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Vapi lets you bring your own speech providers: a custom transcriber (a WebSocket that receives the call audio and returns transcripts) and a custom voice (an HTTP endpoint that returns raw audio for a sentence). Lokutor is not a built-in Vapi provider. The open-source vapi-lokutor-adapter implements both of Vapi’s protocols and forwards to the Lokutor API, so you can use Lokutor in a Vapi assistant today by running that small service yourself.
The adapter was tested against Vapi’s published protocol documentation with a local mock, and against the live Lokutor API. It has not yet been run on a Vapi account. Treat the first call as a test, and tell us if Vapi behaves differently from its documentation.

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How it fits together

Recognition covers English, Spanish, Catalan, Galician, Basque, Portuguese, French, Italian and German (languages). Voices are the slots F1-F5 and M1-M5. Usage is billed to your Lokutor account at the rate card prices: synthesis per character and recognition per second of audio sent. Every transcriber connection also counts as one of your account’s concurrent sessions.

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1. Run the adapter

You need a Lokutor API key and a public HTTPS address for the adapter (Vapi cannot reach localhost; use your own reverse proxy or a tunnel for testing).
Always set ADAPTER_SECRET: without it, anyone who finds the URL can spend your Lokutor balance. The adapter accepts it as an x-vapi-secret header or as Authorization: Bearer <secret>. The other settings (default voice and language, upstream URL, timeout) are in the repository README.

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2. Create the credential in Vapi

In the Vapi dashboard, add a Custom Credential of type Bearer Token whose token is your ADAPTER_SECRET, and note its id.

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3. Configure the assistant

Vapi does not send a voice id to a custom voice, so put the voice and language in the server URL’s query string. For the transcriber, language fixes the recognition language; leave it out and Lokutor detects it.
Replace adapter.example.com with your adapter’s address. Use any language model Vapi supports; the model section is Vapi’s, not Lokutor’s.

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Behaviour to know about

  • The voice is not streamed. The adapter waits for the whole sentence from POST /tts/synthesize and then answers, so time to first audio is the synthesis time of the full sentence plus the network hop to the adapter, not the streaming first-audio latency of WS /ws/tts.
  • Only the customer channel is transcribed. Vapi’s second (assistant) channel is ignored.
  • End of speech is Lokutor’s. An utterance ends after about 0.7 s without speech (Speech-to-Text); Vapi’s own endpointing settings do not apply to a custom transcriber.
  • Errors. If Lokutor refuses a synthesis (unknown voice, balance, capacity) the adapter answers 502, or 503 with Retry-After when synthesis is at capacity, so a Vapi fallbackPlan voice can take over. See Error Handling.
  • Expression tags such as [laugh] in the text Vapi sends are passed to Lokutor unchanged.
If you want a full voice agent rather than only speech providers, the hosted agent API runs recognition, the language model, tools and synthesis in one connection.