name: feature-option description: Guides step-by-step through all 11 touchpoints for adding a new @Fake annotation option (like mutability or callHistory). Covers annotation enum, CLI processor, Gradle plugin, FIR metadata, IR bridge, analysis models, and code generators. Use when adding a new option to @Fake, adding a new enum to the annotation, making something configurable per-interface, implementing a new compiler plugin feature flag, adding a new Gradle extension property, or extending the three-tier resolution (annotation → plugin → default). Make sure to use this skill whenever a change touches more than one layer of the option pipeline, even for small additions like a new enum value. allowed-tools: Read, Grep, Glob, AskUserQuestion

Feature Option Guide

Adds a new option to the @Fake annotation, following the confirmed pattern from mutability and callHistory.

Instructions

1. Gather Option Details

Use AskUserQuestion to collect:

• Option name (e.g., threadSafety)
• Enum values — always DEFAULT plus two semantically opposite values (e.g., ENABLED/DISABLED)
• Plugin-level default — the boolean value when annotation says DEFAULT (e.g., true or false)
• What it controls — which generators are affected

2. Walk Through All 11 Layers

Guide the implementation in this exact order. Each layer depends on the previous.

Layers 1-2 define the user-facing API that developers interact with — the enum they import and the annotation parameter they set.

Layer 1 — Annotation Enum (new file)

File: annotations/src/commonMain/kotlin/com/rsicarelli/fakt/{OptionName}Mode.kt

public enum class {OptionName}Mode {
    DEFAULT,   // Follow plugin-level setting
    {VALUE_A}, // Explicit enable
    {VALUE_B}, // Explicit disable
}

Pattern: MutabilityMode(DEFAULT, IMMUTABLE, MUTABLE), CallHistoryMode(DEFAULT, ENABLED, DISABLED).

Layer 2 — @Fake Annotation Parameter

File: annotations/src/commonMain/kotlin/com/rsicarelli/fakt/Fake.kt

Add parameter with DEFAULT default:

public annotation class Fake(
    val callHistory: CallHistoryMode = CallHistoryMode.DEFAULT,
    val mutability: MutabilityMode = MutabilityMode.DEFAULT,
    val {optionName}: {OptionName}Mode = {OptionName}Mode.DEFAULT, // NEW
)

Naming note: Real field names are descriptive and may not follow the template literally. E.g., mutability uses enableMutableFakes (not enableMutability) and generateMutableBehaviors (not generateMutability). Choose names that best describe the feature's effect.

Layers 3-4 wire the option through the Gradle build system to the compiler — this is how users set project-wide defaults in build.gradle.kts.

Layer 3 — Gradle Plugin Extension

File: gradle-plugin/src/main/kotlin/com/rsicarelli/fakt/gradle/FaktPluginExtension.kt

Add Property<Boolean> with convention default:

public val enable{OptionName}: Property<Boolean> =
    objects.property(Boolean::class.java).convention({defaultValue})

Layer 4 — Gradle Subplugin (passes option to compiler)

File: gradle-plugin/src/main/kotlin/com/rsicarelli/fakt/gradle/FaktGradleSubplugin.kt

In applyToCompilation, add SubpluginOption:

add(SubpluginOption(key = "enable{OptionName}", value = extension.enable{OptionName}.get().toString()))

Layers 5-6 register the option with compiler plugin infrastructure — the CLI processor receives the value from Gradle, and FaktOptions makes it available throughout the compiler.

