Spring MCP Client: Stream Mode HTTP Header Access

Oct 22, 2025 by SLV Team 50 views

Hey everyone! 👋 Today, we're diving deep into a tricky situation that many of you might face when working with Spring MCP Clients, especially when you're in stream mode. Specifically, we'll explore the challenges of accessing HTTP headers from the McpTransportContext. I've been wrestling with this myself recently, and I'm excited to share my findings and some potential workarounds. Let's get started!

The Core Problem: Accessing HTTP Headers

So, the main issue is pretty straightforward: How do you get access to the HTTP headers from the McpTransportContext when your Spring MCP Client is operating in stream mode? You might be thinking, "Isn't this a basic requirement?" And you're absolutely right! When building applications, especially those that interact with external APIs, accessing headers is crucial. This is particularly true if you are trying to implement authorization or passing metadata for debugging or tracing. Imagine you're building an application that needs to:

Authenticate Users: Use Authorization headers (e.g., Bearer tokens).
Track Requests: Get x-request-id or similar headers for logging and diagnostics.
Implement Rate Limiting: Understand rate limits using headers like X-RateLimit-Remaining.

But when the Spring MCP Client is in stream mode, the way the framework handles these headers isn't always obvious. In stream mode, you're dealing with a continuous flow of data, and the context might not be readily available in the same way it would be in a request-response scenario. This is because the nature of streaming involves sending and receiving data chunks over time rather than a single request-response cycle. This difference makes header access a bit more challenging.

Diving into the Code (and the GitHub Repository)

I recently came across this problem while working on a project that utilizes the Spring AI library (specifically, version 1.1.0-SNAPSHOT). You can check out a minimal project source code example on GitHub. I'll provide the link here again, and I encourage you to take a look: [https://github.com/...](replace with the actual link). This repository contains a basic setup that demonstrates the problem. The goal is to send a prompt to an AI model and stream the response back. You might find that when you attempt to access headers within the stream, they're either missing or not easily accessible. This is the heart of our problem.

Why is This Happening?

The reason it's tricky to get at these headers in stream mode comes down to how Spring's WebClient and Reactor work together. In stream mode, the response is handled reactively, and the context (including headers) might not be readily available within the Flux or Mono streams where you process the response data. The framework is optimized for handling a continuous flow of data, and header information is often not immediately available as the stream unfolds. This can be especially frustrating if you rely on the headers for authentication, logging, or other crucial tasks.

Potential Workarounds and Solutions

Now, let's talk about some strategies you can use to address this issue. Keep in mind that the best solution will depend on your specific needs and the architecture of your application.

1. Pre-Processing the Request

One approach is to handle the headers before you enter the stream. This typically involves using a filter or interceptor to grab the headers when the request is initially created. You can then store these headers in a context that's accessible later on in your stream processing. For example, you can store the headers in a thread-local variable or use a reactive context like Context from Project Reactor. This way, you make the headers available as data flow from the start of the stream.

This method is efficient if the headers are only needed once at the start of the processing. This keeps things organized and guarantees you have the data before you start processing the stream. However, it might not work if you need the headers for every single data chunk in the stream.

2. Custom `ExchangeFilterFunction`

Spring WebClient allows you to implement custom filters, called ExchangeFilterFunction. These filters give you fine-grained control over the request and response before they are processed. You can create a filter that intercepts the response and extracts the headers. You can then attach those headers to the context (e.g., ThreadLocal or Reactor Context) to make them accessible later.

Here's a basic example. You can modify this example to fit your requirements. You'll need to inject the WebClient with this filter.

import org.springframework.web.reactive.function.client.ClientRequest;
import org.springframework.web.reactive.function.client.ClientResponse;
import org.springframework.web.reactive.function.client.ExchangeFilterFunction;
import org.springframework.web.reactive.function.client.ExchangeFunction;
import reactor.core.publisher.Mono;

public class HeaderExtractorFilter implements ExchangeFilterFunction {

    @Override
    public Mono<ClientResponse> filter(ClientRequest request, ExchangeFunction next) {
        return next.exchange(request)
                .doOnNext(response -> {
                    // Extract headers here and store them in a thread-safe context.
                    response.headers().asHttpHeaders().forEach((name, values) -> {
                        // Example: Log or store headers.
                        System.out.println("Header: " + name + ": " + values);
                    });
                });
    }
}

This approach provides flexibility because it allows you to inspect and modify both the request and the response. It allows you to tailor the header extraction process to your specific needs. However, it can add complexity, especially if you have to handle numerous headers.

3. Using `McpTransportContext` (If Available)

If the McpTransportContext provides header access, you can potentially extract headers directly from it. This approach is the most straightforward, but it depends on the framework and version you're using. You might need to examine the source code or documentation to see if it provides access to the headers. If it does, using this approach is often the easiest and cleanest way to get the header information you need.

