mitsuhiko: switching reasoning levels busts KV caches; Fable 5.1 is the exception
mitsuhiko · x · 2026-09-06
Simon Willison (mitsuhiko) explains a practical detail: for most models the reasoning level lives in the system prompt, so switching it mid-conversation invalidates the KV cache. The main exception is Fable 5.1, which allows changing reasoning levels mid-conversation without busting the cache.
Related event: Switching reasoning levels invalidates KV cache, developers find(3 posts)→
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