The hidden cost of context compaction: re-prefilling 400K tokens after cache expiry
dotey · x · 2026-10-30
dotey highlights a counterintuitive cost lesson for long agent sessions: compressing after the prompt cache expires is expensive.
The real cost isn't the summary's tokens — the model must fully prefill the entire session (400K tokens in the author's test) before it can write the summary. Starting a fresh session and fetching history on demand skips that full re-read, and that's where the bill difference comes from, not summary length.
Related event: Context Compraction Called Costly and Unnecessary for Long Agent Sessions(2 posts)→
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