fix: generate a new request id on each client request subscription

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

@@ -261,9 +261,9 @@ private String generateRequestId() {
 	 */
 	@Override
 	public <T> Mono<T> sendRequest(String method, Object requestParams, TypeRef<T> typeRef) {
-		String requestId = this.generateRequestId();
-
 		return Mono.deferContextual(ctx -> Mono.<McpSchema.JSONRPCResponse>create(pendingResponseSink -> {
+			// A new id per subscription, so that a retried request does not reuse one
+			String requestId = this.generateRequestId();
 			logger.debug("Sending message for method {}", method);
 			this.pendingResponses.put(requestId, pendingResponseSink);
 			McpSchema.JSONRPCRequest jsonrpcRequest = new McpSchema.JSONRPCRequest(method, requestId, requestParams);

mcp-core/src/test/java/io/modelcontextprotocol/spec/McpClientSessionTests.java

@@ -5,6 +5,8 @@
 package io.modelcontextprotocol.spec;
 
 import java.time.Duration;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Map;
 import java.util.function.Function;
 
@@ -69,6 +71,33 @@ void testSendRequest() {
 		session.close();
 	}
 
+	@Test
+	void testResubscribedRequestUsesNewRequestId() {
+		List<Object> sentRequestIds = new ArrayList<>();
+		var transport = new MockMcpClientTransport((t, message) -> {
+			if (message instanceof McpSchema.JSONRPCRequest request) {
+				sentRequestIds.add(request.id());
+				// Fail the first attempt, answer the retry
+				McpSchema.JSONRPCResponse response = sentRequestIds.size() == 1
+						? McpSchema.JSONRPCResponse.error(request.id(),
+								new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
+										"Temporary failure"))
+						: McpSchema.JSONRPCResponse.result(request.id(), "test response");
+				t.simulateIncomingMessage(response);
+			}
+		});
+		var session = new McpClientSession(TIMEOUT, transport, Map.of(), Map.of(), Function.identity());
+
+		StepVerifier.create(session.sendRequest(TEST_METHOD, "test", responseType).retry(1))
+			.expectNext("test response")
+			.verifyComplete();
+
+		// JSON-RPC request ids must not be reused within a session
+		assertThat(sentRequestIds).hasSize(2).doesNotHaveDuplicates();
+
+		session.close();
+	}
+
 	@Test
 	void testSendRequestWithError() {
 		var transport = new MockMcpClientTransport();