A note on the language of ACP

[danbarua]

Adds docs/agent/acp-language.md: what the Agent Client Protocol's messages mean, and what each one obliges the other side to send, in what order. It is written for whoever builds the bridge between ACP and an agent runtime. It describes ACP itself, not labkit's implementation; packages/app-acp/protocol-reference.md and docs/agent/acp-conformance.md keep covering that.

docs/agent/guidance.md links to it.

Contents

• Types. Where the typed request/response pairs live in @agentclientprotocol/sdk 1.5.0 (AgentRequestParamsByMethod → AgentRequestResponsesByMethod and the client-side maps), and what the types leave out: ordering rules such as session/load replaying before it answers.
• Methods. Every stable method, its direction, its request and response types, and the capability that allows it.
• Rules by area. Capabilities, paths and positions, sessions, the prompt turn and cancellation, session/update kinds, tool calls, permission, client file system, terminals, elicitation, config options and modes, authentication, transports, extensibility and error codes.
• Disagreements and gaps. Where the specification pages, the RFDs and the SDK disagree, and what the specification leaves open.

Sources

The specification repository agentclientprotocol/agent-client-protocol at commit 90f44f4 (stable pages under docs/protocol/v1/, RFDs under docs/rfds/), and the SDK's dist/schema/types.gen.d.ts and dist/acp.d.ts. Three research passes extracted rules with their sources, one each for lifecycle, prompt turn and tools/client services, and the note was written from those results. Rules that exist only in UNSTABLE SDK types, draft pages or RFDs are marked unstable.

Verification

Docs only. The method-to-type table was checked against AgentRequestHandlersByMethod and ClientRequestHandlersByMethod in the installed SDK, and the package exports were checked to confirm the zod schemas are not exported. The PR touches nothing under packages/, so it gets no area label.