Rate Limits

Nineninesix enforces two limits, both per project: a request rate (requests per minute) and a concurrency cap (simultaneous streams). They scale by the project's tier.

Tiers

A long-lived WebSocket connection spends a single request token at handshake; what bounds its work is the concurrency cap.

Handling 429 Responses

When you exceed a limit you'll receive 429 Too Many Requests with a Retry-After header:

{ "error": "rate_limited", "message": "requests/min limit exceeded for your tier" }

Concurrency rejections look like:

{ "error": "concurrent_limit", "message": "concurrent stream limit reached for your tier" }

import time
from cartesia import Cartesia

client = Cartesia(api_key="sk_996_your_api_key", base_url="https://api.nineninesix.ai")

def generate(transcript: str, attempt: int = 0):
    try:
        return client.tts.bytes(
            model_id="gepard-1.0",
            transcript=transcript,
            voice={"mode": "id", "id": "<voice-id>"},
            output_format={"container": "wav", "encoding": "pcm_s16le", "sample_rate": 22050},
        )
    except Exception:
        if attempt >= 3:
            raise
        time.sleep(2 ** attempt)
        return generate(transcript, attempt + 1)

Credits

Billing is separate from rate limits. Each character of transcript costs 1 credit; $5 buys 1,000,000 characters and credits never expire. When you run out you'll get a 402 payment_required — top up on the Billing page.

Best Practices

1. Cache generated audio — don't regenerate the same text repeatedly
2. Respect concurrency — pool and reuse streams rather than opening unbounded connections
3. Back off on 429 — use the Retry-After header
4. Monitor your dashboard — watch usage and balance before hitting limits