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Credit-based quota

Usage is metered in credits, allocated per billing cycle and tracked separately for TTS and ASR. Your allocation depends on your plan: When you exhaust a cycle’s credits, requests return 402 until the cycle resets or you upgrade.

Checking usage

Query your current usage with:
Check this proactively (e.g. before large batches) so you can upgrade or throttle before hitting 402.

Rate limiting

Beyond quota, the API rate-limits bursts of requests. If you send too many too quickly you’ll get 429:
Handle it with exponential backoff (see Errors). For high throughput, queue requests and cap concurrency rather than firing them all at once.
Exact per-second/per-minute rate-limit thresholds depend on the deployment and plan. Design for backoff on 429 rather than a fixed request rate.