feat(transport): add SEP-2243 Mcp-Method / Mcp-Name header mirroring with MCP-Protocol-Version validation

Implement SEP-2243 HTTP header standardization across client and server
servlet transports.

* Client: Emit 'Mcp-Method' on outbound Streamable HTTP requests and
  notifications, and 'Mcp-Name' when targeting named tools, prompts, or
  resources.
* Server: Validate 'Mcp-Method' and 'Mcp-Name' headers against deserialized
  JSON-RPC payloads in HttpServletStreamableServerTransportProvider and
  HttpServletStatelessServerTransport. Reject mismatches with HTTP 400 while
  tolerating absent headers for backward compatibility.
* Versioning: Validate 'MCP-Protocol-Version' against supported protocol
  versions on incoming servlet requests.
* Tests: Add Sep2243ClientRequestHeaderTests and Sep2243ServerHeaderValidationTests
  verifying emission, mismatch rejections, and absent-header tolerance.

Author: slachiewicz

docs/client.md

@@ -165,6 +165,9 @@ McpTransport transport = new StdioClientTransport(params, McpJsonDefaults.getMap
     - Configurable connect timeout
     - Custom HTTP request customization
     - Multiple protocol version negotiation
+    - SEP-2243 header mirroring: every POST carries an `Mcp-Method` header, and requests
+      targeting a tool, prompt, or resource also carry `Mcp-Name` (the name or URI), so
+      servers can validate the headers against the body without parsing it.
 
 === "Streamable WebClient (external)"
 

docs/server.md

@@ -167,6 +167,10 @@ Key features:
     - Configurable keep-alive intervals
     - Security validation support
     - Graceful shutdown support
+    - SEP-2243 validation: the servlet transport rejects requests whose present
+      `Mcp-Method` / `Mcp-Name` headers do not mirror the request body, and rejects
+      unsupported `MCP-Protocol-Version` values. Missing headers are tolerated so legacy
+      clients keep working.
 
 === "Streamable HTTP WebFlux (external)"
 

mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java

@@ -544,10 +544,23 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
 					.header(HttpHeaders.ACCEPT, APPLICATION_JSON + ", " + TEXT_EVENT_STREAM)
 					.header(HttpHeaders.CONTENT_TYPE, APPLICATION_JSON_UTF8)
 					.header(HttpHeaders.CACHE_CONTROL, "no-cache")
-					.header(HttpHeaders.PROTOCOL_VERSION,
-							ctx.getOrDefault(McpAsyncClient.NEGOTIATED_PROTOCOL_VERSION,
-									this.latestSupportedProtocolVersion))
-					.POST(HttpRequest.BodyPublishers.ofString(jsonBody));
+					.header(HttpHeaders.PROTOCOL_VERSION, ctx.getOrDefault(McpAsyncClient.NEGOTIATED_PROTOCOL_VERSION,
+							this.latestSupportedProtocolVersion));
+				// Per SEP-2243, mirror the JSON-RPC method and, where applicable, the
+				// target name/URI in dedicated headers so the server can validate
+				// them without parsing the body.
+				if (sentMessage instanceof McpSchema.JSONRPCRequest jsonrpcRequest) {
+					builder = builder.header(HttpHeaders.MCP_METHOD, jsonrpcRequest.method());
+					String name = extractNameFromParams(jsonrpcRequest.method(), jsonrpcRequest.params());
+					if (name != null) {
+						builder = builder.header(HttpHeaders.MCP_NAME, name);
+					}
+				}
+				else if (sentMessage instanceof McpSchema.JSONRPCNotification jsonrpcNotification) {
+					builder = builder.header(HttpHeaders.MCP_METHOD, jsonrpcNotification.method());
+				}
+
+				builder = builder.POST(HttpRequest.BodyPublishers.ofString(jsonBody));
 				var transportContext = ctx.getOrDefault(McpTransportContext.KEY, McpTransportContext.EMPTY);
 				return Mono
 					.from(this.httpRequestCustomizer.customize(builder, "POST", uri, jsonBody, transportContext));
@@ -740,6 +753,45 @@ public <T> T unmarshalFrom(Object data, TypeRef<T> typeRef) {
 		return this.jsonMapper.convertValue(data, typeRef);
 	}
 
