2026-09-28
The Data Contract Is the Product
Not financial advice. Verify claims independently.
In trading apps, the API contract is the architecture. Define the data first or the UI invents it.
The failure mode we see in every broken trading-app codebase:
UI components invent fields. The P&L display assumes a 'realizedPnl' the API doesn't return; someone computes it client-side from three other fields, wrong in edge cases; the number drifts from the ledger. Now you have a correctness bug that only shows up in production.
The fix: the contract is the product. Define the exact data shape — field names, types, semantics, units — before a single UI component exists. The UI renders what the contract guarantees and nothing more. If a designer wants a field the contract doesn't have, that's an API conversation, not a frontend workaround.
Stock Picks's contract docs ran before the first sprint. Six months in, the ledger and the UI have never disagreed.
From the desk
Want this built into your product?
Open a work order — or try Stock Picks, the paper-trading app we shipped as our flagship case study.