Text-to-Speech & Sound Effects API

Ultra-realistic voice synthesis (ElevenLabs), prompt-directed audio creation (ByteDance Seed Audio), and cinematic sound effects, behind x402. Pay per call in USDC — no subscriptions, no API keys.

ElevenLabs models are billed per input character; ByteDance Seed Audio is billed per second of output audio (quoted from an estimate of your input). Either way the price is quoted up front in the 402 challenge and settlement only happens after the audio is generated. A failed generation is never charged.

Endpoints

POST https://blockrun.ai/api/v1/audio/speech          # text-to-speech
POST https://blockrun.ai/api/v1/audio/sound-effects   # sound effect generation
GET  https://blockrun.ai/api/v1/audio/voices           # list voices (free)

Text-to-Speech

Request

{
  "model": "elevenlabs/flash-v2.5",
  "input": "Hello from BlockRun. Pay per call, no subscription.",
  "voice": "sarah",
  "response_format": "mp3"
}

Parameters

Models

Seed Audio 1.0 (ByteDance)

bytedance/seed-audio-1.0 is not plain TTS — it is prompt-directed audio creation: describe the voice, emotion, and sound staging in natural language inside input, and the model performs it. Example:

{
  "model": "bytedance/seed-audio-1.0",
  "input": "A middle-aged sports commentator, hoarse with excitement, shouts over a roaring stadium crowd: GOAL! Absolutely unbelievable!",
  "response_format": "mp3"
}

Differences from the ElevenLabs models:

• Billing is per second of output audio ($0.003/second). Since the exact duration isn't known before synthesis, the 402 quote prices an estimated duration derived from your input length (CJK text estimates slower speech than Latin text). The 402 body reports the estimate in generation_info.estimated_seconds, and the response reports the actual duration_seconds.
• Output is capped at 120 seconds (so the maximum possible quote is $0.36).
• The voice parameter is ignored — direct the voice in the prompt itself.
• Supports up to 3,000 input characters.

Response

{
  "created": 1733443200,
  "model": "elevenlabs/flash-v2.5",
  "data": [
    {
      "url": "https://blockrun.ai/api/media/audios/2026/06/05/....mp3",
      "format": "mp3",
      "characters": 51
    }
  ]
}

Sound Effects

POST /api/v1/audio/sound-effects
{
  "text": "thunderclap with heavy rain",
  "duration_seconds": 5
}

Flat price: $0.052 / generation (+5% fee).

Voices (free)

GET /api/v1/audio/voices

Returns the available voices with their voice_id, name, and alias (if mapped). Pass the alias or voice_id as the voice field to /v1/audio/speech.

Notes

• Audio is stored by BlockRun and returned as a permanent hosted URL.
• The price is recomputed from the request body on the paid call, so a payment signed for short text cannot be reused to synthesize longer text.
• Settlement happens only after successful synthesis; upstream failures are not charged.

What's next?