# ERROR: Some build dependencies for package are missing: 'cython' is required to build the package but is not installed.

- **ID:** `python/pip-pep517-requires-build-deps`
- **Domain:** python
- **Category:** install_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

The project's build-system.requires lists a build dependency that pip cannot fetch or that is not installed when --no-build-isolation is used.

## Version Compatibility

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

## Workarounds

1. **** (88% success)
   ```
   Install build deps in the host and disable isolation: pip install cython && pip install --no-build-isolation .
   ```
2. **** (92% success)
   ```
   Ensure build-system.requires is correct: [build-system]\nrequires = ["setuptools>=61", "cython>=3.0"]\nbuild-backend = "setuptools.build_meta"
   ```
3. **** (85% success)
   ```
   Check network access to fetch build deps: pip download cython -d /tmp/cython-test
   ```

## Dead Ends

- **** — Installing in the host env does not help when build isolation is enabled; the isolated env still lacks it. (80% fail)
- **** — The build actually needs Cython; removing it causes a different error (missing .c files). (85% fail)
- **** — --no-deps affects runtime deps, not build-time deps. (90% fail)