fix(server): reject a message whose contextId disagrees with its task

[JakubWorek]

What

Spec 3.4.3: "Agents MUST reject messages containing mismatching contextId and taskId (i.e., the provided contextId is different from that of the referenced Task)."

A follow-up carrying a real taskId with someone else's contextId was accepted with HTTP 200, and the client's contextId was then written into the task's history while the Task kept its own - two disagreeing context ids inside one task.

This guard already existed

RequestContext has carried it all along (agent_execution/context.py:77-78). It only fires when a non-None task reaches build(), and v2 passes task=None on purpose:

# We will get the task when we have to process the request to avoid concurrent read/write issues.
task=None,

So the check went dead when V2 became the default. The legacy handler passes the task and still rejects.

Rather than undo that deliberate task=None, the comparison goes where v2 already fetches the task to prove it exists, one line after the TaskNotFoundError it raises there. No extra read, and the task is not retained.

Scoped to requests that name a task

The check only runs under if original_task_id:. A contextId with no taskId is a new task in an existing context, which stays legal - test_scenario_context_id_visibility depends on it, and there is a test here pinning it.

A test that passed for the wrong reason

Worth flagging for review. TaskManager already rejects this mismatch once the agent emits an event, with Context in event doesn't match TaskManager wrong-context : real-context. That message happens to contain both context ids, so the obvious assertion - pytest.raises(InvalidParamsError) plus both ids in the message - passes with or without this change. It did, and mutation testing is what caught it.

The test now asserts the executor was never awaited. That is the actual behavioural difference: the request is refused before any agent work starts, instead of after the agent has already been handed a request whose ids contradict each other.

Result

Clears CORE-MULTI-006 (must) on all three bindings.

[github-actions]

🧪 Code Coverage (vs jakubworek/acts-input-mode-validation)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/request_handlers/default_request_handler_v2.py	92.68%	92.80%	🟢 +0.12%
Total	93.12%	93.12%	⚪️ 0.00%

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