I Built 3 Free AI Tools in 3 Hours - Here Is How

Three free AI tools in three hours using DeepSeek's API: a cover letter generator, a resume analyzer, and a code explainer. The author emphasizes that effective prompt engineering is more important than the choice of AI model, and shares lessons learned including the need for rate limiting, clear instructions for users, and explicitly specifying English responses to avoid language defaults. The tools are offered completely free with no email collection or premium upsells, and the author invites feedback while acknowledging they may have occasional bugs.

Last Saturday, I woke up with that familiar itch. You know the one - where you want to build something, but you're not sure what, and you end up scrolling Twitter for an hour instead. But this time was different. I had this thought: "What if I just pick three annoying tasks people deal with regularly and wrap an AI around them?" Three hours later, I had three working tools. Here's the messy, honest story of how it went down. Honestly? I was tired of seeing AI tools that charge $20/month for something that takes 3 seconds of API calls. The job market is brutal right now. People are applying to hundreds of positions. Friends were asking me to review their resumes. And every single "code explainer" tool wanted my credit card. So I thought: screw it. Let me build the free versions I wish existed. The three that came to mind: Nothing fancy here: The whole thing is literally: form submits → API route calls DeepSeek → returns response → renders on screen. That's it. DeepSeek was the real MVP here. I'd been using OpenAI but kept hitting rate limits on the free tier. DeepSeek's pricing made it a no-brainer for a project I wanted to keep free forever. This was the simplest. A form with: job description, company name, your skills. The prompt is basically: "Write a professional cover letter for company . The job requires description . The applicant is skilled in skills . Keep it under 350 words, sound human, not like a template." The trick? I added "sound human" to the prompt. Without it, the letters were robotic and full of "leveraging synergies" nonsense. This one needed more thought. I built a prompt that evaluates: The response comes back as bullet points with a score. No magic. Just a well-structured prompt and a model that follows instructions. The simplest of all. Paste code, get explanation. But I added a dropdown for "beginner" vs "advanced" mode. Beginner mode explains like you're 12. Advanced mode assumes you know what a closure is. 1. Prompt engineering model choice I spent way too long comparing models. The truth? A good prompt on a cheap model beats a bad prompt on GPT-4. Every time. 2. Rate limiting is your friend DeepSeek is fast, but I still added artificial delays and queuing. Nothing worse than a tool that breaks because 10 people used it simultaneously. 3. People don't read instructions I put "paste your resume here" in big letters. Someone still tried to upload a PDF. So I added a note saying "copy-paste only, I know it sucks." That helped. 4. Free tools attract real feedback Within 2 hours of sharing on Reddit, I got 47 responses. Some bug reports. Some feature requests. One guy said the cover letter "made me sound like a human instead of a LinkedIn bot." That made my day. The first version of the code explainer had a bug where it would sometimes return the explanation in Chinese. Turns out DeepSeek's base model defaults to Chinese if the prompt is ambiguous. I fixed it by explicitly saying "respond in English" at the end of every prompt. Also, I accidentally left a console.log with my API key in the first deploy. Caught it after 11 minutes. No one stole it, thankfully. Don't be me. I'm not trying to build a startup here. I don't want your email. I don't want to sell you premium. I just wanted three tools that work, that are free, and that I'd actually use myself. The code explainer has already saved me twice this week trying to understand legacy code. The resume analyzer caught that I was using "helped with" instead of "implemented" on my own resume. That's worth more than any SaaS subscription. If you want to check them out: → ai-cover-letter-gamma-ruby.vercel.app The cover letter generator is the most polished. The resume analyzer needs you to paste text sorry, no PDF parsing yet . The code explainer works best for Python and JavaScript, but handles most languages okay. If they break and they might - I'm one person , give it a minute and try again. Or DM me on Twitter. I'll probably fix it within a day. One last thing: If you build something similar, don't overthink it. Three hours, three tools, one API key. The barrier to building useful AI stuff is lower than ever. Go make something janky that helps one person. That's enough.