slimtoolkit/slim · error

unexpected target image type - %s (%v)

Error message

unexpected target image type - %s (%v)

What it means

After fetching metadata, the handler expects each referenced object to be a plain image (meta.MediaType.IsImage()). If the descriptor is some other type — e.g. an index/manifest list or an unknown artifact — it fails with this error naming the image and its media type.

Source

Thrown at pkg/app/master/command/registry/handler_image_index.go:147

			basicImageInfo(xc, imgMeta)

			imgConfig, err := imgMeta.ConfigFile()
			if err != nil {
				xc.FailOn(err)
			}
			imgRefMeta, err := partial.Descriptor(imgMeta)
			if err != nil {
				xc.FailOn(err)
			}
			imgRefMeta.Platform = imgConfig.Platform()
			indexImageImgRefs = append(indexImageImgRefs,
				mutate.IndexAddendum{
					Add:        imgMeta,
					Descriptor: *imgRefMeta,
				})
		} else {
			xc.FailOn(fmt.Errorf("unexpected target image type - %s (%v)", imageName, meta.MediaType))
		}
	}

	if cparams.AsManifestList {
		imageIndex = mutate.IndexMediaType(imageIndex, types.DockerManifestList)
	}

	imageIndex = mutate.AppendManifests(imageIndex, indexImageImgRefs...)

	if err := remote.WriteIndex(imageIndexRef, imageIndex, remoteOpts...); err != nil {
		var terr *transport.Error
		if errors.As(err, &terr) && terr.StatusCode == http.StatusUnauthorized {
			xc.Out.Info("registry.auth.error",
				ovars{
					"message": "need to authenticate",
				})

			exitCode := -111

View on GitHub (pinned to 81940d17fa)

Solutions

1. Reference individual single-platform image tags, not existing manifest lists
2. If you need to nest, flatten the existing index into its child image digests first
3. Check the media type with 'docker manifest inspect <ref>' before adding it
4. Skip or handle non-image artifacts separately

Example fix

// before
ImageNames: []string{"registry.example.com/app:v1"} // v1 is already a manifest list
// after
ImageNames: []string{"registry.example.com/app:v1-amd64", "registry.example.com/app:v1-arm64"}

desc, err := remote.Head(ref)
if err == nil && !desc.MediaType.IsImage() {
    return fmt.Errorf("%s is %s, expected an image manifest", ref.Name(), desc.MediaType)
}

func isPlainImage(mtypes types.MediaType) bool {
    return mtypes == types.DockerManifestSchema2 || mtypes == types.OCIManifestSchema1
}

When it happens

Trigger: Passing an already-multi-arch manifest list or an OCI index (or a non-image artifact like an attestation blob) as one of the ImageNames entries; media type not recognized as an image manifest.

Common situations: Nesting indexes (index-of-index) which this code path doesn't support; referencing sboms/signatures/attestation manifests by mistake; registry returning unusual media types for older Docker schema images.

Related errors

• unexpected media type for index
• 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/d120b24ff538a95d. Report an issue: GitHub.