We built 126 browser tools with zero uploads. Here is what broke along the way

We are two friends building Pageonaut , a collection of 126 free browser tools (converters, calculators, PDF and image utilities, dev helpers). Early on we committed to one constraint: everything runs client-side . No file uploads, no accounts, no server-side processing. That one decision shaped the whole architecture, and it broke things in ways I did not expect. Here are the lessons, including the one where our server filled up with 419 GB of cache and took the site down twice. Why client-side only Every time I needed a quick converter, the top search results wanted me to upload my file to their server, create an account, or pay to remove a watermark. For work that a browser can trivially do locally. So the rule became: drop a file into one of our converters and it never leaves your device. You can watch the network tab while using it. This is great for privacy and trust, and it has a nice side effect: our server does almost nothing per user, so hosting stays cheap even if a tool gets popular. The cost: some tools are genuinely harder to build. PDF manipulation in the browser (we use client-side libraries instead of a server queue), image processing on the main thread without freezing the UI, and no "just call an API" escape hatch. When a tool truly needs the network (say, fetching a URL you give it), the page says so explicitly. Lesson 1: Unbounded URL params + ISR = a full disk This is the expensive one. We built shareable challenge pages: beat my score, try this color, that kind of thing. The URLs look like /tools/<slug>/challenge/<value> , where <value> is user-generated. With Next.js ISR, every unique URL that renders gets persisted to the filesystem cache. You can see where this is going. The value space is infinite. Bots found the pattern and started enumerating it. .next/server/app grew to 419 GB . The disk filled up, the site went down, and because we did not understand the root cause immediately, it happened a second time a few days later. The fix was tw