## Why
Clients that create and negotiate realtime calls themselves need Codex to join
those calls without creating another call or overwriting the client-owned
session configuration.
## What changed
- Add the `existingCall` transport to `thread/realtime/start`, accepting a
client-provided `callId` and optional `realtimeSessionId`.
- Attach Codex over the call's sideband WebSocket without SDP negotiation or a
session update, and reconnect that sideband while preserving transcript
state.
- Default existing calls to realtime v1, support v1 and v3, and reject v2 or
options that would reconfigure the client-owned session.
- Encode call IDs as a single URL path segment for v3 sideband connections.
## Testing
Add protocol, app-server, core, reconnection, authentication-header, and URL
encoding coverage for existing-call attachments.
GitOrigin-RevId: 2046b70566f98efa55381bb1e461ea3cc68256c5
## What changed
- Reconnect frameless bidirectional WebRTC sideband sockets after unexpected transport loss with capped exponential backoff.
- Preserve pending text or handoff output and bounded transcript state across reconnects.
- Treat `404 Not Found` and `410 Gone` handshake responses as terminal session completion, and preserve HTTP status information from failed WebSocket handshakes.
- Bound Realtime delegation input and transcript fields while retaining the most relevant portion of each.
## Testing
- Add unit coverage for reconnect backoff, terminal status handling, transport error classification, transcript continuity and truncation, and pending outbound messages.
- Add an end-to-end WebRTC conversation test covering an unclean disconnect, reconnect, transcript reconciliation, outbound progress during sustained inbound traffic, and terminal session completion.
GitOrigin-RevId: 544fd7e542c3434d0bb42cfbcc7143604419a037
## What changed
- Route realtime V3 `bemTags` handoffs using `[ANALYSIS]`, `[COMMENTARY]`, and `[FINAL]` prefixes by default.
- Add `codexResponseHandoffChannelPrefixes` to realtime start requests so clients can replace the accepted prefixes for individual channels. Each channel may accept multiple prefixes, and omitted channels retain their defaults.
- Apply the configured prefixes to both complete and streamed handoff messages, without treating empty prefixes as matches.
## Testing
- Cover request deserialization, default and custom routing, partial streamed prefixes, empty-prefix handling, and end-to-end V3 handoffs.
GitOrigin-RevId: d0aef50aa71362a5beef2c6f6d28b8633da4e529
## What changed
- Add `codexResponseHandoffMode` to `thread/realtime/start` for V3 sessions, with `thinking` as the default and `commentary` and `bemTags` routing modes.
- Map BEM `analysis` and `commentary` output to the commentary channel and `final` output to the speakable channel, while preserving the original BEM envelope. Unrecognized output falls back to speakable.
- Mark explicit speech appends as speakable and stop adding the legacy `"Agent Final Message"` prefix to V3 handoffs. V1 and V2 continue to ignore the new setting.
## Testing
- Cover channel selection for every routing mode, streamed BEM header parsing, fallback behavior, explicit speech, and V1 compatibility.
GitOrigin-RevId: 1974578257488f2914b252c9c1990ba38831c96b
Zahan Malkani