Add or remove tools mid-conversation without busting the prompt cache — `mid-conversation-tool-changes-2026-07-01` is now in beta
Claude Platform release notes·high signal
As of July 24 you can change an agent's tool set between turns while preserving the prompt cache, via the `mid-conversation-tool-changes-2026-07-01` beta header, on Fable 5, Mythos 5, Opus 4.8, and Opus 5. Until now, swapping tools mid-run invalidated the cached prefix, which is why most agent harnesses pre-declare every tool a run might need and eat the token cost. This makes phase-scoped toolsets — read-only tools during planning, write tools only after approval — cheap enough to use as a default pattern.