A platform team rewrites the instructions its agent appends to the system prompt, ships the change, and asks users to resume their existing investigations so the new wording takes effect. Sessions started after the deploy behave as intended; resumed ones keep following the old wording for hours, then abruptly start following the new one. What accounts for both halves of that?
- A.
Resumed sessions read the appended text lazily on the first turn that actually needs it, so the change lands whenever the conversation next touches a topic the instructions cover, which is why the switchover looks abrupt and unrelated to elapsed time.
- B.
Appended instructions are cached by the API for a fixed period keyed on their text, so a resumed session reads the previous cached block until the entry expires; the switchover time is the cache lifetime rather than anything about the session itself.
- C.
The system prompt is recorded on a session's first request and that record is reused until the session is compacted, so the new text reaches a resumed session only after compaction, or in a new session.
- D.
Resumed sessions load the appended text from the transcript rather than from the deploy, and the transcript is rewritten whenever the session is renamed, so the sessions that switched over are the ones whose users happened to rename them.
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Answer: C
A session records its system prompt on the first request and keeps using that record until it is compacted.
- A. System prompt text is not loaded lazily per topic; it is fixed for the request.
- B. Prompt caching affects cost and latency, not which system prompt text a session uses.
- C. Matches the documented recording behaviour and explains the post-compaction switchover.
- D. Renaming a session changes a label and does not rebuild its recorded system prompt.