gemcp

MCP tools for RubyGems package intelligence. Ask about gems, versions, dependencies, and ownership. Get structured, provenance-tagged answers an agent can trust.

gem install gemcp

Add to your MCP client (Claude Desktop, Kiro, etc.):

{
  "mcpServers": {
    "gemcp": {
      "command": "gemcp"
    }
  }
}

That's it. If your Ruby isn't on the default PATH, use the full path:

{
  "mcpServers": {
    "gemcp": {
      "command": "/path/to/gemcp"
    }
  }
}

Find it with gem which gemcp && which gemcp.

gemcp reads from installed gemspecs first, a persistent disk cache second, and the RubyGems.org API only when neither has what you need.

These are the kinds of questions gemcp helps an agent answer with authoritative data:

• "Is sidekiq still actively maintained? Who owns it on RubyGems?"
• "What Ruby version does nokogiri 1.18.8 require?"
• "Can authlogic 3.6 coexist with Rails 8?"
• "What changed in authlogic's dependencies between version 3.6.1 and 6.6.0?"
• "What gems depend on rack?"
• "Show me every release of rails between 7.0.0 and 8.0.0"
• "Compare puma and unicorn — versions, owners, dependency counts"
• "What are the runtime dependencies of rails 8.1?"
• "Build me a dependency graph for sidekiq at depth 2"
• "Find me gems for background job processing"

installed gemspecs
      ↓ miss
fresh disk cache
      ↓ stale/miss
conditional RubyGems.org request (ETag / If-Modified-Since)
      ↓
refresh cache

Local results include metadata_source: "local". On network failure with a stale cache entry, the stale data is served rather than raising.

Search is conservative: when installed gems satisfy a query, no network call is made.

You can pass these through the MCP config:

{
  "mcpServers": {
    "gemcp": {
      "command": "gemcp",
      "env": {
        "GEMCP_LOCAL_FIRST": "true",
        "GEMCP_CACHE_TTL": "7200"
      }
    }
  }
}

Tools receive a Client via constructor injection. Each tool wraps its operation in dry-monads (Success/Failure) and renders the result to MCP response format. No global state.

Tool.new(client:) → with_errors { client.do_thing } → Success(data) / Failure(error:, message:) → render → MCP response

The cache uses atomic temp-file + rename writes. The rate limiter uses a Mutex to serialize sleeps between outbound requests.

get_dependency_graph is exploratory, not authoritative. It follows declared dependencies using the latest release of each, preserving requirement constraints on edges. It does not consider lockfiles, Ruby versions, platforms, or source configuration. It is not a substitute for Bundler.

check_compatibility tests whether two gems' declared dependency constraints overlap by sampling candidate versions against both Gem::Requirement objects. It catches common conflicts (e.g., < 5.0 vs = 8.0.0) but is not a full constraint solver. It does not resolve transitive dependencies.

Registry facts (versions, checksums, dependencies, ownership, yanked state) are structured and authoritative. README, changelog, and documentation content is untrusted prose and never overrides registry facts.

• fast-mcp — MCP server framework (instance-based tools, Rack transport)
• dry-monads — Success/Failure result types for tool error handling
• rack — HTTP transport

git clone https://github.com/baweaver/gemcp.git
cd gemcp
bundle install
bundle exec rake

MIT