What causes "unmarshal host image index: %w" in dagger/dagger?

Remove the corrupted image from the local store and re-pull it (dag.Container().from forces a fresh fetch).

unmarshal host image index: %w

Error message

unmarshal host image index: %w

What it means

After successfully reading the host image index blob, the code unmarshals it into an OCI Index struct. If the blob is not valid JSON or not a well-formed OCI index, json.Unmarshal fails and the error is wrapped with this prefix.

Source

Thrown at core/schema/host.go:729

	switch desc.MediaType {
	case ocispec.MediaTypeImageManifest, images.MediaTypeDockerSchema2Manifest:
		if _, err := store.Info(ctx, desc.Digest); err != nil {
			if cerrdefs.IsNotFound(err) {
				return nil, true, nil
			}
			return nil, false, fmt.Errorf("stat host manifest %s: %w", desc.Digest, err)
		}
		return &desc, true, nil

	case ocispec.MediaTypeImageIndex, images.MediaTypeDockerSchema2ManifestList:
		indexBlob, err := content.ReadBlob(ctx, store, desc)
		if err != nil {
			return nil, false, fmt.Errorf("read host image index blob: %w", err)
		}

		var idx ocispec.Index
		if err := json.Unmarshal(indexBlob, &idx); err != nil {
			return nil, false, fmt.Errorf("unmarshal host image index: %w", err)
		}

		matched := false
		for _, manifest := range idx.Manifests {
			switch manifest.MediaType {
			case ocispec.MediaTypeImageManifest, images.MediaTypeDockerSchema2Manifest,
				ocispec.MediaTypeImageIndex, images.MediaTypeDockerSchema2ManifestList:
			default:
				continue
			}
			if manifest.Platform != nil && !matcher.Match(*manifest.Platform) {
				continue
			}
			target, childMatched, err := resolveHostStoreManifestMatch(ctx, store, manifest, matcher)
			if err != nil {
				return nil, false, err
			}
			matched = matched || childMatched

View on GitHub (pinned to 82ba2681db)

Solutions

Remove the corrupted image from the local store and re-pull it (dag.Container().from forces a fresh fetch).
Verify the blob integrity (crane/digest check) if you suspect a corrupted engine cache; restart the engine afterwards.
If a mirror/proxy serves the blob, check it for corruption or truncation.

Defensive patterns

Strategy: retry

Try / catch

try { img = await dag.host().containerImage(name) } catch (e) { if (String(e).includes("unmarshal host image index")) { /* delete cached image and re-pull */ } throw e }

Prevention

Watch for disk corruption; run fs checks on the engine's data volume.
Avoid third-party tools writing into the Dagger content store.
Verify registry mirrors serve untruncated blobs.

When it happens

Trigger: Calling Host.containerImage where the cached index blob in the host content store is corrupted, truncated, or not actually an OCI image index despite its media type.

Common situations: Disk corruption or interrupted writes to the content store; blobs produced/tampered with by third-party tools; unusual media types with non-index payloads.

Related errors

AI-assisted analysis of dagger/dagger@82ba2681db (2026-09-05). Data as JSON: /api/errors/ffd30e02740bf8e6. Report an issue: GitHub.