Rate Limits

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Tiers

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What counts as a request

• POST /tts/bytes and POST /tts/sse — one request per HTTP call.
• GET /tts/websocket — one request per connection, spent at handshake, not per message. A long-lived socket streaming many turns costs a single request token when it opens; what bounds its ongoing work is the concurrency cap.

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How concurrency is measured

• A slot is held for roughly 0.5–4 seconds per spoken turn — just the generation window — then freed the instant the turn finishes (~1 s after its last audio).
• Idle time between turns (the user talking, silence, thinking) costs nothing.

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Handling 429 Responses

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

{ "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)

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Credits

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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, and retry rejected turns on a fresh context_id
4. Monitor your dashboard — watch usage and balance before hitting limits