Add Check PCKE Support (#563)

Refer To [Issue
473](https://github.com/modelcontextprotocol/go-sdk/issues/473)
Changes:
- Add check for PKCE support via server metadata and return an error if
not supported.

oauthex/auth_meta.go

@@ -139,6 +139,11 @@ func GetAuthServerMeta(ctx context.Context, issuerURL string, c *http.Client) (*
 				// Security violation; don't keep trying.
 				return nil, fmt.Errorf("metadata issuer %q does not match issuer URL %q", asm.Issuer, issuerURL)
 			}
+
+			if len(asm.CodeChallengeMethodsSupported) == 0 {
+				return nil, fmt.Errorf("authorization server at %s does not implement PKCE", issuerURL)
+			}
+
 			return asm, nil
 		}
 		errs = append(errs, err)

oauthex/auth_meta_test.go

@@ -7,9 +7,14 @@
 package oauthex
 
 import (
+	"context"
 	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -28,3 +33,88 @@ func TestAuthMetaParse(t *testing.T) {
 		t.Errorf("got %q, want %q", g, w)
 	}
 }
+
+func TestGetAuthServerMetaPKCESupport(t *testing.T) {
+	ctx := context.Background()
+	tests := []struct {
+		name           string
+		hasPKCESupport bool
+		wantError      string
+	}{
+		{
+			name:           "server_with_pkce_support",
+			hasPKCESupport: true,
+		},
+		{
+			name:           "server_without_pkce_support",
+			hasPKCESupport: false,
+			wantError:      "does not implement PKCE",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Start a fake OAuth 2.1 auth server
+			wrapper := http.NewServeMux()
+			wrapper.HandleFunc("/.well-known/oauth-authorization-server", func(w http.ResponseWriter, r *http.Request) {
+				u, _ := url.Parse("https://" + r.Host)
+				issuer := "https://localhost:" + u.Port()
+				metadata := AuthServerMeta{
+					Issuer:                            issuer,
+					AuthorizationEndpoint:             issuer + "/authorize",
+					TokenEndpoint:                     issuer + "/token",
+					RegistrationEndpoint:              issuer + "/register",
+					JWKSURI:                           issuer + "/.well-known/jwks.json",
+					ScopesSupported:                   []string{"openid", "profile", "email"},
+					ResponseTypesSupported:            []string{"code"},
+					GrantTypesSupported:               []string{"authorization_code"},
+					TokenEndpointAuthMethodsSupported: []string{"none"},
+				}
+
+				// Add PKCE support based on test case
+				if tt.hasPKCESupport {
+					metadata.CodeChallengeMethodsSupported = []string{"S256"}
+				}
+				// If hasPKCESupport is false, CodeChallengeMethodsSupported remains empty
+
+				w.Header().Set("Content-Type", "application/json")
+				json.NewEncoder(w).Encode(metadata)
+			})
+			ts := httptest.NewTLSServer(wrapper)
+			defer ts.Close()
+
+			// The fake server sets issuer to https://localhost:<port>, so compute that issuer.
+			u, _ := url.Parse(ts.URL)
+			issuer := "https://localhost:" + u.Port()
+
+			// The fake server presents a cert for example.com; set ServerName accordingly.
+			httpClient := ts.Client()
+			if tr, ok := httpClient.Transport.(*http.Transport); ok {
+				clone := tr.Clone()
+				clone.TLSClientConfig.ServerName = "example.com"
+				httpClient.Transport = clone
+			}
+
+			meta, err := GetAuthServerMeta(ctx, issuer, httpClient)
+			if tt.wantError != "" {
+				if err == nil {
+					t.Fatal("wanted error but got none")
+				}
+				if !strings.Contains(err.Error(), tt.wantError) {
+					t.Errorf("wanted error to contain %q, but got: %v", tt.wantError, err)
+				}
+			} else {
+				if err != nil {
+					t.Fatalf("unwanted error: %v", err)
+				}
+				if meta == nil {
+					t.Fatal("wanted metadata but got nil")
+				}
+				// Verify PKCE support is present
+				if len(meta.CodeChallengeMethodsSupported) == 0 {
+					t.Error("wanted PKCE support but CodeChallengeMethodsSupported is empty")
+				}
+			}
+		})
+	}
+}