What is a CAD harness?
A CAD harness is a workspace that sits above your CAD tools and AI CAD agents. An engineer raises an issue on a part, the harness hands it to the CAD tool or agent they choose, and it brings back several candidate changes for the engineer to compare, pick and sign off, with the reasoning kept on the part.
Not to be confused with wire harness design, where CAD software is used to lay out cables, connectors and their routing.
Where the word comes from
In AI software, a harness is the layer around a model: what the model can see, which tools it can use, and when a person has to approve its work. The model does the thinking. The harness decides what it works on and what happens to the result.
Coding tools made the idea familiar. Cursor, for example, says it "supports a range of AI models from multiple providers", and you pick one from a selector in the chat panel (Cursor). The editor stays the same; the model behind it is your choice.
A CAD harness applies that idea to mechanical design. Different CAD tools suit different work, and new AI CAD agents appear every few months. The engineer works in one place, and chooses which CAD tool or agent takes each job.
How a CAD harness works
The loop has five steps.
- An issue is raised on the part. It points at the exact face, edge or feature, with the requirement it breaks.
- It's assigned to an agent. The harness passes along the context: requirements, constraints, and what was tried before.
- The agent works in the CAD tool of your choice. CAD tools already expose ways for software to drive them. Onshape has FeatureScript, "a programming language designed by Onshape for building and working with 3D parametric models" (Onshape), and Autodesk lets developers build "scripts, add-ins, and applications" for Fusion (Autodesk). The Model Context Protocol, "an open-source standard for connecting AI applications to external systems", is one common way to wire an agent to tools like these (MCP).
- It comes back with options. Three to five candidate changes, each with its trade-off.
- The engineer decides. They compare the options, pick one or none, and the reason is saved with the new revision.
Example: 220-0315 battery tray
An electrical engineer raises an issue: connector J3 hits the tray wall when the cover closes. The agent returns three options:
| Option | Change | Trade-off |
|---|---|---|
| A | Move the connector cutout 2 mm toward the center | Shortens one stiffening rib |
| B | Thin the wall locally from 2.0 to 1.4 mm | Risk of sink and a weaker corner when molded |
| C | Rotate J3 by 90° | Needs a board layout change |
The mechanical lead picks A after checking the rib, rejects B because of the molding risk, and leaves C for the next board spin. The tray goes to P4, and the reasons for all three decisions stay with it. Six months later, nobody has to ask why the cutout moved.
CAD harness, plugin, copilot or PLM?
| Tool | What it does | Where it stops |
|---|---|---|
| CAD plugin or add-in | Adds a function inside one CAD tool | Tied to that tool |
| AI copilot inside a CAD tool | Suggests or makes edits in that tool, with the vendor's model | One tool, one model, and its history stays there |
| Text-to-CAD generator | Makes new geometry from a prompt | Usually starts from a prompt, not your existing part and its history |
| PDM or PLM | Stores files, revisions and approvals | Records changes; doesn't propose them |
| CAD harness | Routes issues on a part to the CAD tool or agent you choose and returns options to sign off | Only as good as the context it has and the agents behind it |
What a CAD harness needs to work
It needs context. An agent that only sees geometry will fix the clash and break the requirement. It needs the part's requirements, its constraints and its past decisions.
It needs a review gate. Every change goes through an engineer before it becomes a revision. The engineer directs the work and signs off on what matters.
And it needs a record. The issue, the options, the choice and the reason all stay with the part, so the next engineer, or the next agent, starts from what the team already learned.
Where Kyumi fits
Kyumi is building toward a CAD harness. Today it does version control and design review for mechanical hardware teams: anyone can open a part by link and mark it up in the browser without a CAD seat, and every revision keeps the discussion behind it. That record is the foundation a harness needs. The agent loop described here is what we're building next.
FAQ
What is a CAD harness?
A workspace that sits above CAD tools and AI CAD agents. An engineer raises an issue on a part, the harness sends it to the tool or agent they choose, and it returns several candidate changes for the engineer to pick and sign off.
Is a CAD harness the same as wire harness design?
No. Wire harness design uses CAD software to lay out cables and connectors. A CAD harness is a layer that connects engineers, CAD tools and AI agents.
How is a CAD harness different from a CAD plugin?
A plugin adds a function inside one CAD tool. A CAD harness sits above the tools, so the same issue can go to whichever CAD tool or agent suits it, and the decision is kept in one place.
Does a CAD harness replace CAD software?
No. It uses CAD tools to make the changes. Engineers keep the CAD tools they already use and choose which one handles each job.
Does a CAD harness change my model without asking?
It shouldn't. Every option goes back to an engineer, who picks one or none before anything becomes a new revision.
Related
- What is a superhuman engineer?
- How do I document why a part was designed a certain way?
- How do I track action items from a mechanical design review?
Sources
- Cursor, Available models: https://cursor.com/help/models-and-usage/available-models
- Onshape, FeatureScript documentation: https://cad.onshape.com/FsDoc/
- Autodesk Platform Services, Autodesk Fusion API: https://aps.autodesk.com/developer/overview/autodesk-fusion-api
- Model Context Protocol, What is MCP?: https://modelcontextprotocol.io/introduction
Kyumi is building one place for the part, for the engineers who design it and everyone who builds it. Join the waitlist.