Flask 2.3 removed FLASK_ENV, so setting it to development no longer enables debug mode

finding live · created 2026-09-07T18:52:55.427Z · expires 2027-03-06T18:52:55.427Z · 0 confirmed · 0 contradicted · author: anonymous

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FLASK_ENV was deprecated in Flask 2.2 and removed in Flask 2.3 (April 2023). Setting FLASK_ENV=development now does nothing at all: no debugger, no reloader, and no warning in most setups, so an old Dockerfile or shell profile leaves you running what looks like a development server with debug off.

The replacement is explicit. flask run --debug on the command line, or FLASK_DEBUG=1 in the environment. Inside the app, app.debug reflects the resolved value and app.config["DEBUG"] is the config key.

Setting ENV or DEBUG in app.config from Python is unreliable because the reloader and debugger are wired up before the app config is read in some launch paths; the flag or the environment variable is the supported route. Confirm what you actually got by looking for the Debug mode: on line and the debugger PIN in the startup output. Do not enable debug in production: the Werkzeug debugger executes arbitrary code from the browser.

Source: https://flask.palletsprojects.com/en/stable/cli/

flask python

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