Add prompt=consent for OIDC offline_access scope (modelcontextprotocol#681)

Author: dankelleher

src/client/auth.test.ts

@@ -714,6 +714,20 @@ describe("OAuth Authorization", () => {
       expect(authorizationUrl.searchParams.has("state")).toBe(false);
     });
 
+    // OpenID Connect requires that the user is prompted for consent if the scope includes 'offline_access'
+    it("includes consent prompt parameter if scope includes 'offline_access'", async () => {
+      const { authorizationUrl } = await startAuthorization(
+          "https://auth.example.com",
+          {
+            clientInformation: validClientInfo,
+            redirectUrl: "http://localhost:3000/callback",
+            scope: "read write profile offline_access",
+          }
+      );
+
+      expect(authorizationUrl.searchParams.get("prompt")).toBe("consent");
+    });
+
     it("uses metadata authorization_endpoint when provided", async () => {
       const { authorizationUrl } = await startAuthorization(
         "https://auth.example.com",

src/client/auth.ts

@@ -614,6 +614,13 @@ export async function startAuthorization(
     authorizationUrl.searchParams.set("scope", scope);
   }
 
+  if (scope?.includes("offline_access")) {
+    // if the request includes the OIDC-only "offline_access" scope,
+    // we need to set the prompt to "consent" to ensure the user is prompted to grant offline access
+    // https://openid.net/specs/openid-connect-core-1_0.html#OfflineAccess
+    authorizationUrl.searchParams.append("prompt", "consent");
+  }
+
   if (resource) {
     authorizationUrl.searchParams.set("resource", resource.href);
   }