Fitness app rescue and rebuild is a Mercury Development service for teams stuck with a broken build or an AI-generated MVP. We've shipped 1,500 apps since 1999, including work for Tonal and Fitbit, and every rescue starts with a code and security audit.
A growing number of our rescue calls start the same way: a working prototype an AI code generator wrote fast, then handed off once real users showed up, and it is easy to see why those apps need a second look. Veracode's latest testing found that just under half of AI-generated code carries a hidden security flaw. We also hear an older, familiar story: sometimes "a freelance team came in, built something, and when the going got tough, walked away." Either way, the app is usually not a write-off, it just has not been reviewed yet.
AI code generators write fast, not safe. A real audit catches exposed keys, weak auth, and unpatched dependencies before members do.
A demo running for one investor is not the same as a launch day crowd. We test under real device and network conditions.
Fast code often skips real structure. A senior engineer checks the architecture before it has to carry paying members.
AI tools rarely explain their own decisions. We document the system so the next team, ours or yours, can work in it.
Wearable and payment links that worked once in a demo often break on real accounts. We verify every connection end to end.
Some AI tools and platforms blur who owns the output. We confirm you hold the code and the IP before we touch a line.
A full review of architecture, dependencies, and security, whatever shape the code is in. From $5,000, with a written, prioritized risk list at the end.
We close the gaps most likely to hurt members: crashes, security holes, and broken releases. That keeps your app running while the bigger rebuild takes shape.
Weak modules get replaced one at a time, never all at once. Member data, subscriptions, and history migrate on a plan instead of a scramble.
Every wearable, payment, and health data link gets checked against the rebuilt code. Apple HealthKit, Google Fit, and Fitbit sync are verified before launch.
A full review of architecture, dependencies, and security, whatever shape the code is in. From $5,000, with a written, prioritized risk list at the end.
We close the gaps most likely to hurt members: crashes, security holes, and broken releases. That keeps your app running while the bigger rebuild takes shape.
Weak modules get replaced one at a time, never all at once. Member data, subscriptions, and history migrate on a plan instead of a scramble.
Every wearable, payment, and health data link gets checked against the rebuilt code. Apple HealthKit, Google Fit, and Fitbit sync are verified before launch.
You do not get a silent rewrite. Every phase ends with something you can see: a report, a stable release, or working code.
Before any code changes, we spend 1 to 2 weeks going through the codebase end to end. Nothing else starts until that report is in your hands.
Critical fixes ship before the rebuild plan is even finished, on a sprint cadence instead of one long freeze. Members never wait on a full rewrite.
We order modules by risk and dependency, not by what looks easiest. Nothing gets rewritten from scratch unless the audit says it has to be.
Each module goes live and gets tested before the next one starts, so members never sit through one big cutover on launch day.
Functional, security, and device testing run before each release, not just at the end. Bugs get caught before your members ever see them.
You get the source code, full documentation, and a team that stays with your app after launch for monitoring, OS updates, and new features.
A rescue starts with whatever you already have, then moves toward a stack we can support long term. Native code where performance matters, cross-platform where budgets matter, health data handled the way regulators expect.
iOS, Android, watchOS, Wear OS, and the web. We also support desktop companion apps on Windows and macOS when a project already ships with them.
Swift, SwiftUI, Kotlin, and Jetpack Compose for native code. Flutter and React Native when one codebase should cover both the App Store and Google Play.
Apple HealthKit, Google Fit, the Fitbit ecosystem, Garmin Connect, Strava, and MyFitnessPal. Workout, heart rate, and sleep sync stays intact through a rebuild.
Mercury Development engineers use GitHub Copilot, Cursor, and Gemini Code Assist in their own work, with a senior engineer checking every change.
Cloud backends built to survive launch-day traffic spikes, plus admin tools so your team can update content and pricing without a developer.
We build and rebuild to HIPAA, GDPR, and CCPA requirements, with WCAG accessibility standards. Every rebuild documents how data is stored, shared, and deleted.
Mercury Development has shipped software since 1999 for clients from startups to Fortune 500 companies. On Clutch, that work holds a 5.0 rating across 34 verified reviews.
1,500
40M
500
FITNESS
Not every project starts on a blank page. Tonal came to Mercury Development with an existing codebase and multiple engineering teams already in motion, and today a team of 12 Mercury Development engineers works inside its iOS, Android, and Apple Watch codebase. Fitbit has trusted Mercury Development since 2011, when it was a six-person startup, through its growth to more than 50 million users and its acquisition by Google. Inheriting a live codebase is a different skill than starting fresh, and it is one we have built since 1999.
We review code, architecture, and security before changing a line. Within 1 to 2 weeks you get a written, prioritized risk list.
Work runs in two-week sprints, each ending with a demo on working code. You judge progress by what runs, not a status report.
All work is work-for-hire, so the code and IP are yours, even code inherited from a past vendor. The roadmap stays yours too.
Every engineer has 3 or more years of experience, and team leads have 7 or more. A senior engineer checks every change.
We review code, architecture, and security before changing a line. Within 1 to 2 weeks you get a written, prioritized risk list.
Work runs in two-week sprints, each ending with a demo on working code. You judge progress by what runs, not a status report.
All work is work-for-hire, so the code and IP are yours, even code inherited from a past vendor. The roadmap stays yours too.
Every engineer has 3 or more years of experience, and team leads have 7 or more. A senior engineer checks every change.
Mercury Development has met all the client's project goals and exceeded their expectations. The team has managed the project in detail, hit all deadlines, managed expectations, and reprioritized tasks. Moreover, they've understood the client's business and become a true partner.
Mercury Development delivered the project on time with a structured approach and strong technical expertise. The engagement resulted in higher customer satisfaction, improved retention, and better regulatory compliance. Communication was through email, messages, and weekly virtual meetings.
Development provides software integration and mobile app development for turnkey retail platforms, ensuring up-to-date functionality and system cohesion by scaling from 2–10 resources as needed. The team’s precise task breakdowns, affordable price point, and proactive reporting strengthen the partnership.
We’ve actually brought Mercury in across three totally different companies. First it was an app for car sales, then later they stepped in to fix a fire safety app that needed help, and now they’re building stuff for a valve manufacturer. Kinda shows they can jump into all sorts of industries and just get the work done.
Mercury Development consistently delivers high-quality codes that the client has never seen from a contract organization. They've not only contributed to existing features but have also led the development of new initiatives. Overall, the client is impressed with the vendor's talented engineers.
Mercury Development has helped the client with all kinds of developments and acquisition plans. They were quickly able to familiarize themselves with the business, industry, and products. The team maintains strong control of the timelines and budget but remains open to changes and client requests.
Mercury is very organised and there are no hidden surprises. They first scoped the project in detail, questioning the specification supplied as necessary. They also focussed on the success testing right from the start. I was also guided through the process so it was very straight-forward.
Mercury was awesome, they were reasonably priced, and took ownership over the project from the get-go. The code is clean, the app is polished, and they made multiple revisions very easy and at no additional charge. I would highly recommend working with them again, especially if you need an Apple TV app built with an API to your YouTube channel.
Feel free to contact us and we'll respond as soon as possible.
