Spring AI MCP integration using WebMVC SSE - Aurora Blog|java/go/python

Bug description I have implemented both the client and server components for the Model Context Protocol (MCP). Both communicate using Server-Sent Events (SSE), allowing real-time, streaming responses. This setup works as expected in most cases, and the client receives responses from the server as a continuous stream of data.

However, in certain scenarios - depending on the question asked, I encounter an error related to the parsing or serialization while returning the response in streaming.

The error that I face:

2025-05-22T11:03:41.070+05:30 ERROR 117009 --- [spring-boot-ai-mcp-client] [oundedElastic-2] o.s.ai.chat.model.MessageAggregator      : Aggregation Error

io.modelcontextprotocol.spec.McpError: Failed to wait for the message endpoint
    at io.modelcontextprotocol.client.transport.HttpClientSseClientTransport.sendMessage(HttpClientSseClientTransport.java:405) ~[mcp-0.9.0.jar:0.9.0]
    at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$11(McpClientSession.java:232) ~[mcp-0.9.0.jar:0.9.0]
    at reactor.core.publisher.MonoCreate.subscribe(MonoCreate.java:61) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SerializedSubscriber.onNext(SerializedSubscriber.java:99) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onNext(FluxTimeout.java:181) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Operators$MonoInnerProducerBase.complete(Operators.java:2834) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SinkOneMulticast.subscribe(SinkOneMulticast.java:101) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.subscribe(Mono.java:4576) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.block(Mono.java:1778) ~[reactor-core-3.7.5.jar:3.7.5]
    at io.modelcontextprotocol.client.McpSyncClient.callTool(McpSyncClient.java:200) ~[mcp-0.9.0.jar:0.9.0]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:115) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:125) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCall(DefaultToolCallingManager.java:205) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCalls(DefaultToolCallingManager.java:128) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.anthropic.AnthropicChatModel.lambda$internalStream$4(AnthropicChatModel.java:265) ~[spring-ai-anthropic-1.0.0-M8.jar:1.0.0-M8]
    at reactor.core.publisher.FluxDefer.subscribe(FluxDefer.java:46) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37) ~[reactor-core-3.7.5.jar:3.7.5]
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317) ~[na:na]
    at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[na:na]
    at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
    Suppressed: java.lang.Exception: #block terminated with an error
        at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:104) ~[reactor-core-3.7.5.jar:3.7.5]
        at reactor.core.publisher.Mono.block(Mono.java:1779) ~[reactor-core-3.7.5.jar:3.7.5]
        ... 15 common frames omitted

2025-05-22T11:03:41.080+05:30 ERROR 117009 --- [spring-boot-ai-mcp-client] [nio-8081-exec-3] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] threw exception

io.modelcontextprotocol.spec.McpError: Failed to wait for the message endpoint
    at io.modelcontextprotocol.client.transport.HttpClientSseClientTransport.sendMessage(HttpClientSseClientTransport.java:405) ~[mcp-0.9.0.jar:0.9.0]
    at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$11(McpClientSession.java:232) ~[mcp-0.9.0.jar:0.9.0]
    at reactor.core.publisher.MonoCreate.subscribe(MonoCreate.java:61) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SerializedSubscriber.onNext(SerializedSubscriber.java:99) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onNext(FluxTimeout.java:181) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Operators$MonoInnerProducerBase.complete(Operators.java:2834) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SinkOneMulticast.subscribe(SinkOneMulticast.java:101) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.subscribe(Mono.java:4576) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.block(Mono.java:1778) ~[reactor-core-3.7.5.jar:3.7.5]
    at io.modelcontextprotocol.client.McpSyncClient.callTool(McpSyncClient.java:200) ~[mcp-0.9.0.jar:0.9.0]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:115) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:125) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCall(DefaultToolCallingManager.java:205) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCalls(DefaultToolCallingManager.java:128) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.anthropic.AnthropicChatModel.lambda$internalStream$4(AnthropicChatModel.java:265) ~[spring-ai-anthropic-1.0.0-M8.jar:1.0.0-M8]
    at reactor.core.publisher.FluxDefer.subscribe(FluxDefer.java:46) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37) ~[reactor-core-3.7.5.jar:3.7.5]
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317) ~[na:na]
    at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[na:na]
    at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
    Suppressed: java.lang.Exception: #block terminated with an error
        at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:104) ~[reactor-core-3.7.5.jar:3.7.5]
        at reactor.core.publisher.Mono.block(Mono.java:1779) ~[reactor-core-3.7.5.jar:3.7.5]
        ... 15 common frames omitted

