# codex-exec-server

`codex-exec-server` is the library backing `codex exec-server`, a small
JSON-RPC server for spawning and controlling subprocesses through
`codex-utils-pty`.

It provides:

- a CLI entrypoint: `codex exec-server`
- a Rust client: `ExecServerClient`
- a small protocol module with shared request/response types

This crate owns the transport, protocol, and filesystem/process handlers. The
top-level `codex` binary owns hidden helper dispatch for sandboxed
filesystem operations and `codex-linux-sandbox`.

## Transport

The server speaks the exec-specific `codex-exec-server-protocol` message
envelope on the wire.

The CLI entrypoint supports:

- `ws://IP:PORT` (default)
- `--remote URL --environment-id ID [--name NAME]`
- `forward --connect ws://HOST:PORT --remote URL --environment-id ID`

Remote mode registers the local exec-server with the environment registry,
then reconnects to the service-provided rendezvous websocket as the environment.
Remote communication uses the Noise relay contract; the registry and harness
must support it.
Forward mode uses the same registration and Noise relay, but opens an independent
WebSocket connection to the destination exec-server for each authenticated
harness stream. Complete message payloads pass unchanged in both directions;
the forwarder does not parse RPCs, initialize sessions, or execute requests.
The destination owns session IDs, processes, and session resumption.
Disconnecting either side closes its peer and resets the remote stream. The
existing harness reconnect flow can then resume a retained destination session.
The forwarder does not replay requests or persist execution state, so recovery
is limited by the destination's session and process-output retention.
It uses the standard Codex ChatGPT sign-in state; run `codex login` first when
remote registration needs authentication. Containerized callers that receive an
Agent Identity JWT in `CODEX_ACCESS_TOKEN` can opt into that auth path with
`--use-agent-identity-auth`; Codex then registers an Agent task and sends the
derived AgentAssertion headers on the registry request.

Alternatively, API users can instead use `CODEX_API_KEY`;
Codex sends it as a bearer token on the registration request. For example:

```sh
CODEX_API_KEY="$OPENAI_API_KEY" \
codex exec-server \
  --remote ... \
  --environment-id "$ENVIRONMENT_ID"
```

Wire framing:

- local websocket: one JSON-RPC message per websocket message
- Noise remote websocket: binary protobuf relay frames carrying encrypted payloads

## Remote Relay Message Format

In remote mode, the harness and environment communicate through rendezvous using
`codex.exec_server.relay.v1.RelayMessageFrame`; the checked-in schema is in
`src/proto/codex.exec_server.relay.v1.proto`. The relay frame carries stream
identity plus endpoint-owned reliability metadata:

```text
version
stream_id
traceparent       // optional W3C parent on the first frame of a traced request
tracestate        // optional W3C vendor state paired with traceparent
body              // handshake | data | ack_frame | resume | reset | heartbeat
ack               // highest contiguous peer segment seq received
ack_bits          // bitset for peer segment seqs after ack
seq               // data only: segment sequence number
segment_index     // data only: 0-based index within message
segment_count     // data only: number of segments in message
payload           // handshake bytes or encrypted data record
next_seq          // resume only: next sender seq
reason            // reset only: reset reason
```

`stream_id` identifies one virtual harness/environment JSON-RPC session on the
environment websocket. The harness generates a UUIDv4 `stream_id`; the environment
demuxes frames by `stream_id` and runs an independent `ConnectionProcessor` per
stream.

Use segment-level sequence numbers for reliability:

```text
seq = 0, 1, 2, 3, ...
```

Use contiguous segment sequence ranges to identify and stitch a segmented
application message:

```text
message_start_seq = seq - segment_index
segment_index = 0
segment_count = 1
```

`message_start_seq` is derived by the receiver, not sent on the wire. For
unsplit messages, `message_start_seq == seq`, `segment_index == 0`, and
`segment_count == 1`.

