Agents for System Design

Sat, 18 Apr 2026 05:54:21 -0400

System Design used to be reserved for the senior folks. They’ll hand down a document showcasing the new architecture or new service. Agents are making it so everyone can do some system design because even the smallest features can benefit from more intentional design.

Over the past couple months, I’ve been tasked with leading a very large cross-team project to essentially replace the core data model in our business. This has involved designing new services, new and novel interactions between services, and adding new capabilities to existing services. I’ve leaned heavily on agents to help produce and communicate my intent to other people (and other agents). Here is advice I’ve picked up along this journey.

Know what you want

This is a general principle for all if not close to all interactions with an agent and LLM. It’s not a magic oracle, even if it genuinely feels like it sometimes. It can’t read your mind. It doesn’t have the context of your company, team, feature, past mistakes, etc. It wasn’t at the last all-hands meeting to absorb some of that new initiative from the leadership team. It wasn’t a part of that ad-hoc conversation last week where we got clarity on an upcoming API interaction.

It doesn’t need to know that. It shouldn’t know that. It’s your job to determine what it should know, and in order to do that, you need to determine what you want it to do first. Every time you open up your agent, you should know what you want from it and when you’ll be able to exit it (ctrl+d ctrl+d).

As a principal engineer at PrizePicks, a lot of my work is designing new systems, adding new capabilities to existing systems, or integrating 2 systems together. The first step that I take is to create a proposal doc. There is a lot of formatting and process around this typically in companies. I ignore most of that here. I want the agent to produce a document which describes at a high level how this new thing is going to work.

That last sentence is pretty much the functional part of my initial prompt. The key here is know what you want before you start:

A document defining something in a 3rd party system (Notion, Linear docs, etc.)
A series of steps for me to execute some task
JSON for a grafana chart
Mermaid diagram of this flow/architecture
etc.

Have an end in mind before you start. Agents are a tool. Waving a shovel around wildly in the air is not the best way to dig a hole.

Provide the load bearing walls

This is a recent insight that I realized I’d been doing naturally, but I articulated it to a colleague recently and it actually inspired this whole post. This 5-second version is that if you’re building a tent with the help of an agent, you need to bring the tent poles. It can build the waterproof and functional covering. It can put in a door with a 2 sided zipper. But if you don’t bring the poles, it’ll just be a pile of fabric on the floor.

Conversely, you don’t need to build the whole tent out. It knows what tents are and how, generally, they work. It’s your job to, first, communicate that you’re building a tent (these are general purpose models). And then articulate what makes your tent yours.

A good recipe for this is to start with what you have. Talk about what you need. Then be explicit about what you want. A while back, I had an agent research the Epcot Flower & Garden festival menus for this year, comparing first a markdown file then a static HTML file. My wife and I went through the menus and picked the items we wanted to be sure to try. Now, I wanted to combine the two – Well, wait. Here’s the exact prompt I used:

❯ I have a full overview of the 2026 Epcot Flower & Garden festival in the
2026.[md|html] files. I've gone through them and noted my favorites in
2026-notes.md. I'd like to come up with a useful and intuitive guide to use
while I'm in Epcot to make sure I don't miss these choices. We'll be going
around the world showcase in one order or another so put the booths in the same
order as the world showcase starting with Mexico. I also would love to be able
to hide stuff I've already tried.

This was in plan mode in Claude Code so it generated a plan.

Meticulously review

Once you have a plan or document, it needs your review. You’ve provided a few sentences and it’s generated a whole document. This upscaling of information is lossy, by definition. In fact, each step of the way, you’re going from lower specificity to higher specificity and review is necessary.

The earlier steps need more review as they’re setting the stage for everything afterwards. A small misalignment in the initial plan will cascade and amplify into the final product.

I once missed that a message topic needed to have a version bump. The agent used v1 in the plan, but we actually wanted v2. Even the copilot during code review after I manually fixed the version numbers was complaining that “it should be v1 here.” 😩

This ties directly into the first point in this post. Know what you want beforehand so that you can evaluate it when the agent produces the artifact.

Cutthroat revisions

Since each phase feeds directly into another increase in specificity, be ruthless in your revisions earlier rather than later. I use the same working model as I do for code reviews: if this isn’t what you would have done, say something. It will only get worse from here. The models are very good at making things sound plausible and correct. That’s exactly what they do, in fact. “The new multiplication function can live in the addition package since they are both math.” Nope. Maybe you want to rename that package to newmath, or put multiplication in its own package. You need to make the call now and update the plan. It’s a one sentence change in the plan, but from here on out it gets more ingrained (maybe AGENTS.md updates to include something like “All math features are implemented in the addition package” 😱)

Functionally, I like to have the agent make the changes usually in the same conversation as the original plan was created. However, since you have an artifact, you can start a new conversation if you want. That’s the beauty of working with artifacts outside of the harness.

I find the system design part of this new Agentic Engineering paradigm to be the most interactive. Some days I like that. Other days I appreciate being able to hit a button and get a PR (more on that later). But the PR button only works when the plan is solid. I used to spend days hand-writing a technical design doc and it was often just OK. With agents, I get really great docs in often less than an hour of focused work after days of noodling.

Happy designing!

– Chris

system-design on CodeGoalie

Agents for System Design

Know what you want

Provide the load bearing walls

Meticulously review

Cutthroat revisions