2025-05-22T11:03:41.081+05:30 ERROR 117009 --- [spring-boot-ai-mcp-client] [nio-8081-exec-3] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed: io.modelcontextprotocol.spec.McpError: Failed to wait for the message endpoint] with root cause

io.modelcontextprotocol.spec.McpError: Failed to wait for the message endpoint
    at io.modelcontextprotocol.client.transport.HttpClientSseClientTransport.sendMessage(HttpClientSseClientTransport.java:405) ~[mcp-0.9.0.jar:0.9.0]
    at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$11(McpClientSession.java:232) ~[mcp-0.9.0.jar:0.9.0]
    at reactor.core.publisher.MonoCreate.subscribe(MonoCreate.java:61) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SerializedSubscriber.onNext(SerializedSubscriber.java:99) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onNext(FluxTimeout.java:181) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Operators$MonoInnerProducerBase.complete(Operators.java:2834) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SinkOneMulticast.subscribe(SinkOneMulticast.java:101) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.subscribe(Mono.java:4576) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.block(Mono.java:1778) ~[reactor-core-3.7.5.jar:3.7.5]
    at io.modelcontextprotocol.client.McpSyncClient.callTool(McpSyncClient.java:200) ~[mcp-0.9.0.jar:0.9.0]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:115) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:125) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCall(DefaultToolCallingManager.java:205) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCalls(DefaultToolCallingManager.java:128) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.anthropic.AnthropicChatModel.lambda$internalStream$4(AnthropicChatModel.java:265) ~[spring-ai-anthropic-1.0.0-M8.jar:1.0.0-M8]
    at reactor.core.publisher.FluxDefer.subscribe(FluxDefer.java:46) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37) ~[reactor-core-3.7.5.jar:3.7.5]
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317) ~[na:na]
    at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[na:na]
    at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
    Suppressed: java.lang.Exception: #block terminated with an error
        at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:104) ~[reactor-core-3.7.5.jar:3.7.5]
        at reactor.core.publisher.Mono.block(Mono.java:1779) ~[reactor-core-3.7.5.jar:3.7.5]
        ... 15 common frames omitted

2025-05-22T11:03:41.086+05:30  WARN 117009 --- [spring-boot-ai-mcp-client] [nio-8081-exec-3] .w.s.m.s.DefaultHandlerExceptionResolver : Ignoring exception, response committed already: org.springframework.http.converter.HttpMessageNotWritableException: No converter for [class java.util.LinkedHashMap] with preset Content-Type 'text/event-stream'
2025-05-22T11:03:41.086+05:30  WARN 117009 --- [spring-boot-ai-mcp-client] [nio-8081-exec-3] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.http.converter.HttpMessageNotWritableException: No converter for [class java.util.LinkedHashMap] with preset Content-Type 'text/event-stream']

Environment Java: 21 Spring-AI: 1.0.0-M8

Steps to reproduce I have a very simple tool:

@Tool(description = "Get weather information by city name")
    public String getWeather(String cityName) {
        return "The weather in " + cityName + " is sunny with a temperature of 25°C.";
    }

When I give a prompt like What is the weather in ABC city? I get above mentioned error.

