Custom mobile app development company for US startups

A senior React Native team on US business hours, at European rates.

We are a Paris-based senior team that works your hours, ships to the App Store in 3 to 5 months, and costs roughly half of a comparable US agency. Not because we are junior. Because Paris is not San Francisco.

Senior engineers onlyiOS + AndroidOnline 9am–4pm ESTProject budgets from $25,000
Book a 30-min technical call

Calendly direct. You will talk to a senior engineer within 48 hours.

Online 9am–4pm ESTUSD invoicingSource code in your GitHub from day oneIP transferred on each milestone payment

How we cost half of a comparable US agency, without cutting quality.

A common reaction on the first call is some version of "this is suspiciously affordable for what you describe."

It is not. Here is the math.

Operating costs in Paris are structurally lower

A senior React Native engineer with 8 years of experience costs us roughly 40% of what the same profile costs a US agency in San Francisco or New York. Lower base salaries, lower social charges relative to total comp, no employer-paid healthcare, lower office costs. None of that has anything to do with skill.

We run a small senior-only team, no pyramid

US agencies bill you a senior rate but staff your project with juniors managed by one senior. We do not have juniors. Every engineer on your project has shipped at least one production app to the App Store. Fewer people, all senior, fixed scope.

We work async-first, on your hours

We are online 9am to 4pm Eastern Time. Your daily standup, your design reviews, your urgent decisions, all happen during your business day. You do not wait 16 hours for a reply.

What this means in practice

A greenfield iOS + Android app that a top-tier US agency would quote at $180k to $300k, we ship for $50k to $120k. Same App Store quality, same engineering rigor, same senior team. A different cost structure on our side.

10+ yearsbuilding production apps
25+projects shipped to production
4.9/5across Sortlist, Google · 35+ reviews

Selected clients across Europe and the United States

BASFGetYourGuideSigfoxSynapse MedicineLynceus Partners

A few of the products we have shipped to production

Visto cover

Visto

Mobile · Social discovery · Location-based

Mariia cover

Mariia

Mobile · Parenting × neuroscience · AI

Métaforma cover

Métaforma

Internal SaaS · B2B operations

US auto-import platform cover

US auto-import platform

Web · Cross-border commerce · AI agents

Every project on this page is one we built end-to-end and that is in production today.

Custom mobile app development:
cross-platform apps that ship to production.

Our mobile app development services for the US market are built around one stack: React Native, with native modules when the app needs them. iOS and Android from a single codebase. Backends in Node or Postgres-friendly stacks. AI features (LLM integration, RAG, agent workflows) when they genuinely improve the product.

Greenfield builds

You have a product spec or a Figma file. We take it from zero to App Store, typically in 3 to 5 months.

$50k – $120k

Rescue & rebuild

Your current app is slow, crashing, or stuck on a stack the original team moved on from. We take ownership, stabilize, and modernize.

$40k – $90k

AI-native features

You want a chat assistant, document intelligence, or AI-driven UX inside an existing app. We have shipped these on production traffic, including agentic flows.

$25k – $60k

Project budgets start at $25,000. We work on fixed-scope engagements with milestone-based pricing. Typical engagement length: 3 to 5 months for a greenfield app, 6 to 10 weeks for a focused rescue or AI feature build.

Case studies

Four products. Each one we built end-to-end, each one shipping in production.

Visto - Ruji case study cover

Visto

Mobile · Social discovery · Location-based

Stack: React Native · Expo · Geolocation · Push

Instagram meets Happn, a photo-driven social app where the people you cross paths with become matches surfaced through their feed. We owned the full mobile build from spec to App Store, including the geolocation pipeline, native push notifications, and the moderation layer.

Live on iOS and Android · Native geolocation · Production traffic since launch

Mariia - Ruji case study cover

Mariia

Mobile · Parenting × neuroscience · AI

Stack: React Native · OpenAI · RAG · Vector DB

An AI parental copilot blending Montessori pedagogy and neuroscience research, delivering personalized 24/7 advice through a generative engine running RAG over thousands of expert sources. We built the consumer app from scratch alongside the founder, owning iOS and Android delivery end-to-end.

iOS + Android in production · RAG over thousands of expert sources · Serving families daily

Métaforma - Ruji case study cover

Métaforma

Internal SaaS · B2B operations

Stack: React · Node.js · AWS · Stripe · LLM features

An end-to-end ERP redefining how training organizations run. We delivered the critical modules; Qualiopi-compliant admin, billing automation, marketing funnel, integrated LMS; replacing 5+ disconnected SaaS tools. Mobile access for field teams and AI-assisted data entry on top.

