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@ -26,4 +26,4 @@ jobs:
- uses: actions/setup-node@v7 - uses: actions/setup-node@v7
with: with:
node-version: '24' # Specify the latest Node.js version. node-version: '24' # Specify the latest Node.js version.
- run: bundle exec rake conformance:test - run: bundle exec rake conformance

4
.gitignore vendored
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@ -9,7 +9,3 @@
/spec/reports/ /spec/reports/
/tmp/ /tmp/
Gemfile.lock Gemfile.lock
docs/_site/
docs/.jekyll-cache/
docs/.jekyll-metadata
docs/_data/versions.yml

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@ -15,9 +15,7 @@ This is the official Ruby SDK for the Model Context Protocol (MCP), implementing
- `bundle install` - Install dependencies - `bundle install` - Install dependencies
- `rake test` - Run all tests - `rake test` - Run all tests
- `rake rubocop` - Run linter - `rake rubocop` - Run linter
- `rake` - Run linting, tests, and the conformance suite (default task) - `rake` - Run tests and linting (default task)
- `bundle exec rake conformance:test` - Run the MCP conformance suite (see conformance/README.md)
- `bundle exec rake docs:preview` - Serve the documentation site locally at http://localhost:4000 (PORT to override)
- `ruby -I lib -I test test/path/to/specific_test.rb` - Run single test file - `ruby -I lib -I test test/path/to/specific_test.rb` - Run single test file
- `gem build mcp.gemspec` - Build the gem - `gem build mcp.gemspec` - Build the gem
@ -36,14 +34,6 @@ This is the official Ruby SDK for the Model Context Protocol (MCP), implementing
- Keep dependencies minimal - Keep dependencies minimal
- Use lowercase HTTP response header names (e.g. `mcp-session-id`); the Rack 3 SPEC requires this, and the MCP spec's `Mcp-Session-Id` casing is prose convention only - Use lowercase HTTP response header names (e.g. `mcp-session-id`); the Rack 3 SPEC requires this, and the MCP spec's `Mcp-Session-Id` casing is prose convention only
## Documentation
- User-facing documentation lives in `docs/`, one page per topic, published at https://ruby.sdk.modelcontextprotocol.io (deploys only when a release is published)
- Pages live in the `docs/_server/`, `docs/_client/`, and `docs/_extensions/` collections
- Keep README.md slim: quick start and pointers only; document features on the relevant docs page
- Internal links are absolute and extensionless (e.g. `/server/tools/`); front matter is followed by a blank line before the h1
- Callout tiers: `.note` (supplementary), `.important` (spec constraints), `.warning` (deprecated features)
## Commit message conventions ## Commit message conventions
- Use conventional commit format when possible - Use conventional commit format when possible

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@ -7,71 +7,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
## [Unreleased] ## [Unreleased]
## [1.5.1] - 2026-09-09
This release keeps the HTTP client working with json 3.0. That release accepts the options of `JSON.parse`
as keywords only, and Faraday's JSON response middleware, through 2.14.3, passes them as a positional Hash,
which Ruby 3 no longer converts, so every JSON body the client received failed as an internal error.
The client now parses response bodies itself, the way it already handled streamed ones, and no longer
registers the middleware. A malformed JSON body delivered whole by an adapter without streaming support
now raises `RequestHandlerError` with `error_type: :parse_error`, as the streamed path already did.
### Fixed
- Parse HTTP client response bodies without Faraday's JSON middleware, which json 3.0 breaks (#546)
## [1.5.0] - 2026-09-05
This release makes the client answer a server's `ping` with the empty result the specification requires.
Earlier clients replied with Method not found over Streamable HTTP and stayed silent over stdio,
so a server that checks liveness dropped their long-lived sessions. On the OAuth side,
the embedding application can refuse an authorization request through `authorization_request_validator`,
and stored tokens are refreshed only against the authorization server that issued them:
when the server named for a session differs, the client reauthorizes instead of refreshing,
and the validator sees the newly named server. Tokens stored by earlier releases carry no issuer and keep refreshing.
### Added
- Add `authorization_request_validator` to let the embedding application refuse an authorization request (#539)
### Fixed
- Answer server-to-client pings in the client (#541)
## [1.4.0] - 2026-08-28
This release repairs the `subscriptions/listen` stream. A host that buffers its responses,
the documented Rails controller pattern among them, can decline the method with
`serve_subscriptions_listen: false` instead of answering a modern client with a streaming body
it cannot render, and the stream's ordering guarantees are enforced: no notification reaches
a client before the acknowledgement, and none follows the graceful closing result.
### Added
- Add `serve_subscriptions_listen:` for hosts that cannot hold an SSE stream open (#533)
### Fixed
- Deliver `subscriptions/listen` notifications only after the acknowledgement (#532)
- Serialize `subscriptions/listen` writes so the graceful result is the final message (#535)
## [1.3.0] - 2026-08-22
User-facing documentation now lives on the documentation site at https://ruby.sdk.modelcontextprotocol.io,
and README.md keeps the quick start. Two entries under "Changed" reject traffic that earlier releases accepted
and ship in a minor release under the exceptions described in [VERSIONING.md](VERSIONING.md).
### Added
- Add a `resources_list_handler` for context-dependent resource lists (#509)
- Pass a handler-returned `_meta` through the subscribe result (#510)
### Changed
- Bound OAuth response bodies in the client (#520)
- Reject duplicate in-flight JSON-RPC request ids (#521)
- Move the documentation from README.md to the documentation site (#523)
## [1.2.0] - 2026-08-15 ## [1.2.0] - 2026-08-15
This release completes the SEP-2575 stateless lifecycle of the 2026-07-28 specification, together with This release completes the SEP-2575 stateless lifecycle of the 2026-07-28 specification, together with

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@ -5,29 +5,21 @@ It is a living document and will evolve as the SDK matures.
## Current Status ## Current Status
The SDK implements the 2025-06-18, 2025-11-25, and 2026-07-28 spec revisions, and passes all scored server The SDK implements the 2025-06-18 and 2025-11-25 spec revisions, and passes all server and client conformance scenarios.
and client conformance scenarios for each revision's requirement set.
Optional protocol extensions (such as DPoP, workload identity federation, and the tasks extension)
are not yet implemented; SEP-1730 does not require extensions for any tier.
## API Stability ## API Stability
The 1.0.0 release marked the public API as stable: breaking changes ship only in major releases, The 1.0.0 release will mark the public API as stable: breaking changes then ship only in major releases,
apart from the narrow exceptions documented in [VERSIONING.md](VERSIONING.md), apart from the narrow exceptions documented in [VERSIONING.md](VERSIONING.md),
which also defines the Semantic Versioning scheme and breaking-change policy. which also defines the Semantic Versioning scheme and breaking-change policy.
## Deprecated Features ## Deprecated Features
The 2026-07-28 spec revision deprecates Roots, Sampling, and Logging (SEP-2577). The 2026-07-28 spec revision deprecates Roots, Sampling, and Logging (SEP-2577).
These features remain supported throughout 1.x: on the legacy lifecycle they work unchanged These features remain fully supported throughout 1.x, and deprecation warnings will be added in a future minor release.
(the deprecations do not apply there), and the 2026-07-28 lifecycle carries them through its own mechanisms: Under Semantic Versioning their removal requires a major release, but no removal is scheduled yet:
roots-shaped and sampling-shaped input requests embedded in `InputRequiredResult` responses whether 2.0.0 removes them depends on how future MCP spec revisions treat these features
(the SEP-2322 multi round-trip requests pattern) and per-request log delivery through the envelope's `logLevel`. and on adoption of their replacements.
The replacements SEP-2577 names live outside the protocol: tool parameters or configuration for roots,
direct LLM provider APIs for sampling, and stderr or OpenTelemetry for logging.
The client emits a deprecation warning when a modern connect declares the deprecated Roots or Sampling capabilities.
Under Semantic Versioning their removal requires a major release, but no removal is scheduled yet: whether 2.0.0 removes
them depends on how future MCP spec revisions treat these features and on adoption of their replacements.
## Conformance ## Conformance
@ -36,16 +28,12 @@ Legacy SSE transport (2024-11-05) is intentionally out of scope; the SDK provide
## Documentation ## Documentation
Reference documentation covers all non-experimental features with examples. Reference documentation covers all core features.
## Tracking New Spec Revisions ## Tracking New Spec Revisions
The SDK aims to support each new MCP specification revision, with the implementation timeline agreed per The SDK aims to support each new MCP specification revision, with the implementation timeline agreed per
release based on feature complexity. release based on feature complexity.
The 2026-07-28 revision shipped during 1.x: the server serves both lifecycle eras side by side, Additive features from the 2026-07-28 revision (such as `server/discover`, multi round-trip requests,
so the SEP-2575 stateless lifecycle (including `server/discover` and `subscriptions/listen`), and the tasks extension) ship as opt-in functionality during 1.x. Breaking parts of that revision,
SEP-2322 multi round-trip requests, SEP-2549 cache hints, and SEP-2243 custom headers are available alongside such as the SEP-2575 stateless lifecycle rewrite, are reserved for 2.0.
the legacy handshake. The few incompatible adjustments the revision required shipped in
a minor release under the spec-conformance and security exceptions described in [VERSIONING.md](VERSIONING.md).
Removing legacy-era support is reserved for a future major release.
The tasks extension (SEP-2663) remains planned ([#391](https://github.com/modelcontextprotocol/ruby-sdk/issues/391)).

