Understanding stability inference

Understanding Stability Inference — read the compiler's mind

Stability is decided by a 12-phase algorithm baked into the Compose compiler. This skill teaches Claude how the algorithm walks a type so it can explain why a report says what it says, and predict classifications before the report is even generated. Pair this with ../diagnosing-compose-stability/SKILL.md (which generates the report) and ../stabilizing-compose-types/SKILL.md (which fixes obvious unstable types). Reach for this skill when the simpler skills produce a verdict that surprises the developer.

When to use this skill

• The developer asks "why is Foo classified as runtime stable and not stable?"
• A report shows runtime or unknown for a class that "looks fine".
• Generics involved: Box<T>, Wrapper<A, B>, Pair<String, Int>, ImmutableList<User>.
• The class lives in another module and ships as a .class/.kotlin_metadata artifact.
• The developer asks about the $stable: Int field, @StabilityInferred, or cross-module classification.
• A self-referential type (class Node(val children: List<Node>)) is unstable for non-obvious reasons.
• A Java type, an interface, or an abstract base appears in a parameter list and surprises the developer.

When NOT to use this skill

• The fix is mechanical (var → val, List → ImmutableList, Flow parameter removal). Use ../stabilizing-compose-types/SKILL.md.
• No report exists yet. Run ../diagnosing-compose-stability/SKILL.md first.
• The developer wants CI enforcement of stability. Use ../enforcing-stability-in-ci/SKILL.md.

Prerequisites

• Compose Compiler reports already generated, or at least one <module>-classes.txt and <module>-composables.txt available in build/compose_compiler/.
• The developer understands the basic stability vocabulary: stable, unstable, skippable, restartable, @Stable, @Immutable.
• Kotlin 2.0+ with the Compose compiler plugin (Strong Skipping default ON).

Workflow — diagnostic question and answer tree

Walk the type through the same 12 phases the compiler does. For each call site, ask the questions in order; the first matching phase wins.

The canonical phase order (used everywhere in this skill and matching references/twelve-phase-algorithm.md):

1. Phase 1 — primitive / String / function / Unit fast path → Stable. The compiler returns immediately. No field analysis runs. Mention the fast path so the developer knows nothing else was inspected.
2. Phase 2 — type parameter substitution. A bare type variable T becomes Stability.Parameter(T); resolution is deferred to the call site that substitutes it.
3. Phase 3 — nullable unwrap (Int? → analyze Int). Nullability does not change stability; the algorithm strips the ? and recurses.
4. Phase 4 — inline class — check underlying type. value class Wrapper(val raw: T) is exactly as stable as T.
5. Phase 5 — cycle detection (recursive trees → conservative UNSTABLE). The algorithm bails on cycles to guarantee termination. The escape hatch is @Stable/@Immutable on the recursive class, which fires in phase 6 before phase 5 is reached.
6. Phase 6 — annotations check (@Stable, @Immutable, @StableMarker). Yes → Stability.Certain (stable). @Immutable enables additional optimizations beyond @Stable because the compiler can promote reads of properties to static expressions and elide equality checks; @Stable only promises change notification.
7. Phase 7 — Known Stable Constructs registry hit (Pair / Triple / Result / ImmutableList / dagger.Lazy / ClosedRange / etc.). Returns Stability.Parameter with the registry's bitmask. See references/bitmask-encoding.md for the full registry.
8. Phase 8 — external configuration match (stability_config.conf). Returns Stability.Parameter with the bitmask declared in the config file.
9. Phase 9 — external module (@StabilityInferred annotation generated by separate compilation). Returns Stability.Runtime. The compiler emits a $stable: Int field on the JVM (a mangled top-level property on Native/JS) that the runtime queries via Composer.changed. Tell the developer this is not a bug — runtime is the compiler saying "I cannot prove this at compile time, so I will check at runtime".
10. Phase 10 — Java type (default UNSTABLE — fix via config file). Java final fields look like var to the inference because the algorithm has no Kotlin metadata to read. Fix via stabilityConfigurationFiles, not by editing the Java source.
11. Phase 11 — interface (UNKNOWN; runtime ===). The compiler cannot enumerate implementations from a single call site; the runtime falls back to identity (===) for the equality probe.
12. Phase 12 — field-by-field analysis (the slow path):
    • Walks the linearized class hierarchy so inherited fields participate. There is no separate "inheritance" phase — inheritance lives here.
    • Any var property → Unstable (mutation observed without Snapshot integration).
    • Any property whose type is Unstable → Unstable (Combined dominates).
    • Otherwise the class is Stable (Combined of all-stable fields collapses to Stable).

