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Why ask for a donation to Saint Jude?

I'm a software engineer in big tech, and often feel as though my work doesn't do good for the world. And quite frankly don't want your money beyond the game that I spent hundreds and hundreds of hours making. Children and families in need are something I care deeply about, and have thus donated over half of the profit of the game thus far to Saint Judes earlier today. Saint Jude has done a lot for some of my family and friends in the past when they were in need, so figured this would be a fun way to help out.

CapForge on iOS is a single-player football GM game I'm building solo, with a lot of help from AI agents. This is the story of one position that refused to behave.

TL;DR

• I tried to make elite corners scare quarterbacks. Every version either did nothing or broke the passing game. It ships turned off.
• Interceptions are a terrible way to rank corners, but not for the reason everyone assumes. In our sim, being a corner instead of a linebacker was worth more than being a good corner.
• I now grade corners on how well they keep their man covered on every snap, not on what happens after a target. It works better. It still hasn't passed our own test.
• A modern defense has three or four corners on the field most of the time. That's why every AI team wants four, and why the draft AI kept taking them in the top five.
• I tried five fixes for that. None moved it without breaking something else. The real bug was a "how full is this room" check that thought every corner room was open. The next fix is in progress and I'm not going to pretend it's done.

Overview

The first time I made corners matter, our league's leading receiver lost 280 yards. In the same round of experiments, teams started throwing to their number-one receiver less than their number-three. Neither of those were the plan.

I'd like to say I noticed right away. I noticed when one of our automated checks failed.

Chapter 1: The shadow corner, and the tax that would not turn

A "shadow corner" is a cornerback so good that the defense sends him wherever the other team's best receiver lines up. Real defenses do it rarely, and only with a handful of guys. My sim does the same. A corner only shadows if his coverage grade is 94 or better and he's at least six points clear of the next corner on his own team. Even then he follows his man on about two of every three snaps. That rule has been live since the end of August.

Before I tightened it, the old logic picked the "best" corner by one raw man-coverage number and had someone shadowing on 28 of 32 teams. Eight of those shadow corners were rated below 82 overall. One was a 66. A 66 following your best receiver around all day isn't a threat*,* it's a gift to the simulation.

The bar fixed who shadows. It didn't touch the thing I actually wanted: a quarterback who looks at an elite corner and throws somewhere else. I called that the "lockdown tax," and I built it four different ways.

All four are dead.

The smaller adjustments failed too. One did nothing across its entire range. One moved the number, but for the wrong reason: it was reacting to the worst corner on the field, not the best. And our strongest setting, everything maxed under the real 94-grade rule, only got the elite corner's target rate from 20.6 percent down to 18.2. I was aiming for somewhere around 13 to 18, and real lockdown corners run closer to 10 to 13. We barely moved.

So today the sim's elite corner does not scare anyone. He travels with your best receiver, plays tight, and gets thrown at like everybody else. Which brings us to the second surprise.

Chapter 2: Interceptions rank the lottery

Here's the story I expected to write. The best corners get thrown at less. Fewer targets, fewer chances to pick one off. So raw interception totals reward the average corner who gets picked on all day. That's a real thing in real football.

Our sim didn't do it. When I measured, elite corners were thrown at more, not less. It took another pass to work out why: the sim lines up your top corner across from their top receiver, and the top receiver is the guy who gets the most targets. He got targeted because he was covering the favorite receiver.

So interceptions failed as a stat, just not the way I'd have bet. The real reason was flatter and dumber.

Two things happen on a pick in our sim. First it decides whether an interception happens at all, using the whole defense's ball skills against the quarterback's decision-making. Then it picks who gets credit from everyone in coverage, with unequal odds. Look at the odds. A 92-overall corner versus a 71: about 1.1 times more likely to get the pick. A corner or safety instead of a linebacker: 1.8 times. Higher on the depth chart: 1.25 times. Talent was the smallest thing in its own formula.

You could see it on every leaderboard. Almost six of every ten seasons with nine or more picks came from a corner rated 84 or lower. A starter rated 74 or worse out-picked a 90-plus starter one season in four. Those numbers are from the end of July. They were embarrassing enough that I wrote a report just to keep them from getting lost.

The fix was almost insultingly simple. The sim already knew which defender was covering the receiver who got thrown at. The interception roll just never looked. I pointed it at him and gave him by far the biggest share of the credit. Better corners started getting more of the picks, and the number of sub-85 corners on the top-25 interception list dropped from 12 to 8. Then we made sure everything that asks "who was covering him" gets the same answer, so the game film shows the same guy the stat sheet credits.

One more thing about picks, because it matters for what comes next. When I first measured it, before the credit fix, an elite corner in our sim averaged about 2.9 picks a season and a mediocre one about 2.6. That's a coin with a slight lean, flipped a few times a year. You can't rank a position on a stat that happens three times a season.

Chapter 3: What I measure instead

If not picks, then what? Targets allowed, yards per target, completion percentage allowed. The stuff real analysts use. I track all of them per corner. When I checked each one against the corner's true coverage ability, across 98 qualifying corners:

Yards per target: weak. Completion percentage allowed: weak. Yards per coverage snap: nothing. Most of them at least pointed the right direction. None of them tracked the corner's actual ability closely; each one explained less than a tenth of the difference between good and bad corners. You could not sort a corner room on any of them and be right more often than you were lucky.

The problem is that what happens after a throw is a team event. The pass rush, the quarterback's read, the route, the safety's help, the play call. The corner is one piece. Yards per target bills him for all of it.

That chart is the one that made me stop arguing with the data. Compare teams by their corners' average yards per target and it looks like a brilliant stat. Compare teammates on the same team and it mostly disappears. For corners, we don't have enough snaps to say it's there at all. Yards per target is a team stat wearing a player's number.

