After an App Service slot swap, login fails because the identity provider redirects to the wrong callback URL. How would you debug it?

Debug broken authentication callbacks after an App Service swap

Diagnose authentication redirect or callback failures after a slot swap or domain change.

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I would capture the actual redirect URL, the configured callback URLs in the identity provider, and the app settings that control public origin or auth authority. Then I would check whether those settings are sticky to the slot and whether the production slot received a staging callback or base URL during swap. I would also inspect forwarded headers if a proxy or gateway is in front, because the app may generate an HTTP or internal host callback. The immediate fix is to restore the correct callback configuration and verify login end to end.

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Auth callback URLs, slot settings, public origins, reverse proxy headers, identity provider configuration, and release safety.

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Authentication callbacks depend on the real public scheme, host, path, proxy headers, cookie policy, identity-provider registration, and slot-owned configuration. A swap can expose a mismatch even when the application code is unchanged.
I would capture the actual redirect URL, the configured callback URLs in the identity provider, and the app settings that control public origin or auth authority. Then I would check whether those settings are sticky to the slot and whether the production slot received a staging callback or base URL during swap. I would also inspect forwarded headers if a proxy or gateway is in front, because the app may generate an HTTP or internal host callback. The immediate fix is to restore the correct callback configuration and verify login end to end.
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I would trace the login flow from browser to identity provider and back to the API or frontend. The important values are public scheme, host, path base, redirect URI, cookie domain, SameSite behavior, and environment-specific auth settings. If the login works on the Azure default hostname but not the custom domain, I would focus on public origin and proxy configuration. If only staging or production fails after swap, I would focus on slot settings and identity provider app registrations.

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Testing only the API health endpoint and not login.
Forgetting callback URLs during custom domain changes.
Letting staging auth settings swap into production.
Ignoring forwarded headers behind a proxy.

After an App Service slot swap, login fails because the identity provider redirects to the wrong callback URL. How would you debug it?

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