My first startup was a failure, and several neighboring startups failed, too. We had: $100K in GCP credits, a founding engineer who’d built systems in enterprise, and go-to-market. And we failed, not because we built it wrong, but because we built it well. That was the problem. While we spent time wrestling with what felt like an “un-optimal” tech stack, we lost the most important thing: time, momentum, and strategically an opportunity. This story isn’t about people without common sense. I had common sense, and we knew we should keep things simple. But when your mental model doesn’t fit the situation, all your common sense gets swept away. You make “correct” decisions that kill you. This also isn’t a story about bad engineering. It’s about how good engineering kills startups. How the very experience that makes you senior becomes your biggest liability. How “doing it right” or even “doing it simple” is often doing it wrong. This article presents mental models to help you make the right decisions and avoid the wrong ones I made. mental models right Who this is for: senior engineers starting or joining early-stage startups. If you’ve spent 5+ years in enterprise or Big Tech, this is your warning. Who this is for: senior engineers starting or joining early-stage startups. If you’ve spent 5+ years in enterprise or Big Tech, this is your warning. Who this is for: Who this is for: senior engineers starting or joining early-stage startups. If you’ve spent 5+ years in enterprise or Big Tech, this is your warning. Mental Model #1 - “Free” Infrastructure Is The Most Expensive $100K in GCP credits seems like a gift, but it’s a trap. It pushes you toward over-engineering because “it’s already paid for.” You get compute instances, load balancers, container registries, and enterprise tools that require enterprise setup. What do you need to get? A “push to deploy” button. Sure, you can build “deploy from GitHub to VM” workflows on GCP/AWS/Azure. Some products come close. But it requires extra steps: configuring Cloud Build, setting up IAM roles, writing deployment scripts, managing secrets, and configuring health checks. You burn time building deployment infrastructure before deploying actual products. Meanwhile, platforms like Railway or Fly.io give you what startups actually need: a persistent VM with start-and-go deployment from GitHub. Easy as it can be: you push your code, and it deploys. Just ready to use VM with environment variables, SSL, load balancers, logs, etc.  It’s not “free,” but it’s ready. Railway Fly.io Free credits push you toward over-engineering because “it’s already paid for.” You convince yourself you’re saving money while spending your most valuable resource: time. THE MENTAL MODEL
You will spend weeks setting up “proper” infrastructure but competitors launched in two days on Heroku. Or they got 1,000 users while we working on a deployment pipeline. Those users are worth more than free credits. Even when you avoid it, there's a deeper problem: how senior engineers think about simplicity itself. THE MENTAL MODEL You will spend weeks setting up “proper” infrastructure but competitors launched in two days on Heroku. Or they got 1,000 users while we working on a deployment pipeline. Those users are worth more than free credits. Even when you avoid it, there's a deeper problem: how senior engineers think about simplicity itself. Mental Model #2 - “Minimal” <> “Simple” The traditional KISS principle tells us to keep our software simple. But in startups, that's the wrong target. You shouldn't keep your SOFTWARE simple; you should keep your SOLUTIONS simple. Real simplicity should be measured by total effort, not code complexity: Total Effort = Initial Build + Maintenance + Debugging + Feature Addition + Security Updates + Scaling Changes Total Effort = Initial Build + Maintenance + Debugging + Feature Addition + Security Updates + Scaling Changes When you build from scratch, you own all of these forever. When you use a service, most of these become zero. The "bloated" third-party service is actually the simple solution because it minimizes total effort. My OAuth Example My OAuth Example Our founding engineer decided to build OAuth from scratch instead of using an "unknown library." One week later, he submitted a PR: clean OAuth implementation with JWT tokens, refresh token rotation, session management, and role-based access control. No dependencies, no vendor lock-in, just code we controlled. I didn't deny the PR. And this was a mistake. Throwing away a week of work would crush morale. But it creates code complexity and puts it on the wrong rails. Plus, not