sats-price/AGENTS.md

AGENTS.md

Guidance for AI coding agents working in this repository. See README.md for the user-facing project description and build/run/test commands — this file covers the things that aren't obvious from reading the code, or that took real trial-and-error to get right.

What this is

SatsPrice is a Kotlin Multiplatform app (shipped on iOS, macOS, and Android; Web and Desktop/JVM exist as fast dev/preview targets, not shipped products — see data/db/SqlDelightStores.kt's doc comment). It converts between BTC, Sats, and fiat currencies using live exchange rates (Coinbase, CoinGecko, or a manually typed-in rate).

This is a from-scratch rewrite of an earlier Skip-based (Swift-transpiled-to- Kotlin) implementation — main is now this Kotlin Multiplatform project.

Module layout

• shared/ — the actual app: UI, view models, data sources, domain logic. Almost everything happens here.
• androidApp/, desktopApp/, webApp/ — thin launcher shells around shared's Compose UI. Rarely need changes.
• iosApp/ — a native SwiftUI app (shared across iOS and macOS via #if os(macOS)), not Compose. It talks to shared through a hand-written bridge (see "Core architecture" below), not by rendering Compose directly.

Inside shared/src, source sets follow standard KMP conventions: commonMain (all platforms), androidMain, jvmMain (Desktop), appleMain (iOS + macOS, shared), webMain (JS + Wasm, shared), jsMain/wasmJsMain (JS/Wasm-specific only), plus per-target *Main folders for things that need exact target granularity.

expect/actual pairs are named Foo.kt (common) / Foo.jvm.kt / Foo.android.kt / Foo.apple.kt / Foo.web.kt — follow that convention for new ones. Ignore Platform.kt/getPlatform() in every source set: it's unused KMP-wizard scaffolding, not load-bearing.

Core architecture

• ui/PriceViewModel.kt — the single state holder (ConverterUiState), shared by every platform. All business logic (rate fetching, currency selection, amount conversion, manual-rate math) lives here.
• ui/ConverterUiStateDisplay.kt — pure derived-display helpers (ConverterUiState.xyz() extension functions) shared between Compose and the iOS bridge. Read the localization note below before adding anything here that produces user-facing text.
• ui/PriceScreen.kt — the actual Compose UI. Used as-is by Android, Desktop, and Web (same composable, three renderers).
• iOS/macOS doesn't use Compose. The chain is: PriceViewModel (Kotlin) → IosPriceViewModel/IosConverterState (shared/src/appleMain/.../IosPriceViewModel.kt, a flattened Map-free snapshot of the state, since Kotlin/Native's Swift export handles List<DataClass> far better than Map<K,V>) → ConverterViewModel.swift (a thin ObservableObject wrapper) → ContentView.swift / CurrencyPickerSheet.swift (the actual SwiftUI views). When you change something in PriceViewModel/ConverterUiState, check whether IosPriceViewModel.kt needs the same field/method added, and whether ConverterViewModel.swift needs a forwarding method.

Localization — read this before touching display strings

Strings live in shared/src/commonMain/moko-resources/base/strings.xml (moko-resources; only a base/ — no other locales exist yet). There is no synchronous, cross-platform way to resolve a moko-resources string from plain shared Kotlin code:

• Compose can resolve one via stringResource(), but only inside a @Composable function.
• Apple (ui/IosLocalization.kt's localizedString/localizedFormattedString) can resolve one synchronously outside Compose — but that's Apple-only.
• Android has no platform Context available in the shared ViewModel.
• Web resource loading is fetch()-based, i.e. asynchronous — fundamentally incompatible with a plain synchronous function call.

So shared, non-Composable code (ConverterUiStateDisplay.kt, PriceViewModel.kt) must never build a user-facing sentence out of English literals. Return data instead (a formatted number, a formatted date/time, a boolean, an enum) and let each UI layer (PriceScreen.kt via stringResource, ContentView.swift via IosLocalizationKt) assemble the localized sentence around it. ConverterUiState.lastUpdatedDateTime() is the pattern to copy: it returns a locale-formatted String? (using domain/DateFormat.kt, which is fine to call from shared code — it's locale-aware formatting via platform APIs, not translated text), and each UI layer wraps it with its own localized "Updated %1$s" string.

