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Sunday — The AI Concierge That Lives in Your Texts

Inspiration

People are lonelier than ever — the Surgeon General called it an epidemic. It's not that people don't want to hang out. It's that planning is exhausting. Juggling DoorDash, OpenTable, Venmo, and a group chat that can't agree on anything. Most plans die right there — not from lack of desire, but from logistics.

Every friend group has one person who does the work: makes the reservation, remembers the allergies, sends the address. That person burns out. Exec (YC W12) tried solving this with human assistants — didn't scale. Instacart (YC S12) solved grocery delivery but created another app silo. Neither made it easier for people to actually come together.

We asked: what if you rebuilt the concierge from scratch in 2026, with AI at the core? Not as another app to download, but as a friend who lives in your existing group chats, one who remembers everyone's allergies, knows the best spots, and has already book the venues?

That's Sunday. An AI concierge and social connector that lives natively in iMessage. You text it like a friend, and it actually makes things happen.

What it does

Out agent resides inside your existing group chats, understands everyone like a best friend. From Thor's favorite cuisine to Grace's go to hangout spot. It remembers the small and big details and stores it in a graph database, connecting the dots together.

It plans. Tell Sunday "dinner Saturday for 8, half vegetarian, $150 budget" and it researches real restaurants with real prices, remembers that Sarah is gluten-free from three weeks ago, proposes a specific plan (not a list of 10 options), and adapts on the fly when constraints change.

It orders. Sunday opens a real cloud browser, navigates DoorDash, searches for items, adds them to your cart, and walks you through checkout — all from a text message. You get a screenshot of your cart to review before confirming.

It creates events. Say "make a Partiful for Friday night at Rock & Reilly's" and Sunday generates a custom invite poster, creates the event on Partiful with the RSVP link, and shares it in the chat. One text, done.

It calls restaurants. Need a reservation? Sunday's AI voice agent calls the restaurant, handles the back-and-forth ("7pm is full, we have 7:30"), and texts you the confirmation.

It remembers everyone. Sunday builds a persistent social graph across conversations. It knows Thor's birthday is March 15, that Jake is into vinyl and City Pop, and that Emma and Jake are both single and both into hiking. When you're planning an event, it proactively suggests who else might want to come.

It connects people. This is the part we're most proud of. Sunday isn't just a logistics bot, it's a social connector. It notices when two friends share interests, nudges when a group hasn't hung out in a while, and plays matchmaker when the moment is right. Every plan is a chance to bring people together.

How we built it

Sunday is a TypeScript pnpm monorepo with two services communicating over a local HTTP API:

The Bridge handles the iMessage pipeline. It connects to iMessage via the Photon SDK, debounces rapid-fire texts into single requests, classifies messages as new tasks or follow-ups, and delivers responses back as native blue-bubble iMessages.

The Agent runs an HTTP server powered by the Claude Agent SDK with 17 custom MCP tools:

• Perplexity Sonar real-time web search for restaurants, products, prices. All recommendations are grounded in real data with source URLs.
• Browserbase + Stagehand is an AI-controlled cloud browsers that navigate real websites. Stagehand's hybrid mode uses both DOM analysis and screenshot-based coordination to interact with Partiful and DoorDash. Handles login flows, OTP verification, and multi-step checkout.
• Partiful creates real event pages with RSVP links via browser automation. Two-phase OTP flow that fills the form, requests verification, waits for the user to text back the code, then resumes the browser session to finish.
• Vapi is an AI voice agent that places real outbound calls to restaurants for reservations.
• Supermemory persistent cross-conversation memory. Stores facts about people as natural language, retrieves them contextually. Every conversation enriches the social graph.
• AI Gateway generates custom event invite posters and photo collage recap cards.
• Social graph tools searches across known people to find who'd enjoy an event or what two people have in common.
• Lists manage shopping lists, guest lists, planning checklists that persist across conversations.

The personality is carefully crafted. Sunday texts like a real friend — lowercase, emoji when it fits, opinions, gentle roasting. It matches the energy of the conversation and never sounds like a bot. The system prompt has a banned-words list (no "Furthermore," no "I'd be happy to help!") and explicit examples of how to talk.

Tech stack: Claude Agent SDK, Vercel AI SDK for streaming Photon Advanced iMessage Kit, Stagehand/Browserbase for browser automation, Perplexity Sonar for search, Supermemory for persistent memory, Vapi for voice calls, Next.js dashboard deployed on Vercel.

