diff --git a/internal/server/gzip.go b/internal/server/gzip.go
index 95b92e4da..d8157a8a3 100644
--- a/internal/server/gzip.go
+++ b/internal/server/gzip.go
@@ -9,9 +9,9 @@ import (
 	"github.com/photoprism/photoprism/internal/config"
 )
 
-// gzipExcludedExtensions contains file extensions that should never be gzip-compressed.
+// GzipExcludedExtensions contains file extensions that should never be gzip-compressed.
 // These formats are already compressed or typically served as large binary payloads.
-var gzipExcludedExtensions = map[string]struct{}{
+var GzipExcludedExtensions = map[string]struct{}{
 	".png":  {},
 	".gif":  {},
 	".jpeg": {},
@@ -23,6 +23,12 @@ var gzipExcludedExtensions = map[string]struct{}{
 	".gz":   {},
 }
 
+// ShouldExcludeGzipExt returns true if the given file extension should not be gzip-compressed.
+func ShouldExcludeGzipExt(ext string) bool {
+	_, ok := GzipExcludedExtensions[strings.ToLower(ext)]
+	return ok
+}
+
 // NewGzipShouldCompressFn returns a high-performance gzip decision function for PhotoPrism.
 // It mirrors the legacy exclusion rules (extensions and path prefixes) and adds targeted
 // route exclusions for binary/streaming endpoints that must not be compressed.
@@ -61,58 +67,90 @@ func NewGzipShouldCompressFn(conf *config.Config) func(c *gin.Context) bool {
 	}
 
 	return func(c *gin.Context) bool {
-		if c == nil || c.Request == nil {
-			return false
-		}
-
-		// Only compress when the client explicitly accepts gzip and the connection is not upgraded.
-		if !strings.Contains(strings.ToLower(c.GetHeader("Accept-Encoding")), "gzip") {
-			return false
-		}
-		if strings.Contains(strings.ToLower(c.GetHeader("Connection")), "upgrade") {
-			return false
-		}
-
-		path := c.Request.URL.Path
-		if path == "" {
-			return false
-		}
-
-		// Exclude known already-compressed/binary extensions.
-		if ext := strings.ToLower(filepath.Ext(path)); ext != "" {
-			if _, ok := gzipExcludedExtensions[ext]; ok {
-				return false
-			}
-		}
-
-		// Exclude configured prefix groups.
-		for _, prefix := range excludedPrefixes {
-			if prefix != "" && strings.HasPrefix(path, prefix) {
-				return false
-			}
-		}
-
-		// Exclude matched route patterns for dynamic endpoints.
-		if full := c.FullPath(); full != "" {
-			if _, ok := excludedFullPaths[full]; ok {
-				return false
-			}
-		}
-
-		// Fallback exclusions using raw path checks for robustness.
-		// Note: Keep the prefix guard here (not just HasSuffix), as the frontend SPA
-		// wildcard route may include paths ending in "/preview" (HTML) that should
-		// remain compressible (e.g., "/library/.../preview").
-		if path == clusterThemePath {
-			return false
-		}
-		if strings.HasPrefix(path, photoDlPrefix) && strings.HasSuffix(path, "/dl") {
-			return false
-		}
-		if strings.HasPrefix(path, sharePrefix) && strings.HasSuffix(path, "/preview") {
-			return false
-		}
-
-		return true
+		return shouldCompressGzip(c, excludedFullPaths, excludedPrefixes, clusterThemePath, photoDlPrefix, sharePrefix)
 	}
 }
+
+// shouldCompressGzip is the core decision logic for gzip compression.
+// It is separated from NewGzipShouldCompressFn to enable unit testing.
+func shouldCompressGzip(c *gin.Context, excludedFullPaths map[string]struct{}, excludedPrefixes []string, clusterThemePath, photoDlPrefix, sharePrefix string) bool {
+	if c == nil || c.Request == nil {
+		return false
+	}
+
+	// Only compress when the client explicitly accepts gzip and the connection is not upgraded.
+	if !clientAcceptsGzip(c) {
+		return false
+	}
+	if isConnectionUpgrade(c) {
+		return false
+	}
+
+	path := c.Request.URL.Path
+	if path == "" {
+		return false
+	}
+
+	// Exclude known already-compressed/binary extensions.
+	if ext := strings.ToLower(filepath.Ext(path)); ext != "" {
+		if ShouldExcludeGzipExt(ext) {
+			return false
+		}
+	}
+
+	// Exclude configured prefix groups.
+	if matchesPrefixExclusion(path, excludedPrefixes) {
+		return false
+	}
+
+	// Exclude matched route patterns for dynamic endpoints.
+	if full := c.FullPath(); full != "" {
+		if _, ok := excludedFullPaths[full]; ok {
+			return false
+		}
+	}
+
+	// Fallback exclusions using raw path checks for robustness.
+	if matchesFallbackExclusion(path, clusterThemePath, photoDlPrefix, sharePrefix) {
+		return false
+	}
+
+	return true
+}
+
+// clientAcceptsGzip checks if the client accepts gzip encoding.
+func clientAcceptsGzip(c *gin.Context) bool {
+	return strings.Contains(strings.ToLower(c.GetHeader("Accept-Encoding")), "gzip")
+}
+
+// isConnectionUpgrade checks if the connection is being upgraded (e.g., WebSocket).
+func isConnectionUpgrade(c *gin.Context) bool {
+	return strings.Contains(strings.ToLower(c.GetHeader("Connection")), "upgrade")
+}
+
+// matchesPrefixExclusion checks if the path matches any excluded prefix.
+func matchesPrefixExclusion(path string, excludedPrefixes []string) bool {
+	for _, prefix := range excludedPrefixes {
+		if prefix != "" && strings.HasPrefix(path, prefix) {
+			return true
+		}
+	}
+	return false
+}
+
+// matchesFallbackExclusion checks for fallback exclusions using raw path checks.
+// Note: Keep the prefix guard here (not just HasSuffix), as the frontend SPA
+// wildcard route may include paths ending in "/preview" (HTML) that should
+// remain compressible (e.g., "/library/.../preview").
+func matchesFallbackExclusion(path, clusterThemePath, photoDlPrefix, sharePrefix string) bool {
+	if path == clusterThemePath {
+		return true
+	}
+	if strings.HasPrefix(path, photoDlPrefix) && strings.HasSuffix(path, "/dl") {
+		return true
+	}
+	if strings.HasPrefix(path, sharePrefix) && strings.HasSuffix(path, "/preview") {
+		return true
+	}
+	return false
+}
diff --git a/internal/server/gzip_test.go b/internal/server/gzip_test.go
index 9b071e82b..8f3c6f2d6 100644
--- a/internal/server/gzip_test.go
+++ b/internal/server/gzip_test.go
@@ -16,6 +16,72 @@ import (
 	"github.com/photoprism/photoprism/internal/config"
 )
 
