0001 — IFC5 / IFCX is its own standard family

• Status: Accepted
• Date: 2026-10-03
• Deciders: openbimrs contributors
• Supersedes: —

Context

IFC2X3, IFC4, and IFC4X3 are EXPRESS schemas serialized as STEP or XML. openbimrs/ifc models them as an untyped entity graph keyed by a SchemaVersion.

IFC5 / IFCX is not another EXPRESS release. It is a JSON model built from layers of components that compose in a USD-like way. The unit of data, the identity model, and the composition rules all differ from the IFC4 entity graph. Forcing it into a SchemaVersion would make the ifc model carry two unrelated data models behind one enum.

Decision

IFCX lives in openbimrs/ifcx as a separate standard family.

• openbimrs/ifc keeps rejecting the IFC5 schema token with a typed UnsupportedSchema error and points readers here.
• ifcx may depend on step, core, and ifc. Nothing in ifc may depend on ifcx.
• buildingSMART's draft ifcJSON (a JSON encoding of the IFC4 data model) is not pursued. openbimrs/ifc#122 was closed as not planned.

Alternatives considered

Option	Why not
Add IFC5 to ifc's SchemaVersion	Different data model; would leak composition semantics into the EXPRESS entity graph
Sibling crate inside the ifc workspace	Ties IFCX releases and its draft churn to the ifc release cycle
Do nothing and leave IFC5 as an implicit gap	The gap was visible only as a negative test case

Consequences

• IFCX can follow its draft revisions without churning the ifc crates.
• An IFC4 to IFCX bridge, if built, lives here and depends on openbim-ifc.
• Tracking: openbimrs/ifcx#1.