# AN-2026-0770: NumPy 2.0 (Jun 16, 2024) broke the C ABI: modules built for NumPy 1.x fail to import ("A module that was compiled using NumPy 1.x cannot be run in", "_ARRAY_API not found")

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- id: AN-2026-0770 (version 1; cite https://breakwire.org/claim/AN-2026-0770/v/1)
- kind: breaking · status: active · review: editor
- source: <https://numpy.org/doc/stable/user/troubleshooting-importerror.html#:~:text=C%2DAPI%2FABI%20incompatibility>
- more sources: <https://github.com/numpy/numpy/blob/v2.5.4/doc/source/user/troubleshooting-importerror.rst> <https://numpy.org/doc/stable/numpy_2_0_migration_guide.html#:~:text=NumPy%202.0%20has%20a%20new%20ABI>
- event date: 2024-06-16
- services: numpy, python
- affected: pypi:numpy >=2.0.0
- severity: high
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. NumPy's troubleshooting page (opened 2026-11-04) says that a message like "A module that was compiled using NumPy 1.x cannot be run in NumPy 2.0.0 as it may crash. To support both 1.x and 2.x versions of NumPy, modules must be compiled with NumPy 2.0", raised as ImportError or with "AttributeError: _ARRAY_API not found", or errors such as "RuntimeError: module compiled against API version v1 but this version of numpy is v2" or, for Cython packages, "ValueError: numpy.dtype size changed, may indi  [✓0 ✗0]
1. The page says the easiest fix after a recent upgrade to NumPy 2 may be to downgrade to numpy<2, and to find the incompatible package by searching the traceback from the back for the first line outside NumPy.  [✓0 ✗0]
2. Reproduced 2026-11-04: pandas 2.1.4 (wheels built against NumPy 1.x) installed next to numpy 2.5.4 on CPython 3.12 fails at import pandas with ValueError: numpy.dtype size changed, may indicate binary incompatibility. Expected 96 from C header, got 88 from PyObject.  [✓0 ✗0]

## Fix
```text
Upgrade the package named in the traceback to a release built against NumPy 2 (for example pandas >= 2.2.2, and current scipy, scikit-learn, matplotlib, pyarrow, torch wheels); rebuilding from source needs the package's NumPy-2-compatible build (pybind11 >= 2.12 for pybind11 modules). If no compatible release exists, pin numpy<2 for that environment.
```

## Error signatures
- substring: `A module that was compiled using NumPy 1.x cannot be run in` (NumPy troubleshooting-importerror page (v2.5.4 docs))
- substring: `_ARRAY_API not found` (NumPy troubleshooting-importerror page)
- substring: `but this version of numpy is` (NumPy troubleshooting-importerror page (module compiled against API version ... but this version of numpy is ...))
- substring: `numpy.dtype size changed, may indicate binary incompatibility` (NumPy troubleshooting-importerror page; reproduced with pandas 2.1.4 + numpy 2.5.4)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0770/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary source opened 2026-10-28