diff --git a/src/server/auth-client.test.ts b/src/server/auth-client.test.ts
index ed107b41..aeb2cb25 100644
--- a/src/server/auth-client.test.ts
+++ b/src/server/auth-client.test.ts
@@ -5436,6 +5436,260 @@ ca/T0LLtgmbMmxSv/MmzIg==
         });
       });
     });
+
+    describe("scope preservation", async () => {
+      it("should preserve the original scope when refreshing access token", async () => {
+        const secret = await generateSecret(32);
+        const transactionStore = new TransactionStore({
+          secret
+        });
+        const sessionStore = new StatelessSessionStore({
+          secret
+        });
+        const authClient = new AuthClient({
+          transactionStore,
+          sessionStore,
+
+          domain: DEFAULT.domain,
+          clientId: DEFAULT.clientId,
+          clientSecret: DEFAULT.clientSecret,
+
+          secret,
+          appBaseUrl: DEFAULT.appBaseUrl,
+
+          routes: getDefaultRoutes(),
+
+          fetch: getMockAuthorizationServer({
+            tokenEndpointResponse: {
+              token_type: "Bearer",
+              access_token: "new_at_123",
+              refresh_token: "new_rt_123",
+              scope: "openid profile email custom_scope",
+              expires_in: 86400
+            } as oauth.TokenEndpointResponse
+          })
+        });
+
+        const expiresAt = Math.floor(Date.now() / 1000) - 10; // expired 10 seconds ago
+        const tokenSet = {
+          accessToken: DEFAULT.accessToken,
+          refreshToken: DEFAULT.refreshToken,
+          scope: "openid profile email custom_scope",
+          expiresAt
+        };
+
+        const [error, updatedTokenSet] = await authClient.getTokenSet(tokenSet);
+        expect(error).toBeNull();
+        expect(updatedTokenSet?.tokenSet).toEqual({
+          accessToken: "new_at_123",
+          refreshToken: "new_rt_123",
+          scope: "openid profile email custom_scope", // Scope preserved from response
+          expiresAt: expect.any(Number)
+        });
+      });
+
+      it("should fallback to original scope when response does not include scope", async () => {
+        const secret = await generateSecret(32);
+        const transactionStore = new TransactionStore({
+          secret
+        });
+        const sessionStore = new StatelessSessionStore({
+          secret
+        });
+        const authClient = new AuthClient({
+          transactionStore,
+          sessionStore,
+
+          domain: DEFAULT.domain,
+          clientId: DEFAULT.clientId,
+          clientSecret: DEFAULT.clientSecret,
+
+          secret,
+          appBaseUrl: DEFAULT.appBaseUrl,
+
+          routes: getDefaultRoutes(),
+
+          fetch: getMockAuthorizationServer({
+            tokenEndpointResponse: {
+              token_type: "Bearer",
+              access_token: "new_at_123",
+              refresh_token: "new_rt_123",
+              // No scope in response
+              expires_in: 86400
+            } as oauth.TokenEndpointResponse
+          })
+        });
+
+        const expiresAt = Math.floor(Date.now() / 1000) - 10; // expired 10 seconds ago
+        const tokenSet = {
+          accessToken: DEFAULT.accessToken,
+          refreshToken: DEFAULT.refreshToken,
+          scope: "openid profile email custom_scope",
+          expiresAt
+        };
+
+        const [error, updatedTokenSet] = await authClient.getTokenSet(tokenSet);
+        expect(error).toBeNull();
+        expect(updatedTokenSet?.tokenSet).toEqual({
+          accessToken: "new_at_123",
+          refreshToken: "new_rt_123",
+          scope: "openid profile email custom_scope", // Fallback to original scope
+          expiresAt: expect.any(Number)
+        });
+      });
+
+      it("should include scope parameter in refresh token request when scope exists", async () => {
+        const secret = await generateSecret(32);
+        const transactionStore = new TransactionStore({
+          secret
+        });
+        const sessionStore = new StatelessSessionStore({
+          secret
+        });
+
+        let refreshTokenRequestBody: string;
+        const mockFetch = vi.fn(
+          async (
+            input: RequestInfo | URL,
+            init?: RequestInit
+          ): Promise<Response> => {
+            let url: URL;
+            if (input instanceof Request) {
+              url = new URL(input.url);
+            } else {
+              url = new URL(input);
+            }
+
+            if (url.pathname === "/oauth/token" && init?.body) {
+              refreshTokenRequestBody = init.body as string;
+            }
+
+            // Use the default mock behavior
+            return getMockAuthorizationServer({
+              tokenEndpointResponse: {
+                token_type: "Bearer",
+                access_token: "new_at_123",
+                refresh_token: "new_rt_123",
