Unclear What the mode values other than the default "none" actually change in the description is not stated.
What
The internal code that builds the description text for the WebFetch tool used to take a simple true/false flag (leanPrompt) that controlled a single special-case explanation about claude.ai artifacts. It now instead takes a mode, a text value that defaults to "none", passed through to the helper functions that assemble the description.
Why
A mode that can hold more than two values suggests more variations of the WebFetch description are possible than the old on/off flag allowed, though what those other modes do isn't specified. The visible WebFetch description text itself is unchanged in this release.
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What the mode values other than the default "none" actually change in the description is not stated.