The same figure often appears on more than one screen: a dashboard, a mobile view, an API response, a written note. If each computes it separately, sooner or later they disagree. The reader then has two numbers and no way to know which is right.

How copies drift

Disagreement rarely comes from a dramatic bug. It comes from small differences. One screen rounds and another truncates. One uses the midpoint and another the last trade. One refreshed a moment ago and another some minutes back. One was fixed last week and the other was missed. Each difference is defensible alone. Together they produce two values for one thing.

The fix that was applied to one copy and not the other is the classic case. The number was corrected, the correction was verified on the screen someone happened to be looking at, and the old logic kept running elsewhere.

What a single source means

A single source of truth means the number is computed in one place and every display reads the result. The dashboard, the API and the written summary all take the same stored value. None recomputes it. If the calculation changes, it changes once and every consumer follows.

This applies to definitions as much as arithmetic. The price of a stock token might be the live midpoint, the last tight midpoint or a fallback. Choosing among them is a rule, and that rule should live in one place with its choice traveling alongside the number. A wide quote is not a price shows why the choice matters.

Time is part of the number

Two screens can read the same source and still differ if they read at different moments. That is correct behavior and it only looks like a bug. The cure is to show the reading’s time with the reading. Two values with two timestamps are consistent. Two values with no timestamps are a contradiction. Why we publish age_seconds is about making that visible.

Things that must stay separate

A single source does not mean a single number for things that are different. The same ticker can list as a perp on more than one book, each with its own price and open interest. Those are separate markets. Adding them together or averaging them would create a figure that describes none of them. One source of truth per market is the rule. Merging markets is a different act and usually a mistake.

Why a reader should care

For a trader the practical value is trust. If the level on the desktop differs from the level in the push notification, both become suspect. If a written read quotes a figure that the chart beside it contradicts, the read loses its footing. Consistency is what lets a reader stop checking and start using the number.

It also makes errors findable. When one calculation feeds every display, a wrong number is wrong everywhere and gets noticed. When each display has its own, a wrong number can sit on a rarely visited screen for a long time.

A quick test

Pick one figure and find it in every place a product shows it. Compare the values and their times. If they match, or differ only by their stated ages, the product has one source. If they differ with no explanation, it has several. How to check what a markets SI told you applies the same test to a written read.

On the MCP & API, every NoVo reading carries as_of and age_seconds, so two readings taken at different moments can be reconciled by their ages.