run_in_background: false is ignored when the Agent dispatch includes a name — spawn goes async ("will receive instructions via mailbox")

Version: 2.1.200 (also observed on earlier 2.1.x builds, 2026-07-04/05) · Platform: Windows 11, agent-teams features active

Behavior: An Agent tool dispatch with both run_in_background: false and a name parameter does not run synchronously. It returns immediately with "will receive instructions via mailbox," and TaskStop later reports the task type as in_process_teammate. Observed 4/4 times across multiple sessions and different custom subagent types.

Control: The identical dispatch with name omitted blocks correctly and returns the agent's final message as the tool result (reproduced repeatedly, same sessions).

Consequence: the caller waits for a tool result that never comes, and the agent's final text is never auto-delivered (per background semantics), so the run silently degrades into notification-plus-nudge choreography.

Expected: either run_in_background: false wins regardless of name, or the docs state that naming an agent forces teammate/async semantics. Neither the subagents docs nor the changelog documents the current interaction.