ifc-impl-format-transcoding

name	ifc-impl-format-transcoding
description	Use when converting an IFC model between encodings (SPF .ifc, ifcXML, ifcJSON, ifcZIP) and the result must stay faithful, and when deciding which on-disk format to pick for a given exchange. Prevents confusing a format transcode with a schema-version migration, expecting STEP #id numbers to survive a round-trip, treating ifcJSON as a guaranteed lossless format, and assuming DERIVE or INVERSE attributes are lost in conversion. Covers the format comparison matrix, the EXPRESS schema as the single canonical model, SPF to ifcXML conversion (ISO 10303-28 plus the Part 28 configuration file), SPF or ifcXML to ifcJSON (provisional), ifcZIP wrapping, what is and is not preserved, and the transcoding vs version-migration vs geometry-export split. Keywords: IFC format conversion, transcoding, SPF to ifcXML, ifc to json, ifcXML, ifcJSON, ifcZIP, ISO 10303-28, Part 28 configuration file, lossless, round-trip, "which IFC format", "convert ifc file", "ifc file too big", "json instead of ifc", "lost data after conversion", "ids changed", how do I convert an IFC file, what format should I export.
license	MIT
compatibility	Designed for Claude Code. Requires IFC IFC2x3, IFC4, IFC4.3.
metadata	{"author":"OpenAEC-Foundation","version":"1.0"}

IFC Format Transcoding

Convert an IFC model between its on-disk encodings without losing data, and pick the right encoding for an exchange. The governing fact : IFC is one EXPRESS schema with several derived encodings. Transcoding re-renders the SAME model population in a different encoding. It NEVER changes the schema version and NEVER changes the geometry.

This skill is encoding-level. For a schema-version change (IFC2x3 to IFC4) see ifc-impl-version-migration ; that is migration, not transcoding. For the encoding syntax itself see ifc-syntax-step-physical-file, ifc-syntax-ifcxml, ifc-syntax-ifcjson.

Quick Reference

The four encodings

Format	Extension	MIME type	ISO standard	Size vs SPF	Status
SPF / STEP	.ifc	application/x-step	ISO 10303-21	100%	Official
ifcXML	.ifcXML	application/xml	ISO 10303-28	113%	Official
ifcZIP	.ifcZIP	application/zip	none	17%	Official
ifcJSON	.json	application/json	none	148%	Provisional / Candidate

Sizes are the official buildingSMART figures (SPF = 100%). ifcZIP 17% is the compressed size of whichever text payload it wraps.

Core rules

• The EXPRESS schema is the single canonical model. ALWAYS treat a transcode as "read the EXPRESS population, re-render it" : the model is the invariant, the file bytes are not.
• ALWAYS keep the schema version fixed across a transcode. SPF IFC4 transcodes to ifcXML IFC4, NEVER to ifcXML IFC2x3. Changing the version is migration.
• DERIVE and INVERSE attributes are NEVER serialized in ANY encoding, so a transcode can NEVER lose them. They are recomputed by the reader.
• NEVER promise a lossless ifcJSON round-trip. ifcJSON is provisional ; round-trip is a design goal, not a guarantee.
• ifcZIP is pure compression. Wrapping and unwrapping is ALWAYS lossless.

Decision Trees

Is this transcoding or migration

Does the schema version change (FILE_SCHEMA value differs) ?
├─ NO  : same version, different encoding
│        -> TRANSCODING. This skill. Re-render the EXPRESS population.
└─ YES : IFC2x3 to IFC4, IFC4 to IFC4.3, etc.
         -> SCHEMA-VERSION MIGRATION, not transcoding.
            Entities are renamed/removed/added. See ifc-impl-version-migration.
            (A migration may ALSO change encoding ; do the two steps separately.)

Which target encoding to pick

What is the priority for this exchange ?
├─ Widest compatibility, smallest text  -> SPF (.ifc). The default.
├─ XML toolchain (XSLT, XPath, XSD validation), schema validation -> ifcXML
├─ Web / JavaScript app, REST API, JSON-native consumer          -> ifcJSON
│   (accept provisional status and weaker tooling)
└─ Transfer or storage where size dominates -> ifcZIP
    (wraps an SPF or ifcXML payload ; decompress before parsing)

Is a given transcode lossless

Which pair ?
├─ SPF  <-> ifcXML   : lossless AT THE MODEL LEVEL (ISO 10303-28 + Part 28
│                      config). File-level details (#id, layout) are NOT kept.
├─ * <-> ifcZIP      : ALWAYS lossless (pure compression of an unchanged payload).
└─ SPF/ifcXML <-> ifcJSON : model-level fidelity NOT guaranteed (provisional
                            format). Verify against the converter before relying
                            on a round-trip.