Expected behavior Not getting this error. And return the response

Minimal Complete Reproducible example

The client exposes the following endpoint, which sends a prompt to the server and returns the streamed content as a Flux:

@GetMapping(produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<String> ask(@RequestParam String question) {
        return chatClient
                .prompt(question)
                .stream()
                .content();
}

Comment From: checkHup

You need to configure

@Configuration public class WebConfig implements WebMvcConfigurer { @Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter(); converter.setSupportedMediaTypes(Arrays.asList( MediaType.TEXT_EVENT_STREAM, MediaType.APPLICATION_JSON, MediaType.TEXT_PLAIN )); converters.add(converter); } }

Comment From: bhautik-sakhiya

@checkHup Still getting this error

2025-05-22T16:44:08.685+05:30 ERROR 221528 --- [spring-boot-ai-mcp-client] [oundedElastic-2] o.s.ai.chat.model.MessageAggregator      : Aggregation Error

io.modelcontextprotocol.spec.McpError: Failed to wait for the message endpoint
    at io.modelcontextprotocol.client.transport.HttpClientSseClientTransport.sendMessage(HttpClientSseClientTransport.java:405) ~[mcp-0.9.0.jar:0.9.0]
    at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$11(McpClientSession.java:232) ~[mcp-0.9.0.jar:0.9.0]
    at reactor.core.publisher.MonoCreate.subscribe(MonoCreate.java:61) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SerializedSubscriber.onNext(SerializedSubscriber.java:99) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onNext(FluxTimeout.java:181) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Operators$MonoInnerProducerBase.complete(Operators.java:2834) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SinkOneMulticast.subscribe(SinkOneMulticast.java:101) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.subscribe(Mono.java:4576) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.block(Mono.java:1778) ~[reactor-core-3.7.5.jar:3.7.5]
    at io.modelcontextprotocol.client.McpSyncClient.callTool(McpSyncClient.java:200) ~[mcp-0.9.0.jar:0.9.0]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:115) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:125) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCall(DefaultToolCallingManager.java:205) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCalls(DefaultToolCallingManager.java:128) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.anthropic.AnthropicChatModel.lambda$internalStream$4(AnthropicChatModel.java:265) ~[spring-ai-anthropic-1.0.0-M8.jar:1.0.0-M8]
    at reactor.core.publisher.FluxDefer.subscribe(FluxDefer.java:46) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37) ~[reactor-core-3.7.5.jar:3.7.5]
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317) ~[na:na]
    at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[na:na]
    at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
    Suppressed: java.lang.Exception: #block terminated with an error
        at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:104) ~[reactor-core-3.7.5.jar:3.7.5]
        at reactor.core.publisher.Mono.block(Mono.java:1779) ~[reactor-core-3.7.5.jar:3.7.5]
        ... 15 common frames omitted

2025-05-22T16:44:08.693+05:30 ERROR 221528 --- [spring-boot-ai-mcp-client] [nio-8081-exec-3] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] threw exception

io.modelcontextprotocol.spec.McpError: Failed to wait for the message endpoint
    at io.modelcontextprotocol.client.transport.HttpClientSseClientTransport.sendMessage(HttpClientSseClientTransport.java:405) ~[mcp-0.9.0.jar:0.9.0]
    at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$11(McpClientSession.java:232) ~[mcp-0.9.0.jar:0.9.0]
    at reactor.core.publisher.MonoCreate.subscribe(MonoCreate.java:61) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SerializedSubscriber.onNext(SerializedSubscriber.java:99) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onNext(FluxTimeout.java:181) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Operators$MonoInnerProducerBase.complete(Operators.java:2834) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SinkOneMulticast.subscribe(SinkOneMulticast.java:101) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.subscribe(Mono.java:4576) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.block(Mono.java:1778) ~[reactor-core-3.7.5.jar:3.7.5]
    at io.modelcontextprotocol.client.McpSyncClient.callTool(McpSyncClient.java:200) ~[mcp-0.9.0.jar:0.9.0]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:115) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:125) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCall(DefaultToolCallingManager.java:205) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCalls(DefaultToolCallingManager.java:128) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.anthropic.AnthropicChatModel.lambda$internalStream$4(AnthropicChatModel.java:265) ~[spring-ai-anthropic-1.0.0-M8.jar:1.0.0-M8]
    at reactor.core.publisher.FluxDefer.subscribe(FluxDefer.java:46) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37) ~[reactor-core-3.7.5.jar:3.7.5]
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317) ~[na:na]
    at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[na:na]
    at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
    Suppressed: java.lang.Exception: #block terminated with an error
        at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:104) ~[reactor-core-3.7.5.jar:3.7.5]
        at reactor.core.publisher.Mono.block(Mono.java:1779) ~[reactor-core-3.7.5.jar:3.7.5]
        ... 15 common frames omitted

