontextprotocol.spec.McpClientSession.sendRequest(McpClientSession.java:245)
	|_  Mono.handle ⇢ at io.modelcontextprotocol.spec.McpClientSession.sendRequest(McpClientSession.java:245)
	|_ Mono.flatMap ⇢ at io.modelcontextprotocol.client.McpAsyncClient.initialize(McpAsyncClient.java:343)

[gakkiyomi]

The fact that the langchain4j-mcp SDK successfully establishes connections is conclusive proof that this is a bug.

//        var transport = HttpClientSseClientTransport.builder(info.baseUri())
//                .sseEndpoint(info.sseEndpoint())
//                .objectMapper(new ObjectMapper())
//                .requestBuilder(HttpRequest.newBuilder())
//                .clientBuilder(HttpClient.newBuilder().connectTimeout(Duration.ofSeconds(100)))
//                .build();
//        McpSyncClient client = McpClient.sync(transport)
//                .clientInfo(new McpSchema.Implementation("gov-ai-service-platform-proxy", "1.0.0"))
//                .build();
        final var mcpTransport = new HttpMcpTransport.Builder()
                .sseUrl("https://xxx.com/mcp/proxy/testchairstream/init")
                .timeout(Duration.ZERO)
                .logRequests(true)
                .logResponses(true)
                .build();
        McpClient client;
        try {
            client = new DefaultMcpClient.Builder()
                    .transport(mcpTransport)
                    .clientName("gov-ai-service-platform-proxy")
                    .clientVersion("1.0.0")
                    .toolExecutionTimeout(Duration.ZERO)
                    .build();
            List<ToolSpecification> toolSpecifications = client.listTools();
            System.out.println(toolSpecifications);

and print result

2025-07-01 16:25:47.041 [OkHttp https://xxxxx.com/...][] [] INFO  MCP - < {"result":{"protocolVersion":"2024-11-05","capabilities":{"resources":{"listChanged":true},"tools":{"listChanged":true}},"serverInfo":{"name":"chair-mcp-demo-server","version":"1.0.0"}},"jsonrpc":"2.0","id":0}
2025-07-01 16:25:47.162 [OkHttp https://xxxx.com/...][] [] INFO  MCP - < {"result":{"tools":[{"name":"list-npm-package-versions","description":"list npm package all versions","inputSchema":{"type":"object","properties":{"name":{"type":"string","description":"npm package name"}},"required":["name"],"additionalProperties":false,"$schema":"http://json-schema.org/draft-07/schema#"}},{"name":"list-npm-package-dist-tags","description":"list npm package all dist-tags","inputSchema":{"type":"object","properties":{"name":{"type":"string","description":"npm package name"}},"required":["name"],"additionalProperties":false,"$schema":"http://json-schema.org/draft-07/schema#"}},{"name":"get-npm-package-dist-tag-version","description":"get npm package dist-tag version","inputSchema":{"type":"object","properties":{"name":{"type":"string","description":"npm package name"},"distTag":{"type":"string","description":"dist-tag name"}},"required":["name","distTag"],"additionalProperties":false,"$schema":"http://json-schema.org/draft-07/schema#"}},{"name":"雨燕-欢迎语","description":"雨燕用户打招呼欢迎语","inputSchema":{"type":"object","properties":{},"additionalProperties":false,"$schema":"http://json-schema.org/draft-07/schema#"}},{"name":"雨燕-新建迭代","description":"新建雨燕应用迭代","inputSchema":{"type":"object","properties":{"yuyanId":{"type":"string","description":"雨燕应用 id"},"sprintName":{"type":"string","description":"雨燕迭代名称"}},"required":["yuyanId"],"additionalProperties":false,"$schema":"http://json-schema.org/draft-07/schema#"}}]},"jsonrpc":"2.0","id":1}
[ToolSpecification { name = "list-npm-package-versions", description = "list npm package all versions", parameters = JsonObjectSchema {description = null, properties = {name=JsonStringSchema {description = "npm package name" }}, required = [name], additionalProperties = false, definitions = {} } }, ToolSpecification { name = "list-npm-package-dist-tags", description = "list npm package all dist-tags", parameters = JsonObjectSchema {description = null, properties = {name=JsonStringSchema {description = "npm package name" }}, required = [name], additionalProperties = false, definitions = {} } }, ToolSpecification { name = "get-npm-package-dist-tag-version", description = "get npm package dist-tag version", parameters = JsonObjectSchema {description = null, properties = {name=JsonStringSchema {description = "npm package name" }, distTag=JsonStringSchema {description = "dist-tag name" }}, required = [name, distTag], additionalProperties = false, definitions = {} } }, ToolSpecification { name = "雨燕-欢迎语", description = "雨燕用户打招呼欢迎语", parameters = JsonObjectSchema {description = null, properties = {}, required = [], additionalProperties = false, definitions = {} } }, ToolSpecification { name = "雨燕-新建迭代", description = "新建雨燕应用迭代", parameters = JsonObjectSchema {description = null, properties = {yuyanId=JsonStringSchema {description = "雨燕应用 id" }, sprintName=JsonStringSchema {description = "雨燕迭代名称" }}, required = [yuyanId], additionalProperties = false, definitions = {} } }]
2025-07-01 16:25:47.171 [main][] [] ERROR c.a.g.c.c.BuiltInMCPServerRegistry - !!!!! MCP服务 '雨燕' 启动失败，应用将停止启动 !!!!!

