diff --git a/coderd/cachecompress/compress.go b/coderd/cachecompress/compress.go
index 4f3ba5314d..9adff6a4de 100644
--- a/coderd/cachecompress/compress.go
+++ b/coderd/cachecompress/compress.go
@@ -240,9 +240,7 @@ func (c *Compressor) serveRef(w http.ResponseWriter, r *http.Request, headers ht
 		}
 
 		for key, values := range headers {
-			for _, value := range values {
-				w.Header().Add(key, value)
-			}
+			w.Header()[key] = values
 		}
 		w.Header().Set("Content-Encoding", cref.key.encoding)
 		w.Header().Add("Vary", "Accept-Encoding")
diff --git a/coderd/cachecompress/compress_internal_test.go b/coderd/cachecompress/compress_internal_test.go
index 2f90ea8d12..b4756614ba 100644
--- a/coderd/cachecompress/compress_internal_test.go
+++ b/coderd/cachecompress/compress_internal_test.go
@@ -155,6 +155,41 @@ type nopEncoder struct {
 
 func (nopEncoder) Close() error { return nil }
 
+func TestCompressorPresetHeaders(t *testing.T) {
+	t.Parallel()
+
+	logger := testutil.Logger(t)
+	tempDir := t.TempDir()
+	cacheDir := filepath.Join(tempDir, "cache")
+	err := os.MkdirAll(cacheDir, 0o700)
+	require.NoError(t, err)
+	srcDir := filepath.Join(tempDir, "src")
+	err = os.MkdirAll(srcDir, 0o700)
+	require.NoError(t, err)
+	err = os.WriteFile(filepath.Join(srcDir, "file.html"), []byte("textstring"), 0o600)
+	require.NoError(t, err)
+
+	compressor := NewCompressor(logger, 5, cacheDir, http.FS(os.DirFS(srcDir)))
+
+	for range 2 {
+		ctx := testutil.Context(t, testutil.WaitShort)
+		req := httptest.NewRequestWithContext(ctx, "GET", "/file.html", nil)
+		req.Header.Set("Accept-Encoding", "gzip")
+
+		respRec := httptest.NewRecorder()
+		respRec.Header().Set("X-Original-Content-Length", "10")
+		respRec.Header().Set("ETag", `"abc123"`)
+
+		compressor.ServeHTTP(respRec, req)
+		resp := respRec.Result()
+
+		require.Equal(t, http.StatusOK, resp.StatusCode)
+		require.Equal(t, []string{"10"}, resp.Header.Values("X-Original-Content-Length"))
+		require.Equal(t, []string{`"abc123"`}, resp.Header.Values("ETag"))
+		require.NoError(t, resp.Body.Close())
+	}
+}
+
 // nolint: tparallel // we want to assert the state of the cache, so run synchronously
 func TestCompressorHeadings(t *testing.T) {
 	t.Parallel()
diff --git a/site/site_test.go b/site/site_test.go
index 3427a70129..3527f31106 100644
--- a/site/site_test.go
+++ b/site/site_test.go
@@ -562,7 +562,7 @@ func TestServingBin(t *testing.T) {
 					}
 
 					if tr.wantEtag != "" {
-						assert.NotEmpty(t, resp.Header.Get("ETag"), "etag header is empty")
+						assert.Equal(t, []string{tr.wantEtag}, resp.Header.Values("ETag"), "etag header values did not match")
 						assert.Equal(t, tr.wantEtag, resp.Header.Get("ETag"), "etag did not match")
 					}
 
@@ -570,6 +570,8 @@ func TestServingBin(t *testing.T) {
 						// This is a custom header that we set to help the
 						// client know the size of the decompressed data. See
 						// the comment in site.go.
+						headerValues := resp.Header.Values("X-Original-Content-Length")
+						assert.Len(t, headerValues, 1, "X-Original-Content-Length should have exactly one value")
 						headerStr := resp.Header.Get("X-Original-Content-Length")
 						assert.NotEmpty(t, headerStr, "X-Original-Content-Length header is empty")
 						originalSize, err := strconv.Atoi(headerStr)