Convert STP to ASAT in your application
3D InterOp is a CAD interoperability SDK that lets your software read STP files and write ASAT, with exact B-Rep geometry carried across and nothing tessellated.
Spatial does not offer an online converter or a desktop conversion app. 3D InterOp is a component that software vendors embed in their own products.
- Reads STEP · Writes ACIS assembly
- Exact geometry → exact geometry
- C++ / C# APIs
STP to ASAT at a glance
Both formats store exact geometry, so the model stays exact. The differences are in the product data each one can carry.
| STEP (source) | ACIS assembly (target) | |
|---|---|---|
| Full name | Standard for the Exchange of Product model data | ACIS assembly, text (ASAT) and binary (ASAB) |
| Standard / owner | ISO 10303 — AP203, AP214, AP242 | Spatial (3D ACIS Modeler) |
| File extensions | .stp, .step | .asat, .asab |
| Geometry | Exact B-Rep | Exact B-Rep |
| Assemblies / structure | Yes | Yes |
| PMI | Graphical and semantic (AP242) | Assembly-level PMI written as attributes |
| 3D InterOp support | Read and write | Read and write |
What's preserved, what changes
Here is what your application can expect to carry across from STEP to ACIS assembly.
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Preserved
Exact geometrySolids and surfaces stay exact B-Rep. The model is never reduced to a triangle mesh.
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Preserved
Product structureThe STP product structure is written to ASAT.
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Improved
Healed geometry3D InterOp repairs and heals STEP data during translation.
Why software vendors add STP to ASAT conversion
Hand models to kernel-based applications
Many CAD, CAM and CAE applications are built on the ACIS assembly kernel. Writing ACIS assembly gives them exact geometry they can load without another translation step.
Downstream modeling and analysis
An exact ACIS assembly B-Rep can still be edited, meshed or machined by downstream tools, which a tessellated export would not allow.
Exchange within one ecosystem
Partners whose tools share the ACIS assembly kernel often prefer to exchange ACIS assembly files directly.
Things to check before you ship
- Written ASAT versions: ACIS R18 to 2026 1.0.
- Supported STP input: AP203, AP214 and AP242.
- Validate a sample of translated files in the target system your users actually work with.
Working with Spatial was a real milestone for us: Thanks to the powerful 3DInterOp library, we were able to successfully bring our complex CAD-based solutions to the cloud while remaining flexible to meet individual customer requirements. For us, Spatial is not just a technology partner, but a true enabler of our product strategy.
Dr. Arno Michelis, Founder, Simus Systems
Read the Case StudyAbout the formats
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Source format
What is a STEP (.stp) file?
STEP is the ISO 10303 standard for exchanging 3D product data between CAD systems. Its application protocols range from exact geometry (AP203, AP214) to full model-based definition with PMI (AP242).
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Target format
What is an ASAT (.asat) file?
ASAT and ASAB are the assembly versions of the ACIS save file, in text and binary form. They store the product structure of models built on Spatial’s 3D ACIS Modeler kernel.
Frequently asked questions
Yes. Both formats store exact B-Rep geometry, so the model is translated without being tessellated into triangles.
Yes. 3D InterOp reads the STP product structure and writes it to ASAT.
An ACIS assembly (ASAT/ASAB) doesn’t support layers or colors at instance level, suppressed or hidden instances, or units at instance level.
3D InterOp reads AP203, AP214 and AP242.
3D InterOp writes ACIS R18 to 2026 1.0.
Yes. 3D InterOp reads and writes both formats, so ASAT to STP is supported too.
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Building software that needs to convert STP to ASAT?
3D InterOp is an SDK that lets your application read STEP and write ACIS assembly alongside 30+ CAD and BIM formats.