5+ SaaS tools replaced · 3× admin-time reduction reported by users · Field-team mobile access

US auto-import platform - Ruji case study cover

US auto-import platform

Web · Cross-border commerce · AI agents

Stack: TypeScript · LLM matching · Distributed scrapers

A US-based importer of classic American cars for European buyers needed to monitor every public US listing - eBay, Bring a Trailer, dealer sites, Craigslist - and surface vehicles matching each client’s mandate. We built a distributed scraping pipeline ingesting thousands of listings daily, plus an LLM layer that scores every match against the client’s criteria with explainable output.

Thousands of US listings ingested daily · LLM-scored matches · Live in production

What clients say · 4.9/5 across Sortlist and Google · 35+ reviews

Roger and his team are exceptional professionals: competent, thoughtful, responsive, and always delivering on time and to spec. We have several major projects running with them and could not be more satisfied.

Gadiel Ngoma

CEO, Hwell · Sortlist review

We genuinely felt heard, understood, and supported at every step. Concrete advice, smart solutions, fast progress in the right direction with no unnecessary stress. Pretty rare in this kind of collaboration.

Marcel Corrieri

Executive, Izippizi · Sortlist review

Communication with Ruji is easy and fast. Highly responsive team, attentive to client feedback. On a fairly unusual project, every requested change was made so the final result matched exactly what I hoped for. Signing up for V2.

Johann Poisson

Client · Google review

Read all reviews on Sortlist.

AI in production

AI features, when they earn their place

We are React Native specialists first. We ship AI features when they make the product measurably better, not because the deck has the word "AI" on it. Three patterns we have shipped on production traffic for paying users:

RAG over private knowledge

Your domain content, indexed and surfaced through chat or guided flows. Built for Mariia, a parental copilot indexing thousands of expert sources across Montessori pedagogy and neuroscience.

Stack: OpenAI · Vector DB · Python

LLM-driven matching

Score candidates against criteria, with explainable output the operator can trust. Built for a US auto-import platform matching seller listings against client mandates.

Stack: TypeScript · LLM · Scraping pipelines

Agentic data extraction

Pipelines that ingest unstructured input - PDFs, web pages, emails - and produce structured records. Built for B2B SaaS internal tools where data entry is the bottleneck.

Stack: TypeScript · LLM agents · OCR

Across all three: we ship the LLM layer alongside the React Native or web app, not as a bolt-on. We pick models pragmatically (OpenAI, Anthropic, open-source) based on cost, latency, and accuracy for the task, and we tell you when AI is the wrong answer.

Why React Native, and when we would point you elsewhere

We are React Native specialists. Here is our honest take on when it is the right call.

When React Native is the right answer

You need iOS and Android from day one. You ship features weekly. Your team is small, or your engineering capacity is borrowed from another product. You want a single codebase that scales as you hire. You would rather ship in 4 months than in 9.

When we would point you elsewhere

You are building a real-time, latency-critical interaction - high-end games, AR/VR, real-time camera vision pipelines. You need deep platform-specific APIs at the heart of the product, not at the edges. Your team already has senior native engineers and the engineering culture to maintain two codebases. In those cases we will tell you so on the call, and point you to teams better suited to the work.

Not ready to commit to a full build? Start with a Pilot Sprint.

Most of our clients sign the full engagement after a paid 2-week Pilot Sprint. It is the lowest-risk way to work with a senior team you have just discovered.

The Pilot Sprint

$5,000 · 2 weeks · fully credited

Before committing to a $50k+ build, we run a 2-week paid sprint that delivers:

  • Full architecture document (mobile, backend, AI layer if applicable)
  • Technical risk assessment with mitigation plan
  • Sprint-by-sprint roadmap with fixed scope per sprint
  • Working prototype of the riskiest screen, real React Native code, not Figma
  • Final pricing for the full build, locked in writing

If you sign the full project, the $5,000 is credited 1:1 against the first milestone.

