What happened?

AgentEmitter.updateStatus() and AgentEmitter.fail(A2AError) mark the emitter terminal before calling EventQueue.enqueueEvent().

If enqueueing throws, no terminal event is delivered, but the emitter remains marked terminal. DefaultRequestHandler catches the exception and attempts to report an InternalError through emitter.fail(A2AError). That fallback call then throws: IllegalStateException: Cannot update task status - terminal state already reached

The original enqueue failure is logged, but the secondary terminal state exception escapes the error handling path and prevents the fallback error event from being emitted.

Reproduction steps

This behavior is reproducible using the SDK’s built-in EventQueue, including when a terminal enqueue is interrupted while waiting for queue capacity.

1. Create an AgentEmitter backed by an EventQueue that throws from enqueueEvent().
2. Call a terminal operation such as emitter.complete().
3. Catch the resulting exception, as DefaultRequestHandler does.
4. Call emitter.fail(new InternalError()).

The fallback operation throws because the first attempt left terminalStateReached set.
Both final status updates through updateStatus() and protocol errors through fail(A2AError) set the terminal flag before enqueueing and are affected.

Reproduced with the SDK’s built-in EventQueue using a size-one queue at capacity and interrupting the terminal enqueue. This was a focused reproduction, not production log output.

Expected behavior

The SDK should handle a failed terminal enqueue without replacing the original failure with a secondary “terminal state already reached” exception. Its terminal state should remain consistent with whether a terminal event was successfully accepted for processing.

Relevant log output

Initial terminal enqueue failure:

java.lang.RuntimeException: Unable to acquire the semaphore to enqueue the event

Fallback failure:

java.lang.IllegalStateException:
Cannot update task status - terminal state already reached

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