# Custom app development in Miami Know your price before you hire.

> Custom mobile apps, web apps, backends, AI features and Stripe payments, built by a founder-led studio in Miami, Florida. Fixed prices, everything in writing.

Canonical: https://app369.com/

Mobile apps, web apps, AI features and payments, built by Simon Dziak, the senior developer you actually talk to. Fixed prices, everything in writing.

Mobile apps start from $9,000. 5 quick questions, then your price.

[**Get my quote **→](https://app369.com/instant-quote/?need=mobile) [Or see all 19 fixed prices](#prices-heading)

- [Client rating**5.0 out of 5**From 127 public Fiverr reviews, 126 of them copied word for word.](https://app369.com/reviews/)
- [Track record![](https://app369.com/.netlify/images?w=64&fm=webp&url=%2Fportfolio%2Fnew%2Fworkspace369-app-store.jpeg)![](https://app369.com/.netlify/images?w=64&fm=webp&url=%2Fportfolio%2Fprojects%2Fmy3693.jpg)![](https://app369.com/.netlify/images?w=64&fm=webp&url=%2Fportfolio%2F866_1x_shots_so.png)![](https://app369.com/.netlify/images?w=64&fm=webp&url=%2Fportfolio%2Fprojects%2Foss-mcp.jpg)+10**14 projects**Apps, platforms and stores shipped since 2020.](https://app369.com/portfolio/)
- [How you pay50% to start50% on delivery**Pay in halves**Custom projects: half to start, the rest on delivery. Fixed-price offers are paid upfront.](https://app369.com/fee-structure/)
- [Where we are Miami**Eastern Time**Working hours in Miami, Florida.](https://app369.com/contact/)

Free check

## Check your AI-built app

Paste your live app’s address. The check reads only what any visitor’s browser already downloads: the home page, its scripts and its headers. It never logs in, never queries your database and never stores the address.

**What it checks:** secret keys shipped in your JavaScript (shown masked), which Supabase or Firebase project the app uses and the tables its code names, security headers, and the page tags search engines and link previews read.

**What it doesn’t:** anything behind a login, your data rules, payments, or whether your tables can be read. Those need the free table check (after you prove you own the site) and the $450 Launch Check.

Selected work since 2020

## **Apps, AI and platforms we’ve shipped**

[**All 14 projects**](https://app369.com/portfolio/)

- ![Workspace369: SaaS, iOS & web](https://app369.com/.netlify/images?w=2100&h=1312&url=%2Fportfolio%2Fnew%2Fworkspace369-app-store.jpeg)2023–nowOur own product ### [Workspace369](https://app369.com/case-studies/workspace369/)
- ![3693: Music app](https://app369.com/.netlify/images?w=1280&h=800&url=%2Fportfolio%2Fprojects%2Fmy3693.jpg)2026Our own product ### [3693](https://app369.com/case-studies/3693-music-app/)
- ![SaaS MCP Connector: AI integration](https://app369.com/.netlify/images?w=1280&h=800&url=%2Fportfolio%2Fprojects%2Foss-mcp.jpg)2026Open source by Simon Dziak ### [SaaS MCP Connector](https://app369.com/case-studies/saas-mcp-connector/)
- ![BILSTEIN: Mobile & web app](https://app369.com/.netlify/images?w=1280&h=800&url=%2Fportfolio%2F866_1x_shots_so.png)Client project ### [BILSTEIN (opens in a new tab)](https://bilstein.com/en-us/products/)
- ![Claude Agent Reliability Kit: AI agents](https://app369.com/.netlify/images?w=1280&h=800&url=%2Fportfolio%2Fprojects%2Foss-agent.jpg)2026Open source by Simon Dziak ### [Claude Agent Reliability Kit (opens in a new tab)](https://github.com/SzymonDziak/claude-agent-reliability-kit)
- ![Chicago Pneumatic: Mobile & web app](https://app369.com/.netlify/images?w=1280&h=800&url=%2Fportfolio%2Fcp-port.png)Client project ### **Chicago Pneumatic**

## **One senior developer across the whole product**

The app, the web front end, the backend, the AI features and the payments.

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### [Mobile apps](https://app369.com/service/mobile-apps/)

iOS and Android apps, from a first release to fixing one that already has users.