Patterns

Pattern 1 : the canonical-model mental model

A transcode is always two steps over the EXPRESS schema as the pivot:

source encoding  --parse-->  EXPRESS population (entities, explicit attributes,
                                                 references)  --render-->  target encoding

ALWAYS think in terms of the EXPRESS population, never byte-for-byte text. Two files in two encodings are "the same model" when they encode the same population of entities and explicit attribute values. ALWAYS confirm FILE_SCHEMA (SPF) or the schema namespace (ifcXML) or schemaIdentifier (ifcJSON) matches before and after.

Pattern 2 : SPF to ifcXML and back

ifcXML is governed by ISO 10303-28, not ISO 10303-21. The EXPRESS-to-XSD mapping is not deterministic on its own : a published Part 28 configuration file, one per IFC schema version, pins the mapping. ALWAYS use the configuration file that matches the model's schema version.

This pair is lossless at the model level : both encode the same EXPRESS population. What does NOT survive:

• STEP #id integers become XML id / ref / href strings. The numbering is NOT preserved.
• Positional attributes become named XML elements or attributes.
• An unset attribute ($ in SPF) becomes an absent element in ifcXML.
• Comments and exact text layout are lost.

ALWAYS validate the model population after the transcode, NEVER diff the raw bytes.

Pattern 3 : transcoding to and from ifcJSON

ifcJSON is a Provisional / Candidate format with no ISO standard, synchronized to IFC4.3 (and usable for IFC4). It is 148% of SPF size. The cross-reference key is the entity globalId (a UUID) inside a {"type": ..., "ref": ...} object, replacing the SPF #id.

ALWAYS state the provisional status when proposing ifcJSON. NEVER promise a lossless SPF to ifcJSON to SPF round-trip : the ifcJSON specification lists round-trip as a design goal, and converters may apply options (compact formatting, inverse handling) that change output. ALWAYS verify a round-trip against the actual converter for the target IFC version before depending on it.

Pattern 4 : ifcZIP wrapping

ifcZIP is a plain ZIP archive. ALWAYS apply these rules:

• The archive uses PKZip 2.04g (DEFLATE) compression. NEVER use encryption, Unicode archive features, or deflate64 : those break compatibility.
• The archive MUST contain exactly ONE .ifc file OR exactly one .ifcXML file in its main directory. NEVER place multiple model files in one ifcZIP.
• Wrapping and unwrapping change nothing in the payload : ifcZIP is ALWAYS lossless. To "transcode" an ifcZIP, unwrap it, transcode the payload, re-wrap.

Pattern 5 : what is at risk, what is safe

ALWAYS preserve, by an explicit strategy, the items genuinely at risk:

At risk in a transcode	Why	Mitigation
Instance identifiers (#id vs id vs globalId)	each encoding uses its own scheme	rely on GlobalId (the IfcRoot UUID), never on #id
Order of unordered aggregates (SET, BAG)	unordered by definition	NEVER depend on member order of a SET or BAG
Header metadata	not modeled identically across encodings	re-state author, timestamp, view definition after transcode
Comments	not part of the model	accept loss ; move notes into model entities
IfcReal numeric formatting	text representation differs	compare values numerically, not as strings

NEVER-at-risk (because never serialized in any encoding): DERIVE attributes and INVERSE attributes. A transcode cannot lose them ; the reader recomputes them.

Pattern 6 : transcoding vs migration vs geometry export

Three different operations are routinely confused:

Operation	Changes	Routes to
Transcoding	encoding only (SPF to ifcXML to ifcJSON)	THIS skill
Schema-version migration	schema version (IFC2x3 to IFC4) ; entities renamed	ifc-impl-version-migration
Geometry / format export	derived output (meshes, glTF, OBJ, COLLADA)	not IFC at all ; ifc-impl-geometry-extraction

Library tools mirror this split : an ifcpatch Migrate recipe performs schema-version migration ; an IfcConvert-style tool performs geometry export. ALWAYS name the operation correctly before choosing a tool. This skill does NOT teach library APIs ; for tool selection see ifc-impl-library-selection.

Reference Links

• references/methods.md : per-format facts, version coverage, the ISO 10303-28 and Part 28 detail, the preservation matrix.
• references/examples.md : the same model in SPF, ifcXML, and ifcJSON, plus worked transcode walk-throughs.
• references/anti-patterns.md : the conversion failure modes and why each fails.

Related skills

• ifc-syntax-step-physical-file : the SPF .ifc encoding (ISO 10303-21).
• ifc-syntax-ifcxml : the ifcXML encoding (ISO 10303-28, Part 28 config).
• ifc-syntax-ifcjson : the provisional ifcJSON encoding.
• ifc-impl-version-migration : schema-version change, NOT transcoding.
• ifc-impl-library-selection : choosing a library or tool for a conversion.
• ifc-errors-encoding-issues : string-escape and character-encoding failures.

Verified sources

All format facts verified 2026-05-20 via WebFetch against the buildingSMART sources in SOURCES.md : the IFC file-formats page, the ifcXML Implementation Guide v2.0, implementation agreement CV-2x3-154 (ifcZIP), and the buildingsmart-community/ifcJSON repository.