Flex Consumption (Python): azd remote build uses Python 3.11.8, ignores functionAppConfig.runtime.version=3.13

[paulyuk]

Summary

On Flex Consumption (Python), azd deploy triggers a remote Oryx build that uses Python 3.11.8 regardless of the Function App's configured runtime version (functionAppConfig.runtime = { name: python, version: 3.13 }). Because the build runs on 3.11, any dependency that is published as Python-3.13-only cannot be installed and the deploy fails.

Concretely, a 3.13 app that depends on azure-functions==2.1.0 (which has Requires-Python >=3.13) fails the remote build:

/opt/Kudu/Scripts/starter.sh oryx build /tmp/zipdeploy/extracted -o /home/site/wwwroot \
  --platform python --platform-version 3.11.8 -p packagedir=.python_packages/lib/site-packages
ERROR: Could not find a version that satisfies the requirement azure-functions==2.1.0
ERROR: No matching distribution found for azure-functions==2.1.0

The (from versions: ... 1.26.0b3) list confirms pip is running on an interpreter (3.11) where 2.x is filtered out by Requires-Python.

Expected

The Flex Consumption remote build should run on the same Python version configured for the app (functionAppConfig.runtime.version, here 3.13), so 3.13-only wheels install correctly.

Actual

azd deploy POSTs to https://<app>.scm.azurewebsites.net/api/publish?RemoteBuild=true, and the server-side Oryx build is invoked with a hardcoded/defaulted --platform-version 3.11.8. The app's functionAppConfig.runtime.version=3.13 is ignored by the build.

Repro

1. Flex Consumption plan (FC1), Linux, Python.
2. App functionAppConfig.runtime = { python, 3.13 } (verified via az functionapp show).
3. requirements.txt includes a 3.13-only package, e.g. azure-functions==2.1.0 (Requires-Python >=3.13).
4. azd up / azd deploy.
5. Remote build runs on --platform-version 3.11.8 → No matching distribution found.

What did NOT change the build interpreter

• Setting infra/main.bicep runtimeVersion: '3.13' and re-provisioning (server-side runtime confirmed 3.13).
• az functionapp restart + redeploy.
• A repo-root runtime.txt containing python-3.13 (ignored by the Flex publish path).
• No PYTHON_* / ORYX_* / build app settings present.

azd deploy --debug shows azd only sends POST /api/publish?RemoteBuild=true — it does not pass --platform-version, so the 3.11.8 selection happens server-side in the Flex build service. Filing here because azd owns this deploy path and the default; please route to the Functions/Flex build-service team if the fix belongs there.

Workaround (works)

Bypass the remote build and ship a pre-built package built for the target interpreter:

# cross-build Linux cp313 wheels (uv evaluates markers for the target)
uv pip install \
  --target .python_packages/lib/site-packages \
  --python-platform x86_64-unknown-linux-gnu \
  --python-version 3.13 \
  --only-binary :all: \
  -r requirements.txt

# deploy as-is, no remote/local build
func azure functionapp publish <APP_NAME> --no-build

This produced a successful deploy with all functions indexed on the 3.13 runtime.

Environment

• azd: 1.25.4 (67d976f0)
• Azure Functions Core Tools: 4.12.0
• OS: macOS 26.5 (25F71)
• Hosting: Flex Consumption (FC1), Linux, Python 3.13, region eastus2
• Failing dependency: azure-functions==2.1.0 (Requires-Python >=3.13)

[vhvb1989]

Investigation Summary

Root Cause

The issue is server-side in the Flex Consumption build service, not in azd.

Deploy chain:

1. azd sends POST /api/publish?RemoteBuild=true (funcapp_host_client.go:71-84)
2. The Flex Consumption build service receives this and invokes Oryx with --platform python --platform-version 3.11.8 — a hardcoded/defaulted value
3. Oryx builds with Python 3.11.8, ignoring the app's functionAppConfig.runtime.version=3.13

azd-side analysis

• azd does not pass a pythonVersion query parameter — and neither does azure-functions-core-tools (func CLI). Confirmed from func source code (PublishFunctionAppAction.cs — HandleFlexConsumptionPublish only adds RemoteBuild=true to the deployment parameters).
• The /api/publish endpoint has no documented support for a pythonVersion or python-version query parameter.
• azd fetches function app properties via GetFunctionAppProperties(), but the current ARM SDK (armappservice/v2 v2.3.0) does not expose the Flex-specific functionAppConfig.runtime structure needed to read the configured Python version.

Next Steps

• Route to the Functions/Flex build-service team to:
  1. Fix the server-side build to honor the configured functionAppConfig.runtime.version when invoking Oryx (the right fix).
  2. Confirm whether /api/publish supports (or will support) a pythonVersion query parameter that clients can pass.
• If the Functions team adds query param support, azd can wire it through PublishOptions → FuncAppHostClient.Publish() → query string.