Background agents and background commands can now be refused

What

A background agent or command keeps running while Claude carries on with other work. Claude asks for this with run_in_background. This release adds several ways such a request can be turned down, and new wording about it:

• Bash: for a run_in_background command, Claude Code now asks for a background shell reservation (remoteCall.backgroundShells.reserve()). If that returns a message, the call is blocked with error code 12.
• Agent tool parameter: the run_in_background parameter of the Agent tool, which starts a subagent (a helper Claude that works on a side task), now takes its description from a setting (parameterDescription). It is left out when background tasks are disabled. The agent launcher also has a new check on background launches.
• Agent tool wording: a new version of the Agent tool description says a subagent runs in the background only if run_in_background: true is passed, and that even then it may run in the foreground or be refused. A matching "Background agents" usage note was added. The Agent tool description captured under this release is unchanged at 7,081 characters, so this wording was not seen in use.
• Launch refusals: a new rule can refuse background agents with "Background agents cannot be started in this session right now. Run the agent without run_in_background." Resuming an agent in the background can fail with "This agent cannot be resumed in the background in this session. Start a new agent instead."
• Comment agents: a background agent working on an artifact comment is denied starting subagents, monitors, background commands or scheduled work, with a message saying "This comment thread is your only job". If it finishes after launching refused work, it is engaged again.
• Remote hosts: background commands started on a remote host are now tracked per agent. Using a task or shell id that this agent did not start on that host fails with "No background command with that ID was started on".
• Fleet view: typing /stop in the fleet-view peek now stops the session.

Why

Background work can now be limited per session and per agent. If Claude reports that a background agent or command was refused, running it in the foreground is the way forward, as the messages themselves suggest.