dockertag

An image reference looks pinned until you read it closely. repo with no tag at all silently means repo:latest; latest itself moves; a capital letter in the repository breaks pushes to registries that require lowercase; and a published image can be missing the labels that say where its source lives, what license it carries, and — for MCP servers — the server-name label the MCP Registry checks before it will list the image. dockertag takes the middle road: you hand it the references and the labels you found, and it hands back an audit.

Every reference is parsed the same careful way. The digest is everything after the last @. A tag exists only when the final path segment contains a : — so the colon in localhost:5000 is a port, never a tag — and the tag is the text after the last : in that segment. The registry is the first segment when it contains a . or a : or is exactly localhost; otherwise the registry is docker.io and the whole path is the repository.

Against the parsed reference and the supplied labels, nine checks run in a fixed order:

• INVALID_REFERENCE (error) — the name part is empty, the repository is empty, or a present tag is empty. No further checks run for that image and its parsed fields are null.

• INVALID_DIGEST (error) — a digest that is not sha256: followed by 64 lowercase hex characters.

• UPPERCASE_REPOSITORY (error) — an uppercase letter anywhere in the repository.

• UNPINNED (error) — no tag and no digest, so the reference resolves to the mutable latest tag.

• LATEST_TAG (warning) — the tag is exactly latest and no digest pins it.

• MISSING_SOURCE_LABEL (warning) — no org.opencontainers.image.source label.

• MISSING_LICENSE_LABEL (warning) — no org.opencontainers.image.licenses label.

• MISSING_SERVER_NAME_LABEL (error) — only checked when you supply expected_server_name: the io.modelcontextprotocol.server.name label is absent.

• SERVER_NAME_MISMATCH (error) — that label is present but differs from the expected value.

The result carries each image's parsed registry, repository, tag, and digest with its finding codes, the flat list of findings (by image input order, then check order) with exact messages, and counts of images, errors, warnings, and clean images.

dockertag never contacts a registry and never pulls an image: the labels are the ones you supply, which makes the audit usable in CI before anything is pushed. Malformed input — a missing or empty image list, a missing, empty, or duplicated reference, labels that are not a string-to-string map, a bad expected_server_name — is a validation error naming the offending path (for example images[1].labels.custom must be a string), and no partial audit is produced.

CLI

npx @reneturcios/dockertag audit --input args.json
npx @reneturcios/dockertag --version

args.json contains the full arguments object:

{
  "expected_server_name": "io.github.CloudCorpRecords/dockertag",
  "images": [
    {
      "reference": "ghcr.io/cloudcorprecords/dockertag:0.1.0",
      "labels": {
        "org.opencontainers.image.source": "https://github.com/CloudCorpRecords/dockertag",
        "org.opencontainers.image.licenses": "MIT",
        "io.modelcontextprotocol.server.name": "io.github.CloudCorpRecords/dockertag"
      }
    },
    { "reference": "myteam/worker" }
  ]
}

The audit is printed as JSON:

{
  "images": [
    {
      "reference": "ghcr.io/cloudcorprecords/dockertag:0.1.0",
      "registry": "ghcr.io",
      "repository": "cloudcorprecords/dockertag",
      "tag": "0.1.0",
      "digest": null,
      "findings": []
    },
    {
      "reference": "myteam/worker",
      "registry": "docker.io",
      "repository": "myteam/worker",
      "tag": null,
      "digest": null,
      "findings": ["UNPINNED", "MISSING_SOURCE_LABEL", "MISSING_LICENSE_LABEL", "MISSING_SERVER_NAME_LABEL"]
    }
  ],
  "findings": [
    {
      "reference": "myteam/worker",
      "code": "UNPINNED",
      "severity": "error",
      "message": "no tag or digest: the reference resolves to the mutable latest tag"
    }
  ],
  "counts": { "images": 2, "errors": 2, "warnings": 2, "clean": 1 }
}

(The flat findings list above is abbreviated; it holds one entry per finding.) Exit codes: 0 on success, 1 on a runtime error (including an unreadable or invalid input file, or invalid input), 2 on a usage error.

Related MCP server: toolfence

MCP server

npx -y @reneturcios/dockertag mcp

MCP client config:

{
  "mcpServers": {
    "dockertag": {
      "command": "npx",
      "args": ["-y", "@reneturcios/dockertag", "mcp"]
    }
  }
}

Tools

• audit_image_tags — Audit container image references and OCI labels: tag pinning, lowercase repositories, source and license labels, and the MCP Registry server-name label.

  • images (array, required, at least one) — images as {reference, labels?} objects; reference is the full image reference as written and is unique across the set, and labels is the image's OCI labels as a string-to-string map

  • expected_server_name (string, optional, non-empty) — the expected value of the io.modelcontextprotocol.server.name label; the two server-name checks run only when this is supplied

  Returns {images, findings, counts}.

Docker

docker run -i ghcr.io/cloudcorprecords/dockertag
docker run --rm -v "$PWD:/w" ghcr.io/cloudcorprecords/dockertag audit --input /w/args.json

Development

npm install
npm run build   # tsc -> dist/
npm test        # tsx --test test/*.test.ts (pure local, no network)
npm run dev     # run the CLI from source

License

MIT — Copyright (c) 2026 Rene Turcios