Anthropic Files API returns 404 "Not found" through Azure AI Foundry Claude endpoint — Messages API unaffected

János 50 Reputation points
2026-08-12T09:17:18.3966667+00:00

Severity suggestion: B (production feature outage) 

  Service: Azure AI Foundry — Claude (Anthropic) models                                       

                                                                                              

  Description:                                                                                

                                                                                              

  Since 2026-08-11, between approximately 10:38 and 17:37 UTC, all requests to the Anthropic  

  Files API (/v1/files — upload, retrieve metadata, download) made through our Azure AI       

  Foundry endpoint fail with:                                                                 

                                                                                              

  404 {"type":"error","error":{"type":"not_found_error","message":"Not found"}}               

                                                                                              

  The Messages API (/v1/messages) on the same endpoint, with the same credentials, continues  

  to work normally — both streaming and non-streaming, including claude-sonnet-5 and          

  claude-opus-5.                                                                              

                                                                                              

  Impact (production):                                                                        

  1. Direct file uploads (PDFs, images attached by users) fail — 200+ consecutive failures    

  since onset, zero successes.                                                                

  2. Requests using the code execution tool complete generation server-side but fail when we  

  attempt to download the generated files via the Files API — the model streams for ~90       

  seconds, then the final file retrieval 404s, losing the entire response.                    

                                                                                              

  Timeline:                                                                                   

  - Last successful Files API upload: 2026-08-11 10:38 UTC                                    

  - First 404 failure: 2026-08-11 17:37 UTC                                                   

  - Since then: hard failure — no intermittent successes, which suggests a                    

  routing/configuration change rather than a transient issue.
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Answer accepted by question author
Anshika Varshney 15,625 Reputation points Microsoft External Staff Moderator
2026-08-12T10:06:37.58+00:00

Hello @János & @Tom Hughes

Thank you for the update and for confirming that the Anthropic Files API functionality has been restored and is working again.

We understand your request for root cause clarification regarding the temporary 404 errors observed on the /anthropic/v1/files endpoint. We are reviewing the details with the relevant product team and will share any information that becomes available.

We appreciate the troubleshooting information and timelines you provided, as they are helpful for the investigation.

Thank you for your patience and cooperation.

If the above steps resolve your issue, please mark this answer as Accepted so others can easily find the solution.

Thankyou!

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  1. Tom Hughes 5 Reputation points
    2026-08-12T20:51:41.9433333+00:00

    Update — service restored, requesting root cause confirmation

    As of approximately 2026-08-12 20:50 UTC, the Anthropic Files API on our resource in EUS2 is working again. POST and GET on /anthropic/v1/files both return 200, and we verified the full cycle end to end (upload, list, download of tool-generated files, delete). No changes were made on our side, the route began serving again on its own, so this was resolved server-side.

    Confirmed impact window for our resource: 2026-08-12 17:49 UTC to ~20:50 UTC. During that window all requests to /anthropic/v1/files returned 404 not_found while /anthropic/v1/messages worked normally, which broke our production document-generation workloads (file upload and edit-in-place features) firm-wide until we deployed an application-side workaround.

    This appears to be the same incident reported by another customer on Microsoft Q&A a day earlier (question 5973180, "Anthropic Files API returns 404 not found through Azure AI Foundry," first failure 2026-08-11 17:37 UTC, acknowledged by a moderator on 2026-08-12).

    Before closing this, we'd like Microsoft to confirm:

    1. Root cause — what removed the Files API capability from our resource on Aug 12? We received the "Your Claude deployment was restored — some features may be affected" notification on Aug 11 and believe this was collateral from that revert program.
    2. Prevention — whether our resource remains in scope for any further reverts or migrations that could remove capabilities again, and if so, when.
    3. Advance notice — how customers will be notified before any future change that removes a live API surface from a production resource. A silent 404 on a documented, working endpoint cost us several hours of engineering forensics that a targeted notification would have avoided.

