Protocol

Like MCP, codex app-server supports bidirectional communication using JSON-RPC 2.0 messages (with the "jsonrpc":"2.0" header omitted on the wire).

Supported transports:

• stdio (--listen stdio://, default): newline-delimited JSON (JSONL).
• websocket (--listen ws://IP:PORT, experimental and unsupported): one JSON-RPC message per WebSocket text frame.
• Unix socket (--listen unix:// or --listen unix://PATH): WebSocket connections over Codex's default app-server control socket or a custom Unix socket path, using the standard HTTP Upgrade handshake.
• off (--listen off): don't expose a local transport.

When you run with --listen ws://IP:PORT, the same listener also serves basic HTTP health probes:

• GET /readyz returns 200 OK once the listener accepts new connections.
• GET /healthz returns 200 OK when the request doesn't include an Origin header.
• Requests with an Origin header are rejected with 403 Forbidden.

WebSocket transport is experimental and unsupported. Local listeners such as ws://127.0.0.1:PORT are appropriate for localhost and SSH port-forwarding workflows. Non-loopback WebSocket listeners currently allow unauthenticated connections by default during rollout, so configure WebSocket auth before exposing one remotely.

Supported WebSocket auth flags:

• --ws-auth capability-token --ws-token-file /absolute/path
• --ws-auth capability-token --ws-token-sha256 HEX
• --ws-auth signed-bearer-token --ws-shared-secret-file /absolute/path

For signed bearer tokens, you can also set --ws-issuer, --ws-audience, and --ws-max-clock-skew-seconds. Clients present the credential as Authorization: Bearer <token> during the WebSocket handshake, and app-server enforces auth before JSON-RPC initialize.

Prefer --ws-token-file over passing raw bearer tokens on the command line. Use --ws-token-sha256 only when the client keeps the raw high-entropy token in a separate local secret store; the hash is only a verifier, and clients still need the original token.

In WebSocket mode, app-server uses bounded queues. When request ingress is full, the server rejects new requests with JSON-RPC error code -32001 and message "Server overloaded; retry later." Clients should retry with an exponentially increasing delay and jitter.