refactor: standardize protocol version handling and HTTP headers

- Add protocolVersion() method to transport and transport provider interfaces and implementations
- Replace hardcoded HTTP header strings with HttpHeaders constants
- Update protocol versions to be transport-specific rather than global
- Deprecate McpSchema.LATEST_PROTOCOL_VERSION in favor of transport-specific versions
- Standardize header names: MCP-Protocol-Version, mcp-session-id, last-event-id
- Update clients and servers to use transport.protocolVersion() for initialization
- Refactor tests to use transport-specific protocol versions

This change improves maintainability by centralizing header name constants
and allows different transports to support different protocol versions
independently.

Signed-off-by: Christian Tzolov <[email protected]>

Author: tzolov

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java

@@ -23,6 +23,7 @@
 
 import io.modelcontextprotocol.spec.DefaultMcpTransportSession;
 import io.modelcontextprotocol.spec.DefaultMcpTransportStream;
+import io.modelcontextprotocol.spec.HttpHeaders;
 import io.modelcontextprotocol.spec.McpClientTransport;
 import io.modelcontextprotocol.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema;
@@ -66,9 +67,7 @@ public class WebClientStreamableHttpTransport implements McpClientTransport {
 
 	private static final Logger logger = LoggerFactory.getLogger(WebClientStreamableHttpTransport.class);
 
-	private static final String MCP_PROTOCOL_VERSION = "2025-06-18";
-
-	private static final String MCP_PROTOCOL_VERSION_HEADER_NAME = "MCP-Protocol-Version";
+	private static final String MCP_PROTOCOL_VERSION = "2025-03-26";
 
 	private static final String DEFAULT_ENDPOINT = "/mcp";
 
@@ -107,6 +106,11 @@ private WebClientStreamableHttpTransport(ObjectMapper objectMapper, WebClient.Bu
 		this.activeSession.set(createTransportSession());
 	}
 
+	@Override
+	public String protocolVersion() {
+		return MCP_PROTOCOL_VERSION;
+	}
+
 	/**
 	 * Create a stateful builder for creating {@link WebClientStreamableHttpTransport}
 	 * instances.
@@ -134,10 +138,10 @@ private DefaultMcpTransportSession createTransportSession() {
 		Function<String, Publisher<Void>> onClose = sessionId -> sessionId == null ? Mono.empty()
 				: webClient.delete()
 					.uri(this.endpoint)
-					.header(MCP_PROTOCOL_VERSION_HEADER_NAME, MCP_PROTOCOL_VERSION)
+					.header(HttpHeaders.PROTOCOL_VERSION, MCP_PROTOCOL_VERSION)
 					.headers(httpHeaders -> {
-						httpHeaders.add("mcp-session-id", sessionId);
-						httpHeaders.add(MCP_PROTOCOL_VERSION_HEADER_NAME, MCP_PROTOCOL_VERSION);
+						httpHeaders.add(HttpHeaders.MCP_SESSION_ID, sessionId);
+						httpHeaders.add(HttpHeaders.PROTOCOL_VERSION, MCP_PROTOCOL_VERSION);
 					})
 					.retrieve()
 					.toBodilessEntity()
@@ -198,11 +202,11 @@ private Mono<Disposable> reconnect(McpTransportStream<Disposable> stream) {
 			Disposable connection = webClient.get()
 				.uri(this.endpoint)
 				.accept(MediaType.TEXT_EVENT_STREAM)
-				.header(MCP_PROTOCOL_VERSION_HEADER_NAME, MCP_PROTOCOL_VERSION)
+				.header(HttpHeaders.PROTOCOL_VERSION, MCP_PROTOCOL_VERSION)
 				.headers(httpHeaders -> {
-					transportSession.sessionId().ifPresent(id -> httpHeaders.add("mcp-session-id", id));
+					transportSession.sessionId().ifPresent(id -> httpHeaders.add(HttpHeaders.MCP_SESSION_ID, id));
 					if (stream != null) {
-						stream.lastId().ifPresent(id -> httpHeaders.add("last-event-id", id));
+						stream.lastId().ifPresent(id -> httpHeaders.add(HttpHeaders.LAST_EVENT_ID, id));
 					}
 				})
 				.exchangeToFlux(response -> {
@@ -259,14 +263,14 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
 			Disposable connection = webClient.post()
 				.uri(this.endpoint)
 				.accept(MediaType.APPLICATION_JSON, MediaType.TEXT_EVENT_STREAM)
-				.header(MCP_PROTOCOL_VERSION_HEADER_NAME, MCP_PROTOCOL_VERSION)
+				.header(HttpHeaders.PROTOCOL_VERSION, MCP_PROTOCOL_VERSION)
 				.headers(httpHeaders -> {
-					transportSession.sessionId().ifPresent(id -> httpHeaders.add("mcp-session-id", id));
+					transportSession.sessionId().ifPresent(id -> httpHeaders.add(HttpHeaders.MCP_SESSION_ID, id));
 				})
 				.bodyValue(message)
 				.exchangeToFlux(response -> {
 					if (transportSession
-						.markInitialized(response.headers().asHttpHeaders().getFirst("mcp-session-id"))) {
+						.markInitialized(response.headers().asHttpHeaders().getFirst(HttpHeaders.MCP_SESSION_ID))) {
 						// Once we have a session, we try to open an async stream for
 						// the server to send notifications and requests out-of-band.
 						reconnect(null).contextWrite(sink.contextView()).subscribe();

