Feature spec
Goal: Send a clear invoice after finishing a job.
- Add a client, line items, a due date, and payment instructions.
- Preview the total before exporting a PDF.
- Keep an editable draft if export fails.
App Overview
Stark Kettlebell
Optimise your results with Stark Kettlebell!
$4.99 · In-app purchases
PRICE
$4.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
★ 4.7
542 ratings · Top 15%
Evidence scope: No current overall US chart position is available. Category-chart positions shown in discovery are a separate scope. Revenue is a directional model estimate, not verified earnings.
Build your own version
Build Kit
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A product spec, screen plans, and build steps for your coding agent.
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What should your app do differently? Your note travels with the kit so your coding agent builds around your idea.
What's included18 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
Preview a sample Build Kit No email needed
Illustrative example: an invoice app for independent contractors. Your kit is tailored to the app you choose; this is not its generated content.
Screen plan
Invoice editor: Client at the top, editable line items in the middle, total and preview action at the bottom.
States: Empty draft, validation errors beside each field, exporting, and a retry action that preserves the draft.
Differentiate and validate
Hypothesis: Contractors need faster repeat invoices more than more templates.
First experiment: Test duplicating a previous job with five contractors. Watch where they hesitate before expanding the feature set.
The full kit adds a phased build plan, evidence notes, tool prompts, and implementation guidance. It is a plan for your coding agent, not a finished app.
Usage: Mechanics are fair game; never reuse the original name, branding, assets, or verbatim copy.
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Free quick prompt For a smaller starting point
Quick prompt
Good fit for mobile-first iOS and Android prototypes with managed app scaffolding.
// RORK SYSTEM PROMPT
Rork plan preview for Stark Kettlebell: map the core Health & Fitness workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
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Opportunities
AI-GENERATED OPPORTUNITY HYPOTHESES
These are generated suggestions, not verified review quotations. Check current App Store reviews and speak with users before treating a possible gap as a requirement.
Hypothesis 1
Add more diverse workouts and allow users to create custom routines.
Assumption to check: Limited workout variety and customization options
Validate with people who use this workflow before building.
Hypothesis 2
Clarify subscription benefits and simplify the upgrade process.
Assumption to check: In-app purchase confusion
Validate with people who use this workflow before building.
Hypothesis 3
Introduce social sharing and challenges to engage users.
Assumption to check: Lack of social features
Validate with people who use this workflow before building.
Hypothesis 4
Streamline UI for better navigation and usability.
Assumption to check: User interface can be cluttered
Validate with people who use this workflow before building.
App evidence
Original App Store material. Ratings and screenshots describe the existing app, not proof of demand for your version.
App Store description
Optimise your results with Stark Kettlebell! - Be inspired by 51 exercises presented in sharp HD video. - Create your own personal workout. - Get a full body workout with exercises in all ranges of difficulty - While working out, follow the step by step workout-feature which take you through the workout using timers and your chosen specifics. - The timers support different workout types including Tabata, HIIT and Circuit training. - Challenge yourself, get rewards and keep track of your score. - Find exercises targeting specific muscle groups. Integration with Apple Health: Save burned calories and performed workouts. The Kettlebell is well known for the ability to build strength, endurance and explosiveness. Basic exercises like “Swing”, “Snatch” and “Get Up” activate all of your body to give you a great workout! Exercises are performed by the professional instructor, Andreas Lanker.
Written user reviews are not shown here. Check current App Store reviews >
Screenshots
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Build scope
Plan a focused first version with your coding agent. These are planning assumptions, not a delivery guarantee.
Define the first version
Choose one audience and one core workflow. Use the kit to agree on its screens, data, and acceptance criteria before building.
Set boundaries
Decide which secondary features, integrations, and platform support can wait. Your version does not need to reproduce everything in the original.
Check the hard parts
Validate external services, specialist technology, data access, and ongoing costs for your chosen scope.
SUGGESTED BUILD STACK
React Navigation
Firebase Authentication
Supabase
Expo-Video
Zustand
React Native Gesture Handler
BUILD KIT DIFFICULTY
Intermediate
From the Build Kit assessment.
A reliable timeline needs an agreed scope and a technical check. Ask your agent to estimate the phases in BUILD_PLAN.md after that review.
