What
When Claude Code blocks launching a subagent because a duplicate is already running, the instructions it shows no longer mention checking on progress with a dedicated tool. They now simply suggest sending the running subagent a message via SendMessage if you need a progress report.
Why
This simplifies the guidance shown in this situation to a single, consistent way of checking on a running subagent's progress.
Names in the bundleSendMessage
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Added since
A small documentation edit on Create custom subagents touched a line naming SendMessage after this was published.
The second filter applies to subagents running in the background. Apart from `Agent` and `ExitPlanMode`, which follow the first filter's conditions wherever the subagent runs, a background subagent keeps every MCP tool but only these built…sub-agents see the edit
Confirmed since
Anthropic's documentation has since written up SendMessage, on Claude Code changelog.
* Fixed cross-session messages held by the receiving session's permission-mode policy leaving no trace: headless senders now get a delivery notice, and `SendMessage` results no longer imply it was readchangelog see the edit
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
Anthropic's documentation has since written up SendMessage, on Claude Code changelog.