Flex Consumption intermittent slow host initialization

[pl4nty]

Investigative information

• Timestamp: 2025-12-16T05:52:49.3675888Z
• Function App version:
• Function App name:
• Function name(s) (as appropriate):
• Invocation ID: 2721b80b-8fe3-41fb-a917-84fc63bdfed3
  • ec6fc964-7894-4491-babe-669a14946f84@2025-12-16T05:52:49.0296607Z for a working request
  • 57e4009a-381d-4153-8c7b-1b49ecec9175@2025-12-16T05:45:56.3561633Z for a slow request with no client timeout
• Region: Australia East

Repro steps

1. Deploy a Flex Consumption app with a single HTTP trigger
2. Send 10+ HTTP requests with short timeouts. In my case, clients have a 1.5s timeout. I've reproed with curl
3. Review logs - I've seen anywhere up to 10s if I remove the timeout

Expected behavior

Host always starts immediately and function is executed afterwards. Host usually takes <10ms and my function takes ~100ms.

Actual behavior

Intermittently, host initialization (Initializing Warmup Extension. log) starts 400ms-10s after a request is sent. This occurs despite 10 always-ready instances and very low RPS, eg 1 per minute.

In my case, clients are POSTing with a 1.5s timeout, causing 499 errors and lost data. Over 12 hours, I receive ~500 requests and ~50 of those fail.

Known workarounds

I really don't want to swap SKUs... Increasing the client timeout is not possible.

Related information

• Programming language used: Python 3.12
• Bindings used: Microsoft.Azure.Functions.ExtensionBundle.Experimental