Flask route without trailing slash returns 404 when slash is appended — how should both variants be handled?

Answer the question first

In Flask 3.x the following holds for a rule defined as /about/:

• Requesting /about triggers a 308 Permanent Redirect to /about/ (older Werkzeug used 301).
• Requesting /about/ with a rule defined as /about returns 404 Not Found.

Both forms can be served without a redirect by either:

1. Using strict_slashes=False on the slash‑terminated rule:

@app.route('/about/', strict_slashes=False)
def about():
    return "About page"

1. Registering the same view twice:

@app.route('/about')
@app.route('/about/')
def about():
    return "About page"

Both approaches return 200 OK for /about and /about/.

Redirect status and caching

The 308 is a permanent redirect. Browsers and caches may store it indefinitely, so the canonical form should be chosen consciously. If you want /about to remain the canonical URL, keep the rule without a trailing slash and let Flask redirect to /about/.

Reverse‑proxy interaction

When a reverse proxy strips trailing slashes before forwarding the request, a /about/ rule can cause a loop: proxy → Flask (404) → Flask redirect → proxy strips again. The usual fix is to configure the proxy to forward the path unchanged or to use strict_slashes=False so Flask accepts both forms.

Practical steps for your project

1. Decide on a canonical form and document it for future developers.
2. If you need both URLs to be directly accessible, add strict_slashes=False or duplicate decorators.
3. Verify the redirect status by running curl -i http://localhost:5000/about and curl -i http://localhost:5000/about/.
4. Test the proxy configuration locally (e.g., with nginx) to ensure no loop occurs when slash stripping is enabled.

Missing diagnostic detail

Does your reverse proxy rewrite the request path (e.g., strip trailing slashes) or simply pass it through unchanged? Knowing this will determine whether you should rely on Flask’s redirect or serve both forms directly.