You pay for a detailed spec or PRD, everyone nods, and then the build slowly drifts away from it. By launch, the document barely matches the product. This is not a discipline problem; it is a format problem. Here is why.
The short answer
A written spec is ambiguous and disconnected from the build, so it gets misread and then ignored once code starts. A product simulation is an executable spec: the decisions are captured in a runnable codebase and data model, so the specification and the product cannot drift apart.
Why spec documents get ignored
- Ambiguity. Prose leaves gaps. Two developers read the same sentence and build two different things.
- Disconnection. The document lives in one place; the code in another. Nothing keeps them in sync, so they diverge.
- Staleness. The moment building starts, reality changes, and updating a long document is nobody's favorite job, so it rots.
- No feedback. A document cannot be run, so you cannot tell if it is right until something is built from it, by which point it is expensive to change.
This is how a spec becomes discovery debt: paid for, then quietly abandoned.
Why a runnable foundation works better
A product simulation makes the spec executable. Instead of describing the data model, it ships a running one. Instead of describing a flow, the flow works. The documentation explains the why, and the codebase enforces the what. Because it runs, you get immediate feedback on whether it is right, and there is no gap between "the spec" and "the build" for the team to drift into.
You still do the thinking a good spec requires. You just capture it in something that cannot be ignored, because it is the foundation everyone is building on.
If your last project drifted from its spec, this is the fix. Start a project for a clear, flat-rate scope built around a runnable foundation.
Frequently asked
So should I not write a spec at all?
Write one, the thinking matters. The point is that a document should not be the final deliverable. The spec should be captured in something runnable, so it cannot be quietly ignored or misread once building starts.
What is an 'executable spec'?
It is a specification expressed as a working, runnable foundation: code and a data model that embody the decisions, plus documentation. The product runs the way it is supposed to, so the spec and the build cannot drift apart.