Claude API beta lets tools be defined inside mid-conversation system messages

BetaSep 22, 2026VerifiedAdded Sep 22, 2026

With the inline-tools-2026-09-15 beta header, a tool_addition block can carry a full tool definition, so agents can add tools, change schemas or upgrade server tools mid-conversation without invalidating the prompt cache. The same header covers adding and removing tools by reference.

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Read Anthropic's release notes

https://platform.claude.com/docs/en/release-notes/overview#september-22-2026

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