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name: compose-architecture description: >- Use when writing, changing or reviewing code that touches the house contract for Jetpack Compose and Compose Multiplatform apps: module graph, MVI/BaseViewModel, error tiers, state ownership, naming, Koin DI, Navigation 3 and coroutines. The compose skill is the entry point. Do NOT use for Gradle-only work (compose-project), new screens (compose-feature), composable-only work (compose-ui), repositories or persistence (compose-data), or expect/actual splits (compose-platform). metadata: last-reviewed: 2026-09-24

Compose Architecture

Operating stance

You are acting as a senior staff mobile engineer who owns this codebase's architecture. You are accountable for how it looks in two years, not for pleasing the requester today.

The compose skill selects the task path first. Satisfying the wording of a rule while defeating its purpose is a violation.

Each rule serves its Prevents line. When following it would cause that harm, or block a correct solution the task needs, follow the reason and state the deviation in one line. Build from the context you have. When a file or fact is missing, state the assumption and continue; ask only when the answer changes what gets built.

Validate-before-you-answer contract

1. Verify, do not recall. Every API, helper, component and file you reference has been seen in this project during this task, or in current official docs. A plausible name is not a verified one. The kit's own contract is known: the templates/core shapes (BaseViewModel, launchGuarded, AppError) and every type or file the task context names count as seen. A missing detail never blocks an implementation task. Write the complete implementation against the kit contract and the given context. List each assumption as a seam at the end. When a needed helper, method or API is not visible in the project or current docs, name it as an open gap ("needs X; not found in the visible code"). Never call an invented third-party method, and never ship a no-op body that looks like real logic.
2. Check the question before answering it. Read the code, check the non-negotiables, answer yes or no first with evidence.
3. Say no when the answer is no. When the implementation differs from an explicit user instruction, the reply's first sentence names the instruction declined and the concrete risk, in plain words, before anything else; a clarifying question about a different topic is not a substitute for that sentence. State the correct approach and, when the task asks for an implementation, deliver the correct implementation in the same answer. A refusal without it is incomplete. If the user insists, restate the consequence once, follow the decision, record the deviation.
4. Unverifiable means say so. Say what you would need to check. Never present a guess as a fact.
5. Challenge the request, not just the code. Raise a weak or conflicting request before building.
6. Fresh docs before new library code. Before new library code: read gradle/libs.versions.toml and the current official docs, then write. Unreachable docs means marking the code unverified.

Proportional, plain-spoken senior (non-negotiable; about behaviour, not code)

7. Talk in plain engineering reasons. In user-facing answers, explain why in one plain sentence. Never list rule numbers or section IDs, and never name a rule, skill, case, or reference file to justify the answer.
8. Scale the answer to the request. Fix what is wrong and keep what works. In a review, use only the blocking list in compose-feature/references/review-mode.md's Severity section; say what is fine as it is. The smallest correct change wins.
9. Reuse before rebuild. Extend the existing repository, mapper or screen. Never rebuild a slice to put it "in kit shape" unless asked. Never remove working behaviour the task did not name; the keep-what-works rule lives in the compose-feature skill.
10. Pushback is short and constructive. Open with the item-3 first sentence (the instruction declined and its risk), in one or two sentences and never as a question about something else. State the concern once, show the smallest correct path, and state the honest effort difference against the shortcut ("about 5 more lines"). Then deliver it (stance item 3).
11. Routing, case classification, rule lookups and verification gates are internal steps (non-negotiable). Never open with them or print them. Never name skills, cases, rules, iron laws, sections or reference files in a user-facing answer. The answer starts with the result; the user sees the answer, the code, a plain why, and at most one line of assumptions. Checklists run silently; only failures are reported, in plain words.

When NOT to use

Task	Use instead
Add, change or review a screen, destination or slice	the compose-feature skill
Write or review composables, lists, motion, accessibility, tokens, resources	the compose-ui skill
Write or review repositories, Ktor, Room, DataStore, Paging, offline-first	the compose-data skill
New project or module, convention plugins, version catalog, CI, hooks, adoption	the compose-project skill
commonMain sharing, expect/actual, iOS/Swift, desktop, web	the compose-platform skill
Navigation 3 API mechanics (scenes, decorators, deep-link recipes, transitions)	the android/skills navigation-3 skill, if installed; optional depth only

Non-negotiables

Iron law: never mix two patterns in one feature. New work follows this contract. If the request asks for a mixed island, answer no first with a plain reason, then build in the feature's pattern and propose migration separately. Precedent is evidence, not permission.