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/client/transport/WebFluxSseClientTransport.java

@@ -9,6 +9,8 @@
 
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
+
+import io.modelcontextprotocol.spec.HttpHeaders;
 import io.modelcontextprotocol.spec.McpClientTransport;
 import io.modelcontextprotocol.spec.McpError;
 import io.modelcontextprotocol.spec.McpSchema;
@@ -64,8 +66,6 @@ public class WebFluxSseClientTransport implements McpClientTransport {
 
 	private static final String MCP_PROTOCOL_VERSION = "2024-11-05";
 
-	private static final String MCP_PROTOCOL_VERSION_HEADER_NAME = "MCP-Protocol-Version";
-
 	/**
 	 * Event type for JSON-RPC messages received through the SSE connection. The server
 	 * sends messages with this event type to transmit JSON-RPC protocol data.
@@ -170,6 +170,11 @@ public WebFluxSseClientTransport(WebClient.Builder webClientBuilder, ObjectMappe
 		this.sseEndpoint = sseEndpoint;
 	}
 
+	@Override
+	public String protocolVersion() {
+		return MCP_PROTOCOL_VERSION;
+	}
+
 	/**
 	 * Establishes a connection to the MCP server using Server-Sent Events (SSE). This
 	 * method initiates the SSE connection and sets up the message processing pipeline.
@@ -254,7 +259,7 @@ public Mono<Void> sendMessage(JSONRPCMessage message) {
 				return webClient.post()
 					.uri(messageEndpointUri)
 					.contentType(MediaType.APPLICATION_JSON)
-					.header(MCP_PROTOCOL_VERSION_HEADER_NAME, MCP_PROTOCOL_VERSION)
+					.header(HttpHeaders.PROTOCOL_VERSION, MCP_PROTOCOL_VERSION)
 					.bodyValue(jsonText)
 					.retrieve()
 					.toBodilessEntity()
@@ -287,7 +292,7 @@ protected Flux<ServerSentEvent<String>> eventStream() {// @formatter:off
 			.get()
 			.uri(this.sseEndpoint)
 			.accept(MediaType.TEXT_EVENT_STREAM)
-			.header(MCP_PROTOCOL_VERSION_HEADER_NAME, MCP_PROTOCOL_VERSION)
+			.header(HttpHeaders.PROTOCOL_VERSION, MCP_PROTOCOL_VERSION)
 			.retrieve()
 			.bodyToFlux(SSE_TYPE)
 			.retryWhen(Retry.from(retrySignal -> retrySignal.handle(inboundRetryHandler)));

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/server/transport/WebFluxSseServerTransportProvider.java

@@ -79,6 +79,8 @@ public class WebFluxSseServerTransportProvider implements McpServerTransportProv
 	 */
 	public static final String ENDPOINT_EVENT_TYPE = "endpoint";
 
+	private static final String MCP_PROTOCOL_VERSION = "2025-06-18";
+
 	/**
 	 * Default SSE endpoint path as specified by the MCP transport specification.
 	 */
@@ -212,6 +214,11 @@ public WebFluxSseServerTransportProvider(ObjectMapper objectMapper, String baseU
 		}
 	}
 
+	@Override
+	public String protocolVersion() {
+		return "2024-11-05";
+	}
+
 	@Override
 	public void setSessionFactory(McpServerSession.Factory sessionFactory) {
 		this.sessionFactory = sessionFactory;

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/server/transport/WebFluxStatelessServerTransport.java

@@ -21,7 +21,6 @@
 
 import java.io.IOException;
 import java.util.List;
-import java.util.function.Function;
 
 /**
  * Implementation of a WebFlux based {@link McpStatelessServerTransport}.

