RuntimeError: A requested session was not found (Flask-Login)

RuntimeError: session not found

$ flask run
Traceback (most recent call last):
  ...
RuntimeError: A requested session was not found

Why this happens

Flask-Login relies on session and SECRET_KEY. If sessions are disabled, misconfigured, or not accessible (e.g., during API calls without cookies), protected routes will fail.

Fix

• Configure SECRET_KEY and ensure the client sends cookies; use login_required correctly.

Wrong code

@login_required
@app.route('/private')
def private():
    return 'secret'

Fixed code

from flask import Flask
from flask_login import LoginManager, login_required

app = Flask(__name__)
app.config['SECRET_KEY'] = 'dev'
login_manager = LoginManager(app)

@app.route('/private')
@login_required
def private():
    return 'secret'