Moonmates

“We should catch up soon.” Almost everyone says it, and most of the time nothing happens. In a survey of 2,000 UK adults, 26% admitted they often say it but never make plans, and it takes an average of 126 group chat messages before a trip is actually booked. Six in ten adults say they have lost touch with a best friend.

The problem isn’t that people care less. Catching up has become admin. Someone has to pick a date, find a place that’s fair for everyone, chase replies and then split the bill. Busy weeks win, and friendships quietly fade. This affects anyone with a full life: people working across a big city like London, friends who moved to different areas, and old groups who still talk every day in a chat but haven’t been in the same room for months.

Moonmates is a shared Space for your circle of friends, built into your banking app. It turns catching up into a ritual instead of a project.

• Everyone saves the same small amount each day, such as 50p, £1, £3 or £5, chosen together. The money moves quietly into a group Space, and a moon on screen fills as the pot grows.

• Every 30 days the moon is full and the night out is already paid for. The circle chooses what the moon becomes: pizza, sushi, burgers, pasta, coffee or bowling.

• With Crossed Paths turned on, the app picks the spot. It uses TfL commute patterns to find where everyone’s weeks already overlap, such as Liverpool Street on a Thursday evening, and suggests a place nearby. Anyone can change the suggestion, or type their own spot.

• One invite goes out to the circle. When most of the group taps “I’m in”, the plan is booked. Anyone who can’t make it keeps their share for the next moon.

• On the night, a virtual card pays, locked to that venue and that evening.

Moonmates removes the three decisions that kill most plans (when, how to pay and where) and replaces 126 messages with one invite. The app also remembers every moon, so the circle can see how often they’ve actually met.

Moonmates combines Starling Engine with an unexpected non-financial API, then adds an AI layer on top.

Starling Engine

• Spaces hold each circle’s group pot and receive the daily sweeps from every member.

• Virtual cards are issued to the people who said yes, for the night out.

• Card controls lock the card to the chosen venue type, location and time window.

TfL Unified API

• Members opt in to share weekly commute patterns, never live location.

• When friends’ routes pass through the same station at similar times, that’s a crossing, and each one becomes a data point.

AI layer

• A spot model turns crossings into the best meeting place and time for the whole group, with a confidence score.

• It then writes the invite. When Crossed Paths is off, the circle picks the spot themselves.

How the pieces fit together

• The frontend is a mobile banking style app that makes requests to our backend.

• The backend runs four services:

  • a scheduler that triggers the daily Engine transfers into the Space

  • a route matcher that reads TfL data and records crossings

  • the AI spot model

  • an invite and quorum service that issues the virtual card once enough people accept

• The database stores circles, members, savings settings, the moon cycle, route data points, chosen spots and partner venues, plus each circle’s history of past moons.

For the hackathon we built a clickable prototype of the full user journey. The Engine, TfL and AI calls are simulated, and a live log shows every call as it would happen. We followed Sam’s advice and faked the integrations so we could focus on telling the story clearly.

Business Impact

• Banks on Engine get a feature people open daily to watch their moon grow, which drives engagement and more people using it as their main account.

• More money stays in the account through regular contributions and card spend on every night out.

• Venues near busy stations, such as restaurants, cafés and bowling alleys, can partner to appear in the suggested places with a perk for circles, in exchange for predictable group bookings.

• Engine gets a ready made feature it can offer to every bank built on it.

• Choosing the idea. Crazy Eights gave us more than 20 ideas, from an applause fund to a zombie tax, so narrowing down took real discipline.

• Finding the real problem. We first framed it as loneliness, but the statistics didn’t fit, so we pivoted to busy people losing touch.

• Making it work for groups. We started with coffee for two friends, then realised the real pain is getting a whole group to agree, so we redesigned around circles.

• Giving TfL the right job. Our first version added extra money whenever friends crossed paths, which didn’t make sense. We reworked Crossed Paths so it only decides where and when to meet, the step that actually causes 126 message group chats.

• Privacy and fairness. We made Crossed Paths fully opt in, based on weekly patterns rather than live location, and made everyone save the same amount so nobody feels they’re paying more than the others.

• Time and access. Without live API access, we simulated Engine and TfL realistically and showed each call in a behind the scenes log.

• Built an end to end clickable prototype covering the moon filling up, friends as stars crossing paths, AI spot suggestions with confidence scores, choosing a nearby place, the group invite, the virtual card and the circle’s history.

• Designed a product that answers the Engine challenge directly, using Spaces, virtual cards and card controls together with an unexpected non-financial API.

• Grounded the problem in real research on UK friendships and group planning. Link: https://www.butlins.com/press/2026/big-weekenders-celebrant-of-friendship

• Learned how much a clear metaphor helps. Engine calls them Spaces, so our circle is our space, and we’re the stars orbiting it.

• Worked through two big pivots as a team of five in a single weekend, and kept the user experience simple: save, get one invite, show up.

• Test and survey with about 10 real circles.

• Connect to the real Starling Engine sandbox and the TfL Unified API.

• Let circles set a goal (for example “£120 for dinner”) instead of a daily amount.

• Sync invites with calendars so the night lands in everyone’s diary.

• Build a partner network of venues around the busiest stations.

• Measure what changes: moons completed, nights out held, and time from full moon to confirmed plan. Over a longer pilot, add the Office for National Statistics (ONS) social connection questions.