🔮 OAuth2::MCP

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I've summarized my thoughts in this blog post.

oauth2-mcp is a Ruby resource-server toolkit for securing HTTP Model Context Protocol servers with OAuth 2.1-style bearer authorization.

It builds on the oauth2 gem for the Ruby OAuth/OIDC substrate and adds the MCP resource-server pieces that generic OAuth clients do not provide.

The gem is intentionally focused on the MCP server side:

• protected-resource metadata for MCP endpoint discovery;
• WWW-Authenticate bearer challenges;
• scope and capability mapping for application policy layers;
• provider adapters for WorkOS/AuthKit and generic OIDC/JWKS validation;
• explicit audience/resource validation so tokens cannot be replayed against another MCP server.

Token passthrough is not a supported pattern. MCP servers should validate the incoming token for their own resource and issue separate downstream credentials when calling other services. Successful authorization results expose normalized claims and mapped capabilities, not the raw bearer token.

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Compatible with MRI Ruby 3.2.0+, and concordant releases of JRuby, and TruffleRuby. CI workflows and Appraisals are generated for MRI Ruby 3.2.0+. This test floor is configured by ruby.test_minimum in .kettle-jem.yml and may be higher than the gem's runtime compatibility floor when legacy Rubies are not practical for the current toolchain.

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Alternatively:

Install the gem and add to the application's Gemfile by executing:

bundle add oauth2-mcp

If bundler is not being used to manage dependencies, install the gem by executing:

gem install oauth2-mcp

oauth2-mcp does not use global configuration. Build the objects that match the OAuth provider and MCP endpoint you are protecting, then pass those objects to a resource server or Rack middleware.

The core resource metadata is the MCP resource URI, the authorization server issuer(s), and the scopes your endpoint understands:

metadata = OAuth2::MCP::ProtectedResourceMetadata.new(
  resource: "https://brain.example.com/mcp",
  authorization_servers: ["https://example.authkit.app"],
  scopes_supported: %w[memory.read memory.write],
)

Token validation is supplied by one of the validator adapters:

• OAuth2::MCP::JWTValidator for JWT bearer tokens when you already have JWKS.
• OAuth2::MCP::OIDCDiscovery when the authorization server exposes OIDC discovery and JWKS metadata.
• OAuth2::MCP::WorkOSAuthKit for WorkOS AuthKit resource-server JWTs.
• OAuth2::MCP::IntrospectionValidator for opaque tokens validated through an OAuth token introspection endpoint.

Map provider scopes to application capabilities with ScopeMapper when the MCP server needs internal permission names:

scope_mapper = OAuth2::MCP::ScopeMapper.new(
  mapping: {
    "memory.read" => "documents_read",
    "memory.write" => "documents_write",
  },
)

Finally, configure the resource server with the metadata URL you publish to clients. That URL is emitted in WWW-Authenticate challenges so clients can discover how to obtain an acceptable token.

resource_server = OAuth2::MCP::ResourceServer.new(
  resource_metadata: metadata,
  resource_metadata_url: "https://brain.example.com/.well-known/oauth-protected-resource/mcp",
  validator: validator,
  scope_mapper: scope_mapper,
)

Build protected-resource metadata for an MCP endpoint:

metadata = OAuth2::MCP::ProtectedResourceMetadata.new(
  resource: "https://brain.example.com/mcp",
  authorization_servers: ["https://example.authkit.app"],
  scopes_supported: %w[memory.read memory.write],
)

metadata.to_json

Return a bearer challenge when authorization is missing or insufficient:

challenge = OAuth2::MCP::BearerChallenge.new(
  resource_metadata: "https://brain.example.com/.well-known/oauth-protected-resource/mcp",
  scope: %w[memory.read],
  error: "insufficient_scope",
  error_description: "memory.read is required",
)

headers["WWW-Authenticate"] = challenge.to_header

Authorize an MCP HTTP request with a provider-specific token validator:

metadata = OAuth2::MCP::ProtectedResourceMetadata.new(
  resource: "https://brain.example.com/mcp",
  authorization_servers: ["https://example.authkit.app"],
  scopes_supported: %w[memory.read memory.write],
)

validator = OAuth2::MCP::JWTValidator.new(
  jwks: {"keys" => provider_jwks},
  issuer: "https://example.authkit.app",
  audience: "https://brain.example.com/mcp",
)

scope_mapper = OAuth2::MCP::ScopeMapper.new(
  mapping: {
    "memory.read" => "documents_read",
    "memory.write" => "documents_write",
  },
)

resource_server = OAuth2::MCP::ResourceServer.new(
  resource_metadata: metadata,
  resource_metadata_url: "https://brain.example.com/.well-known/oauth-protected-resource/mcp",
  validator: validator,
  scope_mapper: scope_mapper,
)

result = resource_server.authorize(
  request: rack_env,
  scopes: ["memory.read"],
)

halt result.status, result.headers, "" unless result.allowed?

