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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
Proxy authentication is distinct from storage-account authentication. The proxy scheme, challenge, machine-versus-user context, PAC result and credential policy determine whether the engine can establish transport.
Evidence worth collecting
- Proxy URI without secrets.
- HTTP 407 challenge schemes.
- PAC/auto-proxy result.
- Engine identity and credential policy.
A controlled diagnostic sequence
- 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 stable item ID, second supported path | A change separates local file-system/provider state from remote service state. | 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
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