To make OAuth work for an HTTP-based MCP server, follow the full trust chain: discover the authorization server from the protected resource’s metadata, validate that server’s metadata, use authorization code with PKCE and the MCP resource indicator, then validate that the returned access token is intended for your server. A successful login alone is not enough: the resource server must still authenticate the token and check its audience and permissions.
This guide follows the MCP 2026-07-28 authorization specification. Its transport-level authorization flow applies to HTTP-based MCP transports; for STDIO, the specification says implementations should obtain credentials from the environment. See the MCP authorization specification.
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How MCP authorization discovery works
Discovery starts at the protected MCP resource, not with a client guessing an identity provider. The resource publishes OAuth 2.0 Protected Resource Metadata (RFC 9728), which identifies one or more authorization servers. A protected server must implement this metadata and advertise at least one authorization server.
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The server can point to the metadata URL in a WWW-Authenticate challenge returned with HTTP 401, using resource_metadata, or serve the metadata at the applicable well-known URI. MCP clients must support both discovery methods. The full lookup rules, including well-known URI forms, are in the authorization-server discovery specification.
Validate the authorization-server metadata
Once the client has the authorization server’s issuer, it retrieves authorization-server metadata using OAuth Authorization Server Metadata (RFC 8414) or OpenID Connect Discovery. When the issuer contains a path, discovery has distinct path-insertion and path-appending well-known URL forms, tried in the specification’s defined order. Implement that procedure rather than constructing a single guessed URL.
Before using the advertised endpoints, compare the metadata’s issuer value with the issuer used to construct the metadata URL. Reject a mismatch. This prevents a client from fetching metadata at one location and treating endpoints for a different issuer as trusted.
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Run the authorization-code flow with PKCE
After validating discovery data, the client needs a client ID. The current MCP specification lists client ID metadata documents (CIMD), pre-registration, and dynamic client registration (DCR) as mechanisms in a defined priority order. Support varies across deployed servers, so check which mechanisms the particular server accepts. The July 2026 release announcement says DCR remains for backward compatibility and is intended for removal in a future specification version; it is not safe to assume every older deployment has moved to CIMD. See the MCP release announcement.
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minute- Create a transaction-specific PKCE verifier. Keep the verifier associated with this authorization attempt, the validated issuer, and the state value if you use state. Do not reuse a verifier across unrelated transactions. An MCP Ruby SDK authorization guide describes PKCE S256 for its authorization-code flow; confirm support in the SDK and identity provider you use.
- Include the resource indicator in the authorization request. Set
resourceto the canonical URI of the MCP server the client intends to access. This tells the authorization server which protected resource the authorization is for. - Verify the authorization response issuer. Record the validated issuer with the transaction. Before redeeming the code, compare the response’s
issvalue with that issuer. If metadata indicates the issuer response parameter is supported but it is absent, reject the response; reject a present value that does not match as well. - Redeem the code with PKCE and the resource indicator. Include
resourcein the token request too, along with the transaction’s PKCE verifier and the other parameters required by the authorization-code flow. The resource indicator must identify the intended MCP server in both requests.
Validate access tokens at the MCP resource server
For each protected HTTP request, the client sends the access token in the Authorization header as Bearer <access-token>. Do not place bearer tokens in a URL query string. The MCP server must validate the token according to OAuth resource-request requirements and confirm that it was issued for that server as the intended audience.
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Use validation appropriate to the token format
Do not assume every access token is a JWT. The issuer determines the token format and the validation method available to the resource server. For a JWT-based deployment, typical checks include validating the signature with keys from the issuer’s JWKS and checking the expected issuer and audience. The MCP PHP SDK authorization guide demonstrates a validator configured with an issuer, audience, and JWKS provider, along with OIDC discovery and JWKS caching. These are implementation examples, not a requirement that all providers issue JWT access tokens or use identical configuration.
Separate authentication failures from permission failures
| Response | Meaning | Server action |
|---|---|---|
| HTTP 401 | The token is missing, invalid, or expired. | Reject the request; use an appropriate authorization challenge to help the client discover authorization metadata. |
| HTTP 403 | The token is valid, but lacks permission for the requested operation. | Reject the operation and, where appropriate, return error="insufficient_scope" with the required scopes in WWW-Authenticate. |
Check scopes before protected operations. For step-up authorization, clients should preserve previously requested scopes and add scopes specified by the current challenge. They should not assume the challenge’s scope set is necessarily a subset or superset of the authorization metadata’s scopes_supported.
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What changed in MCP 2026-07-28
The current specification hardens issuer binding: authorization servers should return iss, clients must validate it before redeeming an authorization code, and credentials are bound to the issuer that minted them. It also establishes CIMD as the standard direction while retaining DCR for backward compatibility. The transition is not necessarily complete in deployed systems, so use the registration mechanisms each server actually supports. See the July 28, 2026 release announcement and the authorization specification.
Diagnose common integration failures
- The client cannot find an authorization server: Check that the protected resource implements RFC 9728 metadata and that the client handles both a
resource_metadataURL in a 401 challenge and the applicable well-known URI. - Discovery succeeds but authorization fails: Verify the issuer equality check and that the client follows the specified well-known URL construction and lookup order, especially for issuers with a path.
- The code exchange is rejected: Confirm the verifier belongs to the current authorization transaction, the flow uses the matching PKCE challenge, and the authorization response’s
issmatches the recorded issuer. - The resource server rejects a token after login: Check that both authorization and token requests carried the intended canonical
resource, and that token validation checks the audience expected by the MCP server. - A protected operation returns 403: Treat this as a scope or permission issue, not as proof that authentication failed. Inspect the
WWW-Authenticatechallenge and requested permissions.
Choosing an SDK or identity-provider integration
Compare implementations against the behaviors your deployment needs rather than looking for a universal best provider: discovery support and issuer-path handling; CIMD, pre-registration, or DCR support; PKCE S256; resource-indicator handling and audience issuance; token formats and JWKS rotation; and scope challenges and step-up behavior. The PHP SDK guide includes Keycloak, Microsoft Entra ID, Auth0, and Okta as integration examples, but their inclusion is not a comparative endorsement or evidence that they share the same token configuration.
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