diff --git a/src/IIIFPresentation/API.Tests/Integration/GetManifestTests.cs b/src/IIIFPresentation/API.Tests/Integration/GetManifestTests.cs
index 8a1bc34..450f970 100644
--- a/src/IIIFPresentation/API.Tests/Integration/GetManifestTests.cs
+++ b/src/IIIFPresentation/API.Tests/Integration/GetManifestTests.cs
@@ -51,6 +51,7 @@ public async Task Get_IiifManifest_Flat_ReturnsManifestFromS3_DecoratedWithDbVal
         // Assert
         response.StatusCode.Should().Be(HttpStatusCode.OK);
         response.Headers.Should().ContainKey(HeaderNames.ETag);
+        response.Headers.Vary.Should().HaveCount(2);
         manifest.Should().NotBeNull();
         manifest!.Type.Should().Be("Manifest");
         manifest.Id.Should().Be("http://localhost/1/manifests/FirstChildManifest", "requested by flat URI");
@@ -81,9 +82,10 @@ public async Task Get_IiifManifest_Hierarchical_ReturnsManifestFromS3()
         // Assert
         response.StatusCode.Should().Be(HttpStatusCode.OK);
         response.Headers.Should().ContainKey(HeaderNames.ETag);
+        response.Headers.Vary.Should().HaveCount(2);
         manifest.Should().NotBeNull();
         manifest!.Type.Should().Be("Manifest");
         manifest.Id.Should().Be("http://localhost/1/iiif-manifest", "requested by hierarchical URI");
         manifest.Items.Should().HaveCount(3, "the test content contains 3 children");
     }
-}
\ No newline at end of file
+}
diff --git a/src/IIIFPresentation/API/Features/Manifest/ManifestController.cs b/src/IIIFPresentation/API/Features/Manifest/ManifestController.cs
index bf16da3..af17e00 100644
--- a/src/IIIFPresentation/API/Features/Manifest/ManifestController.cs
+++ b/src/IIIFPresentation/API/Features/Manifest/ManifestController.cs
@@ -5,6 +5,7 @@
 using API.Features.Manifest.Validators;
 using API.Features.Storage.Helpers;
 using API.Infrastructure;
+using API.Infrastructure.Filters;
 using API.Infrastructure.Helpers;
 using API.Infrastructure.Requests;
 using API.Settings;
@@ -25,6 +26,7 @@ public class ManifestController(IOptions<ApiSettings> options, IAuthenticator au
 {
     [HttpGet("manifests/{id}")]
     [ETagCaching]
+    [VaryHeader]
     public async Task<IActionResult> GetManifestFlat([FromRoute] int customerId, [FromRoute] string id)
     {
         var pathOnly = !Request.HasShowExtraHeader() ||