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@ -14,11 +14,10 @@ require "rubocop/rake_task"
RuboCop::RakeTask.new RuboCop::RakeTask.new
task default: [:rubocop, :test, "conformance:test"] task default: [:rubocop, :test, :conformance]
namespace :conformance do desc "Run MCP conformance tests (PORT, SCENARIO, SPEC_VERSION, VERBOSE)"
desc "Run MCP conformance tests (PORT, SCENARIO, SPEC_VERSION, VERBOSE)" task :conformance do |t|
task :test do |t|
next unless npx_available?(t.name) next unless npx_available?(t.name)
require_relative "conformance/server_runner" require_relative "conformance/server_runner"
@ -32,67 +31,24 @@ namespace :conformance do
Conformance::ServerRunner.new(**options).run Conformance::ServerRunner.new(**options).run
Conformance::ClientRunner.new(**options.reject { |key, _value| key == :port }).run Conformance::ClientRunner.new(**options.reject { |key, _value| key == :port }).run
end end
desc "List available conformance scenarios" desc "List available conformance scenarios"
task :list do |t| task :conformance_list do |t|
next unless npx_available?(t.name) next unless npx_available?(t.name)
system("npx", "--yes", "@modelcontextprotocol/conformance", "list", "--server") system("npx", "--yes", "@modelcontextprotocol/conformance", "list", "--server")
system("npx", "--yes", "@modelcontextprotocol/conformance", "list", "--client") system("npx", "--yes", "@modelcontextprotocol/conformance", "list", "--client")
end end
desc "Start the conformance server (PORT)" desc "Start the conformance server (PORT)"
task :server do task :conformance_server do
require_relative "conformance/server" require_relative "conformance/server"
options = {} options = {}
options[:port] = Integer(ENV["PORT"]) if ENV["PORT"] options[:port] = Integer(ENV["PORT"]) if ENV["PORT"]
Conformance::Server.new(**options).start Conformance::Server.new(**options).start
end
end
# The other two former names now fail with a "Did you mean?" suggestion, but a bare `conformance`
# would match the `conformance/` directory: Rake synthesizes a file task for it and exits 0 without
# running anything. This keeps the name pointing at the suite instead of at that silent no-op.
desc "Alias for conformance:test"
task conformance: "conformance:test"
namespace :docs do
desc "Serve the documentation site locally at http://localhost:4000 (PORT)"
task :preview do
docs_dir = File.expand_path("docs", __dir__)
generate_docs_versions_data(docs_dir)
env = {
"BUNDLE_GEMFILE" => File.join(docs_dir, "Gemfile"),
"RUBYOPT" => "-r#{File.join(docs_dir, "_preview", "taint_shim.rb")}",
}
port = ENV.fetch("PORT", "4000")
Bundler.with_unbundled_env do
system(env, "bundle", "install", "--quiet", chdir: docs_dir, exception: true)
system(env, "bundle", "exec", "jekyll", "serve", "--port", port, chdir: docs_dir, exception: true)
rescue Interrupt
# Ctrl-C is the way to stop the preview, not an error.
end
end
end
# Mirrors bin/generate-gh-pages.sh: the released site receives `_data/versions.yml` from
# the version tags at deploy time, and the preview generates the same data so the nav footer
# shows the released-gem version line.
def generate_docs_versions_data(docs_dir)
versions = %x(git tag --list).split("\n").filter_map { |tag|
tag[/\A[^0-9]*(\d+\.\d+\.\d+(?:-[a-zA-Z0-9.-]+)?)\z/, 1]
}.sort_by { |version|
Gem::Version.new(version)
}.reverse
return if versions.empty?
mkdir_p(File.join(docs_dir, "_data"))
File.write(File.join(docs_dir, "_data", "versions.yml"), versions.map { |version| "- #{version}\n" }.join)
end end
def npx_available?(task_name) def npx_available?(task_name)

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@ -12,7 +12,7 @@ Validates the Ruby SDK's conformance to the MCP specification using [`@modelcont
### Run all scenarios ### Run all scenarios
```bash ```bash
bundle exec rake conformance:test bundle exec rake conformance
``` ```
Runs both server and client conformance tests in sequence. Server conformance starts the Runs both server and client conformance tests in sequence. Server conformance starts the
@ -31,20 +31,20 @@ are allowed to fail without affecting the exit code.
```bash ```bash
# Run a single server scenario # Run a single server scenario
bundle exec rake conformance:test SCENARIO=ping bundle exec rake conformance SCENARIO=ping
# Use a different port with verbose output # Use a different port with verbose output
bundle exec rake conformance:test PORT=3000 VERBOSE=1 bundle exec rake conformance PORT=3000 VERBOSE=1
# Start the server on a specific port # Start the server on a specific port
bundle exec rake conformance:server PORT=3000 bundle exec rake conformance_server PORT=3000
``` ```
### Start the server and test separately ### Start the server and test separately
```bash ```bash
# Terminal 1: start the server # Terminal 1: start the server
bundle exec rake conformance:server bundle exec rake conformance_server
# Terminal 2: run all scenarios # Terminal 2: run all scenarios
npx @modelcontextprotocol/conformance@alpha server --url http://localhost:9292/mcp npx @modelcontextprotocol/conformance@alpha server --url http://localhost:9292/mcp
@ -59,7 +59,7 @@ on a single scenario. Stop the server with Ctrl+C when done.
### List available scenarios ### List available scenarios
```bash ```bash
bundle exec rake conformance:list bundle exec rake conformance_list
``` ```
Prints all scenario names that can be passed to `SCENARIO`. Prints all scenario names that can be passed to `SCENARIO`.
@ -76,7 +76,7 @@ repository with the conformance server running:
```bash ```bash
# Terminal 1 (this repository): start the conformance server # Terminal 1 (this repository): start the conformance server
bundle exec rake conformance:server bundle exec rake conformance_server
# Terminal 2 (conformance repository): run the tier audit skill as a slash command in Claude Code # Terminal 2 (conformance repository): run the tier audit skill as a slash command in Claude Code
/mcp-sdk-tier-audit /path/to/modelcontextprotocol/ruby-sdk http://localhost:9292/mcp /mcp-sdk-tier-audit /path/to/modelcontextprotocol/ruby-sdk http://localhost:9292/mcp
@ -102,4 +102,4 @@ conformance/
Known-failing scenarios are registered in `conformance/expected_failures.yml`. They are allowed to Known-failing scenarios are registered in `conformance/expected_failures.yml`. They are allowed to
fail without affecting the exit code and are tracked to catch regressions. fail without affecting the exit code and are tracked to catch regressions.
These are shown in the output of `bundle exec rake conformance:test`. These are shown in the output of `bundle exec rake conformance`.

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@ -1,15 +0,0 @@
# frozen_string_literal: true
# Dependencies for the local docs preview (`rake docs:preview`), kept out of the gem's own Gemfile:
# github-pages mirrors the GitHub Pages runtime that builds the released site
# (jekyll-remote-theme, jekyll-redirect-from, and the Jekyll version Pages actually runs).
source "https://rubygems.org"
gem "github-pages", group: :jekyll_plugins
gem "webrick"
# Former default gems that the Jekyll version pinned by github-pages still requires on Ruby 4.0.
gem "base64"
gem "bigdecimal"
gem "csv"
gem "logger"