Protect a Rack-compatible MCP HTTP endpoint:

use OAuth2::MCP::RackMiddleware,
  resource_server: resource_server,
  scopes: ["memory.read"]

Or build the validator from OIDC discovery:

validator = OAuth2::MCP::OIDCDiscovery.new(
  issuer: "https://example.authkit.app",
).jwt_validator(
  audience: "https://brain.example.com/mcp",
)

For WorkOS AuthKit, use the provider adapter. It validates against the AuthKit issuer and /oauth2/jwks endpoint:

validator = OAuth2::MCP::WorkOSAuthKit.new(
  subdomain: "acme",
  audience: "https://brain.example.com/mcp",
)

For opaque access tokens, use OAuth token introspection with an OAuth2::Client configured for the authorization server:

validator = OAuth2::MCP::IntrospectionValidator.new(
  client: oauth_client,
  introspection_url: "https://auth.example.com/oauth2/introspection",
  audience: "https://brain.example.com/mcp",
  issuer: "https://auth.example.com",
)

While ruby-oauth tools are free software and will always be, the project would benefit immensely from some funding. Raising a monthly budget of... "dollars" would make the project more sustainable.

We welcome both individual and corporate sponsors! We also offer a wide array of funding channels to account for your preferences. Currently, GitHub Sponsors, and Liberapay are our preferred funding platforms.

If you're working in a company that's making significant use of ruby-oauth tools we'd appreciate it if you suggest to your company to become a ruby-oauth sponsor.

You can support the development of ruby-oauth tools via GitHub Sponsors, Liberapay, PayPal, and Tidelift.

I’m driven by a passion to foster a thriving open-source community – a space where people can tackle complex problems, no matter how small. Revitalizing libraries that have fallen into disrepair, and building new libraries focused on solving real-world challenges, are my passions. I was recently affected by layoffs, and the tech jobs market is unwelcoming. I’m reaching out here because your support would significantly aid my efforts to provide for my family, and my farm (11 🐔 chickens, 2 🐶 dogs, 3 🐰 rabbits, 8 🐈‍ cats).

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See SECURITY.md.

If you need some ideas of where to help, you could work on adding more code coverage, or if it is already 💯 (see below) check issues or PRs, or use the gem and think about how it could be better.

We so if you make changes, remember to update it.

See CONTRIBUTING.md for more detailed instructions.

See CONTRIBUTING.md.

Everyone interacting with this project's codebases, issue trackers, chat rooms and mailing lists agrees to follow the .

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Also see GitLab Contributors: https://gitlab.com/ruby-oauth/oauth2-mcp/-/graphs/main

This library follows for its public API where practical. For most applications, prefer the Pessimistic Version Constraint with two digits of precision.

For example:

spec.add_dependency("oauth2-mcp", "~> 0.0")

Dropping support for a platform can be a breaking change for affected users. If a release changes supported platforms, it should be called out clearly in the changelog and versioned with that impact in mind.

To get a better understanding of how SemVer is intended to work over a project's lifetime, read this article from the creator of SemVer:

• "Major Version Numbers are Not Sacred"

See CHANGELOG.md for a list of releases.

The gem is available as open source under the terms of the MIT .

See LICENSE.md for the official copyright notice.

• Copyright (c) 2026 Peter H. Boling

Maintainers have teeth and need to pay their dentists. After getting laid off in an RIF in March, and encountering difficulty finding a new one, I began spending most of my time building open source tools. I'm hoping to be able to pay for my kids' health insurance this month, so if you value the work I am doing, I need your support. Please consider sponsoring me or the project.

To join the community or get help 👇️ Join the Discord.

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Thanks for RTFM. ☺️