server: reduce download concurrency for HuggingFace URLs

Reduces concurrent download parts from 16 to 4 for HuggingFace URLs
to avoid triggering rate limits (HTTP 429 errors).

Adds OLLAMA_HF_CONCURRENCY environment variable for users who want
to customize the concurrency level.

Fixes #13297

server/download.go

@@ -96,10 +96,36 @@ func (p *blobDownloadPart) UnmarshalJSON(b []byte) error {
 
 const (
 	numDownloadParts          = 16
+	numHFDownloadParts        = 4
 	minDownloadPartSize int64 = 100 * format.MegaByte
 	maxDownloadPartSize int64 = 1000 * format.MegaByte
 )
 
+// isHuggingFaceURL returns true if the URL is from HuggingFace domains
+func isHuggingFaceURL(u *url.URL) bool {
+	if u == nil {
+		return false
+	}
+	host := strings.ToLower(u.Hostname())
+	return strings.HasSuffix(host, "huggingface.co") ||
+		strings.HasSuffix(host, ".hf.co") ||
+		host == "hf.co"
+}
+
+// getNumDownloadParts returns the number of concurrent download parts to use
+// for the given URL. HuggingFace URLs use reduced concurrency to avoid rate limiting.
+func getNumDownloadParts(u *url.URL) int {
+	if isHuggingFaceURL(u) {
+		if v := os.Getenv("OLLAMA_HF_CONCURRENCY"); v != "" {
+			if n, err := strconv.Atoi(v); err == nil && n > 0 {
+				return n
+			}
+		}
+		return numHFDownloadParts
+	}
+	return numDownloadParts
+}
+
 func (p *blobDownloadPart) Name() string {
 	return strings.Join([]string{
 		p.blobDownload.Name, "partial", strconv.Itoa(p.N),
@@ -271,7 +297,11 @@ func (b *blobDownload) run(ctx context.Context, requestURL *url.URL, opts *regis
 	}
 
 	g, inner := errgroup.WithContext(ctx)
-	g.SetLimit(numDownloadParts)
+	concurrency := getNumDownloadParts(directURL)
+	if concurrency != numDownloadParts {
+		slog.Info(fmt.Sprintf("using reduced concurrency (%d) for HuggingFace download", concurrency))
+	}
+	g.SetLimit(concurrency)
 	for i := range b.Parts {
 		part := b.Parts[i]
 		if part.Completed.Load() == part.Size {