LoggingMessageNotification#data accepts non-String payload

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

@@ -5355,7 +5355,10 @@ static ResourcesUpdatedNotification fromJson(@JsonProperty("uri") String uri,
 	 *
 	 * @param level The severity levels. The minimum log level is set by the client.
 	 * @param logger The logger that generated the message.
-	 * @param data JSON-serializable logging data.
+	 * @param data The logging data, as a string. The MCP specification allows any JSON
+	 * value; when a non-string value is received, it is converted with
+	 * {@link Object#toString()}. The resulting format is intended for human readers and
+	 * is not guaranteed to be valid JSON.
 	 * @param meta See specification for notes on _meta usage
 	 * <p>
 	 * Note: {@code level} and {@code data} are required by the MCP specification.
@@ -5377,8 +5380,9 @@ public record LoggingMessageNotification( // @formatter:off
 
 		@JsonCreator
 		static LoggingMessageNotification fromJson(@JsonProperty("level") LoggingLevel level,
-				@JsonProperty("logger") String loggerName, @JsonProperty("data") String data,
+				@JsonProperty("logger") String loggerName, @JsonProperty("data") Object rawData,
 				@JsonProperty("_meta") Map<String, Object> meta) {
+			String data = rawData != null ? rawData.toString() : null;
 			if (level == null || data == null) {
 				List<String> missing = new ArrayList<>();
 				if (level == null) {

mcp-test/src/test/java/io/modelcontextprotocol/client/McpAsyncClientResponseHandlerTests.java

@@ -559,4 +559,31 @@ void testPingMessageRequestHandling() {
 		asyncMcpClient.closeGracefully();
 	}
 
+	@Test
+	void testLoggingNotificationWithObjectData() throws IOException {
+		MockMcpClientTransport transport = initializationEnabledTransport();
+
+		List<McpSchema.LoggingMessageNotification> receivedNotifications = new ArrayList<>();
+
+		McpAsyncClient asyncMcpClient = McpClient.async(transport)
+			.loggingConsumer(notification -> Mono.fromRunnable(() -> receivedNotifications.add(notification)))
+			.build();
+
+		assertThat(asyncMcpClient.initialize().block()).isNotNull();
+
+		// Logging notification as sent by FastMCP servers, with an object as data
+		McpSchema.JSONRPCMessage notification = McpSchema.deserializeJsonRpcMessage(JSON_MAPPER, """
+				{"jsonrpc":"2.0","method":"notifications/message",
+				"params":{"level":"info","logger":"server","data":{"msg":"working","extra":null}}}""");
+		transport.simulateIncomingMessage(notification);
+
+		assertThat(receivedNotifications).singleElement().satisfies(received -> {
+			assertThat(received.level()).isEqualTo(McpSchema.LoggingLevel.INFO);
+			assertThat(received.logger()).isEqualTo("server");
+			assertThat(received.data()).isEqualTo("{msg=working, extra=null}");
+		});
+
+		asyncMcpClient.closeGracefully();
+	}
+
 }

mcp-test/src/test/java/io/modelcontextprotocol/spec/McpSchemaTests.java

@@ -17,6 +17,7 @@
 import org.assertj.core.api.InstanceOfAssertFactories;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 import org.junit.jupiter.params.provider.NullSource;
 import org.junit.jupiter.params.provider.ValueSource;
 
@@ -2729,6 +2730,60 @@ void testLoggingMessageNotificationDeserializationWithMissingRequiredFields() th
 		assertThat(notification.data()).isEmpty();
 	}
 
+	@Test
+	void testLoggingMessageNotificationDeserializationWithNullData() throws Exception {
+		McpSchema.LoggingMessageNotification notification = JSON_MAPPER.readValue("""
+				{"level":"info","data":null}""", McpSchema.LoggingMessageNotification.class);
+
+		assertThat(notification.data()).isEmpty();
+	}
+
+	@Test
+	void testLoggingMessageNotificationDeserializationWithStringData() throws Exception {
+		McpSchema.LoggingMessageNotification notification = JSON_MAPPER.readValue("""
+				{"level":"warning","logger":"my-logger","data":"plain text"}""",
+				McpSchema.LoggingMessageNotification.class);
+
+		assertThat(notification.level()).isEqualTo(McpSchema.LoggingLevel.WARNING);
+		assertThat(notification.logger()).isEqualTo("my-logger");
+		assertThat(notification.data()).isEqualTo("plain text");
+	}
+
+	@Test
+	void testLoggingMessageNotificationDeserializationWithObjectData() throws Exception {
+		// FastMCP sends {"msg": ..., "extra": ...} for every ctx.log() call
+		McpSchema.LoggingMessageNotification notification = JSON_MAPPER.readValue("""
+				{
+				  "level": "info",
+				  "logger": "server",
+				  "data": {
+				    "msg": "working",
+				    "extra": {
+				      "user": "bob",
+				      "ids": [1, 2 ]
+				    }
+				  }
+				}""", McpSchema.LoggingMessageNotification.class);
+
+		assertThat(notification.level()).isEqualTo(McpSchema.LoggingLevel.INFO);
+		assertThat(notification.logger()).isEqualTo("server");
+		assertThat(notification.data()).isEqualTo("{msg=working, extra={user=bob, ids=[1, 2]}}");
+	}
+
+	@ParameterizedTest
+	@CsvSource(delimiter = '|', textBlock = """
+			[1,"two",true] | [1, two, true]
+			42             | 42
+			1.5            | 1.5
+			true           | true
+			""")
+	void testLoggingMessageNotificationDeserializationWithNonStringData(String data, String expected) throws Exception {
+		McpSchema.LoggingMessageNotification notification = JSON_MAPPER.readValue("""
+				{"level":"debug","data":%s}""".formatted(data), McpSchema.LoggingMessageNotification.class);
+
+		assertThat(notification.data()).isEqualTo(expected);
+	}
+
 	// --- Icon tests (SEP-973) ---
 
 	@Test