Diagnose: Azure Function trigger never fires: binding not registered

TL;DR: Symptom: Azure Function trigger never fires: binding not registered Cause: The Functions host does not know the trigger binding, almost always because the extension bundle is missing from host.json, or FUNCTIONSWORKERRUNTIME is wrong so the worker never starts. CONFIRM IT CONFIRM: host log on func start or in the portal log stream: 'The binding type(s) ... are not registered' = missing bundle. Check host.json for the extensionBundle block and app settings for FUNCTIONSWORKERRUNTIME.

The fix

SYMPTOM Azure Function trigger never fires: binding not registered

CONFIRM IT CONFIRM: host log on func start or in the portal log stream: 'The binding type(s) ... are not registered' = missing bundle. 'Did not find functions with language [x]' = wrong worker runtime. Check host.json for the extensionBundle block and app settings for FUNCTIONSWORKERRUNTIME.

FIX FIX: add the extension bundle to host.json (Microsoft.Azure.Functions.ExtensionBundle, version [4.*, 5.0.0)), set FUNCTIONSWORKERRUNTIME to the right language, redeploy. For Service Bus/Event Hub triggers also confirm the connection app setting exists.

VERIFY VERIFY: host log shows 'Host started' with the function listed; trigger it (HTTP call or queue message) and see the invocation log.

When to use this

• This covers exactly what the title says: Diagnose.
• You are setting this up for the first time, or auditing an existing setup.
• You want the key gotchas in one place before you start.

When not to use this

• You are doing a different workflow with Diagnose; these steps are specific to the title above.
• You need the full reference docs; this is the short path, not the manual.

Why it happens

CAUSE The Functions host does not know the trigger binding, almost always because the extension bundle is missing from host.json, or FUNCTIONSWORKERRUNTIME is wrong so the worker never starts.

Diagnostic skill indexed by the exact error string. Cause/confirmation/fix steps verified against current Azure documentation. Evidence: https://learn.microsoft.com/en-us/azure/azure-functions/functions-reference-python