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@ -1,405 +0,0 @@
---
layout: default
title: Authorization
nav_order: 8
---
# Authorization
Authorization is handled by the transport layer. This page covers authenticating the HTTP transport,
from custom headers such as bearer tokens to the OAuth 2.1 flows the SDK implements
(PKCE with dynamic client registration, the client credentials grant, and cross-app access).
## HTTP Authorization
By default, the HTTP transport layer provides no authentication to the server, but you can provide custom headers if you need authentication. For example, to use Bearer token authentication:
```ruby
http_transport = MCP::Client::HTTP.new(
url: "https://api.example.com/mcp",
headers: {
"Authorization" => "Bearer my_token"
}
)
client = MCP::Client.new(transport: http_transport)
client.tools # will make the call using Bearer auth
```
You can add any custom headers needed for your authentication scheme, or for any other purpose. The client will include these headers on every request.
## OAuth 2.1 Authorization
When an MCP server enforces the [MCP Authorization spec](https://modelcontextprotocol.io/specification/latest/basic/authorization),
pass an `MCP::Client::OAuth::Provider` to the transport instead of a static `Authorization` header. The transport will:
- Send `Authorization: Bearer <access_token>` on every request when a token is available.
- On a `401 Unauthorized`, parse the `WWW-Authenticate` header, discover the authorization server (Protected Resource Metadata + RFC 8414 Authorization Server Metadata),
perform Dynamic Client Registration if needed, run the OAuth 2.1 Authorization Code flow with PKCE (S256), and retry the failed request with the acquired token.
- Fall back to the legacy 2025-03-26 discovery when the server publishes no Protected Resource Metadata, matching the TypeScript and Python SDKs: the MCP server's origin acts
as the authorization base URL, its metadata is fetched from `<origin>/.well-known/oauth-authorization-server` and must name that origin as its `issuer` (RFC 8414 Section 3.3),
and when even that is absent the spec's default endpoints `/authorize`, `/token`, and `/register` at the origin are used with PKCE S256 assumed.
When no Protected Resource Metadata candidate serves a JSON object, a request that could not reach the server, or that returned a `5xx` or `429`,
raises `Flow::MetadataUnreachableError` instead of triggering that fallback, and a body over the response cap is refused outright.
- On subsequent 401s with a saved `refresh_token`, exchange it at the token endpoint before falling back to the full interactive flow (RFC 6749 Section 6).
`ClientCredentialsProvider` and `CrossAppAccessProvider` refresh the same way and fall back to their own grant instead;
their refresh also requires the `issuer` the SDK records on the tokens, so tokens stored without it run the grant again.
- On a `403 Forbidden` whose `WWW-Authenticate` header carries `error="insufficient_scope"` (OAuth 2.0 step-up, RFC 6750 Section 3.1 and the MCP scope-selection-strategy),
run a fresh authorization request for the union of the currently granted scope and the scope named in the challenge, then retry the failed request once.
The refresh path is bypassed because refreshing would re-issue the same scope set the server just rejected. A `403` without that challenge is surfaced unchanged.
- Request the `offline_access` scope when `client_metadata[:grant_types]` includes `refresh_token` and the authorization server advertises `offline_access` in its metadata
`scopes_supported` (SEP-2207). This is what lets the server issue the `refresh_token` used above. As an SDK-level safeguard, when the authorization server does not advertise
`offline_access` the scope is also stripped from any other source (challenge, PRM, or provider-supplied scope) so a server that does not support it never receives it.
```ruby
require "mcp"
provider = MCP::Client::OAuth::Provider.new(
client_metadata: {
client_name: "My MCP App",
redirect_uris: ["http://localhost:3030/callback"],
grant_types: ["authorization_code", "refresh_token"],
response_types: ["code"],
token_endpoint_auth_method: "none",
},
redirect_uri: "http://localhost:3030/callback",
redirect_handler: ->(authorization_url) {
# Send the user to the authorization URL - typically `Launchy.open(authorization_url)`
# or a manual `puts authorization_url` in CLI tools.
},
callback_handler: -> {
# Capture the redirect (for example, by running a small HTTP listener on
# `redirect_uri`) and return [code, state] from the query string.
},
)
transport = MCP::Client::HTTP.new(
url: "https://api.example.com/mcp",
oauth: provider,
)
client = MCP::Client.new(transport: transport)
client.connect # `initialize` is sent here; if the server replies 401 the OAuth flow runs and the handshake is retried with the acquired token
client.tools
```
Required keyword arguments to `Provider.new`:
- `client_metadata`: Hash sent to the authorization server's Dynamic Client Registration endpoint. Must include `redirect_uris`, `grant_types`, `response_types`,
`token_endpoint_auth_method`. `redirect_uri` (below) must appear in this list, otherwise the constructor raises `Provider::UnregisteredRedirectURIError`.
When `application_type` is omitted, the SDK infers `"native"` or `"web"` from `redirect_uris` per SEP-837 before registering (loopback or custom-scheme URIs are native);
an explicit value always wins.
- `redirect_uri`: String. Must use HTTPS or be a loopback URL (`localhost`, `127.0.0.0/8`, `::1`); other values raise `Provider::InsecureRedirectURIError`.
- `redirect_handler`: Callable invoked with the fully-built authorization `URI`. Typically opens the user's browser.
- `callback_handler`: Callable that returns `[code, state]` or `[code, state, iss]` after the user is redirected back to `redirect_uri`. Returning the 3-element form
(with `iss` set to the RFC 9207 `iss` parameter from the redirect, or `nil` when absent) opts into SEP-2468 issuer validation: a present `iss` must match
the authorization server's issuer, and a missing one is rejected when the server advertises `authorization_response_iss_parameter_supported`.
Optional keyword arguments:
- `scope`: Space-separated scopes to request when the server's `WWW-Authenticate` does not specify one.
- `authorization_request_validator`: Callable invoked with an `MCP::Client::OAuth::AuthorizationRequest` before any authorization request is built.
Returning a falsy value abandons the flow with `Flow::AuthorizationRefusedError`. See [Reviewing the authorization request](#reviewing-the-authorization-request).
- `http_client_customizer`: Callable invoked with the Faraday connection the SDK builds for the OAuth flow's own requests, after its defaults and before its origin guard.
See [Customizing the OAuth HTTP Client](#customizing-the-oauth-http-client).
- `storage`: Object responding to `tokens`, `save_tokens(t)`, `client_information`, `save_client_information(info)`. Defaults to `MCP::Client::OAuth::InMemoryStorage`,
which keeps credentials in process memory only. Persisted `client_information` is stamped with an `"issuer"` member binding it to the authorization server that
issued it (SEP-2352): when the server's authorization server changes, the SDK discards the stale registration and its tokens and re-registers automatically
(portable CIMD `client_id`s are kept). Saved `tokens` carry an `"issuer"` member of their own, recording the authorization server that minted them, which is what
lets a later refresh refuse a server the MCP server has since renamed. Treat both hashes as opaque and persist them as-is; a storage that writes out selected members
instead drops these bindings with no error.
- `client_id_metadata_document_url`: URL where you publish a Client ID Metadata Document
(`draft-ietf-oauth-client-id-metadata-document` and the MCP authorization specification).
When the authorization server advertises `client_id_metadata_document_supported: true`,
the SDK uses this URL as the OAuth `client_id` and skips Dynamic Client Registration.
Spec-required: the URL MUST be `https://` with a non-root path and MUST NOT include a fragment,
userinfo, or `.`/`..` segments. The SDK additionally rejects query strings (the draft only marks
them SHOULD NOT include, but the SDK refuses to send any) for `client_id` stability.
Any of these failures raise `Provider::InvalidClientIDMetadataDocumentURLError`. The CIMD document
served at the URL is a separate JSON artifact from the `client_metadata` keyword above:
the DCR `client_metadata` MUST NOT include `client_id`, while the CIMD document MUST include
`client_id` set to the document URL, `client_name`, and `redirect_uris` covering `redirect_uri`.
- `token_request_params`: Hash of String keys and values added to every token request the provider makes,
for parameters the authorization server requires beyond the grant itself, such as Auth0's `audience`.
Defaults to `nil`, which adds nothing. The authorization request is not affected.
A key the SDK sets itself (listed in `Flow::RESERVED_TOKEN_REQUEST_PARAMS`), a Hash that compares keys by identity,
or a value that is not a Hash of Strings is refused with `Flow::InvalidTokenRequestParamsError`, a subclass of `ArgumentError`,
when the provider is built, and the accepted Hash is copied and frozen.
Any provider that defines a `token_request_params` method gets the same treatment on every token request it makes;
the flow checks the returned value by the same rules and raises the same error before the token request is sent.
{: .warning }
> The OAuth 2.0 Dynamic Client Registration Protocol (RFC 7591) is deprecated as a client registration mechanism as of MCP 2026-07-28 in favor of Client ID Metadata Documents,
> while remaining available for authorization servers that do not support them. Publish a CIMD document and set `client_id_metadata_document_url`; the SDK then prefers it
> automatically wherever the authorization server advertises support.
To persist credentials across restarts, supply your own storage:
```ruby
class FileTokenStorage
def initialize(path)
@path = path
end
def tokens
read["tokens"]
end
def save_tokens(value)
write("tokens" => value)
end
def client_information
read["client"]
end
def save_client_information(value)
write("client" => value)
end
private
def read
File.exist?(@path) ? JSON.parse(File.read(@path)) : {}
end
def write(updates)
File.write(@path, JSON.dump(read.merge(updates)))
end
end
provider = MCP::Client::OAuth::Provider.new(
# ... required keywords ...
storage: FileTokenStorage.new(File.expand_path("~/.config/my-app/oauth.json")),
)
```
### Token Endpoint Errors
When a token exchange or refresh fails, `MCP::Client::OAuth::Flow::AuthorizationError` includes the HTTP status and
the authorization server's `error` and `error_description` from [RFC 6749 Section 5.2](https://www.rfc-editor.org/rfc/rfc6749#section-5.2).
For example:
```text
Token endpoint returned status 400. invalid_request: Client must not use multiple authentication methods
```
The exception exposes `http_status`, `error`, and `error_description` readers for structured diagnostics. Missing or non-string
diagnostic fields are `nil`; non-JSON responses retain the status-only message. Other authorization failures have `nil` readers.
An `invalid_grant` response still raises `Flow::InvalidGrantError`, a subclass of `Flow::AuthorizationError`, so refresh-token
recovery behavior is unchanged.
Diagnostic fields are limited to 128 characters for `error` and 512 for `error_description`, including a trailing `...` when
truncated. Characters outside the RFC's printable ASCII set are replaced with spaces, and surrounding whitespace is removed.
The SDK excludes all other response fields, including `error_uri`, and does not include the raw response body in these errors.
Descriptions are provider-controlled text, not guaranteed to be free of sensitive information; apply your application's logging
and redaction policy before persisting them or displaying them to users.
### Client Credentials Grant
For a confidential machine-to-machine client (no user, no browser redirect), use `MCP::Client::OAuth::ClientCredentialsProvider` instead of `Provider`.
The transport discovers the authorization server the same way, then exchanges the OAuth 2.1 `client_credentials` grant (RFC 6749 Section 4.4) at
the token endpoint. There is no authorization request, PKCE, or `offline_access`, because the grant is not expected to issue a refresh token (RFC 6749 Section 4.4.3);
a refresh token the authorization server issues anyway is used on the next `401`.
```ruby
provider = MCP::Client::OAuth::ClientCredentialsProvider.new(
client_id: "my-service",
client_secret: ENV.fetch("MCP_CLIENT_SECRET"),
# token_endpoint_auth_method: "client_secret_basic" (default), "client_secret_post", or "private_key_jwt"
# scope: "mcp:read mcp:write" (optional; used when the server does not advertise scopes)
# token_request_params: { "audience" => "https://api.example.com" } (optional; parameters the authorization server requires)
)
transport = MCP::Client::HTTP.new(url: "https://api.example.com/mcp", oauth: provider)
```
Keyword arguments:
- `client_id`: Required. `client_secret`: Required with the secret-based methods; the grant is
for confidential clients, so a credential is mandatory.
- `token_endpoint_auth_method`: `"client_secret_basic"` (default), `"client_secret_post"`,
or `"private_key_jwt"` (RFC 7523 JWT client assertion per SEP-1046). `"none"` is rejected
with `ClientCredentialsProvider::InvalidCredentialsError`.
- `private_key`, `signing_algorithm`: Required with `private_key_jwt` - the key (a PEM string
or `OpenSSL::PKey::PKey`, never written to `storage`) signs the client assertion with `"ES256"`
or `"RS256"`; `client_secret` must not be set, because the private key is the credential.
- `scope`, `storage`, `authorization_request_validator`, `token_request_params`, `http_client_customizer`: Optional, same meaning as on `Provider`.
Use `token_request_params` for a parameter the authorization server requires on the `client_credentials` grant, such as Auth0's `audience`.
### Cross-App Access (JWT Bearer) Grant
For enterprise MCP deployments where an identity provider (IdP) governs authorization (SEP-990), use `MCP::Client::OAuth::CrossAppAccessProvider` instead of `Provider`.
The client exchanges an IdP-issued ID token for an Identity Assertion Authorization Grant (ID-JAG) at the IdP via RFC 8693 token exchange, then presents the ID-JAG
to the MCP authorization server with the RFC 7523 `jwt-bearer` grant, authenticating with `client_secret_basic`. There is no authorization request, PKCE, DCR, or `offline_access`.
A refresh token the authorization server issues is exchanged on the next `401` with the stored client secret, without calling `assertion_provider` again.
Mirrors `CrossAppAccessProvider` and `requestJwtAuthorizationGrant` in the TypeScript SDK.
`MCP::Client::OAuth::IDJAGTokenExchange.request` performs the RFC 8693 exchange at the IdP token endpoint. Wrap it in a callable so the same provider can plug into
an enterprise secret store or a test double without changing the transport wiring.
```ruby
provider = MCP::Client::OAuth::CrossAppAccessProvider.new(
client_id: "my-mcp-client",
client_secret: ENV.fetch("MCP_CLIENT_SECRET"),
assertion_provider: ->(audience:, resource:) {
MCP::Client::OAuth::IDJAGTokenExchange.request(
token_endpoint: "https://idp.example.com/token",
id_token: ENV.fetch("IDP_ID_TOKEN"),
client_id: "my-idp-client",
audience: audience,
resource: resource,
)
},
# scope: "mcp:read mcp:write" (optional; used when neither WWW-Authenticate nor PRM specify one)
)
transport = MCP::Client::HTTP.new(url: "https://api.example.com/mcp", oauth: provider)
```
Keyword arguments:
- `client_id`, `client_secret`: Required. The `jwt-bearer` grant authenticates with `client_secret_basic` at the MCP authorization server.
- `assertion_provider`: Required. Callable invoked as `call(audience:, resource:)` and returning the ID-JAG assertion.
`audience` is the MCP authorization server's validated issuer identifier; `resource` is the canonical MCP server URL (RFC 8707).
Passing both through to `IDJAGTokenExchange.request` covers the common case.
- `scope`, `storage`, `authorization_request_validator`, `token_request_params`, `http_client_customizer`: Optional, same meaning as on `Provider`.
### Customizing the OAuth HTTP Client
The requests the OAuth flow makes (Protected Resource Metadata discovery on the MCP server's origin, authorization server metadata discovery,
dynamic client registration, and every token request the flow sends, whether the first exchange, a refresh, or a step-up) go over a Faraday connection of their own,
not over the transport's connection: the transport's is bound to the MCP server URL and carries the `headers:` and the customizer block meant for that server.
To add middleware to the OAuth flow's connection, or to swap its adapter, pass `http_client_customizer:` to the provider:
```ruby
provider = MCP::Client::OAuth::ClientCredentialsProvider.new(
client_id: "my-service",
client_secret: ENV.fetch("MCP_CLIENT_SECRET"),
http_client_customizer: ->(faraday) { faraday.use MyApp::Middleware::HttpRecorder },
)
```
The callable receives the `Faraday::Connection` after the SDK has applied its defaults and registered the middleware that records the requested URL,
and before the SDK registers its origin guard, the same position the transport's customizer block has on the MCP server connection.
It may be invoked more than once, and from more than one thread at a time, so keep it free of side effects and safe to run concurrently;
today it runs once per authorization attempt, but that is not a promise.
A few constraints follow from the checks described below:
- Do not add redirect-following middleware. Every destination check runs against the URL as written, so a request that middleware added by the customizer would send
to a different origin after the SDK has recorded the requested URL, whether by following a `3xx` or by rewriting the URL, is refused with `Flow::DestinationMismatchError`
before it reaches the adapter, as is a request that reaches the guard without that record. Middleware inserted ahead of the record with `builder.insert(0, ...)`
that rewrites the URL first or rebuilds the environment is outside the guard, as is following done inside an adapter, so leave both off.
- Leave `Accept-Encoding` unset. The response cap below is measured on decoded bytes, and claiming the header turns Net::HTTP's decoding off.
- Do not add Faraday's `raise_error` middleware. The flow reads statuses itself, both to tell an absent metadata document from a failed request
and to turn a token endpoint error into `Flow::InvalidGrantError`.
- With an adapter that does not stream through `on_data`, the response cap is applied once the body has been buffered rather than as it arrives.
- A middleware that records requests sees the client credentials on token requests (`Authorization: Basic`, `client_secret`, `client_assertion`), refresh tokens,
and the access tokens in token responses; redact them before they reach a log.
### Communication Security
When `oauth:` is set, the MCP transport URL and every OAuth-facing URL (PRM, Authorization Server metadata, `authorization_endpoint`, `token_endpoint`, `registration_endpoint`,
`redirect_uri`) must use HTTPS or a loopback host. Non-loopback `http://` URLs are rejected at the SDK boundary so a bearer token is never sent over plain HTTP to a remote host.
The transport also snapshots the canonicalized origin, path, and query string of the MCP URL at `initialize` time and re-checks them on every outgoing request through
a Faraday middleware that runs after any user-supplied customizer. That means any URL swap raises `MCP::Client::HTTP::InsecureURLError` before the request reaches the adapter,
whether the swap was triggered by
`instance_variable_set(:@url, ...)`, by a Faraday customizer rewriting `url_prefix`, or by a custom middleware rewriting `env.url` (including just `env.url.query`) at request time,
and whether the new URL is `http://` *or* `https://` to a different host or tenant.
### Discovery URL Destinations
The scheme rules above say how a URL is contacted, not where it points. Discovery URLs arrive from the network, so the SDK also constrains their destinations.
Both checks run before the request is sent, and neither is configurable.
- The `resource_metadata` URL in a `WWW-Authenticate` challenge must be on the MCP server's own origin. Protected Resource Metadata describes that server,
so a real deployment publishes it there; requiring it means a `401` cannot aim the first request of the flow at an unrelated host. This is stricter than RFC 9728,
which does not require it.
- The PRM `authorization_servers` entry and the `authorization_endpoint`, `token_endpoint`, and `registration_endpoint` from Authorization Server metadata must not be
IP literals in a private, loopback, link-local, or unique-local range, per the SSRF precaution in [RFC 9728 Section 7.7](https://www.rfc-editor.org/rfc/rfc9728#section-7.7).
The blocked ranges are `0.0.0.0/8`, `10.0.0.0/8`, `100.64.0.0/10`, `127.0.0.0/8`, `169.254.0.0/16`, `172.16.0.0/12`, `192.168.0.0/16`, `::/96`, `fc00::/7`,
and `fe80::/10`, along with the IPv4-mapped IPv6 spellings of each and the `localhost` name.
- That range check is skipped when the MCP server URL you configured is itself on such an address. Pointing the client at a private network is a deliberate act,
and the authorization server for it usually lives on the same network, so `http://localhost` development and deployments that never leave a corporate network keep working.
The range check compares IP literals and does not resolve hostnames, so it cannot recognize an internal service that is named rather than addressed,
such as `https://vault.corp.internal/`. Resolving names here would not close that gap either, because the address the SDK looked up need not be the one
the HTTP client connects to a moment later. The same-origin rule is what protects the `resource_metadata` URL, which is the only one of these a server supplies directly.
On the connection the SDK builds, a middleware that would send a request to a different origin, by following a `3xx` or by rewriting the URL, is refused before the request
goes out (see [Customizing the OAuth HTTP Client](#customizing-the-oauth-http-client)). A connection supplied through `MCP::Client::OAuth::Flow.new(http_client_factory:)`
replaces that one, the provider's `http_client_customizer` and the guard included, so do not add redirect-following middleware to it: every check above runs against
the URL as written, and a connection that follows a `3xx` on its own would reach hosts these rules just refused.
A factory that wants to keep them can return `MCP::Client::OAuth::Flow.build_http_client(customizer)`, the connection the SDK builds for itself.
The SDK also bounds what those endpoints may return. A discovery, dynamic client registration, token, or token exchange response is refused once it passes 4 MiB,
measured as the body arrives rather than after it has been buffered, so a compressed body that expands past the limit is refused partway through the expansion.
Unlike the transport's `max_message_bytes:`, this limit is not configurable: these documents run to kilobytes in normal operation, and a connection supplied through
`http_client_factory:` is bounded as well, so there is no way to opt out of it.
### Reviewing the authorization request
The checks above constrain where the SDK will send a request. What they cannot decide is whether the authorization server an MCP server names is one you want
your users signing in to. That choice belongs to the MCP server: it publishes `authorization_servers` in its Protected Resource Metadata and states the scopes
it wants in `scopes_supported` or in the `WWW-Authenticate` challenge. An authorization server is legitimately a different origin from the resource it protects,
so no origin rule can settle the question, and validating that a token was issued for the intended audience is a responsibility the specification places on
MCP servers rather than on clients.
`authorization_request_validator` is where an application that *does* know which providers its users deal with can say so:
```ruby
ALLOWED_ISSUERS = ["https://login.example.com", "https://accounts.google.com"]
provider = MCP::Client::OAuth::Provider.new(
# client_metadata:, redirect_uri:, redirect_handler:, and callback_handler: as in the first example.
authorization_request_validator: ->(request) {
ALLOWED_ISSUERS.include?(request.authorization_server)
},
)
```
The argument is an `MCP::Client::OAuth::AuthorizationRequest` carrying `authorization_server` (the selected issuer), `scopes` (an Array, empty when neither
the challenge nor the metadata named any), `server_url`, and `resource`. It is one object rather than keyword arguments so that later revisions of the specification
can add to it without changing the shape you wrote. Only the named readers are the contract. The positional access a `Struct` also happens to provide
(`request[0]`, `to_a`, `each`) is not, and can break when the representation changes.
`server_url` is the URL the transport was configured with, verbatim; `resource` is the value actually sent as the RFC 8707 `resource`, which is that URL canonicalized
(fragment and userinfo dropped, scheme and host lowercased, a default port removed) or, when the Protected Resource Metadata advertises one,
the canonicalized value from there. That advertised value may name a parent path, so a server at `https://api.example.com/mcp` can legitimately produce
a `resource` of `https://api.example.com`. Match on `server_url` when you mean the server you configured.
Compare the issuer as a whole string, the way the SDK compares it everywhere else, rather than picking its host out: multi-tenant providers tell tenants apart by path,
and a legacy authorization server whose metadata never named an issuer arrives as `nil`, which an exact comparison refuses instead of raising.
The provider is only half of the decision. The MCP server chose the scopes too, so a request naming a provider you allow can still ask for more than that server has
any business asking for. `scopes` rides on the request so that a host with a policy per server can apply it:
```ruby
ALLOWED_SCOPES = { "https://api.example.com/mcp" => ["mcp:read", "mcp:write"] }
provider = MCP::Client::OAuth::Provider.new(
# client_metadata:, redirect_uri:, redirect_handler:, and callback_handler: as in the first example.
authorization_request_validator: ->(request) {
ALLOWED_ISSUERS.include?(request.authorization_server) && request.scopes.all? { |scope| ALLOWED_SCOPES.fetch(request.server_url, []).include?(scope) }
},
)
```
A host with a user to ask can put the decision to them instead. The request carries what such a prompt has to name: the provider, the scopes, and the server that asked for them.
It runs on all three grants, after that server's metadata has been fetched (which is where the validated issuer comes from) and before any registration, credential,
or user reaches it: on the authorization-code grant before dynamic client registration and before any browser is opened,
and on the JWT bearer grant before `assertion_provider` is invoked, since obtaining an ID-JAG tells your identity provider which authorization server
the assertion is for. Refusing therefore leaves nothing registered at, and no assertion minted for, the authorization server you rejected.
Returning a falsy value raises `Flow::AuthorizationRefusedError`. It subclasses `Flow::AuthorizationError`, so a rescue written for that still catches it,
while rescuing the narrower class tells a refusal by your own policy apart from a network or metadata failure.
The hook decides whether to proceed, not what to ask for: the scopes are passed to the authorization server unchanged either way, because the specification
requires a client to treat the scopes in the challenge as authoritative for the operation. Leaving it unset authorizes whatever the server asked for,
which is what every MCP SDK does today.
It is asked when a new grant is requested, not on every token refresh, which happens unattended and against an authorization server you already answered for.
An authorization server that changes between authorization and refresh is caught instead: the SDK records which one issued the tokens, and `refresh!` refuses to
present a refresh token to a different one, even when the client identity is portable across authorization servers as a Client ID Metadata Document URL is.
The transport answers that refusal by running a full authorization, which brings the new authorization server back here for you to accept or refuse.
Tokens stored before this behavior shipped carry no issuer and keep refreshing; the binding applies from their next authorization.

