Azure Functions host errors: extension bundle and worker runtime first

TL;DR: Function host fails to start or bindings do not fire. The checklist is short: FUNCTIONSWORKERRUNTIME, extension bundle in host.json, and the storage connection the host itself needs. Must match the language (python, node, dotnet). Wrong value = worker never starts.

The error

). Wrong value = worker never starts. Check app settings, not just local.settings.json (which does not deploy).
2. **Extension bundle.** host.json needs:

The fix

1. FUNCTIONSWORKERRUNTIME. Must match the language (python, node, dotnet). Wrong value = worker never starts. Check app settings, not just local.settings.json (which does not deploy).
2. Extension bundle. host.json needs: {"version": "2.0", "extensionBundle": {"id": "Microsoft.Azure.Functions.ExtensionBundle", "version": "[4.*, 5.0.0)"}} Missing bundle = non-HTTP/Timer triggers never register. This is the number one "trigger does not fire" cause.
3. AzureWebJobsStorage. The host itself needs a storage account connection for its internal bookkeeping (locks, keys). Missing or wrong = host will not start. Identity-based: AzureWebJobsStorage__accountName instead of a connection string.
4. Python version pin. linuxFxVersion like PYTHON|3.11 must be a runtime the plan supports. Deploying 3.12 code to a 3.11-only setting fails at import.
5. Cold start vs broken. Consumption plan cold starts add seconds, not errors. If invocations 500 immediately, it is code/config, not cold start. Verify: func start locally reproduces most of these. The host log names the missing piece; read it before redeploying.

When to use this

• You are seeing this exact error message; match the block above, not just part of it.
• The failing call matches the scenario in the title: Azure Functions host errors.
• You want the fastest verified fix before digging through logs.

When not to use this

• Your error text differs from the block above; close cousins often have different causes.
• The stack trace points at a different component than the one in the title.
• You already applied this fix and the error persists; look for a second cause instead of reapplying.

Proactive error-catalog skill. Host startup requirements verified against current Functions docs. Evidence: https://learn.microsoft.com/en-us/azure/azure-functions/functions-reference-python