feat(services/hf): honor HF_HUB_DISABLE_XET to force http download mode

core/services/hf/src/backend.rs

@@ -144,6 +144,20 @@ impl HfBuilder {
             .unwrap_or_else(|| "https://huggingface.co".to_string())
     }
 
+    /// Resolve the download mode: an explicit config value wins; otherwise a set,
+    /// non-empty HF_HUB_DISABLE_XET (a huggingface_hub env var) forces http; default Xet.
+    fn hf_download_mode(&self) -> HfDownloadMode {
+        if let Some(mode) = self.config.download_mode {
+            return mode;
+        }
+        if let Ok(val) = std::env::var("HF_HUB_DISABLE_XET") {
+            if !val.is_empty() {
+                return HfDownloadMode::Http;
+            }
+        }
+        HfDownloadMode::default()
+    }
+
     fn hf_home() -> Option<PathBuf> {
         if let Ok(h) = std::env::var("HF_HOME") {
             return Some(PathBuf::from(h));
@@ -191,6 +205,7 @@ impl Builder for HfBuilder {
 
         let token = self.hf_token();
         let endpoint = self.hf_endpoint();
+        let download_mode = self.hf_download_mode();
 
         let repo_type = self.config.repo_type.ok_or_else(|| {
             Error::new(ErrorKind::ConfigInvalid, "repo_type is required")
@@ -216,7 +231,6 @@ impl Builder for HfBuilder {
         debug!("backend use root: {}", &root);
         debug!("backend use token: {}", token.is_some());
         debug!("backend use endpoint: {}", &endpoint);
-        let download_mode = self.config.download_mode.unwrap_or_default();
         debug!("backend use download_mode: {:?}", download_mode);
 
         let info = ServiceInfo::new(HF_SCHEME, "", "");
@@ -541,6 +555,40 @@ mod tests {
         assert_eq!(result, "https://env.example.com");
     }
 
+    #[test]
+    fn hf_download_mode_defaults_to_xet() {
+        let _guard = ENV_LOCK.lock().unwrap();
+        unsafe { std::env::remove_var("HF_HUB_DISABLE_XET") };
+        assert_eq!(HfBuilder::default().hf_download_mode(), HfDownloadMode::Xet);
+    }
+
+    #[test]
+    fn hf_download_mode_disable_xet_env_forces_http() {
+        let _guard = ENV_LOCK.lock().unwrap();
+        unsafe { std::env::set_var("HF_HUB_DISABLE_XET", "1") };
+        let mode = HfBuilder::default().hf_download_mode();
+        unsafe { std::env::remove_var("HF_HUB_DISABLE_XET") };
+        assert_eq!(mode, HfDownloadMode::Http);
+    }
+
+    #[test]
+    fn hf_download_mode_config_takes_priority_over_env() {
+        let _guard = ENV_LOCK.lock().unwrap();
+        unsafe { std::env::set_var("HF_HUB_DISABLE_XET", "1") };
+        let mode = HfBuilder::default().download_mode("xet").hf_download_mode();
+        unsafe { std::env::remove_var("HF_HUB_DISABLE_XET") };
+        assert_eq!(mode, HfDownloadMode::Xet);
+    }
+
+    #[test]
+    fn hf_download_mode_empty_env_keeps_xet() {
+        let _guard = ENV_LOCK.lock().unwrap();
+        unsafe { std::env::set_var("HF_HUB_DISABLE_XET", "") };
+        let mode = HfBuilder::default().hf_download_mode();
+        unsafe { std::env::remove_var("HF_HUB_DISABLE_XET") };
+        assert_eq!(mode, HfDownloadMode::Xet);
+    }
+
     #[test]
     fn build_accepts_datasets_alias() {
         HfBuilder::default()