Locale-aware formatting

domain/NumberFormat.kt (digit grouping, decimal separator) and domain/DateFormat.kt (date/time) are expect/actual wrappers around each platform's native locale APIs (java.text/java.time on JVM/Android, NSLocale/NSDateFormatter on Apple, Intl.* on web). These are safe to call from anywhere in shared code — they format using the platform's locale, they don't translate UI text.

Kotlin/Native ↔ Swift interop conventions

Functions called from Swift are written as plain top-level functions, not CurrencyInfo/ConverterUiState extension functions — e.g. matchesCurrencySearch(info, query) rather than info.matchesCurrencySearch(query), currencyFlagEmoji(code). Kotlin/Native's Objective-C/Swift export handles extension-function receivers unpredictably (the exported Swift signature's argument label for the receiver isn't obvious ahead of time); plain functions export as FileNameKt.functionName(label: ...) with predictable, positional argument labels. Follow this for anything new that Swift needs to call.

Currency data is inherently messy — expect platform divergence

• SystemCurrencies.kt / CurrencyFlag.kt: issuingCountryCodes() derives a currency's issuing country/countries from its ISO 4217 code (plus a hardcoded map for the handful of currencies shared by multiple countries with no single issuer). currencyFlagEmoji() returns null on web (supportsFlagEmoji() is false there) because Compose for Web renders through Skia onto a <canvas>, not the browser's text stack — no system/browser color-emoji font to fall back to, so a flag's regional-indicator codepoints would draw as empty boxes.
• SystemCurrencies.web.kt needs its own withdrawn-currency filtering (WITHDRAWN_CURRENCY_CODES + a year-annotation regex, e.g. CLDR naming a retired currency "Afghan Afghani (1927–2002)"): Intl.supportedValuesOf ('currency')'s result is ICU-version-dependent per browser and includes currencies retired decades ago. JVM/Android/Apple instead derive "currently used" live from each platform's own per-country currency lookup (java.util.Currency/NSLocale), so they don't need a maintained list. If you need ground truth for "is this currency still active," a quick JVM snippet iterating Locale.getISOCountries() + Currency.getInstance(Locale) is the most reliable source available in this environment — more reliable than trusting any single browser's Intl data or assuming a Wikipedia snapshot is current.
• Region/country display names (used by currency search) are cached (regionDisplayNameCache in SystemCurrencies.kt) and warmed at PriceViewModel startup (warmRegionDisplayNameCache), since some platforms' lookups (especially constructing Intl.DisplayNames on web) are too expensive to redo on every keystroke.

Persistence

SQLDelight (data/db/SqlDelightStores.kt + platform-specific driver actuals) backs ExchangeRateStore/SelectedCurrenciesStore/ SelectedSourceStore, each exposed via an expect fun createXStore() factory. Web has no real SQLDelight driver — it gets an in-memory-only fallback (nothing persists across a page reload), consistent with web not being a shipped platform.

Testing and verification

• shared/src/commonTest holds the real unit test suite (pure Kotlin, runs identically across targets). ./gradlew :shared:jvmTest is the fast one to run during iteration; it's usually sufficient since commonTest code doesn't touch platform actuals directly.
• After any shared-code change, also compile the other targets to catch platform-specific breakage even without running their tests: :shared:compileAndroidMain, :shared:compileKotlinWasmJs, :shared:compileKotlinJs, :shared:compileKotlinMacosArm64.
• For iOS/macOS-touching changes, build both xcodebuild -scheme iosApp -destination 'platform=macOS' and -destination 'generic/platform=iOS Simulator' — they catch different Swift/bridge mismatches.
• Compose for Web renders to a <canvas>, not real DOM — there's no text to inspect or click via DOM selectors. To actually see a Compose UI change, run ./gradlew :webApp:wasmJsBrowserDevelopmentRun --continuous and screenshot it with headless Chrome. Plain --headless=new alone fails to get a WebGL context (Skia needs one); use: --headless=new --disable-gpu-sandbox --use-gl=angle --use-angle=swiftshader --enable-unsafe-swiftshader --ignore-gpu-blocklist. This is also the most practical way to verify a Compose UI change at all, since there's no headless Android/Desktop runner set up here.
• Interactive verification of the native macOS build via AppleScript/System Events GUI scripting works but is genuinely flaky — stale processes can linger across launches, the accessibility tree doesn't always reflect sheets/dialogs, and clicks sometimes don't land. It's worth one clean attempt for a behavior a screenshot alone can't confirm (e.g. confirming a click actually triggers an action), but don't rabbit-hole on it — a successful build plus a static screenshot plus careful code review is often the more reliable combination.