HTTP / WebSocket Transport

agent-client-protocol-http exposes ACP agents over one /acp endpoint.

• POST /acp with initialize creates a connection and returns Acp-Connection-Id.
• Later POST /acp requests include Acp-Connection-Id; session-scoped requests also include Acp-Session-Id or params.sessionId.
• GET /acp with Accept: text/event-stream streams agent messages over SSE. Use a connection-level stream for connection-scoped messages and per-session streams for session-scoped messages.
• GET /acp with a WebSocket upgrade uses text frames for JSON-RPC messages.
• DELETE /acp tears down the connection.

POST /acp request bodies are limited to 16 MiB.

HTTP + SSE Streams

After initialize, clients should open a connection-level SSE stream:

• GET /acp
• Accept: text/event-stream
• Acp-Connection-Id: <connection id>
• no Acp-Session-Id

This stream carries connection-scoped messages.

Session-scoped messages are routed to session-specific SSE streams. For each active session, clients should also open:

• GET /acp
• Accept: text/event-stream
• Acp-Connection-Id: <connection id>
• Acp-Session-Id: <session id>

Open a session stream before sending methods such as session/prompt, session/load, session/resume, or other session-scoped requests. When a session/new or session/fork response returns a new sessionId, open an SSE stream for that returned session before expecting updates or responses for it.

Features

The crate does not enable either transport side by default. Opt into only the side(s) you need.

agent-client-protocol-http = { version = "...", features = ["client"] }
agent-client-protocol-http = { version = "...", features = ["server"] }
agent-client-protocol-http = { version = "...", features = ["client", "server"] }

The client feature exposes HttpClient. The server feature exposes AcpHttpServer, ServerOptions, and CorsOptions.

Request Cancellation

Request cancellation is available through the core SDK:

agent-client-protocol-http = { version = "...", features = ["client", "server"] }

$/cancel_request is connection-scoped. The HTTP transport does not apply Acp-Session-Id to cancellation notifications, and routes outgoing cancellation notifications over the connection stream rather than a session stream.

WebSocket connections can carry cancellation at any point after the socket is open. With HTTP + SSE, cancellation can be sent after initialize completes and the client has received Acp-Connection-Id; an in-flight initialize request cannot be cancelled with a hop-local $/cancel_request on this transport shape.

Server

#![allow(unused)]
fn main() {
use agent_client_protocol_http::AcpHttpServer;

let app = AcpHttpServer::new(|| my_agent()).into_router();
let listener = tokio::net::TcpListener::bind("127.0.0.1:8080").await?;
axum::serve(listener, app).await?;
}

Cross-origin browser access is disabled by default. Enable it by allowlisting the browser origins that should be able to access the ACP endpoint:

#![allow(unused)]
fn main() {
use agent_client_protocol_http::{AcpHttpServer, CorsOptions, ServerOptions};

let app = AcpHttpServer::new(|| my_agent())
    .with_options(ServerOptions {
        cors: CorsOptions::allow_origins(["http://localhost:5173"])?,
        ..ServerOptions::default()
    })
    .into_router();
}

Client

#![allow(unused)]
fn main() {
use agent_client_protocol_http::HttpClient;

let transport = HttpClient::new("http://127.0.0.1:8080")?;
my_client().connect_to(transport).await?;
}

The same HttpClient also speaks WebSocket — pass a ws:// or wss:// URL and it will open a single bidirectional connection instead of using POST + SSE:

#![allow(unused)]
fn main() {
let transport = HttpClient::new("ws://127.0.0.1:8080")?;
my_client().connect_to(transport).await?;
}