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Requests per minute (RPM) or requests per second (RPS) and concurrent-session limits measure different things. RPM/RPS controls how quickly an application can start API operations; concurrency caps how many voice sessions can be active at once. A voice API may enforce either measure or both, depending on the provider, endpoint, model, and account tier.
Requests per minute vs. concurrent sessions: what each limit measures
| Limit | What it counts | What it can constrain |
|---|---|---|
| RPM or RPS | Requests started during a time window: per minute or per second, respectively. | How quickly your application can initiate API operations. A burst of new requests can hit this limit even when few sessions are open. |
| Concurrent sessions | Voice sessions active at the same time. | How many calls or live interactions can remain open simultaneously. A long session occupies capacity for longer than a short one. |
These limits are not interchangeable. An application might have enough request-rate capacity to start calls but run out of session slots during a busy period. Conversely, it might have spare session capacity but be unable to start operations quickly enough because it has reached an RPM or RPS cap.
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For voice traffic, a useful planning approximation is that expected simultaneous sessions depend on the rate at which sessions arrive and their average duration. That estimate is for capacity planning, not a provider quota or guarantee: bursts, setup time, retries, and provider enforcement affect actual usage.
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There is no universal quota model for voice AI APIs. One provider can use different dimensions for different endpoints, and a single endpoint may have more than one limit. Other independent measures, including token-based limits, can also constrain API use; whichever applicable limit is reached first can be the bottleneck.
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xAI documents Voice & Audio endpoints with request-per-second and concurrent-session (CST) limits rather than token limits, while Speech to Speech is limited by concurrent sessions alone. OpenAI’s GPT-Live 1 page measures rate limits in concurrent sessions. OpenAI’s general API guidance describes multiple possible rate-limit metrics, including RPM and token-based limits. Check the documentation for the specific model and endpoint you intend to use rather than assuming that a provider-wide label applies to every voice product.
Published examples: OpenAI GPT-Live 1 and xAI Voice & Audio
The following figures are published in official documentation accessed October 4, 2026. They illustrate product-specific limits, not industry benchmarks. The applicable limits can depend on account details; xAI directs users to its Console for personalized team limits.
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OpenAI GPT-Live 1 concurrent sessions
OpenAI states that GPT-Live 1 rate limits are measured in concurrent sessions. Its model documentation lists these tier limits:
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| Tier 1 | 25 |
| Tier 2 | 50 |
| Tier 3 | 200 |
| Tier 4 | 300 |
| Tier 5 | 500 |
These values apply to GPT-Live 1 as listed on its model documentation page; they should not be generalized to other OpenAI voice models or endpoints.
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xAI Voice & Audio limits by endpoint and tier
xAI’s published table uses tiers 0–4. RPS is requests per second; CST is concurrent sessions. A blank request-rate value in the source is shown below as “not stated.”
| Endpoint | Quota | Tier 0 | Tier 1 | Tier 2 | Tier 3 | Tier 4 |
|---|---|---|---|---|---|---|
| grok-voice-think-fast-2.0 | RPS | Not stated | Not stated | Not stated | Not stated | Not stated |
| grok-voice-think-fast-2.0 | CST | 10 | 20 | 50 | 100 | 200 |
| Text to Speech | RPS | 50 | 50 | 100 | 250 | 500 |
| Text to Speech | CST | 100 | 200 | 200 | 300 | 500 |
| Speech to Text | RPS | 10 | 10 | 20 | 30 | 40 |
| Speech to Text | CST | 100 | 200 | 200 | 300 | 500 |
The figures are from xAI’s Rate Limits documentation. Its published Voice & Audio limits should be checked against the team’s current Console values before capacity decisions.
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How to estimate the capacity your application needs
- Identify the exact API operation. Record the provider, endpoint, model, and account tier. Limits for one model or voice operation do not establish the limits for another.
- Estimate request starts separately from open sessions. Calculate the expected peak rate of operations over the provider’s stated window, then estimate peak simultaneous sessions using arrival rate and typical session duration. Include expected bursts rather than relying only on an average.
- Check every applicable quota. Look for request-rate limits, concurrent-session limits, and any additional token or audio-related dimensions. For xAI, check the personalized team limit in the Console; for OpenAI, check the model’s documentation and the limits applicable to the account.
- Set admission control around session capacity. When session slots are full, queue or defer new session attempts instead of immediately launching repeated attempts. Retries do not increase the provider’s concurrency allowance.
- Exercise throttling and recovery behavior. Load-test the application’s own controls and recovery paths against its expected traffic. No provider’s published quota table establishes how a particular application will behave under load.
What to do when a request is throttled
A rate-limit response means the application should respect the provider’s throttling signal, not retry in a synchronized loop. xAI documents HTTP 429 for exceeding a listed rate limit and recommends exponential backoff. OpenAI recommends following Retry-After when present; if it is absent, increase the delay and add a small random jitter. These measures can reduce repeated collisions, but they do not create extra request or session capacity.
Distinguish a request-rate throttle from a full concurrent-session ceiling in your own handling. Backoff can be appropriate for a transient rate-limit response; when no more session slots are available, control or queue new session attempts so they wait for capacity rather than all retrying at once.
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Do not use old Realtime API launch limits as current quotas
OpenAI’s Realtime API announcement was updated February 3, 2025 with the statement, “We no longer limit the number of simultaneous sessions on the Realtime API.” That is a dated announcement, not a quota statement for every OpenAI voice product today. The current GPT-Live 1 model page, for example, publishes tier-specific concurrent-session limits. Use the documentation for the current model and endpoint rather than treating an earlier launch announcement as a universal or current limit.
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