diff --git a/server-common/src/main/java/org/a2aproject/sdk/server/agentexecution/AgentExecutor.java b/server-common/src/main/java/org/a2aproject/sdk/server/agentexecution/AgentExecutor.java
index aba33913a..a80c13879 100644
--- a/server-common/src/main/java/org/a2aproject/sdk/server/agentexecution/AgentExecutor.java
+++ b/server-common/src/main/java/org/a2aproject/sdk/server/agentexecution/AgentExecutor.java
@@ -97,6 +97,7 @@
  * @see org.a2aproject.sdk.server.requesthandlers.DefaultRequestHandler
  * @see org.a2aproject.sdk.spec.AgentCard
  */
+@FunctionalInterface
 public interface AgentExecutor {
     /**
      * Executes the agent's business logic for a message.
@@ -147,5 +148,7 @@ public interface AgentExecutor {
      * @throws org.a2aproject.sdk.spec.TaskNotCancelableError if this agent does not support cancellation
      * @throws A2AError if cancellation is supported but failed to execute
      */
-    void cancel(RequestContext context, AgentEmitter emitter) throws A2AError;
+    default void cancel(RequestContext context, AgentEmitter emitter) throws A2AError {
+        throw new org.a2aproject.sdk.spec.TaskNotCancelableError("Agent execution is not cancelable.");
+    }
 }