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@ -1,71 +0,0 @@
---
layout: default
title: Cancellation
nav_order: 5
---
# Cancellation
`MCP::Client` lets the caller cancel a request it has already issued,
per the [MCP `notifications/cancelled` utility](https://modelcontextprotocol.io/specification/latest/basic/patterns/cancellation).
The recommended pattern is to pass
an `MCP::Cancellation` token into the request method, run the request on a worker thread, and call
`cancellation.cancel(reason:)` from another thread. The cancelling thread sends `notifications/cancelled` to
the server, and the calling thread is woken up with `MCP::CancelledError`:
```ruby
client = MCP::Client.new(transport: transport)
cancellation = MCP::Cancellation.new
Thread.new do
client.call_tool(name: "slow_tool", arguments: {}, cancellation: cancellation)
rescue MCP::CancelledError
# cleanup
end
# Later, from another thread:
cancellation.cancel(reason: "user pressed cancel")
```
All request methods (`tools`, `list_tools`, `resources`, `list_resources`, `resource_templates`, `list_resource_templates`,
`prompts`, `list_prompts`, `call_tool`, `read_resource`, `get_prompt`, `complete`, `discover`, `ping`) accept the `cancellation:` keyword.
Request ids are managed internally, so the token is the only thing a caller needs to cancel a request.
{: .note }
> When a cancel wins the race, the SDK's worker thread that is blocked on the underlying I/O is *not* force-killed;
> it stays blocked until the transport actually returns (or the user closes the transport). This matches the server-side
> `StreamableHTTPTransport#send_request` trade-off. For `Client::HTTP`
> the leak resolves as soon as the server sends any response; for `Client::Stdio` you may need to call `client.transport.close`
> to free the thread if the server stops responding entirely. The cancel-dispatch thread waits for the worker's send-boundary signal
> (`&on_sent` from `send_request`) before issuing `notifications/cancelled`, so the cancel is held until the worker has at
> least committed to writing the request; while the worker is wedged the cancel notification is deferred along with it.
{: .note }
> On a [modern](/client/lifecycle/) connection the cancel notification cannot reach the in-flight
> request: correlating the two is a session mechanic of the handshake lifecycle, and modern requests
> are sessionless single POST exchanges. The local effect is unchanged - the calling thread still
> raises `MCP::CancelledError` - but the server runs the request to completion.
## Wire-order guarantees
`Client::Stdio` serializes the request write and any subsequent `notifications/cancelled` write through a single `@write_mutex`,
so the server is guaranteed to read the request line before the cancel line.
`Client::HTTP` cannot offer the same wire-arrival guarantee. Faraday's synchronous `post` does not expose a post-write / pre-response hook,
so the SDK yields just before the request POST is dispatched. After the yield, the cancel-dispatch thread issues a separate `notifications/cancelled` POST
on its own connection, and the two POSTs may overlap on the network. The spec is satisfied either way: the sender has already issued the request and
still believes it to be in-progress when issuing the cancel ([MCP cancellation spec](https://modelcontextprotocol.io/specification/latest/basic/patterns/cancellation)),
and on the receiver side, "receivers MAY ignore a cancellation notification whose `requestId` is unknown" covers the case where the cancel POST
happens to arrive first. The calling thread raises `MCP::CancelledError` regardless of network ordering.
## Custom transports
Custom transports that want to support `cancellation:` must implement `send_notification(notification:)` so `notifications/cancelled` can be delivered.
They should also accept the optional block passed to `send_request(request:, &on_sent)` and call it once the request bytes have been handed off to the wire
(under a write-side mutex for stdio-style transports, immediately before the synchronous round-trip for HTTP-style transports).
The cancel-dispatch thread waits on this signal before sending `notifications/cancelled`. Transports that do not invoke the block fall back to waiting for
the worker thread to terminate, which preserves wire-order at the cost of delaying the cancel notification until the request has fully completed.
## Server Side
How servers observe cancellation in their handlers is documented on the server [Cancellation](/server/cancellation/) page.

