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400s to OpenAI service due to top_p parameter being set to 0

Justin Tran 20 Reputation points
2026-04-23T20:18:58.5066667+00:00

We received a number of 400s (a few last week, and then 100% of our requests) due to the error "top_p parameter must be greater than 0 and less than or equal to 1."

We had been setting top_p to 0 previously and did not see issues. Was some new parameter enforcement rolled out this week? We've resolved the issue on our side by not setting top_p to 0, but wanted to confirm that something new had been updated on the OpenAI side.

Azure OpenAI in Foundry Models

Answer accepted by question author

Karnam Venkata Rajeswari 2,890 Reputation points Microsoft External Staff Moderator
2026-05-04T18:14:34.22+00:00

Hello @Justin Tran ,

Thank you for your patience while we were working on the update.

The top_p parameter controls nucleus sampling by defining the cumulative probability mass from which tokens are selected. From a functional and mathematical standpoint, this requires a value strictly greater than 0 and less than or equal to 1 (0 < top_p ≤ 1). A value of 0 represents an empty probability set, which is not meaningful for token selection.

Earlier service behavior did not consistently enforce this constraint at runtime, which allowed requests with top_p = 0 to succeed without error. Recent updates have introduced uniform runtime validation, and such requests are now rejected with an HTTP 400 response indicating an invalid parameter value. This reflects enforcement alignment rather than a change in model behavior.

The concern regarding documentation accuracy is valid. Existing references typically describe top_p as a value “between 0 and 1,” without explicitly clarifying whether 0 is inclusive or exclusive. In addition, some earlier or schema-style specifications listed a minimum value of 0, which can reasonably be interpreted as inclusive.

For reference, the closest official documentation currently available includes:

At present, there is no single authoritative reference that explicitly states the constraint as (0,1].

For tracking API and validation updates

While parameter-level enforcement changes may not always be announced individually, the following official sources provide the most reliable visibility into platform updates:

  1. OpenAI API changelog - Changelog | OpenAI API
  2. Azure OpenAI API version lifecycle - Azure OpenAI in Microsoft Foundry Models v1 API - Microsoft Foundry | Microsoft Learn
  3. Azure service updates (AI & Machine Learning) - Azure updates | Microsoft Azure
  4. SDK and client library release notes (Azure SDK repositories on GitHub)

Monitoring these sources together helps identify upcoming validation changes, deprecations, and behavioral alignments before they impact production workloads.

Thank you

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  1. Karnam Venkata Rajeswari 2,890 Reputation points Microsoft External Staff Moderator
    2026-04-25T19:33:51.2866667+00:00

    Hello @Justin Tran ,

    Welcome to Microsoft Q&A .Thank you for reaching out to us.

     The top_p parameter is currently defined to accept values strictly greater than 0 and less than or equal to 1 (0,1].

    • Requests with top_p = 0 fall outside this range and are now rejected with HTTP 400 due to stricter validation rules.
    • Earlier, such values may have been processed without consistent validation, which is why no errors were observed previously.
    • The current behavior reflects consistent enforcement of documented constraints, rather than a confirmed change to the parameter definition itself.

    The corrective action already taken—removing top_p or setting a valid value within (0,1]—is aligned with the supported API behavior and resolves the issue effectively.

    Regarding tracking API updates and parameter changes , Azure OpenAI and related APIs evolve continuously, and it is important to note that parameter-level validation updates may not always be proactively announced.

    To stay informed and reduce the impact of similar changes, the following sources can be monitored -

    1. API changelog for platform-level updates - Changelog | OpenAI API
    2. Azure OpenAI API lifecycle and updates - Azure OpenAI in Microsoft Foundry Models v1 API - Microsoft Foundry | Microsoft Learn
    3. Azure service updates page - Azure updates | Microsoft Azure
    4. DK and client library release notes (GitHub) - Useful for identifying validation or schema-related changes introduced in client integrations

    These sources together provide visibility into major updates, although continuous review is recommended due to the evolving nature of the service.

    Thank you

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  2. AI answer

    2026-04-23T20:19:06.2633333+00:00

    The error is caused by stricter validation of the top_p parameter. The current contract requires top_p to be strictly greater than 0 and less than or equal to 1. A value of 0 is now rejected with HTTP 400.

    To resolve the 400 errors:

    1. Stop sending top_p = 0.
    2. Either:
      • Remove top_p from the request body entirely (so the service uses its default behavior), or
      • Set top_p to a valid value in the open interval (0, 1], for example top_p = 0.1 or top_p = 1.
    3. As a general guideline, adjust either temperature or top_p, but not both at the same time.

    If top_p was previously accepted as 0, that behavior was permissive and is no longer supported under the documented constraints, which is why all such requests now fail with 400.

    References:

    AI-generated content may be incorrect. Read our transparency notes for more information.

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