Use cumulative `ack` plus fixed-size `ack_bits` instead of variable ack ranges:

```text
ack = highest contiguous received segment seq
bit i in ack_bits acknowledges seq = ack + 1 + i
```

Send `ack` and `ack_bits` redundantly on every outbound frame. Acks are not
themselves acked. Acks, retries, duplicate suppression, segmentation, and
reassembly are endpoint responsibilities; rendezvous only routes relay frames
by `stream_id`.

## Lifecycle

Each connection follows this sequence:

1. Send `initialize`.
2. Wait for the `initialize` response.
3. Send `initialized`.
4. Call process or filesystem RPCs.

Requests run sequentially by default. Pass `--concurrent-requests <COUNT>` to
enable concurrent processing.

If the server receives any notification other than `initialized`, it replies
with an error using request id `-1`.

If the websocket connection closes, the server terminates any remaining managed
processes for that client connection.

## API

### `initialize`

Initial handshake request.

Request params:

```json
{
  "clientName": "my-client"
}
```

Response:

```json
{}
```

### `initialized`

Handshake acknowledgement notification sent by the client after a successful
`initialize` response.

Params are currently ignored. Sending any other notification method is treated
as an invalid request.

### `process/start`

Starts a new managed process.

Request params:

```json
{
  "processId": "proc-1",
  "argv": ["bash", "-lc", "printf 'hello\\n'"],
  "cwd": "file:///absolute/working/directory",
  "env": {
    "PATH": "/usr/bin:/bin"
  },
  "tty": true,
  "pipeStdin": false,
  "arg0": null
}
```

Field definitions:

- `processId`: caller-chosen stable id for this process within the connection.
- `argv`: command vector. It must be non-empty.
- `cwd`: `file:` URI for the child process working directory.
- `env`: environment variables passed to the child process.
- `tty`: when `true`, spawn a PTY-backed interactive process.
- `pipeStdin`: when `true`, keep non-PTY stdin writable via `process/write`.
- `arg0`: optional argv0 override forwarded to `codex-utils-pty`.

Response:

```json
{
  "processId": "proc-1"
}
```

Behavior notes:

- Reusing an existing `processId` is rejected.
- PTY-backed processes accept later writes through `process/write`.
- Non-PTY processes reject writes unless `pipeStdin` is `true`.
- Output is streamed asynchronously via `process/output`.
- Exit is reported asynchronously via `process/exited`.

### `process/read`

Reads buffered output and terminal state for a managed process.

Request params:

```json
{
  "processId": "proc-1",
  "afterSeq": null,
  "maxBytes": 65536,
  "waitMs": 1000
}
```

Field definitions:

- `processId`: managed process id returned by `process/start`.
- `afterSeq`: optional sequence number cursor; when present, only newer chunks
  are returned.
- `maxBytes`: optional response byte budget.
- `waitMs`: optional long-poll timeout in milliseconds.

Response:

```json
{
  "chunks": [],
  "nextSeq": 1,
  "exited": false,
  "exitCode": null,
  "closed": false,
  "failure": null
}
```

### `process/write`

Writes raw bytes to a running process stdin.

Request params:

```json
{
  "processId": "proc-1",
  "chunk": "aGVsbG8K"
}
```

`chunk` is base64-encoded raw bytes. In the example above it is `hello\n`.

Response:

```json
{
  "status": "accepted"
}
```

Behavior notes:

- Writes to an unknown `processId` are rejected.
- Writes to a non-PTY process are rejected unless it started with `pipeStdin`.

### `process/terminate`

Terminates a running managed process.

Request params:

```json
{
  "processId": "proc-1"
}
```

Response:

```json
{
  "running": true
}
```

If the process is already unknown or already removed, the server responds with:

```json
{
  "running": false
}
```

## Notifications

### `process/output`

Streaming output chunk from a running process.

Params:

```json
{
  "processId": "proc-1",
  "seq": 1,
  "stream": "stdout",
  "chunk": "aGVsbG8K"
}
```

Fields:

- `processId`: process identifier
- `seq`: per-process output sequence number
- `stream`: `"stdout"`, `"stderr"`, or `"pty"`
- `chunk`: base64-encoded output bytes

### `process/exited`

Final process exit notification.