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@ -1,47 +0,0 @@
---
layout: default
title: Overview
nav_order: 1
permalink: /client/
redirect_from:
- /building-clients.html
- /building-clients/
---
# Building an MCP Client
The `MCP::Client` class provides an interface for interacting with MCP servers.
This class supports:
- Lifecycle negotiation and connection via `MCP::Client#connect`, adopting the modern lifecycle
when the server serves it; see [Lifecycle](/client/lifecycle/)
- Server discovery via the `server/discover` method (`MCP::Client#discover`); see [Explicit Discovery](/client/lifecycle/#explicit-discovery)
- Liveness check via the `ping` method (`MCP::Client#ping`), and automatic answers to server pings;
see [Answering Server Pings](/client/ping/#answering-server-pings)
- Tool listing via the `tools/list` method (`MCP::Client#tools`)
- Tool invocation via the `tools/call` method (`MCP::Client#call_tool`)
- Resource listing via the `resources/list` method (`MCP::Client#resources`)
- Resource template listing via the `resources/templates/list` method (`MCP::Client#resource_templates`)
- Resource reading via the `resources/read` method (`MCP::Client#read_resource`)
- Prompt listing via the `prompts/list` method (`MCP::Client#prompts`)
- Prompt retrieval via the `prompts/get` method (`MCP::Client#get_prompt`)
- Completion requests via the `completion/complete` method (`MCP::Client#complete`); see [Completion](/server/completion/)
- Automatic driving of multi round-trip `input_required` results once `on_elicitation`, `on_sampling`,
or `on_roots` handlers are registered; see [Multi Round-Trip Requests](/client/mrtr/)
- Cancellation of in-flight requests via the `cancellation:` keyword; see [Cancellation](/client/cancellation/)
- Cursor-based page iteration on the `list_*` methods and whole-collection fetching with
the `max_pages` guard; see [Pagination](/client/pagination/)
- Automatic JSON-RPC 2.0 message formatting
- UUID request ID generation
Clients are initialized with a [transport layer](/client/transports/) instance that handles the low-level communication mechanics.
Authorization is handled by the transport layer; see [Authorization](/client/authorization/).
## Tool Objects
The client provides a wrapper class for tools returned by the server:
- `MCP::Client::Tool` - Represents a single tool with its metadata
This class provides easy access to tool properties like name, description, input schema, and output schema.

