# werkzeug.routing.BuildError: Could not build url for endpoint 'external' with values ['scheme', 'host']. Did you mean 'index' instead?

- **ID:** `python/flask-url-for-external-scheme`
- **Domain:** python
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Attempting to use url_for with an endpoint that doesn't exist or with extra parameters not defined in the route.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |

## Workarounds

1. **** (100% success)
   ```
   Define the route correctly: @app.route('/external') def external(): return 'ok', then use url_for('external')
   ```
2. **** (80% success)
   ```
   If external URL needed, use url_for('index', _external=True) if index exists
   ```

## Dead Ends

- **** — The endpoint still doesn't exist; _external only affects the URL scheme. (90% fail)
- **** — These are not valid parameters for route building unless defined in the route. (80% fail)