Voice agents can now switch language mid-call

The previous answer to a multilingual caller was a sibling agent per language plus a transfer - and a transfer drops context by design, since only a summary crosses the handoff. additional_languages replaces that: one agent, one session, and a switch_language tool it reaches for automatically when the caller speaks another configured language or asks for one.

Each additional language can keep the agent’s own voice for continuity through the switch, or use a different, native-speaking voice. You can also start a session or conversation directly in a configured language via the language field on session and conversation creation.

Multilingual agents need a capable model to detect language shifts and switch cleanly without narrating it to the caller, so the feature is gated to a curated set of premium OpenAI models - which puts it on Pro plans and above.

Full field reference and the model requirement are in the docs changelog entry.