Challenges we ran into

Browser automation is hard. Getting Stagehand to reliably fill out forms on Partiful and DoorDash took serious iteration. The AI agent would click the wrong elements, date pickers were tricky, and we hit Browserbase rate limits from rapid testing. We switched between DOM mode and hybrid mode multiple times, debugged $PARAMETER_NAME schema validation errors in Stagehand's AI SDK layer, and eventually got it working reliably by setting experimental: true to force local agent execution with hybrid tools.

OTP flows across sessions. Partiful and DoorDash both require phone verification. We needed the browser to fill a form, trigger an OTP, pause, wait for the user to text us the code via iMessage, then resume the exact same browser session to enter it. We built persistent Browserbase contexts and session keepalive to make cookies survive across separate browser sessions.

Zombie processes. During development, background agent processes would pile up without being killed properly. At one point we had 5 duplicate agent instances all processing the same iMessage, causing the user to get 2-3 copies of every response. We added EPIPE error handling and global uncaughtException handlers to prevent crashes from dead SSE client connections.

Bridge timeouts. Browser automation takes 2-5 minutes. The bridge's HTTP timeout was set to 2 minutes, so it would abort the request and send a fallback "agent unavailable" message — even though the agent successfully created the event. We bumped the timeout to 6 minutes for tool-heavy flows.

The personality balance. Getting an AI to text like a real friend — not too formal, not too chaotic — took dozens of prompt iterations. The banned-words list alone went through 5 revisions. Getting it to use "yo" occasionally but not every message. Getting it to have opinions without being annoying. Getting it to react to good news with genuine excitement, not corporate enthusiasm.

Accomplishments that we're proud of

It actually works end-to-end in iMessage. We kept the user experience first by not needing users to install another app. It resides in an app they interact with daily. Just text Sunday to create your Sunday plans!

Parallel voice calls. The conversational agent takes numbers and context from perplexity and calls up to 10 venues parallelly, asking for availability, price and finally reserving the booking. This flow saves 1-2 hours per event.

Partiful. "Make a Partiful for Friday night" → Sunday generates a custom poster, creates the event on Partiful via browser automation, and shares the RSVP link, all from one text. The event is real and people can RSVP!

The social connector instinct. Sunday doesn't just handle logistics. It notices that Jake and Emma are both into hiking and Japanese food, and suggests adding Emma to the dinner party. It remembers that a friend group hasn't hung out in a while and nudges.

The voice. Sunday texts like a friend. "what's good" not "Hello! How can I help you today?" People who test it genuinely enjoy the conversation, not just the utility.

What we learned

Important of UX in integrating AI agents. The need to not download an app, not create an account and get started by just adding a number to your group chat is frictionless. Great for distribution.

Browser automation is the new API. Most services don't have public APIs for consumers. DoorDash, Partiful and most restaurants don't let you programmatically place orders or create events. But with Stagehand and Browserbase, any website becomes an API. This is a paradigm shift.

Memory changes everything. The difference between a chatbot and a concierge that one notices and remembers. When Sunday remembers that Sarah is gluten-free from a conversation three weeks ago and automatically factors that into tonight's dinner plan, that's the moment it stops feeling like a tool and starts feeling like a friend.

Personality > capability. We could have shipped more features. Instead, we spent hours on the system prompt, the banned-words list, the emotional range. That investment paid off. People remember how Sunday talks, not just what it does.

AI agents should bring people together, not replace human interaction. The last decade of consumer tech optimized for convenience at the cost of connection. Sunday uses AI but points towards more human interaction.

What's next for Sunday

Visa virtual card payments. We're exploring Visa's B2B Virtual Account Payment Method API to generate single-use virtual Visa cards for each order. Sunday would create a $50-limit virtual card, enter it at DoorDash checkout via Stagehand, and the card auto-expires after the transaction. Real payments, real spending controls, no saved card data.

Group chat intelligence. Sunday already works in group chats, but we want it to understand group dynamics better — who usually organizes, who needs more lead time, which subgroups hang out most. The social graph gets richer with every conversation.

Multi-platform messaging. iMessage is home, but Sunday's architecture (bridge → agent) is platform-agnostic. WhatsApp, Telegram, and Discord bridges are natural extensions.

Proactive outreach. Sunday shouldn't just respond, it should initiate. "Hey, it's been 3 weeks since the crew got together. Want me to put something together for this weekend?" The memory and social graph make this possible.

More commerce integrations. Every website is an API through browser automation. Uber, OpenTable, Amazon, flower delivery, concert tickets — Sunday should be able to handle any "get me X" request through a real browser.

Built With

• anthropic-api
• browserbase
• browserbase-stagehand
• claude-agent-sdk
• next.js
• node.js
• partiful
• perplexity
• perplexity-sonar
• supermemory
• tailwind
• typescript
• vapi
• vercel
• vercel-ai-sdk