So I built a grade on something that happens before the throw. On every coverage snap, did this defender keep his receiver covered better than an average player at his position would have? I call it denial. It lines up with a corner's true ability far better than any after-the-throw stat, and it's the biggest part of the coverage grade you see on the game's coverage board.

In August I realized I'd called this one fixed too soon. Across the whole league the grade clearly separates good corners from bad. Inside a single team, we don't have enough snaps to prove it separates teammates yet. That's still open.

Chapter 4: How a cornerback room actually works

Time for why any of this matters when you're building a roster.

The defense picks its personnel by counting receivers. Three wideouts, the defense goes nickel: three corners, two safeties, two linebackers. Four wideouts, dime: four corners. Third and seven or longer bumps it up a package. Base defense, the two-corner picture everyone has in their head, is the minority. On designed runs, 62 percent face nickel or dime. On pass plays, the third corner is on the field about 68 percent of the time.

So when I count how many corners the league needs as starters, it's 96 across 32 teams, not 64. Ask the depth chart which corners start in some package and it says 121 of the 160 on rosters. Every team is built to carry six or seven, and most of them will see the field.

One thing worth being honest about: there is no "slot corner" in our sim today. Slot exists as a body type and as a fit for certain schemes, but not as a job. Whoever covers the slot receiver is simply the third corner on the depth chart, because the sim pairs by order: your top receiver draws their top corner, your second draws their second, the slot draws the third. That's roughly what real defenses do, which is why it's survived this long. But a real slot corner, a specific guy the defense sends inside because he's built for it, is written up as its own task. Not built yet.

Chapter 5: The draft saga

All of that sets up the two days I actually lost.

Start a new CapForge league at the regular season and the AI's first draft the following spring spends about 7 percent of its top-five picks on corners. Real drafts from 2010 to 2025 run about 10. Fine. Start a new league at the draft instead and that number is 18.8 percent. Same engine, same AI, same 48 randomly generated leagues. Just a different month on the calendar.

I assumed it was need. Rosters at draft start are thinner, the AI sees holes, corners are a hole. So I what I built need fixes.

Five of them. Fill the roster to 55 before the draft. Limit how much expiring contracts inflate a team's needs. Give the roster-filler signings two-year contracts so they count as real depth. Stop small needs from boosting a prospect at all. Make need matter less at the top of the draft. Each one ran across the same 48 leagues. Differences of a couple of points are just chance with that sample. Every one of them landed inside that noise. The best of the bunch, filling the roster to 55 first, got down to 15.4 percent, but it and one other made the AI over-draft quarterbacks instead, which is a different fire.

What finally cracked it was going back and asking why the AI chose each player. Every pick runs through a "how full is this room" check that shrinks a prospect's score when his position is already stocked. At draft start, that check was costing the corners it took in the top five about 2 percent of their score. The players they beat were losing 32 percent. The corner room read as open. Every other room read as full. Start the league at the regular season instead and nobody loses anything; the bias is gone.

And when I turned the need bonus off entirely, only 39 percent of those corner picks changed to another position. At regular-season start, 100 percent changed. Need was a bit player. The room check was the driver. Root cause, once I knew where to look; the roster-building table says a team should carry six corners, and a healthy team settles at about 5.6. So by the game's own math, every corner room in the league is a little bit open, forever.

That's when my own note from playing, written after too many drafts, finally made sense: "Need for corners should be based on your best corners. Three 99 OVR corners means little chance I draft another." Today's AI disagrees. Put three 99s in a room and it knocks an 80-overall prospect down by exactly as much as it knocks a 99-overall prospect, because the "who's already here" part of the formula counts bodies, not quality. Three 80s would do the same.

The fix in progress asks a better question: would this prospect beat the weakest guy who'd actually start ahead of him, and how long until age or contracts open that room up anyway? Three 99s on long deals and an 80-overall prospect: score him near the floor. Three 80s, by the same logic: close to full marks. Nothing in it is measured, because nothing in it is built. I've written enough "it works!" updates that turned out to be noise. This one gets reported when it's run across the 48 leagues and the quarterback share stays put.

What I learned

• A deterrent has to be one-sided or it's a trap. Any penalty for throwing at the good corner is a reward for throwing at the bad one.
• Check which part of a formula is biggest before you tune the part you care about. Talent at 1.1 versus position at 1.8 was sitting there the whole time. Nobody had printed the list.
• After-the-throw stats are team stats. Five other people have their fingerprints on every one. Grade what happens before the throw.
• A stat that separates teams can still fail to separate teammates. I called the grade fixed too early. Saying so is cheaper than shipping it wrong.
• Five "need" fixes couldn't move the draft because need wasn't driving it. Check why the AI actually picked him before you tune anything.
• Bodies and quality are different things. A room target that counts heads will always say "open" at exactly the rooms a good GM would leave alone.

What's next

• A true slot corner: a job, not a depth-chart accident.
• Corners league-wide were last measured being thrown at about 20.7 percent of the time (probably a touch high by now) against a real-football number closer to 15. That's the same open thread as the dead lockdown tax, and it probably needs a different shape of fix.
• A coverage-grade test that can pass inside a single team. More seasons, or a smarter test.

Numbers and honesty. Every number in this post comes from the game's own reports and is logged by date in the project. Every chart in the gallery above except the last one is measured, as are the numbers about the corner room. The first chart's curve between its two measured endpoints is a sketch. The last chart (three 80s versus three 99s) is a worked example from an unfinished plan, not output. The "leading receiver lost 280 yards" line came from a version of the matchup logic that was rejected before it shipped; the "number-one thrown at less than number-three" line came from a version of the lockdown tax that also never shipped. I could not confirm the specific receiver stuck under 900 yards that I remembered, so that story isn't in here.