I'm a self-taught developer from China. Three months ago I had zero passive income. Today I have 21 APIs live on RapidAPI, all running on 0/monthinfrastructure.Here′sexactlyhowIdidit,whatworked,andwhatIwishIknewearlier.TheStack(AllFree)Backend:FastAPI—0/month infrastructure. Here's exactly how I did it, what worked, and what I wish I knew earlier. The Stack (All Free) Backend: FastAPI — 0 Hosting: Render Free Tier — 0HealthCheck:UptimeRobot—0 Health Check: UptimeRobot — 0 Monetization: RapidAPI — 20% commission Total monthly infrastructure cost: 0.00ThePlaybookPureAlgorithmAPIsAreCheatCodesNoexternaldependencies.NoAPIkeys.Noratelimits.Writeonce,runforever.QRCodeGenerator—PurePython—ZeromaintenancePasswordChecker—Entropymath—ZeromaintenanceUUIDGenerator—Pythonuuid—ZeromaintenanceMarkdowntoHTML—Regex—ZeromaintenanceThesetook4hourseach.They′veneverbrokenonce.FreePublicDataSourcesAreGoldminesWikidata→Companyinfoforanybusiness→CompanyInfoAPIRDAPProtocol→Domainregistrationdata→DomainWHOISAPIOpenStreetMap→Businesslistingsworldwide→PlacesSearchAPIAllfree.Allglobalcoverage.NoAPIkeysneeded.CachingPreventsColdStartsRender′sfreetiersleepsafter15minutesofinactivity.Firstrequesttakes50+seconds.Fix:UptimeRobotpings/healthevery5minutes.50monitors,allfree.TheNumbersAfter3months:21APIs,0.00 The Playbook Pure Algorithm APIs Are Cheat Codes No external dependencies. No API keys. No rate limits. Write once, run forever. QR Code Generator — Pure Python — Zero maintenance Password Checker — Entropy math — Zero maintenance UUID Generator — Python uuid — Zero maintenance Markdown to HTML — Regex — Zero maintenance These took 4 hours each. They've never broken once. Free Public Data Sources Are Goldmines Wikidata → Company info for any business → Company Info API RDAP Protocol → Domain registration data → Domain WHOIS API OpenStreetMap → Business listings worldwide → Places Search API All free. All global coverage. No API keys needed. Caching Prevents Cold Starts Render's free tier sleeps after 15 minutes of inactivity. First request takes 50+ seconds. Fix: UptimeRobot pings /health every 5 minutes. 50 monitors, all free. The Numbers After 3 months: 21 APIs, 0 monthly cost, 3+ APIs with growing traffic. Best performer: Domain WHOIS API — 7.6K calls per week from a single user in Germany. People are willing to pay for convenience. RDAP is free and well-documented, but wrapping it in a clean JSON API is worth real money. What I'd Do Differently Start with health endpoints. Lost weeks debugging cold starts that turned out to be missing /health routes. Vary your categories. Data, Finance, Tools, Security — different search terms catch different buyers. Write longer descriptions. RapidAPI search favors APIs with detailed markdown descriptions. The APIs Data: Domain WHOIS, Company Info, Email Validator, IP Geolocation, Places Search Finance: Currency Converter, Stock Price, EU VAT Validator Security: Password Checker Tools: QR Code, Barcode, UUID, Markdown, Slug, User Agent Parser, PDF to Text Search "buigialy30206" on RapidAPI to find all of them. The real lesson: you don't need venture capital, a team, or a SaaS product. You need one free hosting provider, one free data source, one afternoon per API, and the willingness to ship. The best passive income is the one that costs $0 to run.