What happens
On a Prime Agent instance running in ACP compatibility mode (a stock prime-agent on PATH, no Pylon-managed build), a turn whose session/prompt request fails leaves the thread on "Working" indefinitely. The session stays running, nothing is shown to the user, and anything waiting on that thread waits forever.
How it was found
Isolated dev server, 2026-09-18, stock prime-agent 0.8.1, model anthropic/claude-sonnet-5. The provider log for the thread ends with:
turn.started ... "model":"anthropic/claude-sonnet-5"
request session/prompt status=started
request session/prompt status=failed errorTag=Die reasonCount=1 (16 s later)
No turn.completed, no runtime.error, no session state change follows. The projection kept status = running for the rest of the server's life, and the UI showed "Working for 4m 42s … Thinking". The same build paired as an executor made a lead call pair_await in a loop against a thread that would never finish.
The server trace at the same time shows why native mode was not used, which is expected and not the bug:
PrimeAgentDaemonManagerError … (incompatible-hello) … The installed Prime Agent SDK does not provide the required caller-owned session contract.
Expected
A failed or defected session/prompt ends the turn as an error with the cause shown, and the session leaves running, as other ACP failures do.
Notes
What happens
On a Prime Agent instance running in ACP compatibility mode (a stock
prime-agentonPATH, no Pylon-managed build), a turn whosesession/promptrequest fails leaves the thread on "Working" indefinitely. The session staysrunning, nothing is shown to the user, and anything waiting on that thread waits forever.How it was found
Isolated dev server, 2026-09-18, stock
prime-agent0.8.1, modelanthropic/claude-sonnet-5. The provider log for the thread ends with:No
turn.completed, noruntime.error, no session state change follows. The projection keptstatus = runningfor the rest of the server's life, and the UI showed "Working for 4m 42s … Thinking". The same build paired as an executor made a lead callpair_awaitin a loop against a thread that would never finish.The server trace at the same time shows why native mode was not used, which is expected and not the bug:
PrimeAgentDaemonManagerError … (incompatible-hello) … The installed Prime Agent SDK does not provide the required caller-owned session contract.Expected
A failed or defected
session/promptends the turn as an error with the cause shown, and the session leavesrunning, as other ACP failures do.Notes
Diemeans the failure is a defect rather than a typed error, so whatever mapssession/promptfailures to a turn error is probably only handling the typed channel.