[ag91]

I had a similar issue in a k8s setting and the problem was that the sse connection is stateful and you need sessionAffinity set for the service definition.

[gakkiyomi]

I had a similar issue in a k8s setting and the problem was that the sse connection is stateful and you need sessionAffinity set for the service definition.

how to set sessionAffinity in mycode?

[ag91]

I was in a rush when I was writing.
The sessionAffinity is a setting for kubernetes (where my mcp server was running).

The issue I think you may be facing is that the MCP client sets a stateful connection with the MCP server (your https://xxx.com/mcp/proxy/testchairstream/init).
But your server is behind a load balancer and so the traffic doesn't reach always the same server instance.

I am not sure if this applies to you as well, but I had a very similar stack trace and all I needed to do is to force the server and the client to be associated for things to work (via my kubernetes settings, so not something you can change in your client code).

[gakkiyomi]

I was in a rush when I was writing. The sessionAffinity is a setting for kubernetes (where my mcp server was running).

The issue I think you may be facing is that the MCP client sets a stateful connection with the MCP server (your https://xxx.com/mcp/proxy/testchairstream/init). But your server is behind a load balancer and so the traffic doesn't reach always the same server instance.

I am not sure if this applies to you as well, but I had a very similar stack trace and all I needed to do is to force the server and the client to be associated for things to work (via my kubernetes settings, so not something you can change in your client code).

thanks for you replay, but i still understand why python sdk or langchian4j sdk can work but not java sdk

[jaredbrink86]

I am running into the same issue. Trying both stdio and sse (running mcp server on local host). But both end up timing out. There is an NPE somewhere in MCPClientSession#sendRequest, But I am having a hard time figuring out where it's coming from.

I am just using a really simple test mcp server https://www.npmjs.com/package/@modelcontextprotocol/server-everything
I am running it on localhost:3001. Works just fine in my browser but I get a timeout any time I try to run client.initalize()
STDIO

  private McpSyncClient createClient() {
    ServerParameters params = ServerParameters.builder("npx")
                                              .args("-y", "@modelcontextprotocol/server-everything")
                                              .build();

    McpClientTransport transport = new StdioClientTransport(params);

    // Absolutely minimal - no timeout, no capabilities
    McpSyncClient mcpClient = McpClient.sync(transport).build();

    mcpClient.initialize();
    return mcpClient;
}

SSE

  private McpSyncClient createClient() {
    McpClientTransport transport = new HttpClientSseClientTransport("http://localhost:3001/sse");

    McpSyncClient mcpClient = McpClient.sync(transport).build();

    mcpClient.initialize();
    return mcpClient;
  }

[anandkrrai]

+1
I am also facing the same issue, added the details in : #412