Inspiration

Students juggle back-to-back classes across campus, and rain or snow changes how long the walk takes. I wanted one link to answer: when do I leave, which route, and what do I bring?

What it does

Paste your class calendar link. ClassCast reads each event’s location, checks the weather, and gives you:

a leave-by time a walking route on a map a bring list a short AI daily brief How we built it Stack: Next.js on Vercel, with code in GitHub. Data: the calendar link supplies the locations. Open-Meteo gives the forecast. OpenRouteService gives routes. OpenStreetMap gives geocoding and map tiles. AI: Claude writes the bring list and brief. The code does the time math, so the AI can’t invent leave times. Leave-by formula: $$t_{leave} = t_{class} - t_{travel} - t_{buffer} - t_{weather}$$

with a 5 minute buffer and extra time for rain or snow.

Challenges Security: the server fetches any calendar URL a user enters, which is an SSRF risk. We blocked internal IPs and checked every redirect. Prompt injection: calendar text is untrusted, so we treat it that way before it reaches the AI. Map tiles blocked: our privacy header stripped the Referer that OpenStreetMap requires. Sending only the site origin fixed it. Deploy error: Vercel built the empty main branch. Merging the PR fixed it. Live APIs: the build sandbox blocked the weather and map APIs, so we tested with mock data first. What we learned Keep API keys server-side. Treat user URLs and calendar text as hostile input. Let code do the math and AI do the language.