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/server/transport/WebFluxStreamableServerTransportProvider.java

@@ -95,6 +95,11 @@ private WebFluxStreamableServerTransportProvider(ObjectMapper objectMapper, Stri
 
 	}
 
+	@Override
+	public String protocolVersion() {
+		return "2025-03-26";
+	}
+
 	@Override
 	public void setSessionFactory(McpStreamableServerSession.Factory sessionFactory) {
 		this.sessionFactory = sessionFactory;

mcp-spring/mcp-spring-webmvc/src/main/java/io/modelcontextprotocol/server/transport/WebMvcSseServerTransportProvider.java

@@ -209,6 +209,11 @@ public WebMvcSseServerTransportProvider(ObjectMapper objectMapper, String baseUr
 		}
 	}
 
+	@Override
+	public String protocolVersion() {
+		return "2024-11-05";
+	}
+
 	@Override
 	public void setSessionFactory(McpServerSession.Factory sessionFactory) {
 		this.sessionFactory = sessionFactory;

mcp-spring/mcp-spring-webmvc/src/main/java/io/modelcontextprotocol/server/transport/WebMvcStreamableServerTransportProvider.java

@@ -147,6 +147,11 @@ private WebMvcStreamableServerTransportProvider(ObjectMapper objectMapper, Strin
 		}
 	}
 
+	@Override
+	public String protocolVersion() {
+		return "2025-03-26";
+	}
+
 	@Override
 	public void setSessionFactory(McpStreamableServerSession.Factory sessionFactory) {
 		this.sessionFactory = sessionFactory;

mcp-spring/mcp-spring-webmvc/src/test/java/io/modelcontextprotocol/server/WebMvcSseCustomContextPathTests.java

@@ -91,8 +91,15 @@ static class TestConfig {
 		@Bean
 		public WebMvcSseServerTransportProvider webMvcSseServerTransportProvider() {
 
-			return new WebMvcSseServerTransportProvider(new ObjectMapper(), CUSTOM_CONTEXT_PATH, MESSAGE_ENDPOINT,
-					WebMvcSseServerTransportProvider.DEFAULT_SSE_ENDPOINT);
+			return WebMvcSseServerTransportProvider.builder()
+				.objectMapper(new ObjectMapper())
+				.baseUrl(CUSTOM_CONTEXT_PATH)
+				.messageEndpoint(MESSAGE_ENDPOINT)
+				.sseEndpoint(WebMvcSseServerTransportProvider.DEFAULT_SSE_ENDPOINT)
+				.build();
+			// return new WebMvcSseServerTransportProvider(new ObjectMapper(),
+			// CUSTOM_CONTEXT_PATH, MESSAGE_ENDPOINT,
+			// WebMvcSseServerTransportProvider.DEFAULT_SSE_ENDPOINT);
 		}
 
 		@Bean

mcp/src/main/java/io/modelcontextprotocol/client/McpAsyncClient.java

@@ -272,9 +272,8 @@ public class McpAsyncClient {
 				asyncProgressNotificationHandler(progressConsumersFinal));
 
 		this.initializer = new LifecycleInitializer(clientCapabilities, clientInfo,
-				List.of(McpSchema.LATEST_PROTOCOL_VERSION), initializationTimeout,
-				ctx -> new McpClientSession(requestTimeout, transport, requestHandlers, notificationHandlers,
-						con -> con.contextWrite(ctx)));
+				List.of(transport.protocolVersion()), initializationTimeout, ctx -> new McpClientSession(requestTimeout,
+						transport, requestHandlers, notificationHandlers, con -> con.contextWrite(ctx)));
 		this.transport.setExceptionHandler(this.initializer::handleException);
 	}
 

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

@@ -215,6 +215,11 @@ public HttpClientSseClientTransport(HttpClient.Builder clientBuilder, HttpReques
 		this.httpRequestCustomizer = httpRequestCustomizer;
 	}
 
+	@Override
+	public String protocolVersion() {
+		return MCP_PROTOCOL_VERSION;
+	}
+
 	/**
 	 * Creates a new builder for {@link HttpClientSseClientTransport}.
 	 * @param baseUri the base URI of the MCP server