Pass request builder to constructor to support custom headers

mcp/src/main/java/io/modelcontextprotocol/client/transport/FlowSseClient.java

@@ -38,6 +38,7 @@
 public class FlowSseClient {
 
 	private final HttpClient httpClient;
+	private final HttpRequest.Builder requestBuilder;
 
 	/**
 	 * Pattern to extract the data content from SSE data field lines. Matches lines
@@ -92,7 +93,17 @@ public interface SseEventHandler {
 	 * @param httpClient the {@link HttpClient} instance to use for SSE connections
 	 */
 	public FlowSseClient(HttpClient httpClient) {
+		this(httpClient, HttpRequest.newBuilder());
+	}
+
+	/**
+	 * Creates a new FlowSseClient with the specified HTTP client and request builder.
+	 * @param httpClient the {@link HttpClient} instance to use for SSE connections
+	 * @param requestBuilder the {@link HttpRequest.Builder} to use for SSE requests
+	 */
+	public FlowSseClient(HttpClient httpClient, HttpRequest.Builder requestBuilder) {
 		this.httpClient = httpClient;
+		this.requestBuilder = requestBuilder;
 	}
 
 	/**
@@ -109,7 +120,7 @@ public FlowSseClient(HttpClient httpClient) {
 	 * @throws RuntimeException if the connection fails with a non-200 status code
 	 */
 	public void subscribe(String url, SseEventHandler eventHandler) {
-		HttpRequest request = HttpRequest.newBuilder()
+		HttpRequest request = requestBuilder
 			.uri(URI.create(url))
 			.header("Accept", "text/event-stream")
 			.header("Cache-Control", "no-cache")

mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java

@@ -82,6 +82,9 @@ public class HttpClientSseClientTransport implements McpClientTransport {
 	 */
 	private final HttpClient httpClient;
 
+	/** HTTP request builder for building requests to send messages to the server */
+	private final HttpRequest.Builder requestBuilder;
+
 	/** JSON object mapper for message serialization/deserialization */
 	protected ObjectMapper objectMapper;
 
@@ -126,15 +129,32 @@ public HttpClientSseClientTransport(HttpClient.Builder clientBuilder, String bas
 	 */
 	public HttpClientSseClientTransport(HttpClient.Builder clientBuilder, String baseUri, String sseEndpoint,
 			ObjectMapper objectMapper) {
+		this(clientBuilder, HttpRequest.newBuilder(), baseUri, sseEndpoint, objectMapper);
+	}
+
+	/**
+	 * Creates a new transport instance with custom HTTP client builder, object mapper, and headers.
+	 * @param clientBuilder the HTTP client builder to use
+	 * @param requestBuilder the HTTP request builder to use
+	 * @param baseUri the base URI of the MCP server
+	 * @param sseEndpoint the SSE endpoint path
+	 * @param objectMapper the object mapper for JSON serialization/deserialization
+	 * @throws IllegalArgumentException if objectMapper, clientBuilder, or headers is null
+	 */
+	public HttpClientSseClientTransport(HttpClient.Builder clientBuilder, HttpRequest.Builder requestBuilder, 
+		String baseUri, String sseEndpoint, ObjectMapper objectMapper) {
 		Assert.notNull(objectMapper, "ObjectMapper must not be null");
 		Assert.hasText(baseUri, "baseUri must not be empty");
 		Assert.hasText(sseEndpoint, "sseEndpoint must not be empty");
 		Assert.notNull(clientBuilder, "clientBuilder must not be null");
+		Assert.notNull(requestBuilder, "requestBuilder must not be null");
 		this.baseUri = baseUri;
 		this.sseEndpoint = sseEndpoint;
 		this.objectMapper = objectMapper;
 		this.httpClient = clientBuilder.connectTimeout(Duration.ofSeconds(10)).build();
-		this.sseClient = new FlowSseClient(this.httpClient);
+		this.requestBuilder = requestBuilder;
+
+		this.sseClient = new FlowSseClient(this.httpClient, requestBuilder);
 	}
 
 	/**
@@ -159,6 +179,8 @@ public static class Builder {
 
 		private ObjectMapper objectMapper = new ObjectMapper();
 
+		private HttpRequest.Builder requestBuilder = HttpRequest.newBuilder();
+
 		/**
 		 * Creates a new builder with the specified base URI.
 		 * @param baseUri the base URI of the MCP server
@@ -190,6 +212,17 @@ public Builder clientBuilder(HttpClient.Builder clientBuilder) {
 			return this;
 		}
 
+		/**
+		 * Sets the HTTP request builder.
+		 * @param requestBuilder the HTTP request builder
+		 * @return this builder
+		 */
+		public Builder requestBuilder(HttpRequest.Builder requestBuilder) {
+			Assert.notNull(requestBuilder, "requestBuilder must not be null");
+			this.requestBuilder = requestBuilder;
+			return this;
+		}
+
 		/**
 		 * Sets the object mapper for JSON serialization/deserialization.
 		 * @param objectMapper the object mapper
@@ -206,7 +239,7 @@ public Builder objectMapper(ObjectMapper objectMapper) {
 		 * @return a new transport instance
 		 */
 		public HttpClientSseClientTransport build() {
-			return new HttpClientSseClientTransport(clientBuilder, baseUri, sseEndpoint, objectMapper);
+			return new HttpClientSseClientTransport(clientBuilder, requestBuilder, baseUri, sseEndpoint, objectMapper);
 		}
 
 	}
@@ -301,7 +334,7 @@ public Mono<Void> sendMessage(JSONRPCMessage message) {
 
 		try {
 			String jsonText = this.objectMapper.writeValueAsString(message);
-			HttpRequest request = HttpRequest.newBuilder()
+			HttpRequest request = this.requestBuilder
 				.uri(URI.create(this.baseUri + endpoint))
 				.header("Content-Type", "application/json")
 				.POST(HttpRequest.BodyPublishers.ofString(jsonText))