[![](https://app369.com/.netlify/images?w=120&fm=webp&url=%2Fportfolio%2Fprojects%2Fmy3693.jpg)Built by App369**3693**2026Our own product](https://app369.com/case-studies/3693-music-app/)

- Flutter
- Swift
- App Store & Google Play

Fixed price: [App Fix Ticket](https://app369.com/fixed-price/flutter-firebase-bug-fix/) (from $450), [AI-Built App Rescue](https://app369.com/fixed-price/ai-app-rescue/) (from $450)

[**Mobile apps from $9,000 **→](https://app369.com/instant-quote/?need=mobile) [**See the service**](https://app369.com/service/mobile-apps/)

### [Web apps and sites](https://app369.com/service/web-apps/)

Customer portals, dashboards, SaaS front ends and fast marketing sites.

[![](https://app369.com/.netlify/images?w=120&fm=webp&url=%2Fportfolio%2Fnew%2Fworkspace369-app-store.jpeg)Built by App369**Workspace369**2023–nowOur own product](https://app369.com/case-studies/workspace369/)

- Vue
- Nuxt
- React
- TypeScript

Fixed price: [Web App MVP Sprint](https://app369.com/fixed-price/web-app-mvp/) (from $6,000), [AI-Built App Rescue](https://app369.com/fixed-price/ai-app-rescue/) (from $450)

[**Web apps from $6,000 **→](https://app369.com/instant-quote/?need=web) [**See the service**](https://app369.com/service/web-apps/)

### [CRMs and client portals](https://app369.com/fixed-price/custom-crm-build/)

A CRM built around how you sell, or a portal where clients see their files, approvals and invoices.

[![](https://app369.com/.netlify/images?w=120&fm=webp&url=%2Fportfolio%2Fnew%2Fworkspace369-app-store.jpeg)Built by App369**Workspace369**2023–nowOur own product](https://app369.com/case-studies/workspace369/)

- Workspace369
- Vue
- Firebase

Fixed price: [Custom CRM Build](https://app369.com/fixed-price/custom-crm-build/) (from $6,000)

[**CRM scoping $450 **→](https://app369.com/fixed-price/custom-crm-build/) [**See the service**](https://app369.com/fixed-price/custom-crm-build/)

### [Backends and APIs](https://app369.com/service/backend-development/)

The server side: data models, APIs, background jobs, security rules and cloud cost.

[![](https://app369.com/.netlify/images?w=120&fm=webp&url=%2Fportfolio%2Fprojects%2Foss-mcp.jpg)Built by App369**SaaS MCP Connector**2026Open source by Simon Dziak](https://app369.com/case-studies/saas-mcp-connector/)

- Node.js
- TypeScript
- Firebase
- Google Cloud

Fixed price: [Firebase Security Rules Audit + Fix PR](https://app369.com/fixed-price/firebase-security-rules-audit/) ($950), [Firebase Cost-Cut Audit](https://app369.com/fixed-price/firebase-cost-audit/) ($750)

[**Firebase Security Rules Audit + Fix PR $950 **→](https://app369.com/fixed-price/firebase-security-rules-audit/) [**See the service**](https://app369.com/service/backend-development/)

### [AI features and agents](https://app369.com/service/ai-integration/)

LLM features inside your product, tool-using agents, and MCP servers for Claude and ChatGPT.

![](https://app369.com/.netlify/images?w=120&fm=webp&url=%2Fportfolio%2Fprojects%2Foss-agent.jpg)Built by App369**Claude Agent Reliability Kit**2026Open source by Simon Dziak

- Claude
- Gemini
- MCP
- Evals

Fixed price: [AI Feature or Agent Build](https://app369.com/fixed-price/ai-feature-development/) ($2,500), [MCP Connector for your SaaS](https://app369.com/fixed-price/mcp-server-for-saas/) ($2,500), [AI Agent Engineering Setup](https://app369.com/fixed-price/ai-agent-engineering-setup/) (from $750)

[**AI Feature or Agent Build $2,500 **→](https://app369.com/fixed-price/ai-feature-development/) [**See the service**](https://app369.com/service/ai-integration/)

### [Payments and integrations](https://app369.com/service/payments-integrations/)

Subscriptions, checkout, payouts, texting, email and calendar, wired up and tested.

[![](https://app369.com/.netlify/images?w=120&fm=webp&url=%2Fportfolio%2Fnew%2Fworkspace369-app-store.jpeg)Built by App369**Workspace369**2023–nowOur own product](https://app369.com/case-studies/workspace369/)

- Stripe
- Twilio
- SendGrid
- Webhooks

Fixed price: [Stripe Billing Fix and Setup](https://app369.com/fixed-price/stripe-billing-setup/) ($950), [API Integration](https://app369.com/fixed-price/api-integration/) ($1,200)

[**Stripe billing $950 **→](https://app369.com/fixed-price/stripe-billing-setup/) [**See the service**](https://app369.com/service/payments-integrations/)

### [Shopify stores](https://app369.com/service/ecommerce-development/)

Theme work, speed fixes and cleaning out code left behind by old apps.

