An App Service slot swap times out during warm-up and never completes. What would you investigate?

Debug an App Service swap timeout during warm-up

Diagnose a deployment slot swap that fails because the target app does not warm up successfully.

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I would look at the warm-up path, the status code returned by that path, and the App Service logs during the swap. A warm-up endpoint that runs migrations, calls many dependencies, or waits on slow startup work can make a good release look broken. I would also check whether the slot has the same required configuration and secrets it needs after swap. If the swap is stuck in a prepared state, I would cancel or reset the swap cleanly instead of trying unrelated changes.

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Warm-up path design, startup latency, health checks, dependency timeouts, logs, and safe release handling.

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A warm-up timeout means the target slot did not prove it could serve traffic within the release window. Slow startup, excessive warm-up work, missing configuration, secret resolution, or blocked dependencies need to be separated before extending the timeout.
I would look at the warm-up path, the status code returned by that path, and the App Service logs during the swap. A warm-up endpoint that runs migrations, calls many dependencies, or waits on slow startup work can make a good release look broken. I would also check whether the slot has the same required configuration and secrets it needs after swap. If the swap is stuck in a prepared state, I would cancel or reset the swap cleanly instead of trying unrelated changes.
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Then I would separate app startup time from dependency readiness. If the process starts slowly because of cold initialization, large caches, or synchronous startup work, I would move that work out of the request path or make it bounded. If readiness fails because the database, Key Vault, or an external service is unavailable, the warm-up result is useful, but the endpoint needs clear logs and safe timeouts. I would tune the warm-up path so it proves the app can serve real traffic without becoming a fragile integration test.

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Making the warm-up endpoint call every downstream dependency.
Ignoring the swap prepared state after a failed swap.
Treating timeout settings as the only problem.
Not logging why readiness failed.

An App Service slot swap times out during warm-up and never completes. What would you investigate?

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