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@ -1,78 +0,0 @@
---
layout: default
title: Lifecycle
nav_order: 3
---
# Lifecycle
Before sending requests, a client establishes its lifecycle with the server: the classic `initialize` handshake
on legacy protocol versions, or the sessionless modern lifecycle of MCP 2026-07-28.
This page covers `MCP::Client#connect` and how it negotiates between the two.
## Handshake
Call `MCP::Client#connect` to perform the MCP [initialization handshake](https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle#initialization) before sending any other requests. The client sends an `initialize` request through the transport, followed by the required `notifications/initialized` notification, and caches the server's `InitializeResult` (protocol version, capabilities, server info, instructions):
```ruby
client.connect
# => { "protocolVersion" => "2025-11-25", "capabilities" => {...}, "serverInfo" => {...} }
client.connected? # => true
client.server_info # => cached InitializeResult
```
`connect` accepts optional `client_info:`, `protocol_version:`, and `capabilities:` keyword arguments. It is idempotent: a second call returns the cached result without contacting the server. After `close`, state is cleared and `connect` will handshake again.
This applies to both the Stdio and HTTP transports described on the [Transports](/client/transports/) page.
By default `connect` [negotiates the lifecycle](#lifecycle-negotiation) first and performs this handshake
only when the server does not serve the modern lifecycle, or when `mode: :legacy` or a legacy `protocol_version:` forces it.
## Lifecycle Negotiation
`MCP::Client#connect` selects the protocol lifecycle automatically by default: on the bundled
`MCP::Client::HTTP` and `MCP::Client::Stdio` transports it probes [`server/discover`](/server/discover/) first and adopts
the stateless modern lifecycle (MCP 2026-07-28, SEP-2575) when the server serves it, falling back to
the classic `initialize` handshake otherwise. Custom transports whose `connect` does not declare
a `mode:` keyword always receive the classic call shape, unchanged.
```ruby
client.connect # negotiate automatically (default)
client.connect(mode: :modern) # require the modern lifecycle; fails on legacy-only servers
client.connect(mode: :legacy) # force the classic initialize handshake
client.connect(protocol_version: "2025-11-25") # an explicit legacy version pins the handshake, no probe
```
Prefer `mode: :legacy` for spawn-per-invocation CLI tools (the probe adds a round trip per process)
and when using server-initiated requests (`on_elicitation` / `on_sampling`), which exist only on
the legacy lifecycle.
Because the raw `connect` return value and `MCP::Client#server_info` mirror the wire result,
their shape depends on the negotiated lifecycle: `InitializeResult` (`protocolVersion`,
top-level `serverInfo`) on legacy, `DiscoverResult` (`supportedVersions`, `ttlMs`/`cacheScope`)
on modern. Code that should work against both lifecycles can use the era-independent readers instead:
```ruby
client.protocol_version # negotiated or adopted version, either lifecycle
client.server_capabilities # capabilities Hash, either lifecycle
client.instructions # instructions text, either lifecycle
client.server_implementation # server name/version; nil when a modern server does not identify itself
```
Troubleshooting: if `server_info["protocolVersion"]` starts returning `nil` after a server you connect to was upgraded,
the server now serves the modern lifecycle and the automatic negotiation adopted it.
Pass `mode: :legacy` for an immediate return to the previous behavior, or switch to the readers above for a permanent fix.
## Explicit Discovery
`MCP::Client#discover` sends `server/discover` directly: sessionless capability discovery
that works before (or instead of) `connect`. It returns an `MCP::Client::DiscoverResult` struct
exposing `supported_versions`, `capabilities`, `server_info`, `instructions`, and
the `ttl_ms` / `cache_scope` cache hints; see the server [Discovery](/server/discover/) page
for the wire shapes.
```ruby
result = client.discover
result.supported_versions # => ["2026-07-28"]
```

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---
layout: default
title: Multi Round-Trip Requests
nav_order: 4
redirect_from:
- /client/multi-round-trip-results/
---
# Multi Round-Trip Requests
MCP 2026-07-28 replaces in-flight server-to-client requests with Multi Round-Trip Requests (SEP-2322): instead of issuing `sampling/createMessage`, `roots/list`,
or `elicitation/create` while a request is being processed, a server may answer with a result whose `resultType` is `"input_required"`, carrying an `inputRequests` map
and an opaque `requestState`; the client fulfills the requests and re-issues the original request with `inputResponses` and the echoed `requestState`.
## Automatic Driving
The Ruby client drives such results automatically: once a handler is registered through `on_elicitation`, `on_sampling`, or `on_roots`, the `call_tool`, `get_prompt`,
and `read_resource` methods fulfill the embedded requests and re-issue the original request with `inputResponses` plus the echoed `requestState`, capped at `input_required_max_rounds`
(10 by default, matching the TypeScript and Python SDKs).
```ruby
client = MCP::Client.new(transport: transport)
client.connect(capabilities: { elicitation: { form: {} } })
client.on_elicitation do |params|
{ action: "accept", content: { name: "Alice" } }
end
# The input_required round trips are driven automatically; this returns the final result.
response = client.call_tool(name: "collect_name", arguments: {})
```
Declare the capabilities matching the registered handlers on `connect`: a server embeds only the request kinds
the client declared.
## Manual Driving
Without a matching handler, `MCP::Client::InputRequiredError` is raised instead of returning the result as if it were final;
the error exposes `input_requests`, `request_state`, and the raw `result` for manual driving via the `input_responses:` and `request_state:` keywords:
```ruby
begin
client.call_tool(name: "collect_name", arguments: {})
rescue MCP::Client::InputRequiredError => error
answers = error.input_requests.transform_values { |request| answer_for(request) }
client.call_tool(
name: "collect_name",
arguments: {},
input_responses: answers,
request_state: error.request_state,
)
end
```
`MCP::ResultType::COMPLETE` and `MCP::ResultType::INPUT_REQUIRED` are provided for forward compatibility.
Servers on legacy protocol versions never send `resultType`, so existing behavior is unchanged.
## Server Side
Authoring `input_required` results with `InputRequiredResult`, securing `requestState`, `resultType` stamping, and the legacy fulfillment
shim that serves pre-2026 clients are documented on the server [Multi Round-Trip Requests](/server/mrtr/) page.

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@ -1,80 +0,0 @@
---
layout: default
title: Pagination
nav_order: 7
---
# Pagination
Servers may paginate `tools/list`, `prompts/list`, `resources/list`, and `resources/templates/list` responses
per the [MCP pagination utility](https://modelcontextprotocol.io/specification/latest/server/utilities/pagination).
Cursor tokens are opaque to clients: the server decides page size, and the client follows `nextCursor` until the server omits it.
## Iterating Pages
`MCP::Client` exposes `list_tools`, `list_prompts`, `list_resources`, and `list_resource_templates`.
**Each call issues exactly one `*/list` JSON-RPC request and returns exactly one page** - not the full collection.
The returned result object (`MCP::Client::ListToolsResult` etc.) exposes the page items and the next cursor
as method accessors; a `meta` accessor also mirrors the response's `_meta` field:
```ruby
client = MCP::Client.new(transport: transport)
cursor = nil
loop do
page = client.list_tools(cursor: cursor)
page.tools.each { |tool| process(tool) }
cursor = page.next_cursor
break unless cursor
end
```
The same pattern applies to `list_prompts` (`page.prompts`), `list_resources` (`page.resources`), and
`list_resource_templates` (`page.resource_templates`). `next_cursor` is `nil` on the final page.
Because a single call returns a single page, how many items come back depends on the server's `page_size` configuration:
| Server `page_size` | `client.list_tools(cursor: nil)` |
|--------------------|---------------------------------------------------------------------|
| Not set (default) | Returns every item in one response. `next_cursor` is `nil`. |
| Set to `N` | Returns the first `N` items. `next_cursor` is set for continuation. |
If your application needs the complete collection regardless of how the server is configured, either loop on
`next_cursor` as shown above, or use the whole-collection methods described below.
## Fetching the Complete Collection
`client.tools`, `client.resources`, `client.resource_templates`, and `client.prompts` auto-iterate
through all pages and return a plain array of items, guaranteeing the full collection regardless
of the server's `page_size` setting. When a server paginates, they issue multiple JSON-RPC round
trips per call. Two guards keep that loop finite: it stops when the server returns a `nextCursor`
it has already sent, and it stops after `max_pages` pages.
```ruby
tools = client.tools # => Array<MCP::Client::Tool> of every tool on the server.
```
`MCP::Client.new` accepts an optional `max_pages:` keyword that caps how many pages these methods
will walk. It defaults to `1_000`; a server that keeps offering a fresh `nextCursor` past that
point raises `MCP::Client::PaginationLimitError` rather than being followed indefinitely. Raise it
if you legitimately expect more pages than that.
Use these when you want the complete list; use `list_tools(cursor:)` etc. when you need
fine-grained iteration (e.g. to stream-process pages without loading everything into memory).
## Cache Hints
Per SEP-2549, list and read results can carry cache hints telling clients how long a result stays fresh (`ttlMs`)
and whether shared intermediaries may cache it (`cacheScope`); see
[List Result Caching](/server/pagination/#list-result-caching) on the server page for how they are emitted.
On the client, the values are surfaced on the paginated result structs as `ttl_ms` and `cache_scope`:
```ruby
page = client.list_tools
page.ttl_ms # => 60000 (nil when the server sent no hint)
page.cache_scope # => "private"
```
## Server Side
Enabling pagination with `page_size:` is documented on the server [Pagination](/server/pagination/) page.

