RFD: Client-Provided System Prompt#1237
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| ### Agent behavior | ||
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| When an agent receives a `systemPrompt`: | ||
| - It SHOULD use the content as the LLM's system prompt (or incorporate it into its system prompt construction) |
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Should this be clear that its always additional to existing system prompt so its a clear set of additional instructions vs replacing the System Prompt.
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great catch, I think you're right this needs to be cleaner. I just pushed a change to the RFD to clarify that systemPrompt uses additive semantics - the client system prompt is incorporated alongside the agent's own built-in instructions, it doesn't replace them. Agents always retain their own identity, safety guardrails, and internal configuration.
the updated Design Choices and Agent Behavior sections should make this explicit now. thanks for flagging it!
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Proposes adding an optional
systemPromptstring field tosession/new, giving clients a standard way to provide system-level instructions that agents can deliver to the LLM's system prompt slot.ACP is currently the only major agent protocol without a dedicated system prompt field — MCP, OpenAI, and Anthropic's Messages API all have one. Every ACP implementation has independently implemented non-standard workarounds:
_metaextensions, user-message injection, filesystem indirection. This RFD proposes a single interoperable solution.systemPrompt?: stringtoSessionNewParams— additive, fully backward compatiblesession/update_system_promptmethod if neededMotivated by Discussion #414.