Azure Functions: Host not running

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Check host.json for errors
# Verify runtime version
func --version
# Install extensions
func extensions install
# Run locally to debug
func start --verbose
# Check Application Insights for errors

How to diagnose Serverless errors

Serverless errors are shaped by hard platform limits: execution duration, deployment package size, memory, and concurrency. Unlike a server you control, these cannot be tuned away. The fix is to restructure the workload. VPC-attached functions add a second class of problem, because they lose default internet access and depend on ENI capacity and NAT configuration.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Check the platform limit before changing code: timeout, package size and memory are all documented, and the error is usually the limit being reported literally.
2. For timeouts, add timing logs around each external call. A function that times out is nearly always waiting on one downstream dependency.
3. Reduce package size with layers, tree-shaking, and by excluding dev dependencies and the provider SDK that the runtime already includes.
4. For VPC functions with no internet access, confirm there is a NAT Gateway or the relevant VPC endpoints. A function in a private subnet cannot reach the public internet by default.
5. Measure cold start separately from warm invocation in your metrics, and use provisioned concurrency only after confirming cold starts are the actual problem.

Tools worth reaching for

• CloudWatch Logs Insights
• aws lambda get-function-configuration
• X-Ray / distributed tracing
• sam local invoke

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 learn.microsoft.com

Related Serverless errors

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• Cloudflare Worker: Script size exceeds limitThe Cloudflare Worker script exceeds the 1MB (free) or 10MB (paid) compressed size limit.
• GCP Cloud Functions: Deployment failedCloud Function deployment failed during build or initialisation. Could be a dependency issue…

See all 11 Serverless errors →

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Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.