- Shopify
- Liquid
- Core Web Vitals

Fixed price: [Shopify Speed & Checkout Tune-Up](https://app369.com/fixed-price/shopify-speed-optimization/) ($750)

[**Shopify Speed & Checkout Tune-Up $750 **→](https://app369.com/fixed-price/shopify-speed-optimization/) [**See the service**](https://app369.com/service/ecommerce-development/)

### [SEO, GEO and AEO](https://app369.com/service/seo-geo-aeo/)

Getting found and quoted by Google and by AI assistants such as ChatGPT and Perplexity.

- Structured data
- llms.txt
- IndexNow
- Core Web Vitals

Fixed price: [AI Search Visibility Audit](https://app369.com/fixed-price/ai-search-visibility-audit/) ($750), [SEO + GEO Fix Sprint](https://app369.com/fixed-price/seo-geo-fix-sprint/) ($1,950), [Monthly AI Search Growth](https://app369.com/fixed-price/ai-search-growth/) ($1,500/month)

[**AI Search Visibility Audit $750 **→](https://app369.com/fixed-price/ai-search-visibility-audit/) [**See the service**](https://app369.com/service/seo-geo-aeo/)

### [Product video](https://app369.com/fixed-price/saas-launch-video/)

Launch videos and Shorts built from your real product screens.

- Remotion
- Motion graphics
- YouTube Shorts

Fixed price: [SaaS Launch Video](https://app369.com/fixed-price/saas-launch-video/) ($1,500), [Shorts-as-a-Service](https://app369.com/fixed-price/youtube-shorts-for-saas/) ($1,500/month)

[**SaaS Launch Video $1,500 **→](https://app369.com/fixed-price/saas-launch-video/) [**See the service**](https://app369.com/fixed-price/saas-launch-video/)

[**All services**](https://app369.com/service/)

## Work directly with the senior developer who builds your app

“**Here the person you talk to writes the code, from start to finish. **

Simon Dziak, App369’s founder and head developer, leads every project himself. Larger firms hand projects to junior team members. At App369 there is no hand-off.

Meet Simon in 60 seconds: how App369 scopes, prices and builds your app.

[**More about Simon**](https://app369.com/about/szymon-dziak/)

Reviews

## **5.0**Rated 5.0 across 127 Fiverr reviews

127 public Fiverr reviews since 2020, 126 of them copied word for word on our reviews page.

At least 20 of the 77 buyers who left a public Fiverr review ordered again. [**See the review data**](https://app369.com/statistics/app369-client-review-data-2026/)

[**Read them on Fiverr** (opens in a new tab)](https://www.fiverr.com/szymondziak) [**All reviews**](https://app369.com/reviews/)

Fixed-price services

## **Fixed scope. Fixed price. In writing.**

Paid upfront, and the work happens in writing; no call needed.

[**All 19 offers**](https://app369.com/fixed-price/)

