feat: Propagate Context to eager connect via McpClientSession (#339)

* feat: Propagate Context to eager connect via McpClientSession

In order to allow the initial connection to have contextual information
in the reactive chain, the McpClientSession should be able to transform
the McpClientTransport#connect result, e.g. to attach Context items.
This change introduces a new constructor for sessions that makes it
possible.

Signed-off-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>

---------

Signed-off-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>

Author: chemicL

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

@@ -11,7 +11,6 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Function;
-import java.util.function.Supplier;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -42,6 +41,7 @@
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.core.publisher.Sinks;
+import reactor.util.context.ContextView;
 
 /**
  * The Model Context Protocol (MCP) client implementation that provides asynchronous
@@ -161,7 +161,7 @@ public class McpAsyncClient {
 	 * The MCP session supplier that manages bidirectional JSON-RPC communication between
 	 * clients and servers.
 	 */
-	private final Supplier<McpClientSession> sessionSupplier;
+	private final Function<ContextView, McpClientSession> sessionSupplier;
 
 	/**
 	 * Create a new McpAsyncClient with the given transport and session request-response
@@ -268,9 +268,8 @@ public class McpAsyncClient {
 				asyncLoggingNotificationHandler(loggingConsumersFinal));
 
 		this.transport.setExceptionHandler(this::handleException);
-		this.sessionSupplier = () -> new McpClientSession(requestTimeout, transport, requestHandlers,
-				notificationHandlers);
-
+		this.sessionSupplier = ctx -> new McpClientSession(requestTimeout, transport, requestHandlers,
+				notificationHandlers, con -> con.contextWrite(ctx));
 	}
 
 	private void handleException(Throwable t) {
@@ -401,9 +400,8 @@ public Mono<McpSchema.InitializeResult> initialize() {
 		return withSession("by explicit API call", init -> Mono.just(init.get()));
 	}
 
-	private Mono<McpSchema.InitializeResult> doInitialize(Initialization initialization) {
-
-		initialization.setMcpClientSession(this.sessionSupplier.get());
+	private Mono<McpSchema.InitializeResult> doInitialize(Initialization initialization, ContextView ctx) {
+		initialization.setMcpClientSession(this.sessionSupplier.apply(ctx));
 
 		McpClientSession mcpClientSession = initialization.mcpSession();
 
@@ -493,14 +491,14 @@ Mono<Void> closeGracefully() {
 	 * @return A Mono that completes with the result of the operation
 	 */
 	private <T> Mono<T> withSession(String actionName, Function<Initialization, Mono<T>> operation) {
-		return Mono.defer(() -> {
+		return Mono.deferContextual(ctx -> {
 			Initialization newInit = Initialization.create();
 			Initialization previous = this.initializationRef.compareAndExchange(null, newInit);
 
 			boolean needsToInitialize = previous == null;
 			logger.debug(needsToInitialize ? "Initialization process started" : "Joining previous initialization");
 
-			Mono<McpSchema.InitializeResult> initializationJob = needsToInitialize ? doInitialize(newInit)
+			Mono<McpSchema.InitializeResult> initializationJob = needsToInitialize ? doInitialize(newInit, ctx)
 					: previous.await();
 
 			return initializationJob.map(initializeResult -> this.initializationRef.get())

mcp/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java

@@ -6,6 +6,7 @@
 
 import com.fasterxml.jackson.core.type.TypeReference;
 import io.modelcontextprotocol.util.Assert;
+import org.reactivestreams.Publisher;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import reactor.core.publisher.Mono;
@@ -16,6 +17,7 @@
 import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Function;
 
 /**
  * Default implementation of the MCP (Model Context Protocol) session that manages
@@ -99,9 +101,27 @@ public interface NotificationHandler {
 	 * @param transport Transport implementation for message exchange
 	 * @param requestHandlers Map of method names to request handlers
 	 * @param notificationHandlers Map of method names to notification handlers
+	 * @deprecated Use
+	 * {@link #McpClientSession(Duration, McpClientTransport, Map, Map, Function)}
 	 */
+	@Deprecated
 	public McpClientSession(Duration requestTimeout, McpClientTransport transport,
 			Map<String, RequestHandler<?>> requestHandlers, Map<String, NotificationHandler> notificationHandlers) {
+		this(requestTimeout, transport, requestHandlers, notificationHandlers, Function.identity());
+	}
+
+	/**
+	 * Creates a new McpClientSession with the specified configuration and handlers.
+	 * @param requestTimeout Duration to wait for responses
+	 * @param transport Transport implementation for message exchange
+	 * @param requestHandlers Map of method names to request handlers
+	 * @param notificationHandlers Map of method names to notification handlers
+	 * @param connectHook Hook that allows transforming the connection Publisher prior to
+	 * subscribing
+	 */
+	public McpClientSession(Duration requestTimeout, McpClientTransport transport,
+			Map<String, RequestHandler<?>> requestHandlers, Map<String, NotificationHandler> notificationHandlers,
+			Function<? super Mono<Void>, ? extends Publisher<Void>> connectHook) {
 
