**Subject:** C++ API – ACAPI_Property_CheckPropertyExpressionString crashes Archicad and returns inconsistent results
**Environment:** Archicad 29 (build 5101), Windows, Spanish version (regional list separator `;`).
**Bug ID:** AC29-2026-10-28-16-19-41-5295
**How to reproduce**
Call `ACAPI_Property_CheckPropertyExpressionString (&expression)` from an add-on, one expression per call. Reproduced with the open-source Tapir add-on, whose command does nothing else (https://github.com/ENZYME-APD/tapir-archicad-automation/pull/1146).
**1. Crash.** Archicad terminates (reproduced 5 times) with:
- `FOOBAR ( 1 )` (unknown function name) — this is the attached Bug ID
- `CONCATENAR ( "a", "b" )` (the same with `;` returns valid)
- `CONCAT ( ###Property:<Group> / <Name>###, "_X" )`
Expected: an error code (e.g. APIERR_BADEXPRESSION), never a crash.
**2. Wrong results.**
- Reported **invalid**: `1`, `1 + 1`, `"OK"`, `CONCAT ( "a", "b" )`, and `CONCAT ( ###Property:<GUID>###, "_X" )` — the stored form that `ACAPI_Property_CreatePropertyDefinition` / `ChangePropertyDefinition` require and that Archicad itself returns in `propertyExpressions`.
- Reported **valid**: `CONCAT ( "a"; ` (unclosed parenthesis), `<ref> * (`, and an empty string.
- Reported **valid** but rejected by Create/ChangePropertyDefinition: `{Property:<GUID>}`, `{Property:<Group>/<Name>}`, and the `;` separator.
**Expected**
- No crash on any input.
- The checker should validate the same syntax that Create/ChangePropertyDefinition accept, or the documentation should state which syntax each function expects.
Related: https://community.graphisoft.com/t5/Archicad-C-API/Expression-Based-Properties-in-AC-27-API/td-p/603...
Operating system used: Windows 11 24H2
