Describe mid-conversation tool changes in Claude Opus 5?

Mid-conversation tool changes is a beta capability that lets developers add or remove tools available to Claude between turns of an ongoing conversation, without invalidating the prompt cache the way changing the tool list previously would have.

Before this, an agentic session generally needed to commit to a fixed tool list for the life of the conversation, since changing it broke caching and could force expensive cache misses on subsequent turns.

This is particularly useful for agents that need to progressively unlock or restrict capabilities as a task unfolds - for example, granting access to a deployment tool only once a task reaches a later stage - without paying a caching penalty for that change.

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