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
Just Piano
GSi JUST PIANO is a sample-based acoustic piano player, built around the powerful WLF Engine that ensures low latency, low memory and high Direct-From-Disk streaming accuracy
$12.99 · In-app purchases
PRICE
$12.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
★ 4.2
12 ratings · Top 30%
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.
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Build Kit
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A product spec, screen plans, and build steps for your coding agent.
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What's included17 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
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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 Just Piano: map the core Music 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
Expand the library with more popular songs and genres.
Assumption to check: Limited song library compared to competitors.
Validate with people who use this workflow before building.
Hypothesis 2
Introduce a freemium model with more free content.
Assumption to check: Subscription pricing may deter casual users.
Validate with people who use this workflow before building.
Hypothesis 3
Incorporate gamification elements to enhance user engagement.
Assumption to check: Limited interactivity in lessons.
Validate with people who use this workflow before building.
Hypothesis 4
Add options for users to share their progress and invite friends.
Assumption to check: Lack of social sharing features.
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
GSi JUST PIANO is a sample-based acoustic piano player, built around the powerful WLF Engine that ensures low latency, low memory and high Direct-From-Disk streaming accuracy. In order to play this instrument, it is necessary to download one of the Piano sounds made available by GSi. When the App is run for the first time, an alert invites you to make your choice and start downloading the sample data. Downloads can be rather large, between 1 and 4 GB according to the chosen piano sound. Only one piano can be installed but can also be replaced at any time. In case new piano sounds are made available by GSi, these will be be added to the catalog. If you want to download more samples and keep them all on your device to switch between them at any time, check out our other app: "Genuine Sounds Vol. I - Piano Edition" which allows you to download up to 40 GB of piano samples of all kinds. Piano samples available as of Aug 2025: - Grand CFX (~4 GB) - Grand D-274 (~4 GB) - Grand C5 (~2,5 GB) - Grand C1 (~1 GB) - Grand C1 Open (~1 GB) - Grand CB 1898 (~2 GB) - Upright U1 Felt (~2,5 GB) PLEASE NOTE: These are not IAP, you only pay for the App then install the piano sound of your choice. If you want to download more samples and keep them all on your device to switch between them at any time, check out our other app: "Genuine Sounds Vol. I - Piano Edition" which allows you to download up to 40 GB of piano samples of all kinds. Main features: - Powered by the exclusive GSi WLF Engine - Polyphony: 128 stereo voices - Authentic string sympathetic resonances - Internal 32 bit processing with native 32 bit sampled material - User defined Velocity Curves - Built-in semi-parametric equalizer - Built-in Stereo Reverb effect - Built-in programmer with unlimited Programs - Embedded user's' manual - Available as Stand-alone App and AUv3 plugin - Very low CPU and RAM usage
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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.
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BUILD KIT DIFFICULTY
Advanced
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.