For full pseudocode of all 12 phases plus the field-by-field loop, see references/twelve-phase-algorithm.md.

Patterns

Pattern: "Why does Box<String> show as runtime stable?"

// Source — the developer's class, in a library module
class Box<T>(val value: T)

// Call site, in app module
@Composable fun BoxRow(box: Box<String>) { Text(box.value) }

The compiler walks Box<T>:

1. Phase 12 (field-by-field) inside the defining module finds one field value: T; recursion on T hits phase 2 (type parameter) → Stability.Parameter. Combined collapses to Stability.Parameter with bitmask 0b1 (the single type parameter affects stability).
2. Because Box is consumed from a different module, the compiler emits @StabilityInferred(parameters = 0b1) on Box and a $stable: Int field initialized from T's stability at runtime. Downstream call sites pick this up via phase 9.
3. Call-site substitution → T = String → String is Certain Stable → bit 0 satisfied.
4. Final report line: runtime stable class Box<T> and at the call site BoxRow is skippable.

// WRONG mental model
// "runtime stable means there is a runtime cost on every recomposition" — partly true but misleading
// WRONG because: the cost is one Int field load and a bitwise AND, performed once when the runtime
// computes the call-site stability. It is far cheaper than the unskipped recomposition it prevents.

// RIGHT mental model
// runtime stable = "the compiler proved stability conditional on the type arguments, and emitted
// a $stable: Int field whose bits the runtime ANDs against the substituted argument stabilities".
// The skip decision is still made; it is just made at runtime instead of compile time.

Pattern: "Why does @Immutable data class Person(val name: String) enable more optimizations than @Stable?"

@Stable is a contract: "I will notify Compose of changes". @Immutable is a stronger contract: "I will never change". With @Immutable the compiler may promote reads of Person.name to static expressions and elide equality probes for nested usages; with @Stable it must still emit equality checks. Both classify as Stability.Certain, but the downstream optimizer treats @Immutable more aggressively.

// WRONG
@Stable data class Coordinates(val lat: Double, val lng: Double)
// WRONG because: Coordinates never mutates after construction. @Stable understates the contract
// and forfeits static-expression promotion at every read site.

// RIGHT
@Immutable data class Coordinates(val lat: Double, val lng: Double)

Pattern: "Why is my recursive tree unstable even though it looks fine?"

data class Node(val id: String, val children: List<Node>)

Phase 5 (cycle detection) bails. The compiler does not attempt fixed-point analysis because it would have to assume the answer to prove the answer. The conservative verdict is Unstable. Even if every field is otherwise stable, the recursion through children returns Unstable to the parent call.

// WRONG — adds @Stable to "force" stability
@Stable data class Node(val id: String, val children: List<Node>)
// WRONG because: List<Node> is a mutable interface backed by ArrayList in practice. The @Stable
// annotation tells the compiler to trust the contract, but the actual List instance can mutate
// between recompositions without notifying Compose, producing silent missed recompositions.

// RIGHT
import kotlinx.collections.immutable.ImmutableList
@Immutable data class Node(val id: String, val children: ImmutableList<Node>)

ImmutableList is in the Known Stable Constructs registry (phase 7) with bitmask 0b1, so the recursion through children is permitted: cycle detection still triggers in phase 5, but the registry hit short-circuits the conservative verdict.

Pattern: "Why does Set<String> block skipping but ImmutableSet<String> doesn't?"

kotlin.collections.Set is an interface (phase 11 → Unknown) backed in practice by LinkedHashSet, which mutates. kotlinx.collections.immutable.ImmutableSet is in the Known Stable Constructs registry with bitmask 0b1, so it is Stability.Parameter and resolves to stable when the element type is stable.

// WRONG
@Composable fun TagRow(tags: Set<String>) { /* ... */ }
// WRONG because: Set is an interface — phase 11 returns Unknown, the call site is non-skippable.

// RIGHT
@Composable fun TagRow(tags: ImmutableSet<String>) { /* ... */ }

Pattern: "Why is a class from another module runtime stable even when it has only vals?"

Separate compilation. At the time the call site compiles, the compiler does not have the full source AST of the dependency, only its .class files plus the metadata in @StabilityInferred(parameters = ...). Phase 9 reads that annotation; the runtime resolves the bitmask against actual type arguments via the generated $stable: Int field. The classification is correct — there is no extra work to do — but it must be deferred to runtime because cross-module compile-time analysis is impossible without the source.

