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E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED
E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED belongs to Windows cloud-file and sync-engine infrastructure. It marks the sync engine reached a proxy that requires authentication. The useful starting point is the exact API, object and state transition that returned 0x8802D007, because a shell or application message can hide that boundary.
Where this result is raised
For E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, a Windows sync engine coordinates local NTFS objects, provider metadata, remote identifiers, quota and naming rules, placeholder hydration, network authentication, and asynchronous upload or download state. When E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED is returned, the same Explorer action can cross several of those boundaries, so diagnosis must retain the first failing stage rather than treating every code as a generic connectivity problem.
When E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED is returned, proxy authentication is distinct from storage-account authentication. During a E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED investigation, the proxy scheme, challenge, machine-versus-user context, PAC result and credential policy determine whether the engine can establish transport.
For E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, the first owner to inspect is the local sync root, placeholder state, file-system operation, provider request, network exchange, account policy, or remote storage operation that produced the HRESULT.
Evidence worth collecting
- Proxy URI without secrets. For
E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, this separates caller input from environment and service state within the Windows cloud-file and sync-engine infrastructure boundary. - HTTP 407 challenge schemes. For
E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, this creates a stable comparison across retries or another machine within the Windows cloud-file and sync-engine infrastructure boundary. - PAC/auto-proxy result. For
E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, this pins the event to an object or resource generation within the Windows cloud-file and sync-engine infrastructure boundary. - Engine identity and credential policy. For
E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, this places the event on the lifecycle or transaction timeline within the Windows cloud-file and sync-engine infrastructure boundary.
A controlled diagnostic sequence
During a E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED investigation, reduce the case while preserving the condition described by the HRESULT.
- First: Capture the 407 response and selected proxy.
- Next: Use supported integrated or explicit credential flow.
- Then: Test direct and proxy routes separately.
- Finally: Avoid logging proxy passwords or authorization headers.
How to interpret comparison tests
| Control | Interpretation for E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED | Hold constant |
|---|---|---|
| Same input, fresh object or connection | For E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, if the result disappears, retained lifecycle or ownership state is implicated. | While diagnosing E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, keep the original data, account, device, or timeline parameters unchanged. |
| Same object, reduced operation | For E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, if the result follows one specific transition, statement, file, or ceremony step, the failure is localized. | While diagnosing E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, remove only unrelated work and keep the first failing boundary visible. |
| Same stable item ID, second supported path | For E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, a change separates local file-system/provider state from remote service state. | While diagnosing E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, keep account, remote version and client build fixed. |
What this code does not justify
For E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, SERVICE_AUTHENTICATION_FAILED occurs at the storage service; this code occurs before that at the proxy boundary.
For E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED, do not put credentials in URLs, logs or global machine settings unnecessarily.
Verification after correction
Treat E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED as corrected only when the engine authenticates to the intended proxy with least privilege and then reaches the service account boundary; retain the original negative case so fallback cannot be mistaken for repair.
Technical references
The references below define the API family or storage/protocol behavior used to interpret E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED.
- For
E_SYNCENGINE_PROXY_AUTHENTICATION_REQUIRED: Microsoft: Build a Cloud Files sync engine - Microsoft: Restrictions and limitations in OneDrive and SharePoint
- Microsoft: WinHTTP authentication
- Microsoft: Cloud Filter API reference
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