JAVA SDK connectto streamablehttptransport raise "Client failed to initialize by explicit API call"

Start with the provided HttpClientStreamableHttpTransport builder example and reproduce the failure using mcpSyncClient.initialize(). Read LifecycleInitializer.java around line 288 and the initialization path leading to the 20-second timeout, then compare the streamable HTTP behavior with the reported working SSE case. Done means the Java SDK initializes successfully for the supplied streamable HTTP endpoint and has a regression test for the behavior.

JAVA SDK connectto streamablehttptransport raise "Client failed to initialize by explicit API call".
sse is fine.
I searched on issuses, Some similar issues were found, but they were all marked as resolved.

Bug description
JAVA SDK connectto streamablehttptransport raise "Client failed to initialize by explicit API call"

Environment
Please provide as many details as possible: Spring MCP version, Java version, which vector store you use if any, etc
java 17 , main brunch 2026-03-25； v1.1.0

Steps to reproduce

    public static void testmcphttp() {
        String mcpUrl = "http://192.168.190.51:31603/api/mcp/mcpserver/mcp";
        String tk = "gtk849416ca3e3b44a28b9956046895fafe";
        mcpUrl = "http://127.0.0.1:8702/api/mcp/mcpserver/mcp";
        tk = "gtkba06f61af36f4cf597624c73cc23abbc";
        HttpClientStreamableHttpTransport transport = HttpClientStreamableHttpTransport
                .builder("http://127.0.0.1:8702")
                .endpoint("/api/mcp/mcpserver/mcp?tk=" + tk)
                .build();
        McpSyncClient mcpSyncClient = McpClient
                .sync(transport)
                .clientInfo(new McpSchema.Implementation("Sample client", "0.0.0"))
                .build();

        mcpSyncClient.initialize();
    }

Expected behavior

Exception in thread "main" java.lang.RuntimeException: Client failed to initialize by explicit API call
	at io.modelcontextprotocol.client.LifecycleInitializer.lambda$withInitialization$2(LifecycleInitializer.java:288)
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:94)
	at reactor.core.publisher.SerializedSubscriber.onError(SerializedSubscriber.java:124)
	at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.handleTimeout(FluxTimeout.java:296)
	at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.doTimeout(FluxTimeout.java:281)
	at reactor.core.publisher.FluxTimeout$TimeoutTimeoutSubscriber.onNext(FluxTimeout.java:420)
	at reactor.core.publisher.FluxOnErrorReturn$ReturnSubscriber.onNext(FluxOnErrorReturn.java:162)
	at reactor.core.publisher.MonoDelay$MonoDelayRunnable.propagateDelay(MonoDelay.java:270)
	at reactor.core.publisher.MonoDelay$MonoDelayRunnable.run(MonoDelay.java:285)
	at reactor.core.scheduler.SchedulerTask.call(SchedulerTask.java:68)
	at reactor.core.scheduler.SchedulerTask.call(SchedulerTask.java:28)
	at java.base/java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:264)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java)
	at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:833)
	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 io.modelcontextprotocol.client.McpSyncClient.initialize(McpSyncClient.java:189)
		at io.modelcontextprotocol.TLCMCP.testmcphttp(TLCMCP.java:33)
		at io.modelcontextprotocol.TLCMCP.main(TLCMCP.java:57)
Caused by: java.util.concurrent.TimeoutException: Did not observe any item or terminal signal within 20000ms in 'map' (and no fallback has been configured)
	... 14 more

Minimal Complete Reproducible example

    public static void testmcphttp() {
        String mcpUrl = "http://192.168.190.51:31603/api/mcp/mcpserver/mcp";
        String tk = "gtk849416ca3e3b44a28b9956046895fafe";
        mcpUrl = "http://127.0.0.1:8702/api/mcp/mcpserver/mcp";
        tk = "gtkba06f61af36f4cf597624c73cc23abbc";
        HttpClientStreamableHttpTransport transport = HttpClientStreamableHttpTransport
                .builder("http://127.0.0.1:8702")
                .endpoint("/api/mcp/mcpserver/mcp?tk=" + tk)
                .build();
        McpSyncClient mcpSyncClient = McpClient
                .sync(transport)
                .clientInfo(new McpSchema.Implementation("Sample client", "0.0.0"))
                .build();

        mcpSyncClient.initialize();
    }