The Fusion 360 .f3d format

Notes from reverse-engineering Autodesk Fusion 360's native design files. Everything here was derived by reading real documents, and every claim is exercised by a test in tests/ against the sample designs.

Document	What it covers
container.md	The ZIP shell, Zstandard entries, .f3z packages
neutron-streams.md	The length-prefixed serializer, manifests, segment streams
asm.md	ASM BinaryFile — the Shape Manager B-Rep bodies
graphics-cache.md	The OGS display mesh — Fusion's own tessellation
unknowns.md	What is still opaque, and what it would unlock

Shape of a document

Design.f3d                                  ZIP, entries Zstandard-compressed
├── Manifest.dat                            document identity + schema versions
├── Properties.dat                          JSON blob, usually "{}"
├── ComponentReferenceData.json             usually "{}"
└── FusionAssetName[Active]/                an *asset folder*
    ├── Manifest.dat                        asset identity + segment table
    ├── FusionDesignSegmentType1/           a *segment*
    │   ├── MetaStream.dat                  index
    │   └── BulkStream.dat                  payload — the parametric timeline
    ├── FusionACTSegmentType1/              assembly context tree
    ├── FusionBrowserSegmentType1/          browser tree
    ├── Breps.BlobParts/
    │   ├── BREP.<uuid>.smb                 ASM body
    │   └── BREP.<uuid>.smbh                ASM body carrying rollback history
    ├── ProteinAssets.BlobParts/*.protein   materials (a nested ZIP)
    ├── OGS.BlobFolder/OGS/DefaultScene/    tessellated display mesh — optional
    ├── Previews/small.png                  thumbnail
    ├── Images.BlobParts/                   decals and textures
    └── DesignConfigurationTable.BlobParts/ configurations

A document may hold more than one asset folder: an animation lives beside the design as its own Animation/ asset with its own segments, and names its parent design by GUID.

Naming is not stable; types are

Segment folder names drift between Fusion versions — the design segment is FusionDesignSegmentType1 in one document and Design1 in another. The asset manifest's segment table maps folder prefix to segment type, and that mapping is what to key off. Everything in ezf3d resolves segments by type for this reason.

Units

Fusion's kernel works in centimetres. The design stream carries a CmMKS unit system described in the file itself as "cm modeling length with MKS (m, kg, second, Celsius) units". Coordinates in ASM bodies are therefore cm, not mm.