May 13, 2026 | Synthflow

Synthflow TTS voice model

Synthflow TTS is now available as a voice model option in the agent editor. It delivers low-latency, natural-sounding speech across 32 languages with 12 built-in voices.

The GET /voices endpoint now supports search and provider query parameters for filtering, and voice responses include provider, gender, and languages fields.

Outbound number pooling for agents

You can now assign multiple outbound phone numbers to one agent and choose the caller ID for each outbound call. See the Outbound Number Pooling guide.

Endpoint	Change
GET /assistants/{model_id}	Returns `attached_phone_numbers` with each attached number’s `number`, `sid`, and `slug`.
PUT /assistants/{model_id}/phone_numbers	Attaches multiple outbound numbers using `phone_numbers` or `phone_number_slugs`.
DELETE /assistants/…/{phone_number_slug}	Detaches one outbound number from the agent.
POST /calls	Accepts `from_phone_number` to select the outbound caller ID for that call.

Knowledge Base API: New Endpoint

You can now update existing knowledge base sources by source ID. The endpoint supports name, url for link and file sources, and content for text sources.

Endpoint	Change
PUT /sources/{source_id}	Updates the source `name` and type-specific fields.