ACP integration note

Status: gate note (FOUND-03). No ACP code exists in this phase. Blocks: ACP transport code only. The shared contract types (src/contract/, src/core/) proceed without this note, per D-18.

Official sources read

Read date: 2026-10-21 (ISO 8601).

Source	URL
Documentation index	https://agentclientprotocol.com/llms.txt
Introduction	https://agentclientprotocol.com/get-started/introduction
Protocol v1 overview	https://agentclientprotocol.com/protocol/v1/overview
Protocol v1 initialization	https://agentclientprotocol.com/protocol/v1/initialization
Protocol v1 transports	https://agentclientprotocol.com/protocol/v1/transports
Protocol v2 overview	https://agentclientprotocol.com/protocol/v2/overview
Protocol v2 initialization	https://agentclientprotocol.com/protocol/v2/initialization
Protocol v2 transports	https://agentclientprotocol.com/protocol/v2/transports

Not read in this phase: the v2 migration guide (/protocol/v2/migration) and the schema page. Both are listed in the index.

Questions and answers

Q1. Spec version

Sourced answer: two protocol generations are documented.

• v2 pages exist under /protocol/v2/. Its initialization example uses "protocolVersion": 2 and the info field for implementation information (https://agentclientprotocol.com/protocol/v2/initialization).
• v1 pages exist under /protocol/v1/. Its initialization example uses "protocolVersion": 1 and clientInfo/agentInfo (https://agentclientprotocol.com/protocol/v1/initialization).
• The protocol version is a single integer identifying a MAJOR version. The agent selects the highest version it supports that is less than or equal to the requested one (v2 initialization, "Version Negotiation"). If the client does not support the agent's selected version, the client SHOULD close the connection (v1 initialization, "Version Negotiation").

Which generation Realm targets is not decided here. Phase 5 decides and records it.

Q2. Transport

Sourced answer (v2 transports page, https://agentclientprotocol.com/protocol/v2/transports; v1 transports page is the same in substance):

1. stdio (defined): the client launches the agent as a subprocess; newline-delimited JSON-RPC on stdin/stdout. Messages MUST NOT contain embedded newlines. The agent MUST NOT write anything to stdout that is not a valid ACP message. Stderr is for logs. Agents and clients SHOULD support stdio whenever possible.
2. Streamable HTTP (not defined): listed as "draft proposal in progress". No normative binding exists in the pages read.
3. Custom transports: permitted, if they preserve the JSON-RPC message format and lifecycle requirements.

Batch rule (v2 transports): clients and agents SHOULD NOT batch lifecycle-sensitive messages such as initialize, auth/login, session/new, session/resume, and session/prompt.

Q3. Remote transport: a discrepancy in the sources

The introduction page says remote agents "communicate over HTTP or WebSocket" and that "full support for remote agents is a work in progress" (https://agentclientprotocol.com/get-started/introduction). The transports pages define only stdio and list Streamable HTTP as a draft. No WebSocket binding was found in the pages read.

Recorded as found: the remote binding is not documented as normative. Any remote ACP transport would be a custom transport under the transports page's rules.

Q4. Authentication and session methods (informational)

v2 defines auth/login and auth/logout. A client MUST NOT call them when authMethods is omitted or empty (v2 overview and initialization). v2 also defines session/new, session/prompt, session/resume, session/close, and session/list. Their details are not needed for the transport decision in this phase.

Q5. Which generation and method Realm uses

Decided in Phase 5 (P5-D10). Realm speaks protocol generation v1: "protocolVersion": 1 with clientInfo / agentInfo (Q1), over the stdio framing of Q2 by reference (newline-delimited JSON-RPC, stdout carries only ACP messages, stderr for logs). Version negotiation follows Q1: the agent selects the highest version it supports at or below the requested one. Lifecycle-sensitive messages are never batched, per the Q2 batch rule (initialize, auth/login, session/new, session/resume, session/prompt).

Realm's six tools ride one Realm extension method named realm/tool, which carries the whole frozen ToolCall map unchanged. realm/tool is a Realm extension, not standard ACP: no client-invoked tool method exists in the standard protocol pages read. Clients must not treat it as a standard ACP method.

Blocked module

ACP transport code (Phase 5). Nothing else is blocked by this note.

Not in this note

• No transport code, no agent, no client, no stdio handler.
• No guessed URLs. Pages not listed above were not read and are not cited.