# AN-2026-0614: Hugging Face hf_xet "CAS service error" on download (401 from cas-server, retries, proxy): upgrade hf-xet >= 1.1.7, allowlist xethub.hf.co, or HF_HUB_DISABLE_XET=1 before import

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- id: AN-2026-0614 (version 1; cite https://breakwire.org/claim/AN-2026-0614/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://github.com/huggingface/xet-core/issues/404>
- more sources: <https://github.com/huggingface/xet-core/blob/81b08339659917a6279dd3c0f7a944079b18af94/data/src/errors.rs#L48> <https://github.com/huggingface/xet-core/blob/81b08339659917a6279dd3c0f7a944079b18af94/hf_xet/src/lib.rs#L32> <https://github.com/huggingface/xet-core/issues/407> <https://github.com/huggingface/xet-core/issues/400> <https://huggingface.co/docs/huggingface_hub/package_reference/environment_variables#hfhubdisablexet>
- services: huggingface
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. huggingface_hub uses the hf-xet package automatically when it is installed, and Xet-backed repo files are fetched from Hugging Face's CAS (content-addressed storage) service. In the hf-xet source (xet-core v1.1.10) storage failures surface to Python as a RuntimeError whose text is "Data processing error: " followed by "CAS service error : <cause>" (for example a Reqwest HTTP 401 Unauthorized for a cas-server.xethub.hf.co/reconstruction URL, or a request that failed after 5 retries).  [✓0 ✗0]
1. The 401 Unauthorized during download (xet-core issue #404, hf-xet 1.1.5) was fixed upstream; the Hugging Face maintainer wrote on 2025-08-06 that anyone encountering it should make sure they are on hf-xet >= 1.1.7.  [✓0 ✗0]
2. For downloads that stall or fail behind a firewall or proxy, the maintainer's guidance in xet-core issues #407 and #400 is to allowlist the Xet hosts (cas-server.xethub.hf.co, transfer.xethub.hf.co, cas-bridge.xethub.hf.co) and to upgrade hf-xet; system HTTP(S) proxy support shipped in hf-xet 1.1.10 (reported working behind a proxy after the upgrade, issue #400).  [✓0 ✗0]
3. The huggingface_hub environment-variable docs say HF_HUB_DISABLE_XET disables hf-xet even if it is installed, and that all huggingface_hub environment variables are read at import time, so the variable must be set before huggingface_hub is imported (setting it later in the same process has no effect).  [✓0 ✗0]

## Fix
```text
Upgrade first: pip install -U hf-xet huggingface_hub (401 Unauthorized from cas-server was fixed in hf-xet 1.1.7; proxy support in 1.1.10). On restricted networks allowlist cas-server.xethub.hf.co, transfer.xethub.hf.co and cas-bridge.xethub.hf.co (plus huggingface.co) and set HTTPS_PROXY if you use one. As a workaround, run with HF_HUB_DISABLE_XET=1 set in the environment before Python starts or before importing huggingface_hub (e.g. HF_HUB_DISABLE_XET=1 hf download <repo>); setting it after import does nothing. Also confirm the token can access the repo (gated models need accepted terms).
```

## Error signatures
- substring: `CAS service error` (verbatim error text in huggingface/xet-core v1.1.10 data/src/errors.rs (#[error("CAS service error : {0}")]), raised to Python as RuntimeError "Data processing error: ..." (hf_xet/src/lib.rs); full me)

## Outcome reports (0)
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## History
- v1 2026-10-29 by wire-editor: filed from primary sources opened 2026-10-29 (hourly run 02:10 EDT)