2025-05-22T16:44:08.694+05:30 ERROR 221528 --- [spring-boot-ai-mcp-client] [nio-8081-exec-3] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed: io.modelcontextprotocol.spec.McpError: Failed to wait for the message endpoint] with root cause

io.modelcontextprotocol.spec.McpError: Failed to wait for the message endpoint
    at io.modelcontextprotocol.client.transport.HttpClientSseClientTransport.sendMessage(HttpClientSseClientTransport.java:405) ~[mcp-0.9.0.jar:0.9.0]
    at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$11(McpClientSession.java:232) ~[mcp-0.9.0.jar:0.9.0]
    at reactor.core.publisher.MonoCreate.subscribe(MonoCreate.java:61) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:76) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SerializedSubscriber.onNext(SerializedSubscriber.java:99) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onNext(FluxTimeout.java:181) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Operators$MonoInnerProducerBase.complete(Operators.java:2834) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.SinkOneMulticast.subscribe(SinkOneMulticast.java:101) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.subscribe(Mono.java:4576) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.Mono.block(Mono.java:1778) ~[reactor-core-3.7.5.jar:3.7.5]
    at io.modelcontextprotocol.client.McpSyncClient.callTool(McpSyncClient.java:200) ~[mcp-0.9.0.jar:0.9.0]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:115) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.mcp.SyncMcpToolCallback.call(SyncMcpToolCallback.java:125) ~[spring-ai-mcp-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCall(DefaultToolCallingManager.java:205) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.model.tool.DefaultToolCallingManager.executeToolCalls(DefaultToolCallingManager.java:128) ~[spring-ai-model-1.0.0-M8.jar:1.0.0-M8]
    at org.springframework.ai.anthropic.AnthropicChatModel.lambda$internalStream$4(AnthropicChatModel.java:265) ~[spring-ai-anthropic-1.0.0-M8.jar:1.0.0-M8]
    at reactor.core.publisher.FluxDefer.subscribe(FluxDefer.java:46) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84) ~[reactor-core-3.7.5.jar:3.7.5]
    at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37) ~[reactor-core-3.7.5.jar:3.7.5]
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317) ~[na:na]
    at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[na:na]
    at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
    Suppressed: java.lang.Exception: #block terminated with an error
        at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:104) ~[reactor-core-3.7.5.jar:3.7.5]
        at reactor.core.publisher.Mono.block(Mono.java:1779) ~[reactor-core-3.7.5.jar:3.7.5]
        ... 15 common frames omitted

Now the parsing issue is solved, but I still get this error.

Comment From: JoeBerg8

@Configuration public class WebConfig implements WebMvcConfigurer { @Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter(); converter.setSupportedMediaTypes(Arrays.asList( MediaType.TEXT_EVENT_STREAM, MediaType.APPLICATION_JSON, MediaType.TEXT_PLAIN )); converters.add(converter); } }

thank you. i was getting an error no static resource sse found when trying to connect to mcp server running inside of my spring boot app and this suggestion fixed it