I'm Bad at CAD, So I Built an Agent to Do It. Its First Design Looked Like It Was Melting.

The veggie I kept pushing around my plate

I studied Mechanical Engineering for my Bachelor's, and the one thing I never enjoyed was designing. It's not that I couldn't do it. Fiddling with a model until every dimension was exactly right just ate so much of my time that I avoided it whenever I could.

But like everything in life, eventually you gotta finish your veggies. I already had some experience with mechanical design and CAD software, but I'd heard Blender was really cool and easy to get into, with a good community behind it, so I picked that up. I struggled for a year. Then I found out Fusion was easier and better, and started learning that instead. Even then, just making a box took me days to get perfect.

Then Autodesk dropped a plot twist

The latest Fusion update came with a local MCP server.

Now, I might be slow and bad at designing. But what am I good at, you ask? Building agents. And good ones. So I did what any robotics engineer who sucks at designing would do: I built a sub-agent to work with the Autodesk MCP server.

There was a catch. Fusion's MCP is very good for designing, but using it directly in my default setup meant carrying the whole MCP server's context in every single session, whether I was designing or not. That quietly ate tokens in the background. No-go.

So I put it in a sub-agent instead. The harness told it: you help me with design work, nothing more and nothing less. I gave it a bunch of reference links for whenever we design, so everything stays industry-conformant. That was it. What could go wrong?

Attempt one, or: the melting enclosure

What I wanted was simple: a housing for an ESP camera, so I can take pictures of my plants. But a simple box still means a lot of small things. I had to figure out the dimensions, import the existing model of the board, make sure everything fit snugly with no overlaps, and, most importantly, make sure every part snapped together. A professional designer could do this in a couple of hours, and do it better than me.

I started with the import, then spent a couple of hours on the agent. The first few runs were horrific. It tried to do everything at once. It didn't wait, it didn't ask, it just went off on its own journey. Like a hobbit trying to destroy the ring. The result was an ugly enclosure that looked like it was melting away.

And I couldn't fix it. I couldn't make the changes myself, and anything I asked the sub-agent to change, it couldn't do either, even after more than an hour and over 200k of my session tokens. I had built the sub-agent to save tokens. The irony was real.

Attempt two, with a proper harness

So I went back and redesigned the harness. The new rule: no one-shot designs.

I asked the main orchestrator to break the design into parts. Once we were happy with the plan, we went bit by bit: first the bottom, then the top, then the attachment, the joints, and finally the stand. Each iteration took less than 15 minutes. And the fun part: I could change each part exactly the way I wanted before the agent moved on to the next one.

Here's the difference, side by side. Flip between my first attempt and the second, step through the parts, and click the rules to see what each one does:

Steal these three ideas

You don't need Fusion to use any of this. If you're building your own sub-agent, these are the three that mattered:

1. Give it its own room. Attach the heavy tool to the sub-agent only. Your main session then doesn't pay for its tool definitions in every conversation.
2. Make it work in parts. One part at a time, with you checking each one before it moves on. Big one-shot requests are where runaway attempts live.
3. Cap the retries. One real attempt at a fix, two at most, then it stops and hands back what it tried and what your options are. A short human decision is cheaper than a long silent loop.

The part I didn't expect

Within a day I had the housing ready, the way I wanted it. Here it is, pulled apart so you can see every piece:

More than anything, I'm enjoying designing now. I don't feel handicapped by not knowing which button to click for complex geometry. And I'm learning a lot along the way, things that would have taken me days to learn on my own.

If you would like to look at the harness in details or want me to help build your subagents feel free to reach out below. Will be more than happy to help