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  2. Tom Hughes 5 Reputation points
    2026-08-13T21:03:30.9866667+00:00

    Microsoft Support Response:

    We understand the impact this behavior had on your production workloads, especially given the dependency on the Anthropic Files API for your document generation processes. Based on the engineering team review, I would like to share the Root Cause Analysis (RCA) below:

     

    What Happened?

    Between August 12, 2026, and August 13, 2026, the customer experienced intermittent failures when accessing the Anthropic Files API (/anthropic/v1/files) through their Azure AI Services (AI Foundry) resource. During the impacted periods, file upload, file listing, metadata retrieval, and file download operations returned 404 Not Found responses, while model inference requests to /anthropic/v1/messages continued to operate successfully.

     

    What Went Wrong and Why?

    Based on our investigation, the behavior was associated with a broader service-side issue affecting Anthropic functionality within Azure AI Foundry. The issue occurred indirectly as part of a service outage related to a service deployment change. As a result, the Anthropic Files API capability became unavailable for affected requests during the impacted periods. The issue was not caused by customer configuration changes or deployment actions. A rollback of the affected service changes restored functionality.

     

    How Did We Respond?

    The engineering team identified the service regression and mitigated the issue by rolling back the affected deployment to a previous known good configuration. Following mitigation, Anthropic Files API functionality was restored and service behavior returned to normal operation. Internal validation and customer reports confirmed successful recovery after the rollback activities were completed.

     

    How Are We Making Incidents Like This Less Likely or Less Impactful?

    We apologize for the service interruption and understand the impact this had on your production workloads. The engineering team has addressed the underlying issue that contributed to the outage and does not expect customers to encounter the same behavior going forward. Additionally, service teams continue to improve deployment validation and service reliability processes to reduce the likelihood of similar regressions affecting Anthropic-integrated functionality in the future.

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  3. Tom Hughes 5 Reputation points
    2026-08-13T15:44:28.7166667+00:00

    The issue has resurfaced again on our resource, with the second occurrence broader than the first.

    Same signature as the original report, on an Azure AI Foundry (AIServices) resource in eastus2 with Claude deployments showing "Hosted on Anthropic infrastructure":

    • POST/GET https://{resource}.services.ai.azure.com/anthropic/v1/files → 404 {"type":"error","error":{"type":"not_found_error","message":"Not found"}} (Anthropic error format with Anthropic request IDs — the request is reaching the Anthropic backend; this is not an Azure routing 404)
    • /anthropic/v1/messages → 200 throughout, on every Claude deployment, including the server-side code-execution tool, which still runs and mints file IDs that then cannot be retrieved.

    Two distinct outage windows on our resource:

    1. **Aug 12, 17:49 UTC → ~**20:50 UTC — upload and list returned 404, but GET /v1/files/{id}/content still worked. Restored server-side with no action on our part.
    2. Aug 13, ~12:04–12:12 UTC → still active as I post this — the entire files surface is gone this time: upload, list, per-file metadata, and per-file content all 404. Code-execution builds complete and save their output, and the file can never be downloaded.

    Customer-side remediation does not resolve it. We created a brand-new claude-opus-5 deployment (provisioningState Succeeded, inference 200) files still 404. Deployment deletion and quota changes likewise have no effect. Requests match Anthropic's current published spec exactly (path, anthropic-version, anthropic-beta: files-api-2025-04-14, multipart file field), and the identical request shape is accepted at auth level by Anthropic's first-party endpoint.

    Per Anthropic's Foundry documentation, the Files API is supported for deployments hosted on Anthropic, and unsupported features are documented to return 400 by design, not 404, so the current state matches neither documented mode.

    Requesting Microsoft treat this as a platform incident: restore the Anthropic Files API capability on affected resources and prevent the revert process from re-removing it.

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  4. GDK 5 Reputation points
    2026-08-13T13:03:57.9033333+00:00

    The issue seems to be recurring with the same symptom - any further info welcome.

    More details on the same issue appear here https://learn.microsoft.com/en-in/answers/questions/5973304/sweden-central-anthropic-files-api-now-returning-4

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