fix: reject inline tasks during agent handoff

[chenghao-mou]

An outgoing tool can deadlock handoff by awaiting an AgentTask. Reject that task with a transition ToolError from handoff request through drain.

Reproduces @dtran26's report in agents-js#2483. The Python fix was developed independently; the similar JS fix was discovered afterward.

147 focused tests and core type checks pass. Full plugin type checks are blocked by environment errors.

Addresses AGT-3502.

Initial prompt and agent context

Model: GPT-6

can you investigate /tmp/agents-python-2483-investigation.md

Essential follow-up instructions:

how about we raise a tool error if the parent acitivity is draining and let the agent to call it later?

Yeah, it should be obvious to the agent that we are in transition to a new agent, and a tool call is not permitted until the transition is finished

I thought we have handoff first in the initial failure case?

[devin-ai-integration]

Devin Review found 1 new potential issue.

[devin-ai-integration]

🔴 Public drain deadlocks inline tasks

When AgentSession.drain() overlaps a tool awaiting an AgentTask, drain admits that inline task. The task waits for the activity lock while drain waits for its scheduling task, so both hang.

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AgentSession.drain() calls this method directly without going through _update_activity, so no other path sets _new_turns_blocked. An admitted inline AgentTask calls _update_activity(..., previous_activity="pause"), which eventually needs this activity's lock. The ongoing drain owns that lock and waits for _scheduling_atask, which contains the tool awaiting the inline task. Neither operation can finish.

Example: A non-cancellable tool pauses before await InlineTask(). Another coroutine starts session.drain(), then releases the tool. The inline task passes _inline_task_slot, waits to pause the locked activity, and the drain waits for that same tool to finish.

Recommended fix: Preserve admission blocking for a successful public drain, but roll it back if the drain is cancelled before scheduling pauses. Keep handoff blocking synchronous in _update_activity and avoid clearing a flag established by another concurrent transition.

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[chenghao-mou]

The agent_activity.py:2067 permits this because the excluded tool no longer counts as pending work.

The handoff case has an additional dependency: after drain, activity.aclose() waits for the tool executor while the handoff still holds the session’s _activity_lock. That prevents the inline task’s _update_activity() from proceeding. Public drain does neither.