diff --git a/packages/core/src/routes/session/index.ts b/packages/core/src/routes/session/index.ts
index 3c6202fa4..9cde41dce 100644
--- a/packages/core/src/routes/session/index.ts
+++ b/packages/core/src/routes/session/index.ts
@@ -10,11 +10,15 @@ import { assignInteractionResults, saveUserFirstConsentedAppId } from '@/lib/ses
 import assertThat from '@/utils/assert-that';
 
 import { AnonymousRouter } from '../types';
+import koaGuardSessionAction from './middleware/koa-guard-session-action';
 import passwordlessRoutes from './passwordless';
 import socialRoutes from './social';
 import usernamePasswordRoutes from './username-password';
 
 export default function sessionRoutes<T extends AnonymousRouter>(router: T, provider: Provider) {
+  router.use('/session/sign-in', koaGuardSessionAction(provider, 'sign-in'));
+  router.use('/session/register', koaGuardSessionAction(provider, 'register'));
+
   router.post('/session', async (ctx, next) => {
     const {
       prompt: { name },
diff --git a/packages/core/src/routes/session/middleware/koa-guard-session-action.ts b/packages/core/src/routes/session/middleware/koa-guard-session-action.ts
new file mode 100644
index 000000000..ee756e134
--- /dev/null
+++ b/packages/core/src/routes/session/middleware/koa-guard-session-action.ts
@@ -0,0 +1,48 @@
+import { SignInMode } from '@logto/schemas';
+import { adminConsoleApplicationId } from '@logto/schemas/lib/seeds';
+import { MiddlewareType } from 'koa';
+import { Provider, errors } from 'oidc-provider';
+
+import RequestError from '@/errors/RequestError';
+import { findDefaultSignInExperience } from '@/queries/sign-in-experience';
+import assertThat from '@/utils/assert-that';
+
+export default function KoaGuardSessionAction<StateT, ContextT, ResponseBodyT>(
+  provider: Provider,
+  forType: 'sign-in' | 'register'
+): MiddlewareType<StateT, ContextT, ResponseBodyT> {
+  const forbiddenError = new RequestError({ code: 'auth.forbidden', status: 403 });
+
+  return async (ctx, next) => {
+    const interaction = await provider
+      .interactionDetails(ctx.req, ctx.res)
+      .catch((error: unknown) => {
+        // Should not block if interaction is not found
+        if (error instanceof errors.SessionNotFound) {
+          return null;
+        }
+
+        throw error;
+      });
+
+    /**
+     * We don't guard admin console in API for now since logically there's no need.
+     * Update to honor the config if we're implementing per-app SIE.
+     */
+    if (interaction?.params.client_id === adminConsoleApplicationId) {
+      return next();
+    }
+
+    const { signInMode } = await findDefaultSignInExperience();
+
+    if (forType === 'sign-in') {
+      assertThat(signInMode !== SignInMode.Register, forbiddenError);
+    }
+
+    if (forType === 'register') {
+      assertThat(signInMode !== SignInMode.SignIn, forbiddenError);
+    }
+
+    return next();
+  };
+}
diff --git a/packages/core/src/routes/session/username-password.test.ts b/packages/core/src/routes/session/username-password.test.ts
index cf5e5ddb2..f32efcd66 100644
--- a/packages/core/src/routes/session/username-password.test.ts
+++ b/packages/core/src/routes/session/username-password.test.ts
@@ -6,7 +6,7 @@ import { mockUser } from '@/__mocks__';
 import RequestError from '@/errors/RequestError';
 import { createRequester } from '@/utils/test-utils';
 
-import sessionRoutes from '.';
+import sessionRoutes from './username-password';
 
 const insertUser = jest.fn(async (..._args: unknown[]) => ({ id: 'id' }));
 const findUserById = jest.fn(async (): Promise<User> => mockUser);