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Claude Agent SDK 0.2.153 adds a system-prompt snapshot flag so resumed sessions stop busting the prompt cache
Released 2026-09-15, the Python SDK adds a `snapshot` field to SystemPromptPreset plus a new SystemPromptCustom typed dict. With snapshot set True the session keeps the system prompt recorded on its first request, which improves prompt-caching behavior across resumed sessions; set False and the prompt is rebuilt every request, which is what you want while iterating on `append` text. It requires Claude CLI 2.1.257+ and the release bundles CLI 2.1.273. Small surface, but it targets a real cost line for anyone running long-lived or resumed agent sessions where a rebuilt system prompt silently invalidates the cache on every turn.
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