#13 Design week with a Golden Gate view
What I built (and learned) at Presidio Bitcoin Design Week in San Francisco.
Presidio Bitcoin Design Week just wrapped up and it was the best bitcoin event I’ve ever been to. I was invited to host an interactive vibe coding session based on the Cashu wallet I hacked into Bitchat a few weeks back. I also jammed on two new projects during breakout sessions. Here’s a quick recap of what went down.
Interactive Bitchat x Cashu Vibe Coding Session
During my session I walked through my design and development process of the Cashu wallet I vibe coded into Bitchat a few weeks ago. After my walkthrough we jumped into a live vibe coding workshop to improve the embedded ecash UI chip in chat. The format was interactive: audience members threw out ideas, I dropped them into Cursor, and we watched the UI update in real time. Afterwards a few attendees came up to me and said how fun it was to collaborate on design like that.
Finding Inspiration
The week was stacked with inspiring talks. Too many to list but what stood out most was seeing how much left field design CashApp embraces. It reminded me of when I used to spend hours in Illustrator and Photoshop. Tweaking textures, gradients, using the touch-type tool to make fonts feel more human.
Over the years I drifted into tight, clean, minimal UI. Somewhere along the way, I lost a bit of my own playful side. Leaving SF, I’m feeling inspired to bring weirdness back into my work. It’s time to retuvrn. Expect bolder designs from me going forward.
Bitchat Permissioned Channels
The first idea came from a quick jam session about permissioned groups in Bitchat. In an hour we sketched out requirements, edge cases, and rough UX flows. I worked with another designer to mock up a UI concept, while Steve and Daniel focused on functionality.
The concept is simple. A user creates a private channel, then shares access via a one-time invite code. Invites are accepted over NFC. We leaned into NFC because it fits real-world use cases like festivals, protests, or meetups, where people want to spin up a private group on the spot.
I hacked together a quick hardcoded prototype. I didn’t get bluetooth mesh working with private channels yet. I’m planning to revisit this and try for a real demo on two Android devices next week.
BumpCash
The second idea came out of one of the Design Challenges. Which invites us to imagine how existing Silicon Valley products might embrace bitcoin. I went back to the magic of ecash + NFC, and I sketched out a CashApp feature I’m calling BumpCash.
Think Apple Pay’s tap-to-pay, but for sending ecash. I’m going to hand wave the regulatory, legal, and technical stuff here (Apple is notorious for strict NFC policy) and just focus on the upside and design.
The flow:
A CashApp user enters an amount, then selects BumpCash.
CashApp debits the user $17 and issues $17 in Cashu stablecoin ecash.
That balance appears as a bearer asset card in the UI. The card is customizable.
Tap phone and the BumpCash (ecash token) is transferred instantly.
If the receiver is offline, standard Chaumian mint rules kick in. The token just remains in local storage and is redeemed when back online. No reclaim option, no double-spend. Just like handing someone a $20 bill.
BumpCash would allow for a perfectly private digital cash UX. You could even design some nice haptic feedback and dopamine rewarding micro animations. Some small things that mimic the features that make physical cash great. Imagine a demo where 20 CashApp users pass around $20 in BumpCash. They’d nearly duplicate the perfect privacy of handing off a $20 bill in meatspace. The CashApp Cashu mint would have no idea that the $20 cycled through 20 people.
BumpTags
I also jammed on how this might work with hardware and came up with the concept of BumpTags: small Bluetooth/NFC devices that pair with CashApp and generate programmable Cashu requests (NUT-18: Payment Request).
Imagine a dancer performing on the street, wearing a BumpTag ring. They move through the crowd, tapping the phones of people who are already recording the show. With each tap, the audience member instantly gets a $1 CashApp tip request. In a second they can accept and send the requested ecash tip.
I think that BumpTags can play into crowd psychology. When a performer physically interacts with your phone and a request pops up it feels personal. Almost like being called out. I suspect people are far more likely to tip when they’re engaged like that. It turns a street performance into an interactive loop: tap, request, response, reward.
Final settlement on contact. Perfectly private. Chaum’s vision of electronic cash for the streets.
Closing Thoughts
Beyond the sessions, the best part of the week was meeting and hanging out with other designers (and getting Chinese food downtown with my homie Roger). There’s a lot of energy in bitcoin design right now, and I’m fueled by it. I left really inspired by San Francisco, and I’m planning to split my time between Stockholm and San Francisco next year. I feel like I’ll grow so much faster as a designer if I spend time around the energy and skill currently concentrated in SF.
I also recorded a 21 in 21 podcast episode with Haley Berkoe while I was there. I’ve been a fan of the pod for a long time, so it was a real honor to be invited on. I can’t wait to share it with you soon.
A huge thank you to Christoph, Mo, Matt for the organizational effort. All the speakers who volunteered their time and expertise, and the entire time at Presidio Bitcoin for being such gracious hosts.
Until next time,
Erik