Five compiler-level stability types

Cite these by name when answering "why" questions. The compiler stores stability as one of:

• Stability.Certain — primitives, String, Unit, function types, enums, @Stable/@Immutable-annotated classes. Decision is final and compile-time.
• Stability.Runtime — separately compiled class. Compile-time emits a $stable: Int field and @StabilityInferred; runtime ANDs the bits against actual type arguments.
• Stability.Unknown — interface, abstract class without concrete analysis, or anything the compiler refuses to commit on. Runtime falls back to === identity for the equality probe.
• Stability.Parameter — generic. Stability is a function of the type arguments via a bitmask.
• Stability.Combined — aggregate of multiple components (fields of a class, or multiple type arguments). Unstable dominates — any single Unstable component poisons the whole.

Bitmask encoding (preview)

Container<T1, T2, T3> uses an Int bitmask where bit i set means Ti participates in stability:

Type	Bitmask	Reading
kotlin.Pair<A, B>	0b11	both A and B affect stability
kotlin.Triple<A, B, C>	0b111	all three affect stability
kotlinx.collections.immutable.ImmutableList<E>	0b1	only E affects stability
java.math.BigInteger	0b0	no parameters; classified as stable regardless of erased type arguments

The full rules — including how @StabilityInferred(parameters = ...) is generated for separately-compiled types and how the $stable: Int field is laid out on the JVM versus the mangled top-level property used on Kotlin/Native and Kotlin/JS — are in references/bitmask-encoding.md.

Mandatory rules

• MUST teach the developer that runtime stable is not a bug or an unstable verdict — it is the compiler's way of saying "stability is conditional on type arguments and will be checked once at runtime via the $stable: Int field".
• MUST NOT suggest structural changes (changing var to val, swapping collection types) before explaining why the current structure is unstable. Diagnosis before treatment.
• MUST distinguish Stability.Unknown from Stability.Unstable when answering — Unknown means "cannot tell" and falls back to identity equality, Unstable means "proven unstable" and disables skipping outright.
• MUST NOT tell the developer to add @Stable to a type whose contract they cannot guarantee. A stability annotation is a contract; breaking it produces silent missed recompositions, which is worse than a non-skippable composable.
• PREFERRED: cite the Kotlin compiler source when depth helps. Concrete files: Stability.kt (the algebraic data type), KnownStableConstructs.kt (the registry), ClassStabilityTransformer.kt (the $stable field emission), and StabilityConfigParser.kt (the config-file reader).
• PREFERRED: when explaining a generic, walk the bitmask explicitly: "bit 0 of Pair's bitmask is set, A=String is Certain Stable, satisfied; bit 1 is set, B=List is Unstable, fails — Combined collapses to Unstable".

Verification

• Claude can predict, before running the report, whether a candidate type will be classified Certain, Runtime, Unknown, Parameter, or Combined.
• Claude can name which of the 12 phases produced the verdict.
• For a runtime stable class, Claude can explain that the compiler emitted @StabilityInferred(parameters = ...) on the class declaration and a $stable: Int field that the runtime ANDs against substituted type-argument stabilities.
• Claude refuses to recommend @Stable or @Immutable on a type whose mutation contract is not guaranteed.
• Claude correctly identifies Set<T>, List<T>, Map<K, V> as Unknown interfaces (not Unstable) when explaining why they block skipping.

References

• Compose stability overview — https://developer.android.com/develop/ui/compose/performance/stability
• Stability — diagnose — https://developer.android.com/develop/ui/compose/performance/stability/diagnose
• Strong Skipping — https://developer.android.com/develop/ui/compose/performance/stability/strongskipping
• Ben Trengrove, "Jetpack Compose Stability Explained" — https://medium.com/androiddevelopers/jetpack-compose-stability-explained-79c10db270c8
• Ben Trengrove, "New ways of optimizing stability" — https://medium.com/androiddevelopers/new-ways-of-optimizing-stability-in-jetpack-compose-038106c283cc
• Chris Banes, "Composable metrics" — https://chrisbanes.me/posts/composable-metrics/
• skydoves, "Optimize App Performance by Mastering Stability" — https://medium.com/proandroiddev/optimize-app-performance-by-mastering-stability-in-jetpack-compose-69f40a8c785d
• skydoves/compose-stability-inference — https://github.com/skydoves/compose-stability-inference
• skydoves/compose-stable-marker — https://github.com/skydoves/compose-stable-marker

Reference files

• references/twelve-phase-algorithm.md — pseudocode walkthrough of all 12 phases plus the field-by-field analysis pseudocode.
• references/bitmask-encoding.md — generic stability bitmask rules, the Known Stable Constructs registry, the @StabilityInferred(parameters = 0b1) annotation generated by the compiler, the runtime $stable: Int field on JVM, and the mangled top-level property approach on Native and JS.