If you do not, you keep everything we produced and can hand it to any other team.

Book a 30-min technical call to scope a Pilot Sprint

A predictable process for unpredictable problems

Four phases. Fixed scope, named owners, milestone-based pricing.

01Week 1 · Free

Technical discovery

30 to 60 minutes on call. We map your product against the React Native ecosystem: what is standard, what needs a native module, what is risky, where the schedule risk lives. You leave the call with a concrete view, regardless of whether you sign with us.

02Week 2

Scoped proposal

A fixed-scope proposal with a phased timeline, milestone-based pricing, and named senior owners on our side. The number you sign for is the number you pay.

03Typically 3 to 5 months

Build

You get a Slack or Discord channel with the team, a weekly demo of working code, and a shared backlog you can re-prioritize at any time. Releases ship to TestFlight from week 3.

04Launch week and after

Production launch + 60-day warranty

We submit to the App Store and Play Store, run the launch, and stay engaged for 60 days post-launch to fix anything we shipped. After that, you can either keep us on retainer or take ownership of a clean codebase.

The Ruji shipping guarantee

Five commitments, written into every engagement.

Fixed scope, fixed price

Scope changes go through a written change request before any work starts; no surprise invoices, ever.

Source code in your GitHub from day one

You own everything from the first commit. IP transfers on each milestone payment, not at the end.

App Store rejection coverage

We handle every Apple and Google review iteration until your app is approved, at no extra cost, no extra timeline.

60-day post-launch warranty

Any bug we shipped, we fix on our time. No retainer required.

Missed-milestone clause

If we miss a confirmed milestone date by more than one week without a scope change on your side, the next sprint is on us.

Where we fit best

A clear view of who we serve well, and who would be better served elsewhere.

We are the right partner when

  • You want senior US-quality engineering at European pricing, and you understand that the savings come from operating costs in Paris, not from cutting quality.
  • You are a Series A-B startup or a B2B SaaS company shipping a mobile product.
  • You have product clarity; a spec, a Figma, or a working web version to port.
  • Your project budget is $25k or more, with milestone-based pricing.
  • You want a senior team that owns delivery end-to-end.

We are probably not the right call when

  • You are at idea stage and would benefit more from a product strategist first.
  • Your priority is the lowest hourly rate.
  • You need ongoing developer staffing rather than a delivery partner.
  • You need to ship in under six weeks.
  • You need every meeting to happen at 6pm Pacific Time. Our last live overlap is 1pm PST.
Ruji founders
Pair programming at Ruji
Ruji team collaboration
Code review at Ruji
Ruji team building
Ruji founders
Pair programming at Ruji
Ruji team collaboration
Code review at Ruji
Ruji team building

A small, senior team out of Paris, shipping for clients across Europe and the United States

We are Ruji, a custom mobile app development company based in Paris. We work with a focused portfolio; every app in our case studies is one we built end-to-end and that runs in production today.

Our clients sit across the United States and Europe; healthtech, edtech, B2B SaaS, and consumer products. Async communication is the norm, responses are fast.

10+ years

shipping production iOS and Android apps

25+ apps

live in the App Store and Google Play today

Senior-only

every engineer on your project has shipped at least one production app

US business hours

online 9am to 4pm Eastern Time, Monday to Friday

Top-rated

4.9/5 across Sortlist and Google · 35+ reviews

Paris, France · serving the USOnline by 9am EST · 9am PST overlap

Read independent reviews on Sortlist.

Frequently asked questions

Everything you would normally ask on the first call.

Hours of operation

Monday to Friday · 9am to 4pm Eastern Time

That is 3pm to 10pm in Paris. Your morning standup is our afternoon. Your urgent end-of-day decision is our early evening.

Slack and Discord channels with response within 2 business hours, US time.

Ready to ship a mobile app you will be proud of?

30 minutes on Calendly. Bring whatever you have; a spec, a Figma, a working prototype, or a problem statement. You will leave with a clear technical read on the project, regardless of what you do next.

You will talk to a senior engineer, who will come back with a clear technical read and a scoped proposal if it is a fit.