+                scope: "openid profile email custom_scope",
+                expires_in: 86400
+              } as oauth.TokenEndpointResponse
+            })(input, init);
+          }
+        );
+
+        const authClient = new AuthClient({
+          transactionStore,
+          sessionStore,
+
+          domain: DEFAULT.domain,
+          clientId: DEFAULT.clientId,
+          clientSecret: DEFAULT.clientSecret,
+
+          secret,
+          appBaseUrl: DEFAULT.appBaseUrl,
+
+          routes: getDefaultRoutes(),
+
+          fetch: mockFetch
+        });
+
+        const expiresAt = Math.floor(Date.now() / 1000) - 10; // expired 10 seconds ago
+        const tokenSet = {
+          accessToken: DEFAULT.accessToken,
+          refreshToken: DEFAULT.refreshToken,
+          scope: "openid profile email custom_scope",
+          expiresAt
+        };
+
+        const [error, updatedTokenSet] = await authClient.getTokenSet(tokenSet);
+        expect(error).toBeNull();
+        expect(updatedTokenSet?.tokenSet).toEqual({
+          accessToken: "new_at_123",
+          refreshToken: "new_rt_123",
+          scope: "openid profile email custom_scope",
+          expiresAt: expect.any(Number)
+        });
+
+        // Verify that scope parameter was included in the refresh token request
+        const params = new URLSearchParams(refreshTokenRequestBody!);
+        expect(params.get("scope")).toBe("openid profile email custom_scope");
+      });
+
+      it("should not include scope parameter when tokenSet has no scope", async () => {
+        const secret = await generateSecret(32);
+        const transactionStore = new TransactionStore({
+          secret
+        });
+        const sessionStore = new StatelessSessionStore({
+          secret
+        });
+
+        let refreshTokenRequestBody: string;
+        const mockFetch = vi.fn(
+          async (
+            input: RequestInfo | URL,
+            init?: RequestInit
+          ): Promise<Response> => {
+            let url: URL;
+            if (input instanceof Request) {
+              url = new URL(input.url);
+            } else {
+              url = new URL(input);
+            }
+
+            if (url.pathname === "/oauth/token" && init?.body) {
+              refreshTokenRequestBody = init.body as string;
+            }
+
+            // Use the default mock behavior
+            return getMockAuthorizationServer({
+              tokenEndpointResponse: {
+                token_type: "Bearer",
+                access_token: "new_at_123",
+                refresh_token: "new_rt_123",
+                expires_in: 86400
+              } as oauth.TokenEndpointResponse
+            })(input, init);
+          }
+        );
+
+        const authClient = new AuthClient({
+          transactionStore,
+          sessionStore,
+
+          domain: DEFAULT.domain,
+          clientId: DEFAULT.clientId,
+          clientSecret: DEFAULT.clientSecret,
+
+          secret,
+          appBaseUrl: DEFAULT.appBaseUrl,
+
+          routes: getDefaultRoutes(),
+
+          fetch: mockFetch
+        });
+
+        const expiresAt = Math.floor(Date.now() / 1000) - 10; // expired 10 seconds ago
+        const tokenSet = {
+          accessToken: DEFAULT.accessToken,
+          refreshToken: DEFAULT.refreshToken,
+          // No scope in tokenSet
+          expiresAt
+        };
+
+        const [error, updatedTokenSet] = await authClient.getTokenSet(tokenSet);
+        expect(error).toBeNull();
+        expect(updatedTokenSet?.tokenSet).toEqual({
+          accessToken: "new_at_123",
+          refreshToken: "new_rt_123",
+          expiresAt: expect.any(Number)
+        });
+
+        // Verify that scope parameter was NOT included in the refresh token request
+        const params = new URLSearchParams(refreshTokenRequestBody!);
+        expect(params.get("scope")).toBeNull();
+      });
+    });
   });
 
   describe("startInteractiveLogin", async () => {
diff --git a/src/server/auth-client.ts b/src/server/auth-client.ts
index 0fecb253..7af2e171 100644
--- a/src/server/auth-client.ts
+++ b/src/server/auth-client.ts
@@ -824,7 +824,10 @@ export class AuthClient {
           {
             ...this.httpOptions(),
             [oauth.customFetch]: this.fetch,
-            [oauth.allowInsecureRequests]: this.allowInsecureRequests
+            [oauth.allowInsecureRequests]: this.allowInsecureRequests,
+            ...(tokenSet.scope && {
+              additionalParameters: { scope: tokenSet.scope }
+            })
           }
         );
 
@@ -857,6 +860,7 @@ export class AuthClient {
           ...tokenSet, // contains the existing `iat` claim to maintain the session lifetime
           accessToken: oauthRes.access_token,
           idToken: oauthRes.id_token,
+          scope: oauthRes.scope || tokenSet.scope, // Preserve original scope as fallback
           expiresAt: accessTokenExpiresAt
         };