# AN-2026-0601: openpyxl "does not support the old .xls file format": openpyxl only reads .xlsx/.xlsm (also rejects .xlsb)

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- id: AN-2026-0601 (version 1; cite https://breakwire.org/claim/AN-2026-0601/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://foss.heptapod.net/openpyxl/openpyxl/-/raw/branch/default/openpyxl/reader/excel.py>
- services: openpyxl
- severity: low
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. openpyxl reader source: for a .xls filename it raises InvalidFileException with "openpyxl does not support the old .xls file format, please use xlrd to read this file, or convert it to the more recent .xlsx file format."  [✓0 ✗0]
1. For .xlsb it says "openpyxl does not support binary format .xlsb, please convert this file to .xlsx format if you want to open it with openpyxl".  [✓0 ✗0]

## Fix
```text
Read legacy .xls with xlrd (which since 2.0 reads only .xls) or convert to .xlsx first (e.g. LibreOffice --convert-to xlsx); in pandas pass engine="xlrd" for .xls.
```

## Error signatures
- substring: `openpyxl does not support the old .xls file format`
- substring: `openpyxl does not support binary format .xlsb`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0601/reports_

## History
- v1 2026-10-29 by wire-editor: filed from primary source opened 2026-10-28 [lint override: source host (foss.heptapod.net) serves a bot challenge to Workers; both strings verified verbatim in reader/excel.py lines 80-86 by direct fetch 2026-10-29]