- [**App Fix Ticket**from $450One reproducible bug in an existing mobile or web app, diagnosed and fixed.](https://app369.com/fixed-price/flutter-firebase-bug-fix/)
- [**AI-Built App Rescue**from $450A Lovable, Bolt or Cursor app checked end to end in 3 days, then its worst problems fixed at a fixed price.](https://app369.com/fixed-price/ai-app-rescue/)
- [**AI Feature or Agent Build**$2,500One LLM feature or agent wired into your app or tools, with an eval set that shows how well it works.](https://app369.com/fixed-price/ai-feature-development/)
- [**Web App MVP Sprint**from $6,000A scoped web app MVP in Vue/Nuxt or React, with a fixed quote after a $450 written scoping.](https://app369.com/fixed-price/web-app-mvp/)
- [**Custom CRM Build**from $6,000A CRM or client portal built around how you sell, on the Workspace369 backend or fully custom, quoted in writing.](https://app369.com/fixed-price/custom-crm-build/)
- [**Stripe Billing Fix and Setup**$950Subscriptions, webhooks, customer portal and failed payments set up or fixed, tested in Stripe test mode.](https://app369.com/fixed-price/stripe-billing-setup/)
- [**API Integration**$1,200Two systems connected through their APIs, with retries, logs, failure alerts and tests.](https://app369.com/fixed-price/api-integration/)
- [**Firebase Security Rules Audit + Fix PR**$950Tests whether one user can read or edit another customer’s data, and fixes what it finds.](https://app369.com/fixed-price/firebase-security-rules-audit/)
- [**MCP Connector for your SaaS**$2,500An MCP server over your existing REST API, so your product works inside Claude, and in ChatGPT with OAuth sign-in.](https://app369.com/fixed-price/mcp-server-for-saas/)
- [**Firebase Cost-Cut Audit**$750Where your Firebase or Google Cloud bill goes, with savings ranked by dollars a month and risk.](https://app369.com/fixed-price/firebase-cost-audit/)
- [**Shopify Speed & Checkout Tune-Up**$750Mobile speed and cart-to-checkout fixes on a duplicate theme, measured before and after.](https://app369.com/fixed-price/shopify-speed-optimization/)
- [**SaaS Launch Video**$1,500A 60-second motion-graphics product video built from your real screens, plus a 15-second and a vertical cut.](https://app369.com/fixed-price/saas-launch-video/)
- [**Shorts-as-a-Service**$1,500/⁠monthTwelve vertical motion-graphics Shorts a month from your docs, posts or a voice memo. No filming.](https://app369.com/fixed-price/youtube-shorts-for-saas/)
- [**Workspace369 setup, done for you**from $450Proposals, signatures, invoices and payments set up in one place for freelancers and small agencies.](https://app369.com/fixed-price/workspace369-setup/)
- [**AI Agent Engineering Setup**from $750Your repos made ready for Claude Code, Codex and Cursor agents: a written diagnostic first, then a setup sprint.](https://app369.com/fixed-price/ai-agent-engineering-setup/)
- [**Monthly Dev Subscription**from $1,495/⁠monthOne development request at a time, most back in about 2 business days. Pause or cancel anytime.](https://app369.com/fixed-price/monthly-dev-subscription/)
- [**AI Search Visibility Audit**$75030 buyer questions run in ChatGPT, Perplexity, Google AI Overviews and Gemini, who gets cited, and ranked fixes.](https://app369.com/fixed-price/ai-search-visibility-audit/)
- [**SEO + GEO Fix Sprint**$1,950Structured data, answer blocks, crawler rules, internal links, speed and 5 answer pages, in 10 business days.](https://app369.com/fixed-price/seo-geo-fix-sprint/)
- [**Monthly AI Search Growth**$1,500/⁠month4 answer-ready pages, citation tracking on 30 questions and one link-earning asset a month. Cancel anytime.](https://app369.com/fixed-price/ai-search-growth/)

## **From idea to launch: most MVPs take **6–12 weeks

1. 01 ### **Understanding your idea** We start by listening to your idea and understanding your goals, then we create a plan to bring your idea to life.
2. 02 ### **Design & development** We design and build your app or website with you in mind, making sure it's easy to use, visually appealing, and works on all devices.
3. 03 ### **Launch & support** Once your app or website is ready, we launch it and continue to provide support, making sure it works flawlessly as your idea grows.

## **Pick the tool that fits the job**

Not the other way round. The tools across the 14 projects in our portfolio, and how many used each.

- **Shopify**6 projects
- **TypeScript**5 projects
- **Flutter**3 projects
- **Firebase**3 projects
- **Stripe**2 projects
- **Twilio**2 projects
- **Claude**2 projects
- **Nuxt**1 project
- **MCP**1 project
- **SQLite**1 project

Also in the toolbox: Swift, Kotlin, React, Vue, Node.js and Google Cloud.

## **Behind the builds **

[**@app369company** (opens in a new tab)](https://instagram.com/app369company)

[![App369 Instagram post](https://app369.com/.netlify/images?w=400&h=500&fm=webp&q=80&url=%2Fsocialmedia%2Fszymondziak+1.jpg)](https://instagram.com/app369company) [![App369 Instagram post](https://app369.com/.netlify/images?w=400&h=500&fm=webp&q=80&url=%2Fsocialmedia%2Fszymondziak+2.jpg)](https://instagram.com/app369company) [![App369 Instagram post](https://app369.com/.netlify/images?w=400&h=500&fm=webp&q=80&url=%2Fsocialmedia%2Fszymondziak+3.jpg)](https://instagram.com/app369company) [![App369 Instagram post](https://app369.com/.netlify/images?w=400&h=500&fm=webp&q=80&url=%2Fsocialmedia%2Fszymondziak+4+%281%29.jpg)](https://instagram.com/app369company)

![](https://app369.com/.netlify/images?w=44&h=44&url=%2Fimages%2Fszymon.jpg)****Simon Dziak****Founder and head developer

## **Get a personalized assessment by email**

Submit your project details and we’ll reply by email with a personalized assessment, usually within one business day.

[**Get in touch **→](https://app369.com/contact/) [**Instant quote**](https://app369.com/instant-quote/)

Prefer a call? [Consultations](https://app369.com/service/consultations/) are $100 per 15 minutes, paid upfront.