When calling the Agent tool without run_in_background: true, the harness can silently launch the agent as an async background task and return 'Async agent launched successfully' instead of blocking. Expected: defaults to false = blocks. Observed: codex-executor without the flag still ran async (~3m45s), confirmed via background notification. Docs say 'Claude decides' but give no criteria. Impact: callers cannot predict execution mode reliably. Suggested: document which subagent types always run async, or guarantee the default blocks. Priority: low — docs clarity only.
When calling the Agent tool without run_in_background: true, the harness can silently launch the agent as an async background task and return 'Async agent launched successfully' instead of blocking. Expected: defaults to false = blocks. Observed: codex-executor without the flag still ran async (~3m45s), confirmed via background notification. Docs say 'Claude decides' but give no criteria. Impact: callers cannot predict execution mode reliably. Suggested: document which subagent types always run async, or guarantee the default blocks. Priority: low — docs clarity only.