Firestore Error: "16 UNAUTHENTICATED: Request had invalid authentication credentials" even after sending Firebase ID Token in Authorization header

I am building a project where the frontend is React, and the backend nodejs uses. I have used the firebase Admin SDK to use firestore . Authentication is done using Firebase Authentication, and after login I store the Firebase ID token in a cookie.

When trying to write to Firestore from the backend, I always get this error:

[Firestore] Error: 16 UNAUTHENTICATED: Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie, or other valid authentication credential.

Frontend (React)

After Firebase login:

const token = await user.getIdToken();
document.cookie = `token=${token}; path=/`;

When calling my backend API, I send the token manually:

fetch("/api/saveData", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    Authorization: `Bearer ${token}`,
  },
  body: JSON.stringify({ value: "test" }),
});

Backend

I extract the token from the header and set Firestore:

import admin from "firebase-admin";

const serviceAccount = require("./firebase-service.json");

if (!admin.apps.length) {
  admin.initializeApp({
    credential: admin.credential.cert(serviceAccount),
  });
}

const db = admin.firestore();

export async function saveData(req, res) {
  const auth = req.headers.authorization;  // Bearer token

  try {
    const decoded = await admin.auth().verifyIdToken(auth.replace("Bearer ", ""));
    await db.collection("users").doc(decoded.uid).set({ test: "data" });
    res.json({ success: true });
  } catch (error) {
    console.error(error);
    res.status(401).json({ error: error.message });
  }
}

But the server still throws:

Error: 16 UNAUTHENTICATED: Request had invalid authentication credentials.

What I tried

✔ Confirmed the Firebase ID token is valid (checked in jwt.io)
✔ Using admin.initializeApp with service account
✔ Sending token in Authorization header
✔ Verified the token before writing to Firestore (it succeeds!)
✔ Ensured the service account JSON is valid
✔ Tried regenerating API key
✔ Disabled Firestore security rules temporarily (still same error)

Why does Firestore Admin SDK still throw 16 UNAUTHENTICATED even though the Firebase ID token is valid and verified? What am I missing?

Project setup details

• Frontend: React

• Backend: Node.js (Express)

• Using Firebase Admin SDK to write to Firestore (server-side)

• Using Firebase Auth client SDK in frontend