spring-projects/spring-ai · error · IllegalArgumentException

Method must have at least 1 parameter (ElicitRequest):

Error message

Method must have at least 1 parameter (ElicitRequest): 

What it means

AbstractMcpElicitationMethodCallback.validateParameters throws this IllegalArgumentException when a method registered for MCP elicitation has zero parameters. An elicitation handler must accept at least an ElicitRequest parameter so the framework can pass the client's response data.

Source

Thrown at mcp/mcp-annotations/src/main/java/org/springframework/ai/mcp/annotation/method/elicitation/AbstractMcpElicitationMethodCallback.java:93

	 * This method should be implemented by subclasses to handle specific return type
	 * validation.
	 * @param method The method to validate
	 * @throws IllegalArgumentException if the return type is not compatible
	 */
	protected abstract void validateReturnType(Method method);

	/**
	 * Validates method parameters. This method provides common validation logic and
	 * delegates exchange type checking to subclasses.
	 * @param method The method to validate
	 * @throws IllegalArgumentException if the parameters are not compatible
	 */
	protected void validateParameters(Method method) {
		Parameter[] parameters = method.getParameters();

		// Check parameter count - must have at least 1 parameter
		if (parameters.length < 1) {
			throw new IllegalArgumentException(
					"Method must have at least 1 parameter (ElicitRequest): " + method.getName() + " in "
							+ method.getDeclaringClass().getName() + " has " + parameters.length + " parameters");
		}

		// Check parameter types
		if (parameters.length == 1) {
			// Single parameter must be ElicitRequest
			if (!ElicitRequest.class.isAssignableFrom(parameters[0].getType())) {
				throw new IllegalArgumentException("Single parameter must be of type ElicitRequest: " + method.getName()
						+ " in " + method.getDeclaringClass().getName() + " has parameter of type "
						+ parameters[0].getType().getName());
			}
		}
		else {
			// TODO: Support for multiple parameters corresponding to ElicitRequest
			// fields
			// For now, we only support the single parameter version
			throw new IllegalArgumentException(

View on GitHub (pinned to 98a7beda4f)

Solutions

1. Add an ElicitRequest parameter as the first (or only) argument of the method
2. If extra parameters are needed, follow the supported signatures: (ElicitRequest) or (ElicitRequest, McpTransportContext/McpSyncServerExchange) per the library's parameter rules
3. Check validateParameters' type checks so the first parameter is exactly ElicitRequest, not Object or a subclass wrapper

Example fix

// before
@McpElicitation(elicitId = "confirm")
public void onConfirm() { }
// after
@McpElicitation(elicitId = "confirm")
public CompleteResult onConfirm(ElicitRequest request) { return ...; }

for (Method m : bean.getClass().getDeclaredMethods()) {
    if (m.isAnnotationPresent(McpElicitation.class) && m.getParameterCount() < 1)
        throw new IllegalStateException("@McpElicitation method " + m.getName() + " needs an ElicitRequest parameter");
}

static boolean hasElicitRequest(Method m) {
    return m.getParameterCount() > 0 && ElicitRequest.class.isAssignableFrom(m.getParameterTypes()[0]);
}

try { new SyncMcpElicitationMethodCallback(bean, method, elicitId); }
catch (IllegalArgumentException e) { log.error("invalid elicitation signature: {}", e.getMessage()); throw new BeanInitializationException(e.getMessage(), e); }

When it happens

Trigger: Annotating a no-argument method with @McpElicitation; validateParameters is invoked from validateMethod during callback construction.

Common situations: Adding the annotation to an empty callback-style method (e.g. run() or handle()) assuming the framework injects state via fields rather than parameters.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Method must return either CompleteResult, CompleteCompletion
• Method must return either CompleteResult, CompleteCompletion
• Method must not be null
• Single parameter must be of type ElicitRequest:
• Currently only methods with a single ElicitRequest parameter

AI-assisted analysis of spring-projects/spring-ai@98a7beda4f (2026-09-11). Data as JSON: /api/errors/589194c99e5535a3. Report an issue: GitHub.