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@ -1,49 +0,0 @@
---
layout: default
title: Ping
nav_order: 6
---
# Ping
The [MCP `ping` utility](https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/ping)
allows either side of the connection to verify that the peer is still responsive.
{: .note }
> MCP 2026-07-28 removes `ping` from the protocol (SEP-2575): requests of
> the [modern lifecycle](/client/lifecycle/) are single POST exchanges whose connection itself
> signals liveness. `#ping` sends the request regardless of the negotiated lifecycle,
> so on a modern connection a conforming server (the Ruby SDK server included) rejects it
> with `-32601` Method not found and `ServerError` is raised; ping is a handshake-lifecycle utility.
`MCP::Client` exposes `ping` to send a ping to the server:
```ruby
client = MCP::Client.new(transport: transport)
client.ping # => {} on success
```
`#ping` raises `MCP::Client::ServerError` when the server returns a JSON-RPC error.
It raises `MCP::Client::ValidationError` when the response `result` is missing or
is not a Hash (matching the spec requirement that `result` be an object).
Transport-level errors (for example, `MCP::Client::Stdio`'s `read_timeout:` firing)
propagate as exceptions raised by the transport layer.
## Answering Server Pings
On handshake-lifecycle connections a server may ping the client the same way, and the client answers automatically with
the empty result - no handler is needed. Over HTTP a ping reaches the client only on a stream that is already open
(the SSE response of an in-flight POST, or the GET listening stream); the client never opens a stream just to receive pings.
Over stdio, a ping that arrives while a response is awaited is answered inline; between requests it is answered when
the next request starts reading. The answer is best effort: a pong that cannot be written (for example, over a broken pipe) is
dropped rather than failing the request whose response is being read. Independently of pings, consider setting `read_timeout:`
on `MCP::Client::Stdio`, since a server that never answers otherwise holds the read until the process exits.
To answer pings yourself, register `transport.on_server_request("ping")` on `MCP::Client::HTTP`; it replaces
the automatic answer and, like any handler registered there, opens the standalone GET listening stream on `connect`,
as described on the [Transports](/client/transports/#server-to-client-requests-elicitation) page.
## Server Side
How servers answer `ping` requests and ping the client themselves is documented on
the server [Ping](/server/ping/) page.

