A new systemPromptSnapshot option records a conversation's system prompt once and reuses it verbatim on every later request and resume, instead of re-rendering appendSystemPrompt each launch. When a snapshot exists, mid-session model switches or system-prompt changes only take effect after compaction or a new session.
Names in the bundlesystemPromptSnapshotappendSystemPrompt
Documented inclaude-code/agent-sdk/typescript
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed since
Anthropic's documentation has since written up appendSystemPrompt, on Agent SDK reference - TypeScript.
`options.cwd` is required. A claim can also set `additionalDirectories`, `model`, `permissionMode`, `maxThinkingTokens`, a flag-settings overlay in `settings`, `appendSystemPrompt`, `title`, `agents`, and per-session tokens in `env`.agent-sdk/typescript see the edit
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Anthropic's documentation agrees
Anthropic's documentation has since written up appendSystemPrompt, on Agent SDK reference - TypeScript.