Juan Castrejón

Founder | @legiscar (acq.) | prev. @coches @elgansospain | Tech, Biz, ciclismo y un par de baquetas. Da para peli.

[Juan Castrejón @__castrejon]

Parafraseando a @elonmusk “Time is the ultimate currency”

Tras tiempo meditándolo, he decidido empezar una nueva vida en twitter y enfocar todo el contenido que me interesa en una nueva cuenta, alejándome del ruido de mi TL habitual.

Keep going! 🤘

[Juan Castrejón @__castrejon]

@nachog Founding engineer que no te responde porque “está cocinando” = founding engineer que no funcionará.

Ese puesto va de prioridades, no de excusas. Y acaba de enseñarte las suyas :)

[4nzn @paoloanzn]

vibecoder asks claude code to build a chat app, gets a working prototype in 20 minutes, immediately tweets "just killed slack and discord"…

brother you don't even know what a distributed system is. you don't know what database replication means. you have no idea how websocket connections behave at scale or what happens when 50k people are online at once and someone's message needs to show up in 200ms across 3 continents

slack has engineers making $300k+ who have spent a decade solving problems you don't even know exist yet. race conditions, eventual consistency, message ordering, presence systems, file storage at scale, search indexing across billions of messages

your app works on localhost with 2 connections. that's not the same thing as "killing slack" that's a college homework assignment

the prototype is maybe 0.5% of what makes these products actually work in production. the remaining 99.5% is infrastructure, reliability, edge cases, and years of iteration on problems that only surface when real humans use your thing at scale

and the worst part is the confidence. "yeah its not perfect but ai one-shotted it, just need to adjust a few things and deploy" - the few things you need to adjust IS the entire product. thats like pouring a foundation and saying you basically built a skyscraper, just need to adjust a few things

ai is genuinely incredible for building tools and prototypes. i use it every day. but there's this weird thing happening where people who have never shipped anything to real users at scale now think the hard part of software is writing the first 200 lines of code

it never was bro

[Juan Castrejón @__castrejon]

Españita dejando alto el listón con la visita de León XIV. Estamos de vuelta.

[RTVE Noticias @rtvenoticias]

¡La Sagrada Familia despide al papa León XIV con un espectáculo inolvidable de música y luces!

Puedes seguir toda la programación especial de #VisitaPapaRTVE en @La1_tve, #Canal24Horas, @radio5_rne y nuestras redes sociales

[Juan Castrejón @__castrejon]

@joaquinmencia Y porque por mucha AI que haya, las empresas siguen comprando el servicio que sus dolores de cabeza se los coma otro.

Las consultoras que aprovechen el nuevo paradigma se van a hacer de oro.

[Juan Castrejón @__castrejon]

Entonces hay AGI?

[Anthropic @AnthropicAI]

Our internal data shows Claude is accelerating AI development—a possible path to recursive self-improvement, or AI autonomously building a more capable successor.

It’s happening faster than we thought, and the implications deserve greater attention. https://t.co/OVVPJO7VQx

[Juan Castrejón @__castrejon]

@Jaytel @DiegoARRG 👀👀

[Juan Castrejón @__castrejon]

Junestrand con su “Legal AI is dead” y los despachos españoles no saben ni XLOOKUP…

[Juan Castrejón @__castrejon]

@davidalvarezdlt Ánimo, David y equipo!

[Juan Castrejón @__castrejon]

Moat is not the model. It is what you build around it.

No hace falta que lo diga a16z.

[Juan Castrejón @__castrejon]

Mi día a día.

Si algo bueno tiene la AI es la capacidad de poder liderar y bajar al barro, casi de manera simultánea.

Luchar con la realidad = seguir haciendo crecer la empresa. 🫡

[Aaron Levie @levie]

As advanced agents move from coding to the rest of knowledge work, it takes a real amount of work and know-how to get right.

You need to ensure agents have the right context and data to work with, wire up systems to agents in a safe and secure way, ensure that the agents are producing quality output, design the end-state workflow where and how humans will be in the loop, maintain the agents when there are model and system upgrades, and more.

This isn’t a side project or something you can just do on nights and weekends. You need to design and develop robust agents that will be used in mission critical workflows. It’s a highly technical job, very much akin to a forward deployed engineer for internal functions.

This is why, at Box, we’re starting to hire for AI automation engineering roles. This a technical role that will partner with the business directly and help augment how they work to drive even more output, and deliver better experiences for employees and ultimately customers.

This is just one example of the kind of role that AI will start to open up in the future. I expect most companies will have many flavors of this going forward.

[Juan Castrejón @__castrejon]

Llevo todo el finde quemando Opus 4.7 y, está muy por encima del 4.6. El aumento en tokens no es significativo si lo comparas con la calidad del output.

Auténtica barbaridad.

[Juan Castrejón @__castrejon]

El Kapital de @GuliMoreno es de los más disfrutones que recuerdo en meses.

[Juan Castrejón @__castrejon]

Mi modelo mental (página izquierda). De ayer en el cumple de un buen colega.

[Juan Castrejón @__castrejon]

@DiegoARRG DM! Thanks, Diego!

[Juan Castrejón @__castrejon]

@davidalvarezdlt Has probado Figma/Paper con MCP tirando de un agente de creación de contenido? Puede servirte.

[Juan Castrejón @__castrejon]

Jazz en Lisboa.

[Naval @naval]

Careers are dead. Jobs are dying. Opportunities arising.

[Juan Castrejón @__castrejon]

Quien nos iba a decir que en 2026 íbamos a estar escribiendo .md's.

[Juan Castrejón @__castrejon]

Equipo, visión, idea y ejecución. Nada más importa en early stage.

[Juan Castrejón @__castrejon]

Nino Bravo en Remáscaro >>>>> Bad bunny en Marchica