Params:

```json
{
  "processId": "proc-1",
  "seq": 2,
  "exitCode": 0,
  "sandboxDenied": false
}
```

`sandboxDenied` lets streaming clients preserve executor-side sandbox denial
detection without issuing a final `process/read` request. Clients recover it
with `process/read` when an older server omits the field.

### `process/closed`

Notification emitted after process output is closed and the process handle is
removed.

Params:

```json
{
  "processId": "proc-1",
  "seq": 3
}
```

## Filesystem RPCs

Filesystem methods require valid `file:` URI strings and return JSON-RPC errors
for invalid or unavailable paths. Native absolute path strings are rejected;
callers must convert them to `file:` URIs before sending requests:

- `fs/readFile`
- `fs/open`, `fs/readBlock`, and `fs/close` (internal transport for
  `ExecutorFileSystem::read_file_stream`)
- `fs/writeFile`
- `fs/createDirectory`
- `fs/getMetadata`
- `fs/canonicalize`
- `fs/readDirectory`
- `fs/remove`
- `fs/copy`

Each filesystem request accepts an optional `sandbox` object. When `sandbox`
contains a `ReadOnly` or `WorkspaceWrite` policy, the operation runs in a
hidden helper process launched from the top-level `codex` executable and
prepared through the shared sandbox transform path. Helper requests and
responses are passed over stdin/stdout.

## Errors

The server returns JSON-RPC errors with these codes:

- `-32600`: invalid request
- `-32602`: invalid params
- `-32603`: internal error

Typical error cases:

- unknown method
- malformed params
- empty `argv`
- duplicate `processId`
- writes to unknown processes
- writes to non-PTY processes
- sandbox-denied filesystem operations

## Rust surface

The crate exports:

- `ExecServerClient`
- `ExecServerError`
- `ExecServerClientConnectOptions`
- `RemoteExecServerConnectArgs`
- protocol request/response structs for process and filesystem RPCs
- `DEFAULT_LISTEN_URL` and `ExecServerListenUrlParseError`
- `ExecServerRuntimePaths`
- `run_main()` for embedding the websocket server
- `RemoteEnvironmentConfig` and `run_remote_environment()` for embedding remote
  registration mode

Callers must pass `ExecServerRuntimePaths` and an explicitly configured
`HttpClientFactory` to `run_main()`. The top-level `codex exec-server` command
builds these paths from the `codex` arg0 dispatch state and resolves its HTTP
client factory from the effective Codex configuration.
`RemoteEnvironmentConfig::new(...)` also takes the auth provider and HTTP client
factory that remote registration mode should use; the CLI builds the auth
provider from Codex auth state before starting remote mode.

## Example session

Initialize:

```json
{"id":1,"method":"initialize","params":{"clientName":"example-client"}}
{"id":1,"result":{}}
{"method":"initialized","params":{}}
```

Start a process:

```json
{"id":2,"method":"process/start","params":{"processId":"proc-1","argv":["bash","-lc","printf 'ready\\n'; while IFS= read -r line; do printf 'echo:%s\\n' \"$line\"; done"],"cwd":"file:///tmp","env":{"PATH":"/usr/bin:/bin"},"tty":true,"pipeStdin":false,"arg0":null}}
{"id":2,"result":{"processId":"proc-1"}}
{"method":"process/output","params":{"processId":"proc-1","seq":1,"stream":"stdout","chunk":"cmVhZHkK"}}
```

Write to the process:

```json
{"id":3,"method":"process/write","params":{"processId":"proc-1","chunk":"aGVsbG8K"}}
{"id":3,"result":{"status":"accepted"}}
{"method":"process/output","params":{"processId":"proc-1","seq":2,"stream":"stdout","chunk":"ZWNobzpoZWxsbwo="}}
```

Terminate it:

```json
{"id":4,"method":"process/terminate","params":{"processId":"proc-1"}}
{"id":4,"result":{"running":true}}
{"method":"process/exited","params":{"processId":"proc-1","seq":3,"exitCode":0,"sandboxDenied":false}}
{"method":"process/closed","params":{"processId":"proc-1","seq":4}}
```