ExperimentalHow the Agent combines content with its own instructions.
Required on the wire, so every request states its mode. In Rust,
[SystemPrompt::new] starts it as [SystemPromptMode::Append]. Unknown
values parse as [SystemPromptMode::Other]; Agents MUST return an error
for modes they do not support.
The system prompt content.
Agents advertising the capability MUST accept [ContentBlock::Text].
Clients MUST NOT send other content block types until a future
capability allows them, and Agents MUST return an error for content
blocks they do not support.
Optional_The _meta property is reserved by ACP to allow clients and agents to attach additional metadata to their interactions. Implementations MUST NOT make assumptions about values at these keys.
See protocol docs: Extensibility
UNSTABLE
This capability is not part of the spec yet, and may be removed or changed at any point.
Client-provided system-level instructions for a session.
Clients MUST only send this when the Agent advertises
sessionCapabilities.systemPrompt. Agents that cannot apply it, for example because of an unadvertised mode or an unsupported content block type, MUST return an error instead of creating, loading, resuming, or forking the session.