Cartesia Infill API

openapi: 3.0.3
info:
  title: Cartesia Agents Infill API
  description: 'The Cartesia REST API for real-time voice AI - text-to-speech and voice-changer generation (single-shot bytes or Server-Sent Events), batch speech-to-text transcription, voice management and cloning, audio infill, custom datasets, pronunciation dictionaries, API keys and scoped access tokens, the Voice Agents platform (agents, calls, call batches, phone numbers, telephony providers, webhooks, deployments, knowledge base documents/folders, and evaluation metrics), and usage reporting. All requests require a `Cartesia-Version` header and an `Authorization: Bearer` API key (`sk_car_...`) or short-lived access token. The lowest-latency, streaming surface for TTS and STT is a separate WebSocket protocol documented in the companion AsyncAPI document at asyncapi/cartesia-ai-asyncapi.yml.'
  version: '2026-03-01'
  contact:
    name: Cartesia
    url: https://cartesia.ai
  license:
    name: API documentation - Cartesia Terms of Service
    url: https://cartesia.ai/terms-of-service
servers:
- url: https://api.cartesia.ai
  description: Cartesia production API
security:
- bearerAuth: []
tags:
- name: Infill
  description: Generate audio that smoothly connects two existing audio segments.
paths:
  /infill/bytes:
    post:
      operationId: infillBytes
      tags:
      - Infill
      summary: Infill (Bytes)
      description: Generates audio that smoothly connects two existing audio segments. At least one of left_audio or right_audio must be provided.
      parameters:
      - $ref: '#/components/parameters/CartesiaVersion'
      requestBody:
        required: true
        content:
          multipart/form-data:
            schema:
              type: object
              required:
              - transcript
              properties:
                left_audio:
                  type: string
                  format: binary
                  description: Audio preceding the infill text.
                right_audio:
                  type: string
                  format: binary
                  description: Audio following the infill text.
                transcript:
                  type: string
                  description: Text to generate between the two audio segments.
                model_id:
                  type: string
                  enum:
                  - sonic-3
                  - sonic-3-2026-01-12
                  - sonic-3-2025-10-27
                voice_id:
                  type: string
                language:
                  type: string
                output_format[container]:
                  type: string
                  enum:
                  - raw
                  - wav
                  - mp3
                output_format[sample_rate]:
                  type: integer
                  enum:
                  - 8000
                  - 16000
                  - 22050
                  - 24000
                  - 44100
                  - 48000
                output_format[encoding]:
                  type: string
                  enum:
                  - pcm_f32le
                  - pcm_s16le
                  - pcm_mulaw
                  - pcm_alaw
                output_format[bit_rate]:
                  type: integer
                  enum:
                  - 32000
                  - 64000
                  - 96000
                  - 128000
                  - 192000
      responses:
        '200':
          description: Infilled audio.
          content:
            audio/*:
              schema:
                type: string
                format: binary
        '401':
          $ref: '#/components/responses/Unauthorized'
        '422':
          $ref: '#/components/responses/ValidationError'
components:
  responses:
    ValidationError:
      description: The request payload failed validation.
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/Error'
    Unauthorized:
      description: Missing, invalid, or expired API key / access token.
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/Error'
  parameters:
    CartesiaVersion:
      name: Cartesia-Version
      in: header
      required: true
      description: API version date, e.g. 2026-03-01.
      schema:
        type: string
        example: '2026-03-01'
  schemas:
    Error:
      type: object
      properties:
        title:
          type: string
        message:
          type: string
        error_code:
          type: string
        status_code:
          type: integer
        doc_url:
          type: string
        request_id:
          type: string
  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer
      bearerFormat: sk_car_... API key or short-lived access token
      description: 'Cartesia API key (sk_car_...) or a short-lived access token minted via POST /access-token, passed as Authorization: Bearer <token>. Every request also requires the Cartesia-Version header.'