+func TestShouldExcludeGzipExt(t *testing.T) {
+	t.Run("ExcludesCompressedFormats", func(t *testing.T) {
+		excludedExts := []string{".png", ".gif", ".jpeg", ".jpg", ".webp", ".mp3", ".mp4", ".zip", ".gz"}
+		for _, ext := range excludedExts {
+			assert.True(t, ShouldExcludeGzipExt(ext), "extension %s should be excluded", ext)
+		}
+	})
+	t.Run("ExcludesCaseInsensitive", func(t *testing.T) {
+		assert.True(t, ShouldExcludeGzipExt(".PNG"))
+		assert.True(t, ShouldExcludeGzipExt(".Jpg"))
+		assert.True(t, ShouldExcludeGzipExt(".GZ"))
+	})
+	t.Run("DoesNotExcludeTextFormats", func(t *testing.T) {
+		allowedExts := []string{".html", ".css", ".js", ".json", ".xml", ".txt", ".svg"}
+		for _, ext := range allowedExts {
+			assert.False(t, ShouldExcludeGzipExt(ext), "extension %s should not be excluded", ext)
+		}
+	})
+	t.Run("EmptyExtension", func(t *testing.T) {
+		assert.False(t, ShouldExcludeGzipExt(""))
+	})
+}
+
+func TestMatchesPrefixExclusion(t *testing.T) {
+	prefixes := []string{"/api/v1/dl", "/health", "/livez"}
+
+	t.Run("MatchesPrefix", func(t *testing.T) {
+		assert.True(t, matchesPrefixExclusion("/api/v1/dl/file", prefixes))
+		assert.True(t, matchesPrefixExclusion("/health", prefixes))
+		assert.True(t, matchesPrefixExclusion("/healthz", prefixes))
+		assert.True(t, matchesPrefixExclusion("/livez", prefixes))
+	})
+	t.Run("DoesNotMatchNonPrefixes", func(t *testing.T) {
+		assert.False(t, matchesPrefixExclusion("/api/v1/photos", prefixes))
+		assert.False(t, matchesPrefixExclusion("/other", prefixes))
+	})
+	t.Run("EmptyPrefixes", func(t *testing.T) {
+		assert.False(t, matchesPrefixExclusion("/anything", nil))
+		assert.False(t, matchesPrefixExclusion("/anything", []string{}))
+	})
+	t.Run("EmptyPath", func(t *testing.T) {
+		assert.False(t, matchesPrefixExclusion("", prefixes))
+	})
+}
+
+func TestMatchesFallbackExclusion(t *testing.T) {
+	clusterTheme := "/api/v1/cluster/theme"
+	photoDl := "/api/v1/photos/"
+	share := "/s/"
+
+	t.Run("MatchesClusterTheme", func(t *testing.T) {
+		assert.True(t, matchesFallbackExclusion(clusterTheme, clusterTheme, photoDl, share))
+	})
+	t.Run("MatchesPhotoDl", func(t *testing.T) {
+		assert.True(t, matchesFallbackExclusion("/api/v1/photos/abc123/dl", clusterTheme, photoDl, share))
+	})
+	t.Run("MatchesSharePreview", func(t *testing.T) {
+		assert.True(t, matchesFallbackExclusion("/s/token/shared/preview", clusterTheme, photoDl, share))
+	})
+	t.Run("DoesNotMatchOtherPaths", func(t *testing.T) {
+		assert.False(t, matchesFallbackExclusion("/api/v1/photos/abc123", clusterTheme, photoDl, share))
+		assert.False(t, matchesFallbackExclusion("/s/token/shared/view", clusterTheme, photoDl, share))
+		assert.False(t, matchesFallbackExclusion("/library/something/preview", clusterTheme, photoDl, share))
+	})
+}
+
 func TestGzipMiddleware(t *testing.T) {
 	gin.SetMode(gin.TestMode)