+	/**
+	 * Extracts the name or URI of the tool, prompt, or resource referenced by a request,
+	 * used to populate the SEP-2243 {@code Mcp-Name} header.
+	 * @param method the JSON-RPC method of the request
+	 * @param params the request parameters
+	 * @return the target name or URI when the method references one, otherwise
+	 * {@code null}
+	 */
+	private String extractNameFromParams(String method, Object params) {
+		if (params == null) {
+			return null;
+		}
+
+		try {
+			return switch (method) {
+				case McpSchema.METHOD_TOOLS_CALL ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.CallToolRequest>() {
+					}).name();
+				case McpSchema.METHOD_PROMPT_GET ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.GetPromptRequest>() {
+					}).name();
+				case McpSchema.METHOD_RESOURCES_READ ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.ReadResourceRequest>() {
+					}).uri();
+				case McpSchema.METHOD_RESOURCES_SUBSCRIBE ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.SubscribeRequest>() {
+					}).uri();
+				case McpSchema.METHOD_RESOURCES_UNSUBSCRIBE ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.UnsubscribeRequest>() {
+					}).uri();
+				default -> null;
+			};
+		}
+		catch (Exception e) {
+			logger.debug("Failed to extract name from params for method {}: {}", method, e.getMessage());
+			return null;
+		}
+	}
+
 	/**
 	 * Builder for {@link HttpClientStreamableHttpTransport}.
 	 */

mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStatelessServerTransport.java

@@ -14,10 +14,12 @@
 
 import io.modelcontextprotocol.json.McpJsonDefaults;
 import io.modelcontextprotocol.json.McpJsonMapper;
+import io.modelcontextprotocol.json.TypeRef;
 
 import io.modelcontextprotocol.common.McpTransportContext;
 import io.modelcontextprotocol.server.McpStatelessServerHandler;
 import io.modelcontextprotocol.server.McpTransportContextExtractor;
+import io.modelcontextprotocol.spec.HttpHeaders;
 import io.modelcontextprotocol.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema;
 import io.modelcontextprotocol.spec.McpStatelessServerTransport;
@@ -161,6 +163,10 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 			return;
 		}
 
+		if (!validateProtocolVersion(request, response)) {
+			return;
+		}
+
 		try {
 			Map<String, List<String>> headers = HttpServletRequestUtils.extractHeaders(request);
 			this.securityValidator.validateHeaders(headers);
@@ -186,6 +192,12 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 
 			McpSchema.JSONRPCMessage message = McpSchema.deserializeJsonRpcMessage(jsonMapper, body);
 
+			// Per SEP-2243, reject header/body mismatches (missing headers are tolerated
+			// so legacy clients keep working).
+			if (!validateMcpHeaders(request, response, message)) {
+				return;
+			}
+
 			if (message instanceof McpSchema.JSONRPCRequest jsonrpcRequest) {
 				try {
 					McpSchema.JSONRPCResponse jsonrpcResponse = this.mcpHandler
@@ -266,6 +278,118 @@ private void responseError(HttpServletResponse response, int httpCode, McpError
 		writer.flush();
 	}
 
