# werkzeug.routing.exceptions.BuildError: Could not build url for endpoint 'login'

- **ID:** `python/werkzeug-builderror-could-not-build-url`
- **Domain:** python
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Endpoint name does not exist or required URL parameters are missing.

## Version Compatibility

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

## Workarounds

1. **Check endpoint name and provide all parameters** (90% success)
   ```
   url_for('login', next='/dashboard')
   ```
2. **List all endpoints with app.url_map** (85% success)
   ```
   print(app.url_map)
   ```

## Dead Ends

- **Assuming endpoint name is the function name without decorator** — Flask endpoint defaults to function name, but can be overridden. (60% fail)
- **Missing required parameter in url_for** — If route has path parameters, they must be provided. (80% fail)