Add RFC 9728 Protected Resource Metadata for MCP client compatibility

MCP clients (Model Context Protocol, per its Authorization spec at
modelcontextprotocol.io/specification/2025-06-18/basic/authorization)
require an OAuth resource server to implement OAuth 2.0 Protected
Resource Metadata (RFC 9728) and return a WWW-Authenticate header on
401 responses pointing at it. oauth2-proxy currently returns a bare
401 with an empty "{}" JSON body and no WWW-Authenticate header,
which MCP clients cannot parse as a valid OAuth error at all -
confirmed against a real deployment fronting an MCP server.

- New /.well-known/oauth-protected-resource route (host root, not
  under ProxyPrefix - MCP clients build this URL themselves).
- New unauthorizedJSON path used for the existing JSON-error branch
  (forceJSONErrors/isAjax/isAPIPath) in Proxy(), setting
  WWW-Authenticate and a non-empty OAuth-shaped error body.
- Reuses the already-configured OIDC issuer as the advertised
  authorization server - no new CLI flags needed.

See oauth2-proxy/oauth2-proxy#3167.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Mike Costas <mike.costas@gmail.com>

oauthproxy.go

@@ -47,6 +47,7 @@ const (
 	applicationJSON = "application/json"
 
 	robotsPath                    = "/robots.txt"
+	protectedResourceMetadataPath = "/.well-known/oauth-protected-resource"
 	signInPath                    = "/sign_in"
 	signOutPath                   = "/sign_out"
 	oauthStartPath                = "/start"
@@ -118,6 +119,12 @@ type OAuthProxy struct {
 	appDirector       redirect.AppDirector
 
 	encodeState bool
+
+	// oidcIssuerURL is the configured OIDC issuer, exposed via
+	// /.well-known/oauth-protected-resource (RFC 9728) so MCP-spec OAuth
+	// clients can discover the authorization server. See
+	// ProtectedResourceMetadata.
+	oidcIssuerURL string
 }
 
 // NewOAuthProxy creates a new instance of OAuthProxy from the options provided
@@ -253,6 +260,7 @@ func NewOAuthProxy(opts *options.Options, validator func(string) bool) (*OAuthPr
 		redirectValidator:  redirectValidator,
 		appDirector:        appDirector,
 		encodeState:        opts.EncodeState,
+		oidcIssuerURL:      opts.Providers[0].OIDCConfig.IssuerURL,
 	}
 	p.buildServeMux(opts.ProxyPrefix)
 
@@ -325,6 +333,12 @@ func (p *OAuthProxy) buildServeMux(proxyPrefix string) {
 	// Register the robots path writer
 	r.Path(robotsPath).HandlerFunc(p.pageWriter.WriteRobotsTxt)
 
+	// RFC 9728 Protected Resource Metadata MUST live at the host root, not
+	// under proxyPrefix (unlike sign_in/start/callback below) - MCP clients
+	// build this well-known URL themselves and don't know about our
+	// ProxyPrefix. See ProtectedResourceMetadata.
+	r.Path(protectedResourceMetadataPath).HandlerFunc(p.ProtectedResourceMetadata)
+
 	// The authonly path should be registered separately to prevent it from getting no-cache headers.
 	// We do this to allow users to have a short cache (via nginx) of the response to reduce the
 	// likelihood of multiple requests trying to refresh sessions simultaneously.
@@ -1057,7 +1071,7 @@ func (p *OAuthProxy) Proxy(rw http.ResponseWriter, req *http.Request) {
 		if p.forceJSONErrors || isAjax(req) || p.isAPIPath(req) {
 			logger.Printf("No valid authentication in request. Access Denied.")
 			// no point redirecting an AJAX request
-			p.errorJSON(rw, http.StatusUnauthorized)
+			p.unauthorizedJSON(rw, req)
 			return
 		}
 
@@ -1344,6 +1358,49 @@ func (p *OAuthProxy) errorJSON(rw http.ResponseWriter, code int) {
 	rw.Write([]byte("{}"))
 }
 
+// unauthorizedJSON returns a 401 with a WWW-Authenticate header pointing at
+// this server's OAuth 2.0 Protected Resource Metadata document (RFC 9728),
+// and a non-empty OAuth-shaped JSON error body (RFC 6750 section 3) -
+// required for MCP clients (modelcontextprotocol.io/specification/
+// 2025-06-18/basic/authorization) to discover how to authenticate. Plain
+// errorJSON's empty "{}" body and missing header left MCP clients unable
+// to parse the response as a valid OAuth error at all.
+func (p *OAuthProxy) unauthorizedJSON(rw http.ResponseWriter, req *http.Request) {
+	metadataURL := url.URL{
+		Scheme: requestutil.GetRequestProto(req),
+		Host:   requestutil.GetRequestHost(req),
+		Path:   protectedResourceMetadataPath,
+	}
+	rw.Header().Set("WWW-Authenticate", fmt.Sprintf(`Bearer resource_metadata=%q`, metadataURL.String()))
+	rw.Header().Set("Content-Type", applicationJSON)
+	rw.WriteHeader(http.StatusUnauthorized)
+	rw.Write([]byte(`{"error":"unauthorized","error_description":"authentication required"}`))
+}
+
+// ProtectedResourceMetadata serves OAuth 2.0 Protected Resource Metadata
+// (RFC 9728) at the well-known path MCP clients fetch after receiving a 401
+// with a WWW-Authenticate header (see unauthorizedJSON). Points clients at
+// the OIDC issuer this proxy is already configured with - the issuer's own
+// RFC 8414 Authorization Server Metadata (a document oauth2-proxy does not
+// need to serve itself) tells the client where to actually authenticate.
+func (p *OAuthProxy) ProtectedResourceMetadata(rw http.ResponseWriter, req *http.Request) {
+	resource := url.URL{
+		Scheme: requestutil.GetRequestProto(req),
+		Host:   requestutil.GetRequestHost(req),
+	}
+	body, err := json.Marshal(map[string]interface{}{
+		"resource":              resource.String(),
+		"authorization_servers": []string{p.oidcIssuerURL},
+	})
+	if err != nil {
+		p.errorJSON(rw, http.StatusInternalServerError)
+		return
+	}
+	rw.Header().Set("Content-Type", applicationJSON)
+	rw.WriteHeader(http.StatusOK)
+	rw.Write(body)
+}
+
 // LoggingCSRFCookiesInOAuthCallback Log all CSRF cookies found in HTTP request OAuth callback,
 // which were successfully parsed
 func LoggingCSRFCookiesInOAuthCallback(req *http.Request, cookieName string) {