What probably matters to youSection of the release
What
The skill documents two beta headers that did not appear in the previous build's docs.
Details
fallbacks: "default" sits behind server-side-fallback-2026-07-01 and lets the server pick the fallback by refusal category; the array form stays on server-side-fallback-2026-06-01, and pairing either header with the other form returns 400.
A new tool-use-concepts section covers mid-conversation-tool-changes-2026-07-01: tool_addition and tool_removal blocks on a role: "system" message change the tool set between turns without invalidating the prompt cache.
Tools involved must be declared with "defer_loading": True.
Evidence
mid-conversation-tool-changes-2026-07-01
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
Read from
Names in the bundleserver-side-fallback-2026-07-01mid-conversation-tool-changes-2026-07-01defer_loading
