MCP & ChatGPT apps guide

MCP server authentication: an OAuth 2.1 guide for SaaS teams

Updated 2026-10-26

Short answer: A remote MCP server is an OAuth resource server. It advertises its authorization server via protected resource metadata, returns 401 with a WWW-Authenticate header, and validates access tokens issued for it. The client handles the PKCE flow.

The flow

The client calls your MCP endpoint without a token and gets 401 with a resource_metadata link.
It reads /.well-known/oauth-protected-resource to find your authorization server.
It reads the authorization server metadata and registers itself (client ID metadata document or dynamic client registration, depending on spec version and your provider).
The user signs in and consents; the client exchanges the code with PKCE.
Every MCP request carries Authorization: Bearer <token>; you validate issuer, audience (your resource URL), expiry and scopes.

Protected resource metadata

GET /.well-known/oauth-protected-resource
{
  "resource": "https://mcp.acme.com/mcp",
  "authorization_servers": ["https://auth.acme.com"],
  "scopes_supported": ["invoices:read", "invoices:write"]
}

# Unauthenticated request to /mcp returns:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer resource_metadata="https://mcp.acme.com/.well-known/oauth-protected-resource"

Practical choices

Reuse your existing identity provider (Auth0, Clerk, WorkOS, Cognito, Keycloak) if it supports the needed metadata and registration.
Map scopes to tool groups, such as invoices:read for read tools and invoices:write for writes.
Never pass the MCP access token straight through to your upstream API unless it was issued for it; exchange it or call with your own service credentials scoped to that user.
Short-lived access tokens, refresh tokens, and a way for users to revoke the connection.

For a pilot

Use stdio and an API key in an env variable. It is fine for a developer audience and lets you validate tools before investing in OAuth.

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Agencies in our index that mention MCP work

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Teamvoy (Ukraine)
AiAdvisors (Poland)
Context Studios (Germany)
AI Agency Manchester (United Kingdom)
Focused (United States)
Autor Technologies (Canada)
Pentridge (United States)
Hourglass AI (Australia)
Enabled Minds (United States)
AppStream Studio (United States)

Questions

Do I need dynamic client registration?

Clients need some way to register. Depending on the spec version, that is a client ID metadata document, dynamic client registration, or a pre-registered client. Check what your target clients support.

Can I use API keys for a remote server?

Technically yes, but users then paste secrets into an AI client. OAuth is the expected pattern for multi-user remote servers.

What is the most common bug?

Not checking the token audience, which lets tokens issued for another service work on yours.

More MCP guides

Cost and effort figures on this page are rough estimates, not quotes.