slimtoolkit/slim · error

unexpected media type for index

Error message

unexpected media type for index

What it means

The fetched descriptor for the index reference is not actually an index: indexMeta.MediaType().IsIndex() returned false. The remote object exists but is a single image manifest (or other type), so treating it as an image index would be invalid and the handler aborts.

Source

Thrown at pkg/app/master/command/registry/handler_image_index.go:186

					"exit.code": exitCode,
					"version":   v.Current(),
				})
			xc.Exit(exitCode)
		} else {
			xc.FailOn(fmt.Errorf("saving image index error - %s (%v)", cparams.ImageIndexName, err))
		}
	}

	indexMeta, err := remote.Index(imageIndexRef, remoteOpts...)
	if err != nil {
		xc.FailOn(fmt.Errorf("index reference metadata get error - %s (%v)", cparams.ImageIndexName, err))
	}

	indexMediaType, err := indexMeta.MediaType()
	xc.FailOn(err)

	if !indexMediaType.IsIndex() {
		xc.FailOn(fmt.Errorf("unexpected media type for index"))
	}

	indexDigest, err := indexMeta.Digest()
	xc.FailOn(err)

	indexManifest, err := indexMeta.IndexManifest()
	xc.FailOn(err)
	xc.Out.Info("index.info",
		ovars{
			"reference":                imageIndexRef,
			"digest":                   indexDigest.String(),
			"manifest.schema":          indexManifest.SchemaVersion,
			"manifest.media_type":      indexManifest.MediaType,
			"manifest.image.ref.count": len(indexManifest.Manifests),
		})

	if cparams.DumpRawManifest {
		if rm, err := indexMeta.RawManifest(); err == nil {

View on GitHub (pinned to 81940d17fa)

Solutions

1. Push the assembled index to a dedicated tag and verify that tag
2. Re-run the index create so the index overwrites the colliding image tag
3. Confirm the remote type with 'docker manifest inspect <ref>' (look for manifests list vs single manifest)
4. Use distinct tags/names for single-arch images vs the multi-arch index

Example fix

// before
ImageIndexName: "registry.example.com/app:v1" // tag also used for amd64 image
// after
ImageIndexName: "registry.example.com/app:v1" // ensure only the index is pushed to this tag; use v1-amd64 for the single arch

desc, err := remote.Head(ref)
if err == nil && !desc.MediaType.IsIndex() {
    return fmt.Errorf("%s is not an index (media type %s)", ref.Name(), desc.MediaType)
}

func isIndexMediaType(mt types.MediaType) bool {
    return mt == types.OCIImageIndex || mt == types.DockerManifestList
}

When it happens

Trigger: The tag pointed to by ImageIndexName resolves to a plain image manifest instead of a manifest list/OCI index — typically because a single-platform image was pushed to the same tag, or the wrong tag name was given for verification.

Common situations: Tag collision where a later single-arch push overwrote the index tag; verifying against a stale tag; registries that rewrite media types; user confusing image tag with index tag.

Related errors

• unexpected target image type - %s (%v)
• no image references for image index
• podPort cannot be empty
• file path is not absolute
• file is not a binary

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/98f29c4d480c4f1c. Report an issue: GitHub.