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Geralt knew it had been too good to be true. He'd known bringing an outsider to the keep would disrupt the balance, especially someone as sociable as Jaskier - his friend wasn't exactly known for his subtlety, and it had been unsurprising he'd butted heads with Lambert even on his first day.
He followed the sounds of shouting and madness coming from somewhere in the keep, hoping that whatever Jaskier had done was at least fixable.
That somewhere ended up being the main hall, and Geralt opened the door expecting chaos - he found it, but what he didn't expect was finding Jaskier, shirtless, sitting cross-legged on one of the long tables while Lambert despairingly pulled at his hair (impressive, as it was so short) and paced in front of him. Even Eskel was looking remarkably less put together than usual, staring intensely at the bard with his hands pressed together like he was an impossible, irritatingly smug enigma he desperately wanted to understand, and Geralt could almost see the question marks around his head.
He immediately understood both reactions deep in his soul, and dryly wondered why he'd expected anything less before clearing his throat loudly.
"Oh, hey Geralt!" Jaskier waved cheerfully, and both his brothers swung around as they noticed him.
"YOUR BARD IS FUCKING WEIRD!" Lambert bellowed immediately, and Eskel nodded furtively. Jaskier's grin got minutely wider.
Only years of practice stopped Geralt's matching smile from betraying him at Lambert's outburst. "Oh?"
Eskel looked almost haunted as he pointed at Jaskier, who was looking very pleased with himself. "Do the thing, show him."
Jaskier waved him over airily, and Geralt made his way over with increasing regret in every step until he was standing beside Eskel, arms crossed.
"I'm a blacksmith, you know. I made a sword last week - vesemir taught me."
Geralt frowned, that didn't sound right. "Jaskier, we would've noticed if you-"
"THAT'S THE THING! HE'S LYING!"
Geralt nodded slowly, coming to the conclusion he must be missing some important context here as Lambert cursed again. "...Yes?"
"Geralt, we can't tell." He looked quizzically over at Eskel, who gestured despairingly at the bard. "We can't tell he's lying."
Wait, what?
"Want me to do it again?" Jaskier seemed far too smug, geralt thought, but nodded anyway. Lambert grabbed his arm and pressed his hand flat against Jaskier's chest, and wouldn't let him pull away. "Lambert, what the fuck?"
"Just feel his heartbeat, trust me."
Geralt rolled his eyes as Jaskier waggled his eyebrows suggestively at the contact.
"I'm... your father. I've missed you, Gerald, my only son."
Geralt paused, straining his senses.
"...holy shit, you really can't tell."
The room erupted into chaos once more, Jaskier laughing openly as the three witchers panicked and shouted amongst themselves, geralt pulled into their madness easily as he tried to wrap his head around it.
How long had Jaskier, a non-magical human, been able to fool a witcher? This was a serious security issue!
"Listen, as funny as it is hearing you theorise about my untapped magical potential, it's literally just practice."
"Practice? Jaskier, very few people on the continent would ever need you to display this level of control to believe a lie, where the fuck would you have learnt to-"
"Wait, witchers! Have you been lying to Geralt for twenty years?!"
Jaskier grinned. "Not on purpose, at first, but somehow he always knew when I was lying or being sarcastic - you still don't pick up on the difference sometimes - and eventually I learnt how to lie well enough that he'd never know if I'd snuck out to a hunt if he didn't catch me first."
They're still reeling; Geralt's reputation is in shambles.
Jaskier, for his part, still just looked smug. "It's basically just imagining I'm telling a story, which doesn't set off the lie-thing you can pick up on. I'm glad somebody is finally appreciating my skill."