Update authorization.mdx

Author: localden

docs/specification/draft/basic/authorization.mdx

@@ -178,19 +178,36 @@ sequenceDiagram
         A->>C: Client Credentials
     end
 
-    Note over C: Generate PKCE parameters
-    C->>B: Open browser with authorization URL + code_challenge
-    B->>A: Authorization request
+    Note over C: Generate PKCE parameters<br/>Include resource parameter
+    C->>B: Open browser with authorization URL + code_challenge + resource
+    B->>A: Authorization request with resource parameter
     Note over A: User authorizes
     A->>B: Redirect to callback with authorization code
     B->>C: Authorization code callback
-    C->>A: Token request + code_verifier
+    C->>A: Token request + code_verifier + resource
     A->>C: Access token (+ refresh token)
     C->>M: MCP request with access token
     M-->>C: MCP response
     Note over C,M: MCP communication continues with valid token
 ```
 
+#### Resource Parameter Implementation
+
+MCP clients **MUST** implement Resource Indicators for OAuth 2.0 as defined in [RFC 8707](https://www.rfc-editor.org/rfc/rfc8707.html)
+to explicitly specify the target resource for which the token is being requested. The `resource` parameter:
+
+1. **MUST** be included in both authorization requests and token requests.
+2. **MUST** identify the MCP server that the client intends to use the token with.
+3. **MUST** use the canonical URI of the MCP server as defined in [RFC 8707 Section 2](https://www.rfc-editor.org/rfc/rfc8707.html#name-access-token-request).
+
+For example, if accessing an MCP server at `https://mcp.example.com`, the authorization request would include:
+
+```
+&resource=https%3A%2F%2Fmcp.example.com
+```
+
+MCP clients **MUST** send this parameter regardless of whether authorization servers support it.
+
 ### Access Token Usage
 
 #### Token Requirements
@@ -317,4 +334,5 @@ MCP servers **MUST** only accept tokens specifically intended for themselves.
 
 If the MCP server makes requests to upstream APIs, it may act as an OAuth client to them. The access token used at the upstream API is a seperate token, issued by the upstream authorization server. The MCP server **MUST NOT** pass through the token it received from the MCP client.
 
-If the authorization server supports the `resource` parameter, it is recommended that implementers follow [RFC 8707](https://www.rfc-editor.org/rfc/rfc8707.html) to prevent token misuse.
+MCP clients **MUST** implement and use the `resource` parameter as defined in [RFC 8707 - Resource Indicators for OAuth 2.0](https://www.rfc-editor.org/rfc/rfc8707.html) to explicitly specify the target resource for which the token is being requested.
+This ensures that access tokens are bound to their intended resources and cannot be misused across different services.