The App We're Building

Today's project comes from a real customer ask.

The brief

A car rental company wants to run its operations in Business Central:

• Fleet management — maintain a register of cars available for rent
• Booking & availability — manage future bookings with a visual calendar so staff can instantly see which cars are free and avoid double-booking
• Pricing & Billing — charge customers per day of rental; prices are effective on the rental start date, not the booking date; apply duration discounts (7 days → 10 %, 30 days → 20 %); bill long-term customers every month and short-term customers on car return
• Customer records — store driver's licence and passport details per customer
• Pickup & return protocols — guide employees through a structured handover checklist:
  • Pickup:
    • attach car photos
    • attach customer identity documents
    • print agreement
    • receive security deposit in cash (20 % of the total booking value)
  • Return:
    • attach car photos
    • add any expenses
    • return security deposit minus expenses
• Live car tracking — see the real-time location of all cars on a map directly inside Business Central, fed from the GPS service at cartrack-simulator-demo.onrender.com

Don't start coding yet

Resist the urge to open the agent and ask it to "build this". You already saw what happens when you do. Today's discipline is different — you'll plan, then build, one feature at a time.

What will be your task

Two phases. Top to bottom, then feature by feature.

flowchart TD
    Brief([Brief])

    subgraph Loop ["Feature loop — repeat per feature · <i>feature branch</i>"]
        direction LR
        Spec --> Implement --> Test --> Docs
    end

    subgraph Constitution ["Constitution · <i>main branch</i>"]
        direction LR
        TD["tech-design.md"] --- RM["roadmap.md"]
    end

    Done([Extension shipped])

    Brief --> Constitution
    Constitution --> Loop
    Loop --> Done
    Loop -.->|Merge & Replan| Constitution

First — design the whole thing.

Work with the agent on the entire brief at a high level. No AL yet. The job here is design:

• Break the brief into features.
• For each feature, decide which standard BC pieces cover it and where you actually need an extension.
• Push the agent to use what BC already gives you. Minimise custom development. The goal is the thinnest possible custom layer on top of standard.
• Lock the decisions into specs/tech-design.md and the feature list into specs/roadmap.md.

Then — build feature by feature.

Walk down the roadmap one feature at a time. For each feature you run the same short loop:

1. Spec — agree with the agent on what this feature does and how to build it.
2. Implement — let the agent write the code while you review.
3. Test — let the agent run the tests until they're green.
4. Docs & Merge — generate the user documentation, then merge.

Replan between features if anything shifts. Then move to the next one. Same loop, all the way down.

What's next

Before any code, before any spec, you'll create the project constitution — the document that turns this brief into a real plan, captures the design decisions, and becomes the agent's permanent memory of what you're building.

Ready? Create the constitution →