VectorMCP

VectorMCP is a Ruby implementation of the Model Context Protocol (MCP) server-side specification. It gives you a framework for exposing tools, resources, prompts, roots, sampling, middleware, and security over the MCP streamable HTTP transport.

Highlights

• Streamable HTTP is the built-in transport, with session management, resumability, and MCP 2025-11-25 compliance
• Class-based tools via VectorMCP::Tool, plus the original block-based register_tool API
• Rack and Rails mounting through server.rack_app
• Opt-in authentication and authorization, structured logging, and middleware hooks
• Image-aware tools/resources/prompts, roots, and server-initiated sampling
• Token-based field anonymization middleware to keep sensitive values out of LLM context

Requirements

• Ruby 3.2+

Installation

gem install vector_mcp

gem "vector_mcp"

Quick Start

require "vector_mcp"

class Greet < VectorMCP::Tool
  description "Say hello to someone"
  param :name, type: :string, desc: "Name to greet", required: true

  def call(args, _session)
    "Hello, #{args["name"]}!"
  end
end

server = VectorMCP::Server.new(name: "MyApp", version: "1.0.0")
server.register(Greet)
server.run(port: 8080)

The class-based DSL is optional. The existing block-based API still works:

server.register_tool(
  name: "echo",
  description: "Echo back the supplied text",
  input_schema: {
    type: "object",
    properties: { text: { type: "string" } },
    required: ["text"]
  }
) { |args| args["text"] }

Rack and Rails

VectorMCP can run as a standalone HTTP server or be mounted inside an existing Rack app:

require "vector_mcp"

server = VectorMCP::Server.new(name: "MyApp", version: "1.0.0")
server.register(Greet)

MCP_APP = server.rack_app

In Rails, mount it in config/routes.rb:

mount MCP_APP => "/mcp"

For ActiveRecord-backed tools, opt into VectorMCP::Rails::Tool:

require "vector_mcp/rails/tool"

class FindUser < VectorMCP::Rails::Tool
  description "Find a user by id"
  param :id, type: :integer, required: true

  def call(args, _session)
    user = find!(User, args[:id])
    { id: user.id, email: user.email }
  end
end

See docs/rails-setup-guide.md for a full setup guide.

Tools, Resources, and Prompts

Expose callable tools:

server.register_tool(
  name: "calculate",
  description: "Performs basic math",
  input_schema: {
    type: "object",
    properties: {
      operation: { type: "string", enum: ["add", "subtract", "multiply"] },
      a: { type: "number" },
      b: { type: "number" }
    },
    required: ["operation", "a", "b"]
  }
) do |args|
  case args["operation"]
  when "add" then args["a"] + args["b"]
  when "subtract" then args["a"] - args["b"]
  when "multiply" then args["a"] * args["b"]
  end
end

Expose readable resources:

server.register_resource(
  uri: "file://config.json",
  name: "App Configuration",
  description: "Current application settings"
) { File.read("config.json") }

Define prompt templates:

server.register_prompt(
  name: "code_review",
  description: "Reviews code for best practices",
  arguments: [
    { name: "language", description: "Programming language", required: true },
    { name: "code", description: "Code to review", required: true }
  ]
) do |args|
  {
    messages: [{
      role: "user",
      content: {
        type: "text",
        text: "Review this #{args["language"]} code:\n\n#{args["code"]}"
      }
    }]
  }
end

VectorMCP::Tool also supports type: :date and type: :datetime, which are validated as strings in JSON Schema and coerced to Date and Time before #call runs.

Security and Middleware

VectorMCP keeps security opt-in, but the primitives are built in:

server.enable_authentication!(
  strategy: :api_key,
  keys: ["your-secret-key"]
)

server.enable_authorization! do
  authorize_tools do |user, _action, tool|
    user[:role] == "admin" || !tool.name.start_with?("admin_")
  end
end

Custom authentication works too:

server.enable_authentication!(strategy: :custom) do |request|
  api_key = request[:headers]["X-API-Key"]
  user = User.find_by(api_key: api_key)
  user ? { user_id: user.id, role: user.role } : false
end

For MCP clients that speak OAuth 2.1 (e.g. Claude Desktop), pass a resource_metadata_url: to turn on RFC 9728 discovery. Unauthenticated requests to /mcp return 401 with a WWW-Authenticate header pointing at the configured metadata document, and the client drives the rest of the OAuth dance automatically. See docs/oauth_resource_server.md for the feature reference and docs/rails_oauth_integration.md for a full Rails + Doorkeeper recipe.

Middleware can hook into tool, resource, prompt, sampling, auth, and transport events, including before_auth, after_auth, on_auth_error, before_request, after_response, and on_transport_error.

See security/README.md for the full security guide.

Field Anonymization

Keep sensitive string values out of the LLM context by substituting them with stable opaque tokens. Values are tokenized on outbound tool results and restored on inbound tool arguments, so the LLM sees only tokens while your handlers receive the original data.

anonymizer = VectorMCP::Middleware::Anonymizer.new(
  store: VectorMCP::TokenStore.new,
  field_rules: [
    { pattern: /email/i, prefix: "EMAIL" },
    { pattern: /\bssn\b/i, prefix: "SSN" }
  ]
)
anonymizer.install_on(server)

Transport Notes

• VectorMCP ships with streamable HTTP as its built-in transport
• POST /mcp accepts a single JSON-RPC request, notification, or response; batch arrays are rejected
• GET /mcp opens an SSE stream for server-initiated messages
• DELETE /mcp terminates the session
• The server advertises MCP protocol 2025-11-25 and accepts 2025-03-26 and 2024-11-05 headers for compatibility
• Default allowed origins are restricted to localhost and loopback addresses

Initialize a session with curl:

curl -X POST http://localhost:8080/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"curl","version":"1.0"}}}'

More Features

• Roots via register_root and register_root_from_path
• Image resources and image-aware tools/prompts
• Structured logging with component loggers
• Server-initiated sampling with streaming/tool-call support
• Middleware-driven request shaping and observability

Documentation

• CHANGELOG.md
• examples/
• docs/rails-setup-guide.md
• docs/rails_oauth_integration.md
• docs/oauth_resource_server.md
• docs/streamable-http-spec-compliance.md
• security/README.md
• MCP Specification

Contributing

Bug reports and pull requests are welcome on GitHub.

License

Available as open source under the MIT License.