diff --git a/packages/template/src/components-page/oauth-callback.test.tsx b/packages/template/src/components-page/oauth-callback.test.tsx
index 072b7fdf5..4b5e75dc5 100644
--- a/packages/template/src/components-page/oauth-callback.test.tsx
+++ b/packages/template/src/components-page/oauth-callback.test.tsx
@@ -5,6 +5,7 @@ import React, { act } from "react";
 import { createRoot, type Root } from "react-dom/client";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import type { StackClientApp } from "../lib/hexclave-app/apps/interfaces/client-app";
+import { hexclaveAppInternalsSymbol } from "../lib/hexclave-app/common";
 import { TranslationProviderClient } from "../providers/translation-provider-client";
 import { OAuthCallback } from "./oauth-callback";
 
@@ -40,6 +41,9 @@ function createAppTestDouble(options: {
     callOAuthCallback: options.callOAuthCallback,
     redirectToSignIn: vi.fn(async () => {}),
     redirectToHome: vi.fn(async () => {}),
+    [hexclaveAppInternalsSymbol]: {
+      awaitPendingAuthResolutions: vi.fn(async () => {}),
+    },
   };
 
   // This test double intentionally implements only the StackClientApp surface
diff --git a/packages/template/src/components-page/oauth-callback.tsx b/packages/template/src/components-page/oauth-callback.tsx
index 7b19d635a..56d512e81 100644
--- a/packages/template/src/components-page/oauth-callback.tsx
+++ b/packages/template/src/components-page/oauth-callback.tsx
@@ -8,6 +8,7 @@ import { useEffect, useRef, useState } from "react";
 import { useStackApp } from "..";
 import { MaybeFullPage } from "../components/elements/maybe-full-page";
 import { StyledLink } from "../components/link";
+import { hexclaveAppInternalsSymbol } from "../lib/hexclave-app/common";
 import { useTranslation } from "../lib/translations";
 import { ErrorPage } from "./error-page";
 
@@ -22,6 +23,11 @@ export function OAuthCallback({ fullPage }: { fullPage?: boolean }) {
     if (called.current) return;
     called.current = true;
     try {
+      // The startup handler in StackClientApp's constructor may have already consumed the
+      // one-time OAuth params (code + state cookie) via a microtask that fires before this
+      // macrotask-scheduled useEffect. Await its completion so we don't race: if it succeeds
+      // it will redirect and this page tears down; if it fails we fall through below.
+      await app[hexclaveAppInternalsSymbol].awaitPendingAuthResolutions();
       const hasRedirected = await app.callOAuthCallback();
       if (!hasRedirected) {
         await app.redirectToSignIn({ noRedirectBack: true });
diff --git a/packages/template/src/lib/hexclave-app/apps/implementations/client-app-impl.ts b/packages/template/src/lib/hexclave-app/apps/implementations/client-app-impl.ts
index c20bd6ff7..e7e4e7fb5 100644
--- a/packages/template/src/lib/hexclave-app/apps/implementations/client-app-impl.ts
+++ b/packages/template/src/lib/hexclave-app/apps/implementations/client-app-impl.ts
@@ -4204,6 +4204,9 @@ export class _HexclaveClientAppImplIncomplete<HasTokenStore extends boolean, Pro
       signInWithTokens: async (tokens: { accessToken: string, refreshToken: string }) => {
         await this._signInToAccountWithTokens(tokens);
       },
+      awaitPendingAuthResolutions: async () => {
+        await this._awaitPendingAuthResolutions();
+      },
     };
   };
 
diff --git a/packages/template/src/lib/hexclave-app/apps/interfaces/client-app.ts b/packages/template/src/lib/hexclave-app/apps/interfaces/client-app.ts
index 4f15aa325..53d75d20e 100644
--- a/packages/template/src/lib/hexclave-app/apps/interfaces/client-app.ts
+++ b/packages/template/src/lib/hexclave-app/apps/interfaces/client-app.ts
@@ -136,6 +136,7 @@ export type StackClientApp<HasTokenStore extends boolean = boolean, ProjectId ex
       redirectToUrl(url: string | URL, options?: { replace?: boolean }): Promise<void>,
       redirectToHandler(handlerName: keyof HandlerUrls, options?: RedirectToOptions): Promise<void>,
       signInWithTokens(tokens: { accessToken: string, refreshToken: string }): Promise<void>,
+      awaitPendingAuthResolutions(): Promise<void>,
     },
   }
   & AsyncStoreProperty<"project", [], Project, false>