Anthropic released code-modernization v1.0.0, a Claude Code plugin for modernizing legacy codebases

Save this post if you have a legacy project worth modernizing

What I find interesting is that it doesn’t just rewrite code. It tries to prove the new implementation behaves like the original

Here’s how:

• Analyzes the legacy system and extracts business rules

• Generates a modernization plan for your approval

• Rewrites or upgrades the code, even across languages

• Runs the old and new implementations with identical inputs and compares outputs

• Generates additional edge cases to test behavior

• Introduces deliberate bugs to verify tests actually catch them

• Produces a verification report with PROVEN, PARTLY PROVEN, or NOT PROVEN results

It also keeps the original code untouched and requires human approval at critical steps

I think this is the right direction for AI-assisted modernization, generating new code is one thing

Proving that you haven’t broken existing behavior is a completely different challenge

Install: /plugin install code-modernization@claude-plugins-official

Repo:

anthropics/claude-plugins-official

Source: https://github.com/anthropics/claude-plugins-official

Claude Code Plugins Directory

A curated directory of high-quality plugins for Claude Code.

⚠️ Important: Make sure you trust a plugin before installing, updating, or using it. Anthropic does not control what MCP servers, files, or other software are included in plugins and cannot verify that they will work as intended or that they won’t change. See each plugin’s homepage for more information.

Structure

• /plugins - Internal plugins developed and maintained by Anthropic
• /external_plugins - Third-party plugins from partners and the community

Installation

Plugins can be installed directly from this marketplace via Claude Code’s plugin system.

To install, run /plugin install {plugin-name}@claude-plugins-official

or browse for the plugin in /plugin > Discover

Contributing

Internal Plugins

Internal plugins are developed by Anthropic team members. See /plugins/example-plugin for a reference implementation.

External Plugins

Third-party partners can submit plugins for inclusion in the marketplace. External plugins must meet quality and security standards for approval. To submit a new plugin, use the plugin directory submission form.

Plugin Structure

Each plugin follows a standard structure:

plugin-name/
├── .claude-plugin/
│   └── plugin.json      # Plugin metadata (required)
├── .mcp.json            # MCP server configuration (optional)
├── commands/            # Slash commands (optional)
├── agents/              # Agent definitions (optional)
├── skills/              # Skill definitions (optional)
└── README.md            # Documentation

Plugin names are immutable

The name field in a marketplace entry is an immutable slug. Once a plugin has been published, its name must not change — users have it installed under that slug, and renaming it breaks their install with a plugin-not-found error.

• To change how a plugin is labeled in the UI, set or update displayName instead.
• If a rename is genuinely unavoidable, add an entry to the top-level renames map in .claude-plugin/marketplace.json so existing installs auto-migrate:

"renames": {
  "old-name": "new-name"
}

The Claude Code plugin loader reads this map and transparently rewrites the old slug to the new one on the user’s next sync.

Skill-bundle plugins

When a plugin’s source repository ships skills (SKILL.md files) without a .claude-plugin/plugin.json manifest, the marketplace entry can declare the skills directly using strict: false and an explicit skills array.

{
  "name": "example-bundle",
  "description": "Brief description of the bundled skills.",
  "author": { "name": "Author Name" },
  "category": "development",
  "source": {
    "source": "git-subdir",
    "url": "https://github.com/example-org/sdk.git",
    "path": "packages/agent-skills",
    "ref": "main",
    "sha": "<commit sha>"
  },
  "strict": false,
  "skills": [
    "./skill-a",
    "./skill-b",
    "./skill-c"
  ],
  "homepage": "https://github.com/example-org/sdk"
}

Each path in skills is relative to source.path and points at a directory containing a SKILL.md. Paths can reach deeper than a single level — for example, ["./libA/skill-1", "./libB/skill-2"] exposes a curated subset across multiple library subdirectories. Each skill is registered as <plugin-name>:<skill-name> in Claude Code.

For the underlying schema, see Strict mode in the marketplace documentation.

License

Please see each linked plugin for the relevant LICENSE file.

Documentation

For more information on developing Claude Code plugins, see the official documentation.