fix: reject SendMessage to tasks in terminal state with UnsupportedOperationError

[brucearctor]

Summary

SendMessage to a task in a terminal state (completed, canceled, rejected, failed) succeeds instead of returning UnsupportedOperationError. Fails on all three transports (JSON-RPC, gRPC, HTTP+JSON).

Requirement

• ID: CORE-SEND-002
• Section: 3.1.1 — SendMessage rejects messages to terminal tasks
• Level: MUST

Changes

Added a guard in DefaultRequestHandler.initMessageSend(), which is invoked by both onMessageSend (blocking) and onMessageSendStream (streaming). The guard checks whether the referenced task is in a final state before forwarding the message to AgentExecutor:

if (task.status().state().isFinal()) {
    throw new UnsupportedOperationError(null,
        "Cannot send message to task " + task.id() +
        " - task is in a terminal state: " + task.status().state(), null);
}

Tests

Added 5 new tests in DefaultRequestHandlerTest:

• testSendMessage_ToCompletedTask_ThrowsUnsupportedOperationError
• testSendMessage_ToCanceledTask_ThrowsUnsupportedOperationError
• testSendMessage_ToRejectedTask_ThrowsUnsupportedOperationError
• testSendMessage_ToFailedTask_ThrowsUnsupportedOperationError
• testSendMessageStream_ToCompletedTask_ThrowsUnsupportedOperationError (streaming path)

All 11 tests in DefaultRequestHandlerTest pass.

This fixes #741

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical compliance issue by ensuring that the system correctly rejects attempts to send messages to tasks that have reached a final, non-active state. This change aligns the system's behavior with the A2A specification, improving robustness and predictability by preventing operations on tasks that are no longer capable of processing new messages.

Highlights

• Error Handling for Terminal Tasks: Implemented a guard in DefaultRequestHandler.initMessageSend() to prevent sending messages to tasks that are already in a terminal state (completed, canceled, rejected, or failed), ensuring an UnsupportedOperationError is thrown as per specification CORE-SEND-002.
• Comprehensive Test Coverage: Added five new unit tests in DefaultRequestHandlerTest to validate the correct behavior for SendMessage and SendMessageStream when targeting tasks in various terminal states.

Changelog

• server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java
  • Modified the initMessageSend method signature to declare throws A2AError.
  • Added a conditional check within initMessageSend to throw an UnsupportedOperationError if the target task's status indicates a final state.
• server-common/src/test/java/io/a2a/server/requesthandlers/DefaultRequestHandlerTest.java
  • Imported the UnsupportedOperationError class.
  • Updated the testRejectMismatchingContextId test to ensure the task is not in a terminal state, preventing the new terminal state guard from firing prematurely.
  • Introduced a private helper method assertSendMessageToTerminalStateThrows to encapsulate common test logic for terminal states.
  • Added testSendMessage_ToCompletedTask_ThrowsUnsupportedOperationError to verify rejection for completed tasks.
  • Added testSendMessage_ToCanceledTask_ThrowsUnsupportedOperationError to verify rejection for canceled tasks.
  • Added testSendMessage_ToRejectedTask_ThrowsUnsupportedOperationError to verify rejection for rejected tasks.
  • Added testSendMessage_ToFailedTask_ThrowsUnsupportedOperationError to verify rejection for failed tasks.
  • Added testSendMessageStream_ToCompletedTask_ThrowsUnsupportedOperationError to test streaming message rejection for completed tasks.

Activity

• This pull request addresses and fixes issue [Bug]: It must not be possible to send a message to a task is in a terminal state #741.

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[gemini-code-assist]

Code Review

This pull request implements a new validation rule in DefaultRequestHandler.java to prevent sending messages to tasks that are in a terminal state (completed, canceled, rejected, or failed), as per A2A spec section 3.1.1 (CORE-SEND-002). The initMessageSend method now throws an UnsupportedOperationError in such cases, and its signature has been updated to declare throws A2AError. Comprehensive unit tests have been added and modified in DefaultRequestHandlerTest.java to verify this behavior for all terminal states and for both standard and streaming message sending operations, ensuring the correct error is thrown.