You'll noticeTier: how much it should matter to you
3Useful: my rating, 1 to 5
2Signal: worth watching, 1 to 5
SDKArea: what it touches
ImprovementsKind: in v2.1.280,
What probably matters to youSection of the release
What
For Claude Code SDK sessions that replace the system prompt via MCP, updating that custom system prompt no longer takes effect immediately. Previously the new prompt text applied starting from the next turn. Now the session keeps using its already-recorded system prompt, and the new text is only sent after the next compaction (the point where Claude Code condenses the conversation history to save space). Setting systemPromptSnapshot: false restores the old behavior, applying the new prompt from the next turn.
Why
This changes when SDK users should expect a system prompt update to actually reach the model, so anyone relying on immediate effect needs to either wait for compaction or set systemPromptSnapshot: false.