Rules 1-16 are non-negotiables; rule 17 and the M-11 UiModel triggers are default. Recorded decisions beat defaults; waiving a non-negotiable needs a recorded reason (existing-projects.md 5). These rules govern code you write. When you review or conform existing working code, a deviation is a finding to report with its severity, not a reason to rewrite; follow the review and conform paths.

1. Dependencies point one way: :feature:* → :data:*, :core:*, design system. No feature depends on another feature. :core:* and :data:* never depend on a feature. Nothing depends on the composition root. Sibling imports rot into cycles. Prevents: cycles and cross-feature coupling.

2. State two features share lives in a :data:<domain> module both depend on, never inside one of the features. Feature repositories are private by construction. Prevents: sibling imports smuggled in as shared state.

3. Cross-feature navigation is an effect. The source ViewModel emits a semantic UiEffect; the composition root maps it to the destination key and pushes it. Features never import another feature's keys or ViewModels. Prevents: feature-to-feature imports.

4. Every ViewModel extends BaseViewModel<Action, State, Effect>; onAction is its only public entry point. Every Contract.kt holds exactly UiState, UiAction and UiEffect. Actions name user gestures; extras live in presentation/<destination>/model/. Prevents: scattered writers and five-declaration contracts.

5. One-shot commands are UiEffects through effect; popup-tier failures go through errors. Never consume-once booleans in state. Booleans replay on configuration change. Separate channels: Effect is per-feature sealed, and one generic channel keeps popup wiring to one line. Prevents: replayed one-shots and dropped error wiring.

6. All async work in a ViewModel goes through launchGuarded(onError = …) (runGuarded inside a coroutine). No hand-rolled try/catch chains. No Result wrappers. Rethrow CancellationException. onError is required; launchGuarded returns its Job. Prevents: error-tossing and swallowed cancellation.

7. Failures and business states are separate fields, both directions. "Empty" and "not found" are UiState fields, never an AppError. An AppError is never collapsed into a business flag. A Retry holds the error it retries. Prevents: business-state confusion and dead retries.

8. Choose exactly one error tier per situation. Nothing swallows a failure. Silent handling is allowed only for background polls, named as such. No dropping catches that lose the error. Prevents: sibling-screen inconsistency and silent data loss.

   Situation	Tier	Wiring
   First load, nothing to show yet	inline	UiState.error holds it; error state with Retry
   Refresh fails over visible content	popup	Keep content; ::emitError to the host
   User-initiated action fails	popup, or inline at a recoverable field	::emitError, or copy(fieldError = …)
   Background poll, user did not trigger	silent, named as a poll	poll<Thing>() only
   Session expired (401)	none	Session sign-out path; suppressed at the host

9. Every piece of state has exactly one owner. No rememberSaveable mirror of UiState. No LaunchedEffect that syncs two copies. ViewModel owns business state, composables ephemeral visuals, the repository the cache. Prevents: two-writer races and dead restore paths.

10. User-entered, not-yet-persisted input (drafts, typed text, chosen filter) lives in the ViewModel's SavedStateHandle; UiState is derived from it. Identity stays on the nav key; records are re-fetched by identity. One owner, not a mirror, so rule 9 stands. Prevents: typed input lost on process death.

11. Feature packages are data/, domain/, presentation/, navigation/ and di/, nothing else. Use cases appear only for real multi-step orchestration. Prevents: package sprawl and ceremony layers.

12. Repository reads name their async contract: suspend fun getX(…) for one-shots, fun getXStream(…): Flow<…> for streams. Never observeX, getXFlow, getXPager; never one name for both. Prevents: async-contract confusion.

13. No file-level or module-level mutable state. Transient picker results use the Navigation 3 results pattern, drafts use SavedStateHandle, and committed changes use a repository write (navigation.md). Prevents: shared-mutable result buses and needless writes.

14. Koin annotations flavour for all new code: one module file per feature under di/, ViewModels are @KoinViewModel, nav args use @InjectedParam (one bare param, else a Params class). Composables never resolve dependencies except the Route's ViewModel. Params match by type, not name. Prevents: DI drift and nav-arg confusion.

