What they did
u/Greengage1 planned a trip to Japan with Claude and then asked it to build a travel companion app that ran inside Claude itself, not as a standalone app. It started with tabs for the itinerary, bookings, a shopping list, phrases and taxi cards in Japanese. The poster then had it split the plan into one page per day with map links, step by step directions through stations, travel times, a suggested time to leave the hotel, nearby shopping list items and dinner picks near the hotel. Claude went through the poster's email for every booking reference, check-in time and breakfast detail and linked each booking email. During the trip it kept checking the inbox for trip changes, translated Japanese emails and drafted replies in Japanese, and the poster added a low walking mode and a photo tab for translating menus or getting help with ticket machines.
How it went
The poster called it extremely useful and could not recall any major mistakes, only small ones that probably came from outdated web pages, such as the wrong floor for hotel breakfast. They used it to re-route after a missed train, swap a tiring activity for something lighter, and work out how much yen to load on a travel card. In the comments they said Opus caught errors in an earlier itinerary drafted with Sonnet, such as a dish that was out of season.
Worth knowing
The app lived inside Claude, so no API keys or hosting were needed, but it is not something you can open offline. The post does not say which email provider or connector was used. A commenter turned the same idea into a reusable GitHub template driven by Claude skills.
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