 		Assert.notNull(requestTimeout, "The requestTimeout can not be null");
 		Assert.notNull(transport, "The transport can not be null");
@@ -113,7 +133,7 @@ public McpClientSession(Duration requestTimeout, McpClientTransport transport,
 		this.requestHandlers.putAll(requestHandlers);
 		this.notificationHandlers.putAll(notificationHandlers);
 
-		this.transport.connect(mono -> mono.doOnNext(this::handle)).subscribe();
+		this.transport.connect(mono -> mono.doOnNext(this::handle)).transform(connectHook).subscribe();
 	}
 
 	private void dismissPendingResponses() {

mcp/src/test/java/io/modelcontextprotocol/client/McpAsyncClientTests.java

@@ -0,0 +1,81 @@
+package io.modelcontextprotocol.client;
+
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.modelcontextprotocol.spec.McpClientTransport;
+import io.modelcontextprotocol.spec.McpSchema;
+import org.junit.jupiter.api.Test;
+import reactor.core.publisher.Mono;
+
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Function;
+
+import static org.assertj.core.api.Assertions.assertThatCode;
+
+class McpAsyncClientTests {
+
+	public static final McpSchema.Implementation MOCK_SERVER_INFO = new McpSchema.Implementation("test-server",
+			"1.0.0");
+
+	public static final McpSchema.ServerCapabilities MOCK_SERVER_CAPABILITIES = McpSchema.ServerCapabilities.builder()
+		.build();
+
+	public static final McpSchema.InitializeResult MOCK_INIT_RESULT = new McpSchema.InitializeResult(
+			McpSchema.LATEST_PROTOCOL_VERSION, MOCK_SERVER_CAPABILITIES, MOCK_SERVER_INFO, "Test instructions");
+
+	private static final String CONTEXT_KEY = "context.key";
+
+	private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
+
+	@Test
+	void validateContextPassedToTransportConnect() {
+		McpClientTransport transport = new McpClientTransport() {
+			Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchema.JSONRPCMessage>> handler;
+
+			final AtomicReference<String> contextValue = new AtomicReference<>();
+
+			@Override
+			public Mono<Void> connect(
+					Function<Mono<McpSchema.JSONRPCMessage>, Mono<McpSchema.JSONRPCMessage>> handler) {
+				return Mono.deferContextual(ctx -> {
+					this.handler = handler;
+					if (ctx.hasKey(CONTEXT_KEY)) {
+						this.contextValue.set(ctx.get(CONTEXT_KEY));
+					}
+					return Mono.empty();
+				});
+			}
+
+			@Override
+			public Mono<Void> closeGracefully() {
+				return Mono.empty();
+			}
+
+			@Override
+			public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
+				if (!"hello".equals(this.contextValue.get())) {
+					return Mono.error(new RuntimeException("Context value not propagated via #connect method"));
+				}
+				// We're only interested in handling the init request to provide an init
+				// response
+				if (!(message instanceof McpSchema.JSONRPCRequest)) {
+					return Mono.empty();
+				}
+				McpSchema.JSONRPCResponse initResponse = new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION,
+						((McpSchema.JSONRPCRequest) message).id(), MOCK_INIT_RESULT, null);
+				return handler.apply(Mono.just(initResponse)).then();
+			}
+
+			@Override
+			public <T> T unmarshalFrom(Object data, TypeReference<T> typeRef) {
+				return OBJECT_MAPPER.convertValue(data, typeRef);
+			}
+		};
+
+		assertThatCode(() -> {
+			McpAsyncClient client = McpClient.async(transport).build();
+			client.initialize().contextWrite(ctx -> ctx.put(CONTEXT_KEY, "hello")).block();
+		}).doesNotThrowAnyException();
+	}
+
+}