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@ -1,285 +0,0 @@
---
layout: default
title: Transports
nav_order: 2
---
# Transports
`MCP::Client` is transport-agnostic: it sends every request through the transport instance it is built with.
This page covers the bundled [stdio](#stdio-transport-layer) and [Streamable HTTP](#http-transport-layer) transports,
their sessions and server-to-client request handling, and the interface a custom transport must implement.
## Stdio Transport Layer
Use the `MCP::Client::Stdio` transport to interact with MCP servers running as subprocesses over standard input/output.
`MCP::Client::Stdio.new` accepts the following keyword arguments:
| Parameter | Required | Description |
|---|---|---|
| `command:` | Yes | The command to spawn the server process (e.g., `"ruby"`, `"bundle"`, `"npx"`). |
| `args:` | No | An array of arguments passed to the command. Defaults to `[]`. |
| `env:` | No | A hash of environment variables to set for the server process. Defaults to `nil`. |
| `read_timeout:` | No | Timeout in seconds for waiting for a server response. Defaults to `nil` (no timeout). |
| `max_line_bytes:` | No | Maximum byte length of a single newline-delimited response frame. A frame that reaches this limit without a newline is rejected as a transport error, preventing unbounded memory growth from a server that never emits a newline. Defaults to `4 * 1024 * 1024` (4 MiB). |
Example usage:
```ruby
stdio_transport = MCP::Client::Stdio.new(
command: "bundle",
args: ["exec", "ruby", "path/to/server.rb"],
env: { "API_KEY" => "my_secret_key" },
read_timeout: 30
)
client = MCP::Client.new(transport: stdio_transport)
# Perform the MCP initialization handshake before sending any requests.
client.connect
# List available tools.
tools = client.tools
tools.each do |tool|
puts "Tool: #{tool.name} - #{tool.description}"
end
# Call a specific tool.
response = client.call_tool(
tool: tools.first,
arguments: { message: "Hello, world!" }
)
# Close the transport when done.
stdio_transport.close
```
The stdio transport automatically handles:
- Spawning the server process with `Open3.popen3`
- MCP protocol initialization handshake (`initialize` request + `notifications/initialized`)
- JSON-RPC 2.0 message framing over newline-delimited JSON
- Answering server `ping` requests; see [Answering Server Pings](/client/ping/#answering-server-pings)
{: .note }
> `ping` is the only server-to-client request answered over stdio. A wire-level `elicitation/create`
> or `sampling/createMessage` is ignored on this transport, so a server that sends one waits for an answer
> that never comes; use Streamable HTTP for those, as described below. The SEP-2322 `input_required` route,
> which is how the modern lifecycle asks for the same input, works on every transport, so `on_elicitation`
> and `on_sampling` handlers still fire over stdio; see [Multi Round-Trip Requests](/client/mrtr/).
## HTTP Transport Layer
Use the `MCP::Client::HTTP` transport to interact with MCP servers using simple HTTP requests.
You'll need to add `faraday` as a dependency in order to use the HTTP transport layer, and `event_stream_parser`
to read SSE (`text/event-stream`) responses.
Whether a response is JSON or SSE is the server's choice, made for each response, and a client must accept both,
so a client written for arbitrary servers needs both gems. This SDK's own server picks SSE by default and always
does on the modern lifecycle, and the listening stream that `on_elicitation` and `on_sampling` open is SSE as well.
`event_stream_parser` is loaded on the first SSE response, so it can be left out only against a server known to
answer with JSON alone, such as this SDK's server in [JSON response mode](/server/transports/#json-response-mode)
serving handshake-lifecycle clients, and only while no listening stream is opened:
```ruby
gem "mcp"
gem "faraday", ">= 2.0"
gem "event_stream_parser", ">= 1.0" # optional, required only for SSE responses
```
Example usage:
```ruby
http_transport = MCP::Client::HTTP.new(url: "https://api.example.com/mcp")
client = MCP::Client.new(transport: http_transport)
# Perform the MCP initialization handshake before sending any requests.
client.connect
# List available tools
tools = client.tools
tools.each do |tool|
puts <<~TOOL_INFORMATION
Tool: #{tool.name}
Description: #{tool.description}
Input Schema: #{tool.input_schema}
TOOL_INFORMATION
end
# Call a specific tool
response = client.call_tool(
tool: tools.first,
arguments: { message: "Hello, world!" }
)
# Call a tool with progress tracking.
response = client.call_tool(
tool: tools.first,
arguments: { count: 10 },
progress_token: "my-progress-token"
)
```
The server sends `notifications/progress` during execution; the bundled transports do not currently
expose these notifications to application code, so the token's effect is visible on the server side.
See the [Progress](/server/progress/) page.
`MCP::Client::HTTP.new` accepts an optional `max_message_bytes:` keyword that caps the bytes buffered in memory for a single message from the server -
an SSE event or a JSON response body. A message that reaches this limit before completing is rejected as a transport error, preventing unbounded memory growth from
a server that never terminates an SSE event. It defaults to `4 * 1024 * 1024` (4 MiB); raise it if your server returns larger responses.
`MCP::Client::HTTP.new` also accepts `max_reconnection_wait:`, a budget in seconds for resuming a closed SSE stream. It gates every wait between reconnection attempts,
and what is left of it becomes the read timeout of each resumed stream. The server chooses that wait through the SSE `retry:` field, and resuming happens on the calling thread,
so without a budget a server answering with a large `retry:` parks a thread of your application for as long as it likes. It defaults to `300` (5 minutes).
The server's `retry:` is never shortened: when honoring it would run past the budget, the client stops trying to resume and raises instead,
the same thing it already does once the reconnection attempts are used up. A floor of 100ms applies to each wait, so a `retry: 0` cannot spin
the listening stream's reconnect loop; waiting longer than the server asked for is explicitly allowed by the SSE reconnection algorithm the spec points at.
### Sessions
After `connect` succeeds, the HTTP transport captures the `Mcp-Session-Id` header and `protocolVersion` from the response and includes them on subsequent requests. Both are exposed on the transport as transport-specific state:
```ruby
http_transport.session_id # => "abc123..."
http_transport.protocol_version # => "2025-11-25"
```
If the server terminates the session, subsequent requests return HTTP 404 and the transport raises `MCP::Client::SessionExpiredError` (a subclass of `RequestHandlerError`). Session state is cleared automatically; callers should start a new session by calling `connect` again.
To explicitly terminate a session (e.g., when the client application is shutting down), call `close`. The transport sends an HTTP DELETE to the MCP endpoint with the session header and clears local session state. A `405 Method Not Allowed` response (server doesn't support client-initiated termination) or `404 Not Found` (session already terminated server-side) is treated as success. Other errors - 5xx, authentication failures, connection errors - propagate to the caller. Local session state is cleared either way. Calling `close` without an active session is a no-op.
```ruby
http_transport.close
```
These are handshake-lifecycle mechanics: on a [modern](/client/lifecycle/) connection there is
no session to capture - `session_id` stays `nil` and requests carry the `_meta` envelope instead.
### Server-to-Client Requests (Elicitation)
Servers can send requests back to the client while one of the client's own requests is in flight - for example,
[`elicitation/create`](https://modelcontextprotocol.io/specification/2025-11-25/client/elicitation) to ask the user for additional input during a tool call.
Register a handler and advertise the capability on `connect` to respond to them:
```ruby
client.connect(capabilities: { elicitation: {} })
client.on_elicitation do |params|
{
action: "accept",
# Fill fields omitted by the user with the schema's `default` values (SEP-1034)
content: MCP::Client::Elicitation.apply_defaults(params["requestedSchema"]),
}
end
```
Registering a handler opens a standalone HTTP GET SSE stream on a background thread
([listening for messages from the server](https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#listening-for-messages-from-the-server)),
since servers deliver requests that are not tied to a client request on that stream. Server requests with no registered handler are answered with
a JSON-RPC `-32601` (method not found) error. To handle methods other than `elicitation/create`, register directly on the transport with
`http_transport.on_server_request("method/name") { |params| ... }`.
On a [modern](/client/lifecycle/) connection servers cannot send requests at all; the same
registered handlers instead drive the requests embedded in `input_required` results,
as documented on [Multi Round-Trip Requests](/client/mrtr/).
### Server-to-Client Requests (Sampling)
Servers can also request an LLM completion from the client with [`sampling/createMessage`](https://modelcontextprotocol.io/specification/2025-11-25/client/sampling),
letting a server leverage the client's model access without its own API keys.
{: .warning }
> MCP Sampling is deprecated as of protocol version `2026-07-28` (SEP-2577), while remaining fully supported under `2025-11-25`.
> Register this handler to interoperate with servers that still send sampling requests during the deprecation window;
> new servers should call LLM provider APIs directly.
Register a handler and advertise the capability on `connect`:
```ruby
client.connect(capabilities: { sampling: {} })
client.on_sampling do |params|
completion = my_llm.complete(params["messages"], max_tokens: params["maxTokens"])
{
role: "assistant",
content: { type: "text", text: completion.text },
model: completion.model,
stopReason: "endTurn",
}
end
```
For trust and safety, the spec recommends a human in the loop able to review, edit, or reject the request and the generated response.
To reject a request, raise `MCP::Client::ServerRequestError` with the spec's user-rejection code `-1`:
```ruby
client.on_sampling do |params|
raise MCP::Client::ServerRequestError.new("User rejected sampling request", code: -1) unless approved?(params)
generate_completion(params)
end
```
Use `capabilities: { sampling: { tools: {} } }` to receive tool-enabled sampling requests. Like elicitation, this uses the same standalone GET SSE listening stream.
### Custom Headers from Tool Parameters
On a modern `MCP::Client::HTTP` connection, `tools/call` mirrors arguments whose `inputSchema` property carries
an `x-mcp-header` annotation into `Mcp-Param-{Name}` request headers per SEP-2243, so intermediaries can route
on the values without parsing bodies. The declarations are learned from `tools/list` responses:
list the tools before calling one to enable the mirroring. Values that cannot ride as plain ASCII header values
(non-ASCII, control characters, edge whitespace, empty strings) are wrapped as `=?base64?...?=`,
and a `null` or absent argument omits its header.
Per the specification, a tool definition whose `x-mcp-header` annotations are invalid (empty or non-token names,
duplicate names, non-primitive properties, annotations outside a chain of `properties` keys) is excluded from
`tools/list` results on modern connections, with a warning naming the tool.
Legacy connections are unaffected: nothing is learned, mirrored, or excluded.
### Customizing the Faraday Connection
You can pass a block to `MCP::Client::HTTP.new` to customize the underlying Faraday connection.
The block is called after the default middleware is configured, so you can add middleware or swap the HTTP adapter:
```ruby
http_transport = MCP::Client::HTTP.new(url: "https://api.example.com/mcp") do |faraday|
faraday.use MyApp::Middleware::HttpRecorder
faraday.adapter :typhoeus
end
```
The block customizes only the connection to the MCP server. The connection the OAuth flow uses for its own requests is customized through
the provider's `http_client_customizer:` keyword instead; see [Customizing the OAuth HTTP Client](/client/authorization/#customizing-the-oauth-http-client).
{: .note }
> Answers to server-to-client requests (a pong, an elicitation result) are POSTed from inside
> the SSE streaming callback of another response, re-entering the connection on the same thread.
> The default Net::HTTP adapter opens a connection per request, which makes this safe; an adapter
> with persistent connections must tolerate that re-entry.
## Custom Transports
If the transport layer you need is not included in the gem, you can build and pass your own instances so long as they conform to the following interface:
```ruby
class CustomTransport
# Sends a JSON-RPC request to the server and returns the raw response.
#
# @param request [Hash] A complete JSON-RPC request object.
# https://www.jsonrpc.org/specification#request_object
# @return [Hash] A hash modeling a JSON-RPC response object.
# https://www.jsonrpc.org/specification#response_object
def send_request(request:)
# Your transport-specific logic here
# - HTTP: POST to endpoint with JSON body
# - WebSocket: Send message over WebSocket
# - stdio: Write to stdout, read from stdin
# - etc.
end
end
client = MCP::Client.new(transport: CustomTransport.new)
```
Custom transports that need to support client-side cancellation have additional requirements;
see [Custom transports](/client/cancellation/#custom-transports) on the Cancellation page.

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@ -2,44 +2,18 @@
title: "MCP Ruby SDK" title: "MCP Ruby SDK"
description: "The official Ruby SDK for Model Context Protocol servers and clients." description: "The official Ruby SDK for Model Context Protocol servers and clients."
remote_theme: just-the-docs/just-the-docs remote_theme: just-the-docs/just-the-docs
color_scheme: ruby-light
plugins: # Include generated files and directories which may start with underscores
- jekyll-redirect-from include:
- "_*"
# Sidebar sections: each collection renders as a plain category heading
# with its pages listed beneath it, instead of a clickable parent page.
collections:
server:
permalink: "/server/:name/"
output: true
client:
permalink: "/client/:name/"
output: true
extensions:
permalink: "/extensions/:name/"
output: true
just_the_docs:
collections:
server:
name: Building Servers
client:
name: Building Clients
extensions:
name: Extensions
# Search # Search
search_enabled: true search_enabled: true
# Callouts # Footer
callouts: gh_edit_link: true
note: gh_edit_link_text: "Edit this page on GitHub."
title: Note gh_edit_repository: "https://github.com/modelcontextprotocol/ruby-sdk"
color: blue gh_edit_branch: "main"
important: gh_edit_source: "docs"
title: Important gh_edit_view_mode: "edit"
color: yellow
warning:
title: Warning
color: red

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@ -1,35 +0,0 @@
---
layout: default
title: Capability Extensions
nav_order: 2
---
# Capability Extensions
Per SEP-2133, both clients and servers can declare protocol extensions under the `extensions` member of their capabilities.
Keys are extension identifiers using the reverse-DNS prefix convention (e.g. `"io.modelcontextprotocol/tasks"`, `"com.example/feature"`);
values are extension-defined configuration objects, with `{}` meaning "supported with no settings".
On the server, declare extensions through the `capabilities` keyword, either as a plain hash or via the `MCP::Server::Capabilities` builder:
```ruby
capabilities = MCP::Server::Capabilities.new
capabilities.support_tools
capabilities.support_extensions("com.example/feature" => { enabled: true })
server = MCP::Server.new(name: "my_server", capabilities: capabilities)
```
The declared extensions appear in the `initialize` result's `capabilities.extensions`. Extensions the client declared during `initialize` are
readable via `server.client_capabilities[:extensions]` (or `session.client_capabilities[:extensions]` for per-session transports).
On the [modern lifecycle](/server/discover/#the-stateless-modern-lifecycle), the same declarations appear in the `server/discover` result,
and the client's extensions ride each request's `_meta` envelope instead of an `initialize` handshake.
Inside a handler, `server_context.client_capabilities[:extensions]` reads the current request's declarations
with envelope-first resolution, on either lifecycle.
On the client, pass extensions through `connect`:
```ruby
client.connect(capabilities: { extensions: { "com.example/feature" => {} } })
```

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@ -1,17 +0,0 @@
---
layout: default
title: Overview
nav_order: 1
permalink: /extensions/
---
# Extensions
Extensions add optional functionality on top of the core MCP protocol.
Support for an extension is negotiated per connection:
clients and servers declare the extensions they speak under the `extensions` member of their capabilities,
as described on [Capability Extensions](/extensions/capability-extensions/).
The extensions this SDK ships support for:
- [MCP Apps](/extensions/mcp-apps/) (SEP-1865) - interactive HTML user interfaces rendered by the host for tool results

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