Web

Building a Real-Time Cross-Platform Chat App with Replit Agent 4

Why this matters for builders

Replit Agent 4 lets you go from a blank workspace to a full realtime chat app that runs on both web and mobile — all inside one Replit project — by describing what you want on the new Agent Canvas.

The launch removes the traditional multi-repo, multi-framework friction that has historically made cross-platform realtime apps painful. You no longer need separate frontend, backend, and mobile codebases or wrestle with Firebase + React Native + Next.js configuration. One workspace, one AI coding partner, and a live demo tomorrow means you can ship production-grade realtime features in hours instead of weeks.

When to use it

• You want a quick realtime collaboration or messaging surface (internal tools, customer support, community features)
• You need the same chat experience on web, iOS, and Android without maintaining three codebases
• You prefer to stay inside a single cloud IDE and let an agent handle scaffolding, state management, and deployment
• You are iterating fast and want the AI to own the boring plumbing while you focus on product decisions

The full process

1. Define the goal (30 minutes)

Start by writing a one-page spec. Good specs for Agent 4 are concrete, scoped, and include success criteria.

Example spec (copy-paste ready):

Build a realtime chat application called "VibeChat" with these requirements:

- Users can create or join rooms by a shareable code
- Messages appear instantly across all connected clients
- Support for text + simple emoji reactions
- Web version using modern React + Tailwind
- Mobile version using React Native (Expo) that shares business logic
- Backend using Replit's built-in database + WebSockets (or Supabase if easier)
- Authentication: simple anonymous login with username choice
- Responsive design that feels native on mobile and clean on desktop

Success criteria:
- Two browser tabs and one mobile simulator all see messages in < 500ms
- Code lives in a single Replit workspace
- Deployable with one click to Replit static + mobile preview

Keep this spec in a file called SPEC.md at the root. Agent 4 performs dramatically better when it can read a clear spec.

2. Shape the prompt for Agent Canvas

Open the new Agent Canvas in your Replit workspace.

Use this starter prompt (customize the project name):

You are an expert full-stack and mobile engineer. We are building "VibeChat" exactly to the spec in SPEC.md.

First, explore the workspace and read SPEC.md.

Then create the project structure using best practices for a shared-logic monorepo inside Replit:
- /web for the web React app
- /mobile for the Expo React Native app
- /shared for types and business logic
- /server for the realtime backend

Use Replit's native realtime primitives or the simplest production-ready stack that works in one workspace.

Show me the folder structure and your technology choices before writing any code.

Agent 4 usually responds with a plan. Review it, then reply:

Looks good. Now implement the backend first, then the shared types, then the web client, then the mobile client. Work one layer at a time and ask for feedback after each major piece.

3. Scaffold and implement iteratively

Follow this rhythm:

1. Let Agent 4 generate the backend (database schema + WebSocket handler).
2. Review the code. Run it locally in the workspace.
3. Ask for the shared types and hooks.
4. Move to the web frontend.
5. Finally tackle the mobile app.

Useful follow-up prompts:

Add end-to-end typing using TypeScript/Zod. Make sure the mobile and web clients import the same message types from /shared.

Implement optimistic updates on the web client so messages appear instantly before the server round-trip.

Add a simple "is typing" indicator that broadcasts via the same realtime channel.

Use the canvas’s ability to edit multiple files at once. After each major feature, run the relevant preview:

npm run dev --prefix web

# Mobile
cd mobile && npx expo start

4. Validate

Create a validation checklist inside the workspace:

• Two web clients see messages instantly
• Mobile client receives and sends messages
• Room code sharing works (copy-to-clipboard + join by code)
• Messages persist when page is refreshed
• No console errors on web or mobile
• Works on mobile device (not just simulator) via Expo Go

Test with real devices: open the web version on your laptop and the mobile build on your phone simultaneously.

Prompt for fixing issues:

The mobile client is not receiving messages. Debug the realtime subscription logic and fix it. Show me the diff.

5. Ship safely

Once everything works:

• Add a simple README with one-click run instructions
• Use Replit Deployments for the web backend and static frontend
• Generate a shareable mobile preview link via Expo
• (Optional) Add basic rate limiting or moderation hooks if this will be public

Final prompt to Agent 4:

Prepare this workspace for public sharing:
- Write clear run instructions in README.md
- Add environment variable placeholders
- Make sure the production build works
- Generate a one-line deploy command for web

Pitfalls and guardrails

### What if Agent 4 suggests three different tech stacks?
Force convergence early. Reply: “Stick to the simplest stack that works inside one Replit workspace. Prefer native Replit primitives over adding new external services unless absolutely necessary.”

### What if the mobile and web logic drift apart?
Always keep business logic in /shared. If you see duplicated code, stop and say: “Refactor so both web and mobile import the same sendMessage and subscribeToRoom functions from /shared.”

### What if realtime latency feels bad?
Ask the agent to switch from polling to proper WebSockets or use Replit’s built-in realtime database. Prompt: “Replace any polling with proper realtime subscriptions and measure round-trip time.”

### What if the workspace gets messy?
Agent 4 can clean up. Prompt: “Organize the codebase following the structure in SPEC.md. Remove any unused files and update imports.”

What to do next

1. Join the live stream tomorrow to see Agent 4 in action on this exact use case.
2. After the stream, duplicate the public template Replit will likely publish.
3. Add one new feature (file uploads, threads, or presence) using the same iterative prompting pattern.
4. Measure how long the entire cycle took you — this becomes your new baseline for realtime features.
5. Consider turning the shared logic into a small open-source package for other Replit builders.

The combination of Agent Canvas + single-workspace web + mobile is a genuine productivity leap. Builders who master this workflow will ship interactive features 5-10× faster than traditional multi-repo setups.

Sources

• Replit announcement: https://x.com/Replit/status/2031868720700854400
• Live stream description detailing the realtime chat build across web and mobile in one workspace