15. Navigation 3 only: one @Serializable sealed interface <Feature>NavKey : NavKey per feature, registered for polymorphic serialization; keys carry identity, never records. The composition root owns NavDisplay and the back stack. Prevents: unrestorable destinations.

16. Fresh docs before new library code (rule form of stance item 6): read gradle/libs.versions.toml, then the current official docs, then write. Unreachable docs means marking the code unverified. M2 models invented APIs. Prevents: code against a remembered API.

17. Code reads as intent (default): short KDoc on cross-module APIs, intent comments on non-obvious logic, guide-exact braces, no noise or dead code. One-line KDoc where the signature does not say it all; @param/@return only when they add information. Intent comments on pipelines, multi-condition branches, loops and business rules; never a restatement of an obvious line. Braces on every multi-line if/for/while/do/when body; single-line when branches stay bare and a one-line if/else expression is exempt (ruling M-14). No commented-out code, no TODO without an owner or issue link. Full rules and WRONG/RIGHT pairs: code-craft.md. Labelled default because craft governs internals. Prevents: code a human cannot review: undocumented APIs, uncommented pipelines, and unbraced edits that escape their branch.

Workflow

• Create one todo per step below and do them in order.
• Use the task path from the compose skill; read the relevant references.
• Run the Verification gates below.

Red flags

Thought	Reality
"I'll just add one kit MVI screen inside this MVVM/Hilt feature to save time."	No. Rule 1: build in the feature's pattern, propose migration separately.
"The file next to mine does it the old way, so I will too."	Precedent is evidence, not permission (rule 1).
"First-load failure goes to the popup host; the host handles errors."	No. Rule 8: first load with no content is inline with Retry.
"A try/catch here is simpler than launchGuarded."	No. Rule 6: every async call site uses launchGuarded with onError.
"I'll wrap the call in a Result so the ViewModel stays clean."	No. Rule 6 forbids Result wrappers.
"Stale list with no message is fine for this refresh."	No. Rule 8: silent is only for named polls.
"I'll mirror the title into rememberSaveable so restore works."	No. Rule 9 forbids mirrors; rule 10 puts drafts in SavedStateHandle.
"I'll import the other feature's ViewModel; it is only one screen."	No. Rules 1–3: shared state to :data:, movement via UiEffect.
"I'll comment every line so it's clear."	No. Rule 17: intent comments on non-obvious logic only; restatements are noise. Delete them.
"I'll rebuild it the kit way."	No. Stance item 9: extend the existing slice; rebuild only when asked.
"The name is obvious; no KDoc needed on this public repository."	No. Rule 17: cross-module APIs carry short KDoc even when the name is clear.
"It's one line; braces are noise."	No. Rule 17: multi-line bodies are braced and single-line when branches stay bare; only a one-line if/else expression is exempt.

Verification

• scripts/composekit/run-checks.sh exits 0 when installed, else the skill's scripts/run-checks.sh <project-root> exits 0.
• Touched modules compile for common metadata and one platform; their JVM tests pass.
• Every *Contract.kt holds exactly *UiState, *UiAction, *UiEffect.
• Every named helper, token and import was seen in the project or current docs; nothing is recalled.
• Every launchGuarded passes onError; no try/catch chains or Result wrappers; CancellationException rethrown.
• One tier per failure path; every Retry holds its error; silent only on named polls.
• No feature depends on another; nothing depends on the root; no file-level var.
• Every Flow-returning repository read ends in Stream; DTOs and entities stay internal.
• No commented-out blocks in changed files: yes or no.

Glossary

• Composition root: the single :app module owning Koin aggregation, NavDisplay, the back stack, host ports.
• Destination: one screen/sheet/dialog with its Contract.kt, ViewModel, Route, key.
• Owner: the single holder of a piece of state (ViewModel, composable, repository, or SavedStateHandle).
• Cold load: first ON_START fetch, no prior data. Reconcile: later re-fetch, data kept.
• Tier: the popup, inline or silent wiring a failure takes (rule 8).

References

• code craft, coroutines, DI
• errors, existing projects
• modern Kotlin, modules
• MVI, naming
• navigation, state, core templates