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Your account is subject to two types of rate limits: WebSocket limits and generation concurrency limits.

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Concurrency limits by subscription plan

Your subscription plan determines how many requests can be processed simultaneously. Sonic Text-to-Speech (TTS) and Ink Speech-to-Text (STT) each have separate concurrency limits with the same values per plan.
PlanTTS Concurrent RequestsSTT Concurrent Requests
Free28
Pro312
Startup520
Scale1560
EnterpriseCustomCustom
Sonic (Text-to-Speech) and Ink (Speech-to-Text) services have separate concurrent request limits. For example, if you’re on the Scale plan, you can have up to 15 concurrent TTS requests AND 60 concurrent STT requests running simultaneously.

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Text-to-Speech (TTS) Concurrency

We measure TTS generation concurrency in terms of the number of unique contexts active at a given time.
  • For HTTP endpoints, each request is treated as a separate context and counts toward your concurrency limit.
  • For WebSockets, a unique context_id defines a context—sending additional requests with the same context_id does not increase your concurrency usage. This is because requests to the same context are processed sequentially.
  • STT does not count towards your TTS concurrency limit
If you exceed your TTS concurrency limit, you will receive a 429 Too Many Requests error. You can check your concurrency limit and upgrade it on the playground at play.cartesia.ai.

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Interpreting TTS concurrency limits

How you interpret your TTS concurrency limit depends on how you’re using the Sonic model family.
For real-time conversational use cases, such as powering voice agents, we’ve found that the number of parallel conversations you can support is effectively 4X your concurrency limit. This is just a rule of thumb, and depends on the types of conversations you’re supporting. You can reach out to us to discuss your specific use case.For example, if you have a TTS concurrency limit of 15, you can typically support 60 parallel conversations.
For non-conversational use cases, such as generating speech in batch jobs, there is a more direct relationship between your concurrency limit and the number of parallel generations you can support.For example, if you have a TTS concurrency limit of 15, you can typically support 15 parallel TTS generations. You can use a connection pool to ensure you don’t exceed your concurrency limit.

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TTS WebSocket limits

We limit the number of parallel TTS WebSocket connections to 10X your concurrency limit. For example, if you have a concurrency limit of 15, you can have up to 150 parallel TTS WebSocket connections. If you exceed your WebSocket limit, you will receive a 429 Too Many Requests error on trying to open a new WebSocket connection. Usually, when users run into TTS WebSocket limits (even at scale), it’s because they’re not properly closing idle connections. Beyond closing idle connections, you can also create a connection pool to ensure you don’t exceed your WebSocket limit.

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TTS WebSocket timeouts

We close idle TTS WebSocket connections after 5 minutes. We recommend closing and re-opening a new websocket connection when connections stay idle for long periods of time.

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Speech-to-Text (STT) Concurrency

Each active transcription stream counts as one concurrent request, regardless of whether you’re using HTTP or WebSocket connections.
  • Each concurrent HTTP or WebSocket connection counts toward your STT concurrency limit
  • Idle STT WebSockets still count towards your STT concurrency limit
  • TTS does not count towards your STT concurrency limit
If you exceed your STT concurrency limit, you will receive a 429 Too Many Requests error.

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STT WebSocket timeouts

We close idle STT WebSocket connections after 3 minutes. We recommend closing and re-opening a new websocket connection when connections stay idle for long periods of time.