discussing the approach beforehand was our real mistake. We let engineering pride make a strategic decision. Then, a client needed Microsoft OAuth and Google OAuth. Custom implementation meant days of refactoring, refresh token rotation, edge cases, RBA, and other things. Each "simple" addition required a deep understanding of our custom auth. Every security update was ours to implement. Every new requirement was ours to code. Principles Principles Classic senior engineer mistake: optimizing for control instead of outcomes. In startups, reality requires completely inverting how senior engineers think: STOP thinking: "This is just a few days of coding"START thinking: "This is a few days NOT coding my actual product"
STOP thinking: "I can build this simply"START thinking: "I can solve this simply by not building"
STOP thinking: "Third-party services add complexity"START thinking: "Third-party services absorb complexity" STOP thinking: "This is just a few days of coding"START thinking: "This is a few days NOT coding my actual product" STOP thinking: "I can build this simply"START thinking: "I can solve this simply by not building" STOP thinking: "Third-party services add complexity"START thinking: "Third-party services absorb complexity" THE MENTAL MODEL
In startups, KISS means being lazy about everything except your core product. Not lazy in quality, BUT lazy in effort. Because every hour spent rebuilding solved problems is an hour stolen from the unsolved problems that actually determine if your startup lives or dies. This mindset shift is hard enough. But it gets harder when your experience actively works against you, when the tools you've mastered become the tools that slow you down. THE MENTAL MODEL In startups, KISS means being lazy about everything except your core product. Not lazy in quality, BUT lazy in effort. Because every hour spent rebuilding solved problems is an hour stolen from the unsolved problems that actually determine if your startup lives or dies. This mindset shift is hard enough. But it gets harder when your experience actively works against you, when the tools you've mastered become the tools that slow you down. Mental Model #3 - Comfort choices We chose Angular because our founding engineer knew it deeply. Smart decision, right? Use your strengths, ship quality code. The framework was fine, BUT the problem was its ecosystem. The Ecosystem Trap The Ecosystem Trap Angular is excellent and our engineer could build anything with it. But "anything" took time just to start. Setting up deployment, authentication, and basic UI components meant endless configuration before writing a single feature. While we debugged Angular Material themes, competitors can (and will) use Next.js + Vercel were already onboarding users. Just compare that to the Next.js + Vercel path: deploy a skeleton app with npx create-next-app on day one, add Clerk authentication and shadcn/ui components on, ship actual features on day one. Same destination, completely different journey. Why does this happen? Why does this happen? The difference isn't framework quality, it's ecosystem optimization. Next.js/React is surrounded by venture-backed startups building tools for other startups: UI: shadcn/ui - copy, paste, ship
Auth: Clerk/Supabase - configure in minutes
Deploy: Vercel - git push equals production
Everything else: If startups need it, someone has built a service UI: shadcn/ui - copy, paste, ship Auth: Clerk/Supabase - configure in minutes Deploy: Vercel - git push equals production Everything else: If startups need it, someone has built a service Angular's ecosystem serves enterprises: powerful, flexible, infinitely customizable. Perfect(?) for teams of 50 and a poison for teams of 3. THE MENTAL MODEL
This choice revealed a crucial distinction:
Framework Senior: "I've mastered Angular. I can build anything with it." They optimize for code quality and "proper" engineering practices. Need a data table? They'll spend two weeks building a perfectly abstracted, paginated component -- for your 50 rows of data. Meanwhile, competitors grabbed shadcn's DataTable and shipped to users on day one.
Applied Senior: "I know Angular, but Next.js ships 3x faster. Let's use that." They optimize for market feedback. They'll load all data client-side because "we won't hit scale for months" and ship with off-the-shelf components. Their code might be forgettable, but they're iterating on feature three while Framework Senior is still perfecting their pagination abstraction.
The truly senior move is asking yourself: "Am I solving tomorrow's problems or today's opportunities?"
