> ## Documentation Index
> Fetch the complete documentation index at: https://agentclientprotocol.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Transports

> Mechanisms for agents and clients to communicate with each other

ACP uses JSON-RPC to encode messages. JSON-RPC messages **MUST** be UTF-8 encoded.

The protocol currently defines the following transport mechanisms for agent-client communication:

1. [stdio](#stdio), communication over standard in and standard out
2. *[Streamable HTTP](#streamable-http) (draft proposal in progress)*

Agents and clients **SHOULD** support stdio whenever possible.

It is also possible for agents and clients to implement [custom transports](#custom-transports).

## stdio

In the **stdio** transport:

* The client launches the agent as a subprocess.
* The agent reads JSON-RPC messages from its standard input (`stdin`) and sends messages to its standard output (`stdout`).
* Messages are individual JSON-RPC requests, notifications, or responses.
* Messages are delimited by newlines (`\n`), and **MUST NOT** contain embedded newlines.
* The agent **MAY** write UTF-8 strings to its standard error (`stderr`) for logging purposes. Clients **MAY** capture, forward, or ignore this logging.
* The agent **MUST NOT** write anything to its `stdout` that is not a valid ACP message.
* The client **MUST NOT** write anything to the agent's `stdin` that is not a valid ACP message.

```mermaid theme={null}
sequenceDiagram
    participant Client
    participant Agent Process

    Client->>+Agent Process: Launch subprocess
    loop Message Exchange
        Client->>Agent Process: Write to stdin
        Agent Process->>Client: Write to stdout
        Agent Process--)Client: Optional logs on stderr
    end
    Client->>Agent Process: Close stdin, terminate subprocess
    deactivate Agent Process
```

## *Streamable HTTP*

*In discussion, draft proposal in progress.*

## Custom Transports

Agents and clients **MAY** implement additional custom transport mechanisms to suit their specific needs. The protocol is transport-agnostic and can be implemented over any communication channel that supports bidirectional message exchange.

Implementers who choose to support custom transports **MUST** ensure they preserve the JSON-RPC message format and lifecycle requirements defined by ACP. Custom transports **SHOULD** document their specific connection establishment and message exchange patterns to aid interoperability.