## What changed
- Add a `--listen` option that accepts `stdio`, `stdio://`, or a
`ws://IP:PORT` endpoint, while retaining stdio as the default.
- Serve the existing length-prefixed protocol in binary WebSocket messages,
with isolated connections, shared host limits, and a `/readyz` endpoint.
- Reject browser-origin handshakes and contain malformed frames to the affected
connection.
## Testing
- Cover listen URL parsing and complete-frame encoding and decoding.
- Exercise readiness, cell execution, tool callbacks, large frames, concurrent
connections, malformed frames, and origin rejection through the WebSocket
listener.
GitOrigin-RevId: 01c8be4c6256b8ce4a3a0002440dcb3294e5f887
## Summary
- add an `EncodedFrame` type so IPC payloads are serialized and
size-checked before entering bounded queues
- add the V1 `operation/cancel` client-to-host message
- pin the new wire shape with protocol tests
## Why
The process-owned code-mode host needs bounded, pre-encoded outbound
messages and a best-effort cancellation signal. Keeping these wire
primitives in a protocol-only change lets their compatibility contract
be reviewed independently from either endpoint.
## Stack
This is **1 of 4** in the process-owned code-mode session stack. The
next PR targets this branch.
## Validation
- `just test -p codex-code-mode-protocol` — 22 passed
- `just fix -p codex-code-mode-protocol`
- `just fmt`
## Why
The process-owned code mode implementation needs an explicit, bounded
wire contract before either side depends on it. Keeping framing and
message semantics in `codex-code-mode-protocol` gives the client and
sidecar one shared source of truth and makes compatibility failures
detectable during connection setup.
## What changed
- adds a versioned client/host handshake with required and optional
capabilities
- defines operation requests and responses for session lifecycle and
cell control
- defines reverse delegate request, response, cancellation, and
cell-closure messages
- adds a four-byte little-endian length-prefixed JSON codec with a hard
frame cap
- rejects malformed frames, unknown fields, invalid identifiers, and
unsupported protocol states
- locks the wire representation down with explicit JSON round-trip tests
## Testing
- `just test -p codex-code-mode-protocol`
## Stack
Part 1 of 6. Followed by
[#29805](https://github.com/openai/codex/pull/29805).
