What
When reporting the status of a background task, Claude Code now checks which tools are available to it. If that check fails, the report omits the output file path, forcing the task to report back only via SendMessage rather than pointing to a saved output file. This is in addition to an existing check based on webFetchSavedFiles.
Why
This prevents background tasks from referencing an output file path in cases where the tools needed to make use of it aren't actually available.
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.