codex/codex-rs/exec-server/README.md

# 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 shared `codex-app-server-protocol` message envelope on
the wire.

The CLI entrypoint supports:

- `ws://IP:PORT` (default)
- `--remote URL --executor-id ID [--name NAME]`

Remote mode registers the local exec-server with the executor registry,
then reconnects to the service-provided rendezvous websocket as the executor.
It requires a bearer token in `CODEX_EXEC_SERVER_REMOTE_BEARER_TOKEN`.

Wire framing:

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

## Remote Relay Message Format

In remote mode, the harness and executor 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
body              // 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           // data only: JSON-RPC message bytes or segment bytes
next_seq          // resume only: next sender seq
reason            // reset only: reset reason
```

`stream_id` identifies one virtual harness/executor JSON-RPC session on the
executor websocket. The harness generates a UUIDv4 `stream_id`; the executor
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.

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": "/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`: absolute working directory used for the child process.
- `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
}
```

### `process/closed`

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

Params:

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

## Filesystem RPCs

Filesystem methods use absolute paths and return JSON-RPC errors for invalid
or unavailable paths:

- `fs/readFile`
- `fs/writeFile`
- `fs/createDirectory`
- `fs/getMetadata`
- `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
- `RemoteExecutorConfig` and `run_remote_executor()` for embedding remote
  registration mode

Callers must pass `ExecServerRuntimePaths` to `run_main()`. The top-level
`codex exec-server` command builds these paths from the `codex` arg0 dispatch
state.

## 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":"/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}}
{"method":"process/closed","params":{"processId":"proc-1"}}
```