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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.
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 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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WebSocket limits

You don’t need to worry about WebSocket limits if you’re only using the HTTP API.
We limit the number of parallel WebSocket connections to 10X your concurrency limit. For example, if you have a concurrency limit of 15, you can have up to 150 parallel 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 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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WebSocket timeouts

We close idle WebSocket connections after 5 minutes. We recommend closing and re-opening a new websocket connection when connections stay idle longer than 5 minutes.

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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.
  • One stream equals one HTTP or WebSocket connection
  • Each concurrent transcription counts toward your STT concurrency limit
If you exceed your STT concurrency limit, you will receive a 429 Too Many Requests error.
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