Choosing the right ecosystem matters. THE MENTAL MODEL This choice revealed a crucial distinction: Framework Senior: "I've mastered Angular. I can build anything with it." They optimize for code quality and "proper" engineering practices. Need a data table? They'll spend two weeks building a perfectly abstracted, paginated component -- for your 50 rows of data. Meanwhile, competitors grabbed shadcn's DataTable and shipped to users on day one. Framework Senior shadcn's Applied Senior: "I know Angular, but Next.js ships 3x faster. Let's use that." They optimize for market feedback. They'll load all data client-side because "we won't hit scale for months" and ship with off-the-shelf components. Their code might be forgettable, but they're iterating on feature three while Framework Senior is still perfecting their pagination abstraction. Applied Senior The truly senior move is asking yourself: "Am I solving tomorrow's problems or today's opportunities?" The truly senior move is asking yourself Am I solving tomorrow's problems or today's opportunities? Choosing the right ecosystem matters. Mental Model #4 - Build Core, Rent Context But even with the right tools, there's one final trap: the compulsion to build things because you can, not because you should. This trap kills technically strong teams and more startups than we can imagine: building things nobody asked for because you can, not because you should. We spent at least a month in total on features nobody needed. Custom OAuth when Auth0 existed. A Postgres-based job queue when Redis + Celery existed. Terraform from day one, when the console worked fine. Each decision felt productive, but each was self-sabotage to face real challenges like talking to customers or doing other customer development. The pattern is simple: if customers won't choose you for it, don't build it. My $50 Rule My $50 Rule If a SaaS costs less than $50/month, you can't afford to build it. Your time is too expensive. Building custom OAuth takes 1-2 weeks in total maintenance and adding different OAuth providers. At startup burn rates, that's $5,000-$15,000 in engineering time, or in a losing opportunity time. Auth0 is free for up to 25,000 active users, then $35/month. You could pay for Auth0 for 35 years with what it costs to build it once. So, this isn't about money but about priorities and opportunity cost. Exception Exception In my opinion, build only if you can't learn about users without it.  A simple example is when you need to test whether users will pay for AI-generated reports. Build the simplest version that proves demand. And everything else tries to slip. Yes, skip infrastructure, skip "doing it right", skip best practices that don't ship features, skip tests. Again, be as lazy as possible in writing code. Again, be as lazy as possible in writing code. What I Actually Use What I Actually Use Auth: Clerk (React-native, better DX) or Auth0 (B2B-focused, enterprise-ready)
Email: Resend (developer-first) or SendGrid (battle-tested)
Analytics: PostHog (free until scale)
Monitoring: Sentry (unbeatable for errors)
Hosting: Cloudflare or Vercel (if all-in on Next.js) Auth: Clerk (React-native, better DX) or Auth0 (B2B-focused, enterprise-ready) Email: Resend (developer-first) or SendGrid (battle-tested) Analytics: PostHog (free until scale) Monitoring: Sentry (unbeatable for errors) Hosting: Cloudflare or Vercel (if all-in on Next.js) These aren't endorsements but my own choices optimized for speed. I guess your stack will differ but this principle won't. THE MENTAL MODEL
If customers see it and choose you for it, build it. Everything else, try to rent: auth, emails, payments, monitoring….whatever. Your startup won't die from using Auth0, but it will die from spending three weeks building authentication while competitors ship features users want. The most senior engineering decision isn't what to build. It's what not to build. THE MENTAL MODEL If customers see it and choose you for it, build it. Everything else, try to rent: auth, emails, payments, monitoring….whatever. Your startup won't die from using Auth0, but it will die from spending three weeks building authentication while competitors ship features users want. The most senior engineering decision isn't what to build. It's what not to build. Bottom Line LLMs have commoditized building. Any junior with Claude can create that custom auth system you're so proud of. Your value isn't in what you can build anymore, BUT it's in knowing what not to build. Leadership is the ability to separate signals from noise. True seniority means having the discipline to ignore 90% of what you know and to ship today's solution, not tomorrow's architecture.

This story contains new, firsthand information uncovered by the writer.

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KISS or Die: Why Senior Engineers Fail at Startups

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