Over the SDK, set_model can also swap your custom system prompt from the next turn.
What's wrong with this entry?
The set_model control request accepts an internal system_prompt field that replaces the custom system prompt (the same slot as --system-prompt) starting from the next turn. It applies only if the model change itself is accepted and the value is non-empty; older transports report success without applying it.
@internal Replaces the custom system prompt (the --system-prompt / initialize systemPrompt slot) from the next turn on.
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
Related
Other releases about the same thing. Found by shared names or similar wording; neither means one caused the other.
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v2.1.219
Opus 5 gets an extra system-prompt instruction restricting AgentTool, workflows and deep-research
Both mention system prompt
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v2.1.219
Opus 5 carries an extra system-prompt block and suppresses subagent nudges
Both mention system prompt
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v2.1.229
Bash tool prompt always carries the full git commit instructions
Both mention system prompt