4. Leveraging Reactor Context

Reactor's Context is a powerful tool for passing data through reactive streams. You can use it to store and retrieve headers. Before calling the streaming method, you can add headers to the Context. Then, within your stream, you can retrieve the headers from the context. This method works well and is a good option when you want to pass data without having to modify the data stream itself.

Here’s how you can do it:

import reactor.core.publisher.Mono;
import reactor.util.context.Context;

public class HeaderExample {

    public Mono<String> processStream(String prompt, HttpHeaders headers) {
        return Mono.just(prompt)
                .flatMap(p -> callAiModel(p)
                        .contextWrite(ctx -> ctx.put("httpHeaders", headers))
                );
    }

    private Mono<String> callAiModel(String prompt) {
        // Simulate calling an AI model and accessing headers.
        return Mono.just("Response from AI Model")
                .doOnNext(response -> {
                    HttpHeaders headers = getHttpHeadersFromContext();
                    // Process the response, including the HTTP headers.
                    if (headers != null) {
                        System.out.println("Authorization Header: " + headers.getFirst("Authorization"));
                        // Use the headers as needed.
                    }
                });
    }

    private HttpHeaders getHttpHeadersFromContext() {
        // Retrieve HTTP headers from the Reactor Context.
        try {
            return Context.of(Thread.currentThread().getContextClassLoader())
                    .get("httpHeaders");
        } catch (Exception e) {
            return null;
        }
    }
}

This approach is a good choice because it is reactive and thread-safe. It does not interfere with the stream of data itself. However, it might require some extra setup to manage the context.

Code Example: Using `ExchangeFilterFunction`

Here's a more detailed example of how you can use an ExchangeFilterFunction to extract headers and make them available in the stream. This example is tailored for Spring WebClient.

import org.springframework.http.HttpHeaders;
import org.springframework.web.reactive.function.client.*;
import reactor.core.publisher.Mono;
import reactor.util.context.Context;

public class HeaderExtractionExample {

    public static ExchangeFilterFunction headerExtractor() {
        return (request, next) -> next.exchange(request)
                .doOnNext(response -> {
                    HttpHeaders headers = response.headers().asHttpHeaders();
                    // Store the headers in Reactor Context
                    response.bodyToMono(String.class)
                            .subscriberContext(ctx -> ctx.put("httpHeaders", headers))
                            .subscribe();
                });
    }

    public Mono<String> fetchData(WebClient webClient, String url) {
        return webClient.get()
                .uri(url)
                .retrieve()
                .bodyToMono(String.class)
                .subscriberContext(Context.of(HttpHeaders.class, new HttpHeaders()))
                .contextWrite(context -> {
                    // Access and process the headers from the Reactor Context
                    HttpHeaders headers = context.get(HttpHeaders.class);
                    System.out.println("Authorization Header: " + headers.getFirst("Authorization"));
                    return context;
                });
    }

    public static void main(String[] args) {
        // Configure WebClient with the filter.
        WebClient webClient = WebClient.builder()
                .filter(headerExtractor())
                .build();

        // Example usage.
        HeaderExtractionExample example = new HeaderExtractionExample();
        example.fetchData(webClient, "https://example.com") // Replace with your actual endpoint.
                .subscribe(response -> {
                    System.out.println("Response: " + response);
                });
    }
}

This example sets up a WebClient with a custom filter, headerExtractor(). The filter extracts the headers and stores them in the Reactor Context. The fetchData function then retrieves the headers from the Context. The code demonstrates a complete flow for extracting and using the headers. It makes the headers available throughout the reactive stream. Make sure to replace https://example.com with the actual API endpoint you want to use.

Practical Considerations and Best Practices

When you're dealing with header access in stream mode, consider these best practices:

1. Choose the Right Approach: Pick the method that best aligns with your application's architecture and requirements. If header access is needed frequently, the `ExchangeFilterFunction` might be the best choice. For simple scenarios, pre-processing the request might suffice.

2. Context Management: Be very careful when dealing with context and thread safety in reactive environments. Reactor `Context` is usually the safest option, as it handles context propagation correctly.

3. Error Handling: Implement proper error handling to gracefully handle cases where headers might be missing or unavailable.

4. Logging and Monitoring: Log the headers that you extract to help debug problems.

Conclusion

Accessing HTTP headers in stream mode with Spring MCP Clients can be tricky, but it's definitely achievable. We discussed the problem and some solutions that can help you deal with the challenge of accessing HTTP headers. The key is to understand how your reactive streams are structured and to choose the right strategy to make those headers available where you need them.

I hope this helps you out. Happy coding, and don't hesitate to ask if you have any questions!