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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
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., 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 it is returned, proxy authentication is distinct from storage-account authentication. During a this result investigation, the proxy scheme, challenge, machine-versus-user context, PAC result and credential policy determine whether the engine can establish transport.
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 it, this separates caller input from environment and service state within the Windows cloud-file and sync-engine infrastructure boundary.
- HTTP 407 challenge schemes. for it, 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 it, 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 it, 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 this result 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 | Hold constant |
|---|---|---|
| Same input, fresh object or connection | If the result disappears, retained lifecycle or ownership state is implicated. | While diagnosing this result, keep the original data, account, device, or timeline parameters unchanged. |
| Same object, reduced operation | If the result follows one specific transition, statement, file, or ceremony step, the failure is localized. | While diagnosing this result, remove only unrelated work and keep the first failing boundary visible. |
| Same stable item ID, second supported path | A change separates local file-system/provider state from remote service state. | While diagnosing it, keep account, remote version and client build fixed. |
What this code does not justify
SERVICE_AUTHENTICATION_FAILED occurs at the storage service; this code occurs before that at the proxy boundary.
Do not put credentials in URLs, logs or global machine settings unnecessarily.
Verification after correction
Treat it 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 it.
- 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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