The anomaly walked in from the data's side, on the morning Sigma had finally come back to life.
Even after the people disappeared, Mio's AI agents kept operations running. A muscular company, a few people running a massive operation. The balance that had nearly been swept away by the wads of cash had turned not toward mere survival, but into the black. On that morning, when they had only just survived.
Every news channel was telling the same story. Social-media investment scams were surging. Losses in six months were double the previous year's. They borrowed the faces of celebrities, lured people into outside chats with "guaranteed profits," and took everything. Delete them and delete them, and they keep springing up, someone on the other side of the screen said with a tired face.
Mio didn't spare the TV a glance. What she pulled toward her instead was what she had at hand—Makoto's logs.
Their own social network, flying the goal of "zero scams." The records of what the detection engine's net strung across it rejected every day—scam accounts and ads, bounced. Who they'd tried to target, when, and with what face. Every trace of the enemies that had tried to flow into Makoto remained inside Sigma.
"If the world's in an uproar, the sensible thing is to check your own footing first," Mio said. Aya set down a coffee for her. "So what turned up at your feet?"
"I'm in the middle of looking."
Mikoshiba came over, coffee in hand. "By your footing, you mean our net?"
"Yes." Mio called up the pile of rejection records on her screen. The carcasses of scams, caught in Makoto's net and killed each day, lined up in a long row. "Whether this net is actually holding the line right now. I want to confirm that first."
"Isn't it a perfect defense? Zero scams and all."
"Perfect, hm." Mio let out a faint breath. "I've told you before. There's no such thing as perfect. It was built on that premise from the start. With the current system, there are two cheap nets." Mio pointed to the two in turn. "The first is a net that watches for what's 'not like the others.' Text, images, domains—for each item, it learns what the usual shape looks like. When something comes in that deviates from that, it raises a warning. Even without being taught what a scam is, it'll catch something just for being an unfamiliar foreign object. It's cheap, and it's fast."
"Like a watchdog, then. It barks when it smells something it doesn't know."
"Close. But since it barks at anything unfamiliar, legitimate new things get barked at too. It's a noisy watchdog." Mio pointed to the second. "That's why I've layered another net on top. A net that bundles weak clues. A new domain, words that are too strong, odd behavior from the advertiser—clues that might each be clean on their own, collected by the handful and multiplied together. The 'likelihood that this ad is a scam' turned into a single number, and a line drawn."
"So—in terms I can understand?"
"You don't arrest a suspect on a single piece of testimony," Mio said evenly. "Eyewitness testimony, fingerprints, holes in an alibi. Each one may be weak, but if ten or twenty point the same way, the probability of guilt shoots up. It's a net that calculates that probability and cuts at a threshold."
"That's the second one."
"Yes. These two—the net that watches and the net that bundles—I've trained on the data we've had so far. They're cheap, fast, and can run on every ad placement. What's knocking the scams out of Makoto's ad placements right now is these two cheap nets." Mio traced the pile of rejection records with a finger. "They're working properly. Most scams can't get through these nets."
"Most, meaning—" Mikoshiba picked up on the word.
Without answering, Mio began re-sorting the logs.
The anomaly wasn't in the rejection records. It was in the records that hadn't been rejected—the handful that had slipped through and made it out into the open.
"...They're getting in."
"Getting in?" Mikoshiba's brows drew together.
"There are some slipping through our net." Mio indicated the handful on screen, evenly. "Makoto runs scam accounts and ads alike through those two nets before placement. Even so, a tiny number have leaked through the mesh and made it out into the open. I picked up a few through user reports. Not many. But—they've been increasing lately."
"Meaning your engine is being beaten?"
"Yes."
Mio opened the ones that had slipped through, one at a time. The faces weren't familiar scam faces. The words, too, were shifted bit by bit from the usual "guaranteed" and "principal protected." The angle of the hype was subtly different. Each had been made to deviate, ever so slightly, from the shape Mio's engine remembered as "this is a scam." Deviating on purpose.
"They're paraphrasing," Mio said, her voice going a little flat. "They're studying what my engine reacts to. They're building words and faces right at the edge where it won't react. This isn't an accidental miss. The other side is getting past my algorithm."
Mikoshiba slowly chewed over what that meant. "...So the enemy has studied your detection, and they're starting to get ahead of it."
"Yes. And they're good." Mio traced the phrasing of one of the ones that had slipped through once more. "They're walking precisely a few centimeters outside the line I drew. You can't walk like that unless you know where the line is."
"What's troublesome," Mio went on, "is that this isn't a one-time thing. If I redraw the line, they'll walk just outside it again. It never ends. It's a game of chase."
Mio didn't shout. She didn't pound the desk, either. Her expression had simply, quietly vanished. Different, too, from her usual hard blankness. A quiet face, seized by something and descending into its interior.
"Mio," Mikoshiba called. "So what do we do?"
Mio rearranged the phrasing of the ones that had slipped through so that they filled the screen. Every one of them walking just outside the line she'd drawn. An enemy that studied her detection, detoured around it ever so slightly, and slipped out into the open.
"How interesting, this problem."
Mikoshiba said nothing. The line was the same as always. But the temperature was different. Not anger, not righteous indignation. Something purer: an engineer's curiosity. There was someone out there tracing the line she'd drawn, precisely, from outside, and getting past it—that single point had seized her from within.
"You're doing it," Mikoshiba said. It wasn't a question.
"What choice is there?" Mio pulled her laptop toward her.
"The current detection isn't enough anymore. If I just redraw the line, they'll walk outside it again. —I'm going to reforge the weapon."
There was no sentiment in her voice. It was the face of someone who had evenly begun leafing, in her head, through the blueprints of what she was about to build.
Mikoshiba took out his phone. "Then I'll shore things up on the outside. While you're reforging the net, I'll handle explanations to the advertisers and arrangements on the ground, so the company doesn't get shaken. So that whatever you design, it'll be in a form we can actually use. That's my job."
"Yes."
Mio was no longer listening.
On the screen, the footprints of the enemy that had walked outside the line glowed, one after another. Beyond them was an opponent who didn't yet have a face. Someone who measured the line she'd drawn from the outside, traced it, and got past it.
"—Next time, they won't get through."
Mio murmured it to no one in particular. Outside the window, the night was beginning to deepen.