Fix LoggingMessageNotification when data is a JSON object (e.g. FastMCP)

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

@@ -18,6 +18,7 @@
 import com.fasterxml.jackson.annotation.JsonProperty;
 import com.fasterxml.jackson.annotation.JsonSubTypes;
 import com.fasterxml.jackson.annotation.JsonTypeInfo;
+import io.modelcontextprotocol.json.McpJsonDefaults;
 import io.modelcontextprotocol.json.McpJsonMapper;
 import io.modelcontextprotocol.json.TypeRef;
 import io.modelcontextprotocol.util.Assert;
@@ -5355,7 +5356,8 @@ 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 Logging data as text. The wire format may carry any JSON value; on
+	 * deserialization non-string values are coerced to their JSON text form.
 	 * @param meta See specification for notes on _meta usage
 	 * <p>
 	 * Note: {@code level} and {@code data} are required by the MCP specification.
@@ -5375,24 +5377,46 @@ public record LoggingMessageNotification( // @formatter:off
 			Assert.notNull(data, "data must not be null");
 		}
 
+		/**
+		 * Coerce notification {@code data} to a string while keeping the public API as
+		 * {@link String}. The MCP spec allows any JSON value; servers such as FastMCP
+		 * send objects (for example {@code {"msg":"...","extra":null}}). Non-string
+		 * values are serialized to JSON text so deserialization does not fail.
+		 */
+		private static String coerceDataToString(Object data) {
+			if (data == null) {
+				return null;
+			}
+			if (data instanceof String stringData) {
+				return stringData;
+			}
+			try {
+				return McpJsonDefaults.getMapper().writeValueAsString(data);
+			}
+			catch (IOException ex) {
+				throw new IllegalArgumentException("Failed to serialize logging notification data to JSON text", ex);
+			}
+		}
+
 		@JsonCreator
 		static LoggingMessageNotification fromJson(@JsonProperty("level") LoggingLevel level,
-				@JsonProperty("logger") String loggerName, @JsonProperty("data") String data,
+				@JsonProperty("logger") String loggerName, @JsonProperty("data") Object data,
 				@JsonProperty("_meta") Map<String, Object> meta) {
-			if (level == null || data == null) {
+			String dataText = coerceDataToString(data);
+			if (level == null || dataText == null) {
 				List<String> missing = new ArrayList<>();
 				if (level == null) {
 					missing.add("level -> INFO");
 					level = LoggingLevel.INFO;
 				}
-				if (data == null) {
+				if (dataText == null) {
 					missing.add("data -> ''");
-					data = "";
+					dataText = "";
 				}
 				McpSchema.logger.warn("LoggingMessageNotification: missing required fields during deserialization: {}",
 						String.join(", ", missing));
 			}
-			return new LoggingMessageNotification(level, loggerName, data, meta);
+			return new LoggingMessageNotification(level, loggerName, dataText, meta);
 		}
 
 		// backwards compatibility constructor

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

@@ -2729,6 +2729,59 @@ void testLoggingMessageNotificationDeserializationWithMissingRequiredFields() th
 		assertThat(notification.data()).isEmpty();
 	}
 
+	@Test
+	void testLoggingMessageNotificationDeserializationWithObjectData() throws Exception {
+		// FastMCP ctx.info()/ctx.log() sends data as an object; the MCP spec allows any
+		// JSON
+		// value. Keep the public API as String by coercing non-string data to JSON text.
+		McpSchema.LoggingMessageNotification notification = JSON_MAPPER.readValue("""
+				{"level":"info","logger":"server","data":{"msg":"working","extra":null}}""",
+				McpSchema.LoggingMessageNotification.class);
+
+		assertThat(notification).isNotNull();
+		assertThat(notification.level()).isEqualTo(McpSchema.LoggingLevel.INFO);
+		assertThat(notification.logger()).isEqualTo("server");
+		assertThatJson(notification.data()).isObject().containsEntry("msg", "working").containsEntry("extra", null);
+	}
+
+	@Test
+	void testLoggingMessageNotificationDeserializationWithStringDataStillWorks() throws Exception {
+		McpSchema.LoggingMessageNotification notification = JSON_MAPPER.readValue("""
+				{"level":"warning","logger":"my-logger","data":"plain text"}""",
+				McpSchema.LoggingMessageNotification.class);
+
+		assertThat(notification.data()).isEqualTo("plain text");
+		assertThat(notification.level()).isEqualTo(McpSchema.LoggingLevel.WARNING);
+	}
+
+	@Test
+	void testLoggingMessageNotificationDeserializationWithArrayAndNumberData() throws Exception {
+		McpSchema.LoggingMessageNotification arrayNotification = JSON_MAPPER.readValue("""
+				{"level":"debug","data":[1,"two",true]}""", McpSchema.LoggingMessageNotification.class);
+		assertThatJson(arrayNotification.data()).isArray().containsExactly(1, "two", true);
+
+		McpSchema.LoggingMessageNotification numberNotification = JSON_MAPPER.readValue("""
+				{"level":"debug","data":42}""", McpSchema.LoggingMessageNotification.class);
+		assertThat(numberNotification.data()).isEqualTo("42");
+
+		McpSchema.LoggingMessageNotification boolNotification = JSON_MAPPER.readValue("""
+				{"level":"debug","data":true}""", McpSchema.LoggingMessageNotification.class);
+		assertThat(boolNotification.data()).isEqualTo("true");
+	}
+
+	@Test
+	void testLoggingMessageNotificationConvertValueWithObjectData() {
+		Map<String, Object> params = new HashMap<>();
+		params.put("level", "info");
+		params.put("logger", "server");
+		params.put("data", Map.of("msg", "working"));
+
+		McpSchema.LoggingMessageNotification notification = JSON_MAPPER.convertValue(params,
+				McpSchema.LoggingMessageNotification.class);
+
+		assertThatJson(notification.data()).isObject().containsEntry("msg", "working");
+	}
+
 	// --- Icon tests (SEP-973) ---
 
 	@Test