spring-projects/spring-ai · error · IllegalArgumentException

Third parameter must be of type String:

Error message

Third parameter must be of type String: 

What it means

Thrown during parameter validation of an @McpLogging method callback when the callback declares multiple parameters and its third parameter's type is not assignable to String. The multi-parameter form must be (LoggingLevel, String, String), where the third parameter carries the log message payload; a non-String third parameter means the callback can never be invoked with a decoded notification. This fails fast at registration time via validateMethod rather than deferring the mismatch to message delivery.

Source

Thrown at mcp/mcp-annotations/src/main/java/org/springframework/ai/mcp/annotation/method/logging/AbstractMcpLoggingMethodCallback.java:123

				throw new IllegalArgumentException("Single parameter must be of type LoggingMessageNotification: "
						+ method.getName() + " in " + method.getDeclaringClass().getName() + " has parameter of type "
						+ parameters[0].getType().getName());
			}
		}
		else {
			// Three parameters must be LoggingLevel, String, String
			if (!LoggingLevel.class.isAssignableFrom(parameters[0].getType())) {
				throw new IllegalArgumentException("First parameter must be of type LoggingLevel: " + method.getName()
						+ " in " + method.getDeclaringClass().getName() + " has parameter of type "
						+ parameters[0].getType().getName());
			}
			if (!String.class.isAssignableFrom(parameters[1].getType())) {
				throw new IllegalArgumentException("Second parameter must be of type String: " + method.getName()
						+ " in " + method.getDeclaringClass().getName() + " has parameter of type "
						+ parameters[1].getType().getName());
			}
			if (!String.class.isAssignableFrom(parameters[2].getType())) {
				throw new IllegalArgumentException("Third parameter must be of type String: " + method.getName()
						+ " in " + method.getDeclaringClass().getName() + " has parameter of type "
						+ parameters[2].getType().getName());
			}
		}
	}

	/**
	 * Builds the arguments array for invoking the method.
	 * <p>
	 * This method constructs an array of arguments based on the method's parameter types
	 * and the available values (exchange, notification).
	 * @param method The method to build arguments for
	 * @param exchange The server exchange
	 * @param notification The logging message notification
	 * @return An array of arguments for the method invocation
	 */
	protected Object[] buildArgs(Method method, Object exchange, LoggingMessageNotification notification) {
		Parameter[] parameters = method.getParameters();

View on GitHub (pinned to 98a7beda4f)

Solutions

1. Change the third parameter to String (the logger name)

Example fix

// before
@McpLogging(clients = "client1")
public void onLog(LoggingLevel level, String logger, Map<String, Object> data) { ... }
// after
@McpLogging(clients = "client1")
public void onLog(LoggingLevel level, String logger, String message) { ... }

if (m.getParameterCount() == 3
        && !String.class.isAssignableFrom(m.getParameterTypes()[2])) {
    throw new IllegalStateException("third param must be String");
}

static boolean thirdParamIsString(Method m) {
    return m.getParameterCount() == 3
        && String.class.isAssignableFrom(m.getParameterTypes()[2]);
}

When it happens

Trigger: Registering a @McpLogging method like (LoggingLevel, String, Object) or (LoggingLevel, String, Map).

Common situations: Developer expects structured payload data as the third argument; message passed as a custom type; refactoring changed the parameter type after registration.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Single parameter must be of type LoggingMessageNotification:
• First parameter must be of type LoggingLevel:
• Second parameter must be of type String:
• Parameter must be of type List<McpSchema.Resource>:
• Method must not be null

AI-assisted analysis of spring-projects/spring-ai@98a7beda4f (2026-09-11). Data as JSON: /api/errors/f64c57c637332d9f. Report an issue: GitHub.