A 429 response means a provider has refused a request under a limit or temporary service condition—but it does not tell you which limit was reached. Check the response body and headers first: the cause may be request or token volume, too many simultaneous operations, endpoint throughput, temporary service load, or an exhausted credit or spending allowance. Then reduce traffic or correct the account condition before deciding whether a retry is appropriate.
What a 429 means for a voice AI integration
HTTP 429 is a signal to stop and diagnose, not a universal diagnosis. The same status can represent different constraints depending on the provider, product, endpoint and account. A request-rate limit, token limit, concurrency cap and account spending limit are separate: hitting one does not mean the others are exhausted.
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Voice integrations can trigger limits in several places: speech generation or transcription requests, realtime sessions, SDK operations, outbound call starts, or repeated status polling. Providers may enforce limits over short windows, so a burst can fail even when the average over a full minute appears acceptable. OpenAI also distinguishes temporary rate limiting from an overloaded-model response: its documentation identifies 429 errors such as rate_limit_error and slow_down, while 503 with server_is_overloaded is a separate capacity condition (OpenAI rate limits).
Diagnose the limit before changing retry behavior
- Capture the error details. Record the HTTP status, exact response message and error code, request ID, timestamp with timezone, endpoint or model, and the organization, project or account used. Do not put API keys in logs or support tickets.
- Read server hints. Check for
Retry-Afterand provider-specific headers. OpenAI documents headers for limits, remaining capacity and reset windows; Twilio documentsTwilio-Concurrent-Requests. Header names and meanings differ by service, so do not assume one provider’s format applies to another. - Identify the unit and scope. Determine whether the error concerns requests, tokens, audio usage, simultaneous jobs, call-start throughput, or credits/spend. Check whether the limit belongs to an organization, project, model family, endpoint, subscription or phone number, and consult the provider dashboard for the account’s current values.
- Look for bursty or redundant work. Review concurrent requests, rapid call starts, repeated SDK operations, polling frequency and duplicate submissions. A minute-level average can hide a short spike.
- Reduce avoidable load. Smooth traffic with a queue, cap concurrency, remove redundant calls, and poll less often. If token usage is the constraint, trim unnecessary prompt content or output-token allowances. Twilio recommends webhooks instead of repeatedly polling resources that change.
- Retry only a transient, safe-to-repeat operation. Use the policy below for temporary throttling or overload. Do not loop on errors that identify a credit balance, usage or spending limit.
- Escalate with evidence if it continues. After reducing traffic, collect request IDs and the error/limit details, check current account limits and the provider’s status information, then contact support or ask about higher throughput if the product supports it.
Choose the fix based on the error class
Temporary throttling or service load
Honor a valid Retry-After value as the minimum delay. If the response supplies no valid delay, use exponential backoff with random jitter, and set both a maximum attempt count and an overall deadline. Coordinate application retries with SDK-internal retries: nested loops can multiply attempts, and unsuccessful requests may still contribute to rate limits. If an SDK cannot wait as long as the server requests, defer the work or return the error rather than retrying early.
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Retries are a safety net, not a traffic plan. A queue, request/token budget and concurrency limit help prevent a burst from becoming a thundering herd of clients retrying together.
Credits, spend caps or usage allowances
OpenAI documents 429 error codes including credit_balance_exhausted, organization_usage_limit_exceeded, organization_spend_limit_exceeded and project_spend_limit_exceeded. These call for the matching account action—such as checking the balance, changing the relevant limit or confirming the intended organization and project—not repeated requests. Consult the current OpenAI error-code guide and account controls for the specific message.
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Duplicate or non-idempotent voice actions
Before retrying a call start or another action with an external effect, establish that replaying it will not create a duplicate. Use the provider’s supported idempotency mechanism where available, or track the operation so a timed-out response does not cause a second call to start. Backoff alone cannot prevent duplicate actions.
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| Provider | Relevant 429 causes or clues | Practical response |
|---|---|---|
| OpenAI API | Request, token, image or audio-related limits; temporary rate limiting; or exhausted balance/usage/spend allowances. Rate limits may be scoped to organization or project and vary by model or shared model family. A rapid increase in request rate may produce slow_down even when displayed RPM and TPM limits appear to have room. |
Inspect the error code and rate-limit headers, confirm the organization/project and model, and distinguish transient throttling from account limits. Follow the rate-limit guide for current limit semantics. |
| ElevenLabs | too_many_concurrent_requests indicates the subscription concurrency limit; system_busy indicates high service traffic. |
For concurrency errors, queue or pace work and check the current plan documentation. For service busy errors, use bounded retries with backoff and jitter. |
| Twilio REST API | Account concurrency limits and product-specific causes can produce error 20429. Twilio-Concurrent-Requests reports current account concurrent requests; subaccount requests do not roll up to the primary account, and requests receiving 429 are included in the count. |
Use the concurrency header, slow spikes and queue requests. Avoid duplicate starts and excessive verification-status polling; see Twilio error 20429. |
| Twilio Programmable Voice | Error 31206 means the client request rate exceeded an authorized limit. Causes can include rapid Voice SDK operations, bursts of outbound call starts, mixed SDK and REST activity, or rapid Call Message Events. | Pace or queue operations, use exponential backoff where appropriate, inspect logs/events, and ask Twilio about higher sustained throughput if available. The applicable throughput is account- and product-specific; there is no universal call-per-second value in the cited guidance. |
ElevenLabs concurrency limits are plan-specific
ElevenLabs’ current API documentation lists these concurrent-request values by subscription plan. The provider says it may revise the values; ElevenAgents has different limits, so check the live documentation for the product and account you use.
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| ElevenLabs subscription plan | Listed concurrent requests |
|---|---|
| Free | 2 |
| Starter | 3 |
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| Scale | 15 |
| Business | 15 |
These are mutable plan-specific limits listed in ElevenLabs’ concurrency documentation, accessed October 4, 2026; they are not a general capacity guarantee for every endpoint or account.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Build retries that reduce pressure rather than amplify it
- Classify first: retry only transient throttling or overload responses, not billing, spending, authorization or exhausted-quota errors.
- Respect the server’s wait: treat a valid
Retry-Afterdelay as a minimum. Never retry sooner because a client setting caps its wait; defer the job or surface the failure. - Add jitter: when there is no valid server delay, exponential backoff plus a random component reduces synchronized retries.
- Bound the policy: specify a maximum number of attempts and a total deadline so a voice job does not remain stuck indefinitely.
- Count all attempts: include SDK retries when setting application retry limits. Failed requests can add load and count against per-minute limits.
- Protect side effects: make sure an operation is safe to replay or use an idempotency strategy before retrying call starts or other actions.
- Control normal traffic: queue work and budget concurrency and request volume rather than depending on retries to manage routine peaks.
OpenAI’s current guidance gives an illustrative ramping rule for a specific high-volume situation: once traffic reaches 1 million input tokens per minute, increase by no more than 50% every 15 minutes. The documentation calls this a rule of thumb, not a universal threshold; the applicable point varies by workload (OpenAI rate-limit guide).
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When to change account settings or contact support
Change an account setting only when the error identifies an account-level allowance and the relevant dashboard confirms the affected organization, project or subscription. For throughput limits, first show that you have smoothed bursts, controlled concurrency and reduced unnecessary polling. If the same properly paced workload still exceeds the documented capacity, provide support with request IDs, timestamps, endpoint/model, error code and relevant limit headers. Do not send credentials.
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