A .NET API deployment to Azure App Service completed, but the app returns errors or will not start. How do you debug it?

How would you diagnose a failed Azure App Service deployment?

Covers deployment logs, startup failures, runtime stack, app settings, missing files, Key Vault references, and post-deployment diagnosis.

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TL;DR

I would first separate deployment failure from application startup failure. I would check deployment logs, whether the expected artifact reached the slot, App Service runtime version, startup logs, app settings, Key Vault references, and recent database migrations. If the app starts but fails requests, I would move to request logs, dependency failures, and Application Insights traces.

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Deployment diagnosis, App Service logs, startup config, runtime mismatch, secret resolution, health checks, and rollback decisions.

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Why this question matters
A completed deployment can still fail at artifact extraction, process startup, configuration resolution, identity, dependency access, migration compatibility, or request handling. Diagnosis starts by locating that boundary before changing the system again.
I would first separate deployment failure from application startup failure. I would check deployment logs, whether the expected artifact reached the slot, App Service runtime version, startup logs, app settings, Key Vault references, and recent database migrations. If the app starts but fails requests, I would move to request logs, dependency failures, and Application Insights traces.
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Stronger answer
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I would compare the broken deployment with the last known good one: commit, package, runtime, settings, slot, identity, database state, and dependency endpoints. A failed deploy can be caused by the build artifact, missing files, wrong framework version, bad config, unresolved secret, startup exception, blocked dependency, or a migration mismatch. I would use the staging slot, log stream, deployment history, and telemetry to classify the failure before choosing rollback, re-deploy, config fix, or hotfix.

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Details

Debugging code before checking whether the app started.
Ignoring slot settings and Key Vault reference resolution.
Making untracked portal changes during an incident.
No mitigation while users keep hitting errors.

A .NET API deployment to Azure App Service completed, but the app returns errors or will not start. How do you debug it?

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