Text-to-Speech & Sound Effects API

Ultra-realistic voice synthesis (ElevenLabs models), 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 and stored. A failed generation is never charged. All three endpoints are synchronous — one paid POST returns the finished audio URL.

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, a final rate — the 5% margin is not added on top; only the $0.001 transaction fee is). 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 $0.361).
• The voice parameter is ignored — direct the voice in the prompt itself.
• Supports up to 3,000 input characters.

The 402 challenge

An unpaid POST returns 402 with the x402 requirement in the X-Payment-Required / PAYMENT-REQUIRED / WWW-Authenticate headers, mirrored at the top of the body as x402Version/accepts; the rest of the body is informational:

{
  "x402Version": 2,
  "accepts": [{ "scheme": "exact", "network": "eip155:8453", "amount": "53500", "asset": "0x8335…", "payTo": "0x…", "maxTimeoutSeconds": 300 }],
  "error": "Payment Required",
  "message": "This endpoint requires x402 payment",
  "price": { "amount": "0.053500", "currency": "USD" },
  "generation_info": {
    "characters": 1000,
    "model": "elevenlabs/flash-v2.5",
    "note": "Price scales with input character count. Synthesis is synchronous (typically <1s for Flash)."
  },
  "paymentInfo": { "network": "base", "asset": "USDC", "x402Version": 2 }
}

For bytedance/seed-audio-1.0 the generation_info block instead carries estimated_seconds and a note that billing is by output duration. price.amount already includes the $0.001 fee. Sign the requirement and re-send with the signature in X-Payment (also accepted: Payment-Signature).

Response

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

credits (ElevenLabs models) is the upstream-reported character cost when available; bytedance/seed-audio-1.0 returns duration_seconds (the actual billed length) instead. The response carries PAYMENT-RESPONSE (base64 JSON {success, transaction, network, payer}) and X-Payment-Receipt (the settlement tx hash) headers.

Sound Effects

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

Flat price: $0.0535 / generation ($0.05 base + 5% margin + the $0.001 transaction fee), regardless of duration. The 402 body is { error, message, price: { amount, currency }, paymentInfo }.

Response:

{
  "created": 1733443200,
  "model": "elevenlabs/sound-effects",
  "data": [{ "url": "https://blockrun.ai/api/media/audios/2026/06/05/....mp3", "format": "mp3" }]
}

Voices (free)

GET /api/v1/audio/voices

Returns the available voices with their voice_id, name, category, labels (accent, gender, age, use case, language), preview_url, and alias (if mapped). Pass the alias or voice_id as the voice field to /v1/audio/speech. No payment; rate-limited to 60 requests/minute per IP (429 with Retry-After) and cached for 5 minutes.

{
  "object": "list",
  "endpoint": "/v1/audio/speech",
  "note": "Pass a voice's `alias` (if present) or `voice_id` as the `voice` field to /v1/audio/speech.",
  "data": [
    { "voice_id": "EXAVITQu4vr4xnSDxMaL", "name": "Sarah - Mature, Reassuring, Confident", "category": "premade", "labels": { "accent": "american", "gender": "female", "age": "young", "language": "en" }, "preview_url": "…", "alias": "sarah" }
  ]
}

Error Codes

Every failure after a verified payment carries a PAYMENT-RESPONSE header with success: false so you can confirm nothing was settled.

Notes

• Audio is stored by BlockRun and returned as a permanent hosted URL. If storage fails there is nothing to return, so the request fails without settling — retry for free.
• 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.
• Each signed authorization is single-use (nonce claim before synthesis); reusing one returns 402 with code: "PAYMENT_REPLAY".
• Settlement happens only after successful synthesis and storage; upstream failures are not charged.

What's next?