Audio - Opper

The gateway covers both directions of audio with two synchronous endpoints:

Text to speech — POST /v3/audio/speech turns text into spoken audio.
Speech to text — POST /v3/audio/transcriptions turns audio into a transcript.

Both are provider-agnostic: model selects the provider, a small set of fields is normalized for you, and anything model-specific goes in parameters. For live, two-way voice conversations, use Realtime instead.

Text to speech

Pass a model and the input text. The response carries the audio inline as base64 and, by default, a stored file_id + presigned url.

curl -sX POST https://api.opper.ai/v3/audio/speech \
  -H "Authorization: Bearer $OPPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/tts-1",
    "input": "Hello from Opper.",
    "voice": "alloy",
    "format": "mp3"
  }'

import os, base64, requests

r = requests.post(
    "https://api.opper.ai/v3/audio/speech",
    headers={"Authorization": f"Bearer {os.environ['OPPER_API_KEY']}"},
    json={
        "model": "openai/tts-1",
        "input": "Hello from Opper.",
        "voice": "alloy",
        "format": "mp3",
    },
)
r.raise_for_status()
audio = r.json()["audio"]
open("speech.mp3", "wb").write(base64.b64decode(audio["b64_json"]))
print("saved speech.mp3", "·", audio.get("file_id"))

Response

{
  "id": "spch_...",
  "model": "openai/tts-1",
  "created": 1716124800,
  "audio": {
    "b64_json": "SUQzBAAAAAA...",
    "mime_type": "audio/mpeg",
    "file_id": "file_...",
    "url": "https://...presigned..."
  },
  "usage": { "cost": 0.0009, "characters": 17 }
}

Field	What it does
`model`	Required. The TTS model, e.g. `openai/tts-1`.
`input`	Required. The text to speak.
`voice`	Provider voice; empty uses the model’s default. Validated against the model’s declared voices.
`format`	`mp3` (default), `wav`, `opus`, `aac`, `flac`, or `pcm`.
`speed`	`0.25`–`4.0` where the model supports it.
`store`	Persist the audio and return a `file_id` + `url`. Defaults to true; the stored file is permanent until deleted. Skipped on zero-data-retention projects.
`parameters`	Opaque per-provider passthrough.

Speech is billed per input character.

Speech to text

Pass a model and an audio source: a file_id from a previous generation or upload, an https URL, or a data-URI (max 25 MB decoded).

A file_id lets you transcribe audio you already have on Opper — a clip you uploaded, or speech you just generated with /v3/audio/speech — without re-sending the bytes. See Files for how files work, lifecycle, and storage quotas.

Transcribe a file_id

curl -sX POST https://api.opper.ai/v3/audio/transcriptions \
  -H "Authorization: Bearer $OPPER_API_KEY" -H "Content-Type: application/json" \
  -d '{ "model": "openai/whisper-1", "audio": "file_abc123" }'

curl -sX POST https://api.opper.ai/v3/audio/transcriptions \
  -H "Authorization: Bearer $OPPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/whisper-1",
    "audio": "https://example.com/meeting.mp3",
    "language": "en"
  }'

import os, requests

r = requests.post(
    "https://api.opper.ai/v3/audio/transcriptions",
    headers={"Authorization": f"Bearer {os.environ['OPPER_API_KEY']}"},
    json={
        "model": "openai/whisper-1",
        "audio": "https://example.com/meeting.mp3",
        "language": "en",
    },
)
r.raise_for_status()
print(r.json()["text"])

Response

{
  "id": "trsc_...",
  "model": "openai/whisper-1",
  "created": 1716124800,
  "text": "Full transcript here...",
  "language": "en",
  "duration": 42.5,
  "segments": [
    { "start": 0.0, "end": 3.2, "text": "Full transcript here..." }
  ],
  "usage": { "cost": 0.004, "seconds": 42.5 }
}

Field	What it does
`model`	Required. The transcription model, e.g. `openai/whisper-1`.
`audio`	Required. A `file_id`, https URL, or data-URI to transcribe.
`language`	ISO-639-1 hint (e.g. `en`); the provider validates it.
`prompt`	Context or vocabulary hint to bias the transcript.
`diarize`	Request speaker labels on segments. Rejected on models that don’t support diarization.
`parameters`	Opaque per-provider passthrough.

Transcription is billed per provider-reported audio duration. Segment and word timestamps are returned when the model provides them.

Discover models

GET /v3/audio/models lists the audio models available, tagged tts or stt, with their voices and formats. Each model’s params.tts / params.stt block also carries a parameters list of the native passthrough keys it accepts (e.g. ElevenLabs’ exact output_format preset, or a language_code), so you can discover the model-specific knobs for parameters rather than guessing:

# all audio models, or filter by type
curl -s "https://api.opper.ai/v3/audio/models?type=tts" \
  -H "Authorization: Bearer $OPPER_API_KEY"

What’s next

Realtime voice

Live, two-way voice over WebSocket.

Models

Which models do speech and transcription.

Control Plane

Govern providers, regions, and spend on every call.

Video

Generate video from a prompt or image.

​Text to speech

​Speech to text

​Discover models

​What’s next

Realtime voice

Models

Control Plane

Video

Text to speech

Speech to text

Discover models

What’s next