How to pass data from an MCP client to an MCP server in Java with Spring AI?

I worked around this by providing a WebClient.Builder bean with an ExchangeFilterFunction that adds context-related headers to each request. On the server side, I intercept those requests and use ThreadLocal appropriately, ensuring it’s cleaned up at the end of processing.

\> For the transport layer, you must use the WebFlux-based client (either SYNC or ASYNC mode) to ensure these headers propagate correctly.

\> This approach is designed for a Spring MVC architecture (one thread per request). For non-blocking/reactive architectures, you should use Reactor’s Context instead of ThreadLocal`.

at MCP Server side
A holder for the data (in my case, the userId). Use these static methods to access the context within your tool calls.

public class CustomVarHolder {
    private static final ThreadLocal userId=new ThreadLocal();
    public static void setUserId(String uString){
        userId.set(uString);
    }
    public static String getUserId(){
        return userId.get();
    }
    public static void clear(){
        userId.remove();
    }
}

Interceptor for the request:

@Component
public class CustomHandlerInterceptor implements HandlerInterceptor{
    public static final String USERHEADERNAME = "X-User-Header";
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
        String userId=request.getHeader(USERHEADERNAME);
        if(userId!=null){
            CustomVarHolder.setUserId(userId);
        }
        else{
            System.out.println("no header found");
        }
        return true;
    }
//Clear the value in the context holder
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception{
        CustomVarHolder.clear();
        System.out.println("thread local cleared");
    }
}

Configuring the interceptor

@Configuration
public class CustomWebMVCConfigurer implements WebMvcConfigurer{
    private final CustomHandlerInterceptor interceptor;
    CustomWebMVCConfigurer(CustomHandlerInterceptor interceptor){
        this.interceptor=interceptor;
    }
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(interceptor).addPathPatterns("/");
        System.out.println("interceptor is added");
    }
}

at MCP Client side:
Configuring a Webclient builder bean to inject to WebFluxSseClientTransport

@Configuration
public class CustomUserHeader {
    private static ThreadLocal userHeader = new ThreadLocal();
    public static final String USERHEADERNAME = "X-User-Header";
    public static void setUserHeader(String user) {
        userHeader.set(user);
    }
    public static String getUserHeader() {
        return userHeader.get();
    }
    public static void clear() {
        userHeader.remove();
    }
    @Bean
    @Primary
    public WebClient.Builder webClientBuilder() {
        System.out.println("Using WebClient from builder: ");
        return WebClient.builder().filter(filterFunction());
    }
    private ExchangeFilterFunction filterFunction(){
        return ExchangeFilterFunction.ofRequestProcessor(req->{
            String user = CustomUserHeader.getUserHeader();
            System.out.println("from filter function "+user);
            if(user != null) {
                ClientRequest modifiedClientRequest = ClientRequest.from(req).headers(
                                        h->{
                                            h.add(USERHEADERNAME, user);
                                        }
                                    ).build();
                return Mono.just(
                    modifiedClientRequest
                );
            }
            System.out.println("user id is null from filter function");
            return Mono.just(req);
        });
    }
}

creating and handling context

@GetMapping("chat")
    public String getMethodName(@RequestParam(value = "message",defaultValue = "tell a joke") String param,@RequestParam(name = "user",defaultValue = "12323")String userId) {
        try{
        //setting the value to be send as context
        CustomUserHeader.setUserHeader(userId);
        ToolCallingChatOptions options =ToolCallingChatOptions.builder().toolCallbacks(syncMcpToolCallbackProvider.getToolCallbacks()).internalToolExecutionEnabled(true).build();
        Prompt prompt = Prompt.builder().chatOptions(options).messages(List.of(UserMessage.builder().text(param).build())).build();
        return chatClient.prompt(prompt).system("give direct answers nothing more nothing less").call().content();
        }
        finally{
//must clear the context value to prevent reusing the value in re use of thread
            CustomUserHeader.clear();
        }
    }

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