RuntimeError: The session is unavailable because no secret key was set

RuntimeError: session secret key missing

$ flask run
Traceback (most recent call last):
  ...
RuntimeError: The session is unavailable because no secret key was set.

Why this happens

Flask uses SECRET_KEY to sign session cookies. If it’s not configured, any access to session will fail. This often happens in new projects or when environment variables aren’t loaded.

Fix

• Set a SECRET_KEY in config and load it before handling requests.
• Use a strong random key for production and avoid committing secrets to source control.

Wrong code

from flask import Flask, session
app = Flask(__name__)

@app.route('/')
def index():
    session['x'] = 1  # fails
    return 'OK'

Fixed code

from flask import Flask, session
app = Flask(__name__)
app.config['SECRET_KEY'] = 'dev-change-me'

@app.route('/')
def index():
    session['x'] = 1
    return 'OK'