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VectleSkillsPython: Function Calling Error when Using APIM Managed Azure OpenAI

Python: Function Calling Error when Using APIM Managed Azure OpenAI

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Fixes 400 'unable to parse' errors on function calling through APIM-managed Azure OpenAI by setting estimate-prompt-tokens to false in the azure-openai-token-limit policy. Use when Python Semantic Kernel function calls fail behind Azure APIM. Not for direct-to-Azure OpenAI calls or non-APIM gateways.

Fix first: in your APIM instance, open the azure-openai-token-limit policy and set estimate-prompt-tokens to false. Two reporters confirmed function calling started working right after the change.

400 unable to parse
  1. Open the azure-openai-token-limit policy in Azure APIM.
  2. Set estimate-prompt-tokens to false. When it is true, APIM estimates tokens from the prompt schema and throws the 400; when false, tokens are calculated from actual model usage.
  3. Retry the function call. Expected: the call succeeds and returns tool calls with no 400.

When to use: function calling fails with a 400 through Azure APIM, in Python Semantic Kernel or similar setups.

When not to use: calls that go straight to Azure OpenAI without APIM, or 400s that come from somewhere other than the token-limit policy.

Compatibility: Semantic Kernel (Python), Azure API Management, Azure OpenAI.

Variants:

  • "apim function calling 400 unable to parse"
  • "azure-openai-token-limit policy function calling"
  • "estimate-prompt-tokens false"

Root cause: with estimate-prompt-tokens set to true, APIM tries to estimate tokens from the function-calling prompt schema and fails to parse it, so the request is rejected before it ever reaches the model.

Edge cases: if other APIs on the same APIM instance depend on estimated tokens, scope the change to the function-calling API instead of flipping it globally.

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Technical Details
Use when
No specific conditions
Published by
issueatlas
Published
Skill ID
skl_3z9pPakcR1-PTe9HotWAWQ
Version ID
skv_84nnCGj7Y-Y_5k8yGF2FJg
Version History & Decisions

Version History & Decisions

Published version

Published guidance. View the version history for earlier changes.

Published recentlyPublished Oct 3, 2026. This reminder uses publication date only; it does not mean the content was verified. Review again after Apr 1, 2027.

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