Every prop firm rule book reads simply on paper. Lose no more than this much in a day. Never fall more than this far below your peak. Don't make most of your profit on one lucky day. Keep positions under this size. Writing the rules is the easy part. Enforcing them, consistently and instantly, for every trader, while the market moves, is the hard part.
This post explains how a risk-rule engine evaluates the four rules most prop firm programs use: what each one measures, when it is checked, and what happens on a breach. It stays conceptual, with no code and no integration details, and it describes the approach the Vera Charts engine takes. Partner firms set the values; the engine enforces them.
Start with equity, not balance
Balance is what the account holds after closed trades. Equity is balance plus the unrealized P&L of every open position at current prices. The difference decides whether a rule engine works.
Picture an account that has closed no trades today and holds one open position deep in the red. Measured on balance, nothing has happened. Measured on equity, the trader may already be past the daily limit. If the position is closed an hour later at a worse price, a balance-based check discovers the breach only then, after the damage is done.
So the engine evaluates on equity, open positions included, and re-evaluates as prices move. That is also why Vera Charts shows real-time unrealized P&L on the chart and updates the daily loss line tick by tick: the number the trader watches is the number the engine judges.
Daily loss limit
What it measures: how far equity has fallen during the current session, against the start-of-day reference the firm defines.
When it is checked: continuously. Every price move changes the equity of an account with open positions, so every price move is a potential breach.
What happens on a breach: the moment the day's drawdown crosses the threshold, the engine liquidates open positions and disables new entries, then reopens the account at the next session boundary. On Vera Charts the limit is configurable per seat.
Two things operators should decide explicitly. First, where the session boundary sits. Crypto trades around the clock and forex trades across time zones, so a daily rule needs a precise definition of a day. Second, what a breach means for the program: a stopped day or a failed evaluation. The engine halts trading and reports the state change to the firm's backend. What that breach means for the trader is the firm's call.
Trailing drawdown
What it measures: how far equity sits below its high-water mark, the highest equity the account has reached.
How it moves: the floor follows equity up. When equity makes a new high, the floor rises with it. When equity falls, the floor stays where it is. On Vera Charts, the trailing drawdown follows equity up, locks at the ceiling the firm configures, and fires an account-state webhook to the firm the moment it is breached.
A hypothetical example makes it concrete. An account starts at $100,000 with a $5,000 trailing drawdown, so the floor sits at $95,000. Equity climbs to $103,000 and the floor rises to $98,000. Equity then falls back to $98,000 and the account breaches, even though it is still $3,000 up overall.
Because the high-water mark follows equity, open profit counts. A trader who runs a position up and gives the gain back has raised the floor on the way up, even though the gain was never banked. That is the point of the rule: it measures how much of a peak the trader surrenders. It also surprises traders who expect only closed trades to count, which is why the drawdown band belongs on the chart where they can see it.
The lock changes the rule's character over time. Once the floor reaches the configured ceiling it stops trailing, and from then on it behaves like a static floor. A static drawdown is the simplest case: a fixed level that never moves. Whichever model a program uses, the engine asks the same question as prices move: where is equity relative to the floor right now?
Consistency cap
What it measures: how much of the account's cumulative profit came from a single day. The cap limits any one day's contribution to a fraction of cumulative P&L that the firm sets.
When it is checked: at session close. Loss rules are about risk in the moment. Consistency is about the shape of the result. A day's share of total profit keeps changing until the day ends, so the engine evaluates it once the day is final.
Why firms use it: an evaluation is meant to find traders who can repeat a result, and one outsized day can carry an account to a profit target on luck. Another hypothetical: with a 40 percent cap, an account showing $10,000 of cumulative profit, $6,000 of it from one day, is outside the rule, because that day is 60 percent of the total. On Vera Charts the cap is optional: a firm switches it on and sets the percentage.
Position and instrument limits
What they measure: the size of a proposed order, per symbol and per side, and whether the instrument is allowed in the trader's program at all.
When they are checked: before the trade. This is the one family of rules that is fully preventive. An order that would exceed a cap never reaches the matching simulator. It is rejected with a structured reason on the trader's screen, and the firm's back office gets a delivery receipt. Instrument whitelists work the same way: a symbol outside the program can't be traded.
Size caps also make the loss rules more predictable, because they bound how much one adverse move can cost.
Why enforcement can't live in the browser
It is tempting to enforce rules where the trader is, in the browser. The browser is the right place to show a rule. It is the wrong place to be the only thing enforcing it.
- The tab can close. Open positions keep moving when the trader shuts the laptop. A daily loss limit that needs the tab open to fire is only a suggestion.
- The client belongs to the trader. Code on the trader's machine can be modified, throttled or paused. The decision that ends an evaluation has to be made somewhere the trader can't edit.
- There has to be one answer. The trader, the firm's support team and the firm's back office need the same verdict at the same moment from the same source. Two systems computing a breach independently will eventually disagree.
- Breaches need a record. A dispute is settled by what the engine saw and decided, not by a screenshot.
So a serious engine works in three layers. In the chart, the trader sees the boundary before crossing it: the daily loss line updates tick by tick and the trailing drawdown band follows the high-water mark. In the order path, every order passes a synchronous rule check before it reaches the matching simulator. In the ledger, every violation has a permanent record the firm can audit, and every state change reaches the firm's backend as a signed webhook. The chart shows the rule. The engine enforces it.
Partners set the values. The engine enforces.
A firm's rule book is how it competes, so the engine doesn't standardize it. It provides the primitives (daily loss limits, trailing drawdowns, consistency caps, position and instrument limits) and enforces whatever values each firm sets, with different rule sets for different cohorts inside the same firm, including scaling plans. Partner firms such as Funded With Forex and Funded With Perps run their programs this way, each under its own brand and terms. Vera Charts is not a prop firm, and every account on the platform is simulated.
If you're evaluating prop firm risk management software, our For prop firms page covers the platform as a whole, “What a prop firm trading platform needs” is a checklist for the rest of the stack, and “What a real risk-rule engine looks like” goes deeper on the in-chart layer. Integration mechanics aren't published: book a partnership call and we'll walk your team through the architecture and a working integration.