Anthropic added the ability to define tools in the system message midway through a conversation to Claude API
Anthropic has made it possible in the beta version of Claude API (header inline-tools-2026-09-15) to define and modify tools directly in the system message midway through a conversation, without having to edit the tools array or invalidate the prompt cache.
Developers working with Claude API and MCP tools can now change tool definitions and schemas during a conversation without having to rewrite the entire call and without losing the prompt cache.