Diagnose: App Service slot swap stuck or 409: previous deployment still running

TL;DR: Symptom: App Service slot swap stuck or 409: previous deployment still running Cause: Kudu runs one deployment at a time; a second push 409s, and a swap-with-preview left open blocks the next swap. Locked files on Windows plans add a third flavor. CONFIRM IT CONFIRM: az webapp deployment list shows an in-progress deployment; portal deployment center shows a swap in Preview state. VERIFY VERIFY: deployment list shows Success for the new build; the app serves the new version (check a version/health endpoint).

The error

App Service slot swap stuck or 409: previous deployment still running Cause: Kudu runs one deployment at a time; a second push 409s, and a swap-with-preview left open blocks the next swap. Locked files on Windows plans add a third flavor. CONFIRM IT CONFIRM: az webapp deployment list shows an in-pro

The fix

TL;DR: Symptom: App Service slot swap stuck or 409: previous deployment still running Cause: Kudu runs one deployment at a time; a second push 409s, and a swap-with-preview left open blocks the next swap. Locked files on Windows plans add a third flavor. CONFIRM IT CONFIRM: az webapp deployment list shows an in-progress deployment; portal deployment center shows a swap in Preview state. VERIFY VERIFY: deployment list shows Success for the new build; the app serves the new version (check a version/health endpoint).

The fix

SYMPTOM App Service slot swap stuck or 409: previous deployment still running

CAUSE Kudu runs one deployment at a time; a second push 409s, and a swap-with-preview left open blocks the next swap. Locked files on Windows plans add a third flavor.

CONFIRM IT CONFIRM: az webapp deployment list shows an in-progress deployment; portal deployment center shows a swap in Preview state. Windows plan + zip deploy failing on DLL locks = file-lock flavor.

FIX FIX: wait for or cancel the in-progress deployment, then redeploy; complete or cancel the stuck swap preview; set WEBSITERUNFROM_PACKAGE=1 to eliminate file locks (recommended deploy mode); do not combine auto-swap with manual pushes to the staging slot.

VERIFY VERIFY: deployment list shows Success for the new build; the app serves the new version (check a version/health endpoint).

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.

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/app-service/deploy-staging-slots