+	/**
+	 * Validates the {@code MCP-Protocol-Version} header against the protocol versions
+	 * supported by this transport. A missing header is allowed and falls back to the
+	 * negotiated protocol version, while a header carrying an unsupported version is
+	 * rejected with a 400 Bad Request.
+	 * @param request the HTTP servlet request
+	 * @param response the HTTP servlet response
+	 * @return true if the header is missing or contains a supported version, false if a
+	 * 400 error response has been written
+	 * @throws IOException if an I/O error occurs
+	 */
+	private boolean validateProtocolVersion(HttpServletRequest request, HttpServletResponse response)
+			throws IOException {
+		String protocolVersion = request.getHeader(HttpHeaders.PROTOCOL_VERSION);
+		if (protocolVersion == null || this.protocolVersions().contains(protocolVersion)) {
+			return true;
+		}
+		this.responseError(response, HttpServletResponse.SC_BAD_REQUEST,
+				McpError.builder(McpSchema.ErrorCodes.METHOD_NOT_FOUND)
+					.message("Unsupported protocol version (supported versions: "
+							+ String.join(", ", this.protocolVersions()) + ")")
+					.build());
+		return false;
+	}
+
+	/**
+	 * Validates the SEP-2243 {@code Mcp-Method} and {@code Mcp-Name} request headers
+	 * against the deserialized message body. Missing headers are permitted for backwards
+	 * compatibility with legacy clients, but any header that is supplied must match the
+	 * corresponding payload attribute. Mismatches are rejected with a 400 Bad Request.
+	 * @param request the incoming servlet request
+	 * @param response the servlet response used to write an error payload if validation
+	 * fails
+	 * @param message the parsed JSON-RPC message
+	 * @return {@code true} if validation passed, {@code false} if a 400 response was
+	 * written
+	 * @throws IOException if writing the error response fails
+	 */
+	private boolean validateMcpHeaders(HttpServletRequest request, HttpServletResponse response,
+			McpSchema.JSONRPCMessage message) throws IOException {
+		String method = message instanceof McpSchema.JSONRPCRequest req ? req.method()
+				: message instanceof McpSchema.JSONRPCNotification notif ? notif.method() : null;
+
+		if (method == null) {
+			return true;
+		}
+
+		String methodHeader = request.getHeader(HttpHeaders.MCP_METHOD);
+		if (methodHeader != null && !methodHeader.isBlank() && !method.equals(methodHeader)) {
+			this.responseError(response, HttpServletResponse.SC_BAD_REQUEST,
+					McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+						.message("Mcp-Method header mismatch: expected '" + method + "' but was '" + methodHeader + "'")
+						.build());
+			return false;
+		}
+
+		Object params = message instanceof McpSchema.JSONRPCRequest req ? req.params()
+				: message instanceof McpSchema.JSONRPCNotification notif ? notif.params() : null;
+		String name = extractNameFromParams(method, params);
+		if (name != null) {
+			String nameHeader = request.getHeader(HttpHeaders.MCP_NAME);
+			if (nameHeader != null && !nameHeader.isBlank() && !name.equals(nameHeader)) {
+				this.responseError(response, HttpServletResponse.SC_BAD_REQUEST,
+						McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+							.message("Mcp-Name header mismatch: expected '" + name + "' but was '" + nameHeader + "'")
+							.build());
+				return false;
+			}
+		}
+
+		return true;
+	}
+
+	/**
+	 * Extracts the name or URI of the tool, prompt, or resource referenced by a request,
+	 * as used to validate the SEP-2243 {@code Mcp-Name} header.
+	 * @param method the JSON-RPC method of the request
+	 * @param params the request parameters
+	 * @return the target name or URI when the method references one, otherwise
+	 * {@code null}
+	 */
+	private String extractNameFromParams(String method, Object params) {
+		if (params == null) {
+			return null;
+		}
+
+		try {
+			return switch (method) {
+				case McpSchema.METHOD_TOOLS_CALL ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.CallToolRequest>() {
+					}).name();
+				case McpSchema.METHOD_PROMPT_GET ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.GetPromptRequest>() {
+					}).name();
+				case McpSchema.METHOD_RESOURCES_READ ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.ReadResourceRequest>() {
+					}).uri();
+				case McpSchema.METHOD_RESOURCES_SUBSCRIBE ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.SubscribeRequest>() {
+					}).uri();
+				case McpSchema.METHOD_RESOURCES_UNSUBSCRIBE ->
+					this.jsonMapper.convertValue(params, new TypeRef<McpSchema.UnsubscribeRequest>() {
+					}).uri();
+				default -> null;
+			};
+		}
+		catch (Exception e) {
+			logger.debug("Failed to extract name from params for method {}: {}", method, e.getMessage());
+			return null;
+		}
+	}
+
 	/**
 	 * Cleans up resources when the servlet is being destroyed.
 	 * <p>