Bash gains a keep-in-background path, and MCP results not meant for the model skip size handling

What

The Bash tool, which runs shell commands for Claude, gains a new way to run commands in the background.

• When this path is used, run_in_background is forced to true, and Bash gets a keptBackgroundShells starter to work with. One finding reports that run_in_background is removed from the tool's options on this path. The other reports that it is kept in the input.
• The tool setup now passes a leanRewrite option into tool descriptions, and the Bash description accepts it. Descriptions can now differ depending on that option.
• The stock Bash description captured under this release is 9,857 characters in its longest of 3 versions, unchanged from the previous release.
• tengu_structured_output_strict is unchanged in this build and still defaults to false.

Separately, MCP tool calls gain a resultNotForModel option, false by default. MCP tools come from add-on servers. When the option is set, the result's content is returned as is, and the usual image and size checks are skipped, along with saving large output to a file.

Why

MCP results that are not meant for the model should no longer be shortened or moved into files. The Bash changes lay the groundwork for keeping shells running in the background and for a different Bash description, without visible effect described yet.