Layer 5 — Compiler CLI Processor

File: compiler/src/main/kotlin/com/rsicarelli/fakt/compiler/core/config/FaktCommandLineProcessor.kt

Three additions in companion object + one in processOption:

// Companion:
val ENABLE_{OPTION}_KEY = CompilerConfigurationKey<Boolean>("fakt.enable{OptionName}")
val ENABLE_{OPTION}_OPTION = CliOption(
    optionName = "enable{OptionName}",
    valueDescription = "true|false",
    description = "Enable {description} by default (default: {defaultValue})",
    required = false,
)

// Add to pluginOptions list:
ENABLE_{OPTION}_OPTION

// In processOption when-branch:
"enable{OptionName}" -> configuration.put(ENABLE_{OPTION}_KEY, value.toBoolean())

Layer 6 — FaktOptions Data Class

File: compiler/src/main/kotlin/com/rsicarelli/fakt/compiler/core/config/FaktOptions.kt

Add field + read in load():

data class FaktOptions(
    // ...existing...
    val enable{OptionName}Default: Boolean = {defaultValue},
)

// In load():
val enable{OptionName} = configuration.get(FaktCommandLineProcessor.ENABLE_{OPTION}_KEY) ?: {defaultValue}
return FaktOptions(/* ...existing... */, enable{OptionName}Default = enable{OptionName})

Layers 7-8 extract the annotation value at compile time during the FIR phase — mirror enums avoid a runtime dependency on the annotations module, and checkers read the actual @Fake parameter value from source code.

Layer 7 — FIR Mirror Enum + Validated Metadata

File: compiler/src/main/kotlin/com/rsicarelli/fakt/compiler/fir/metadata/FirFakeMetadata.kt

// Mirror enum (avoids runtime dep on annotations):
enum class Fir{OptionName}Mode { DEFAULT, {VALUE_A}, {VALUE_B} }

// Add to BOTH ValidatedFakeInterface and ValidatedFakeClass:
val {optionName}Mode: Fir{OptionName}Mode = Fir{OptionName}Mode.DEFAULT,

Layer 8 — FIR Checkers (annotation extraction)

Files: compiler/.../fir/checkers/FakeInterfaceChecker.kt + FakeClassChecker.kt

Add private extraction method in both:

private fun extract{OptionName}Mode(declaration: FirClass, session: FirSession): Fir{OptionName}Mode {
    val fakeAnnotation = declaration.annotations.find { annotation ->
        annotation.toAnnotationClassIdSafe(session) == FAKE_ANNOTATION_CLASS_ID
    } ?: return Fir{OptionName}Mode.DEFAULT

    val arg = fakeAnnotation.argumentMapping.mapping.entries
        .find { it.key.asString() == "{optionName}" }?.value

    return when (arg) {
        is FirPropertyAccessExpression -> {
            val enumEntry = arg.calleeReference.toResolvedEnumEntrySymbol()
            when (enumEntry?.name?.asString()) {
                "{VALUE_A}" -> Fir{OptionName}Mode.{VALUE_A}
                "{VALUE_B}" -> Fir{OptionName}Mode.{VALUE_B}
                else -> Fir{OptionName}Mode.DEFAULT
            }
        }
        else -> Fir{OptionName}Mode.DEFAULT
    }
}

Call in analyzeMetadata() and pass to ValidatedFakeInterface/ValidatedFakeClass constructor.

Layers 9-10 bridge the FIR metadata to code generation in the IR phase — the resolver function implements the three-tier resolution (annotation → plugin → default), and analysis models expose the final resolved boolean that generators consume.

Layer 9 — IR Generation Metadata Bridge

File: compiler/.../ir/transform/IrGenerationMetadata.kt

Three additions:

a) Field in IrGenerationConfig:

val {optionName}Mode: Fir{OptionName}Mode = Fir{OptionName}Mode.DEFAULT,

b) Accessor on IrGenerationMetadata and IrClassGenerationMetadata:

val {optionName}Mode: Fir{OptionName}Mode get() = config.{optionName}Mode

c) Resolver function + usage in toInterfaceAnalysis() and toClassAnalysis():

private fun resolve{OptionName}Enabled(
    annotationMode: Fir{OptionName}Mode,
    pluginDefault: Boolean,
): Boolean = when (annotationMode) {
    Fir{OptionName}Mode.{VALUE_A} -> true
    Fir{OptionName}Mode.{VALUE_B} -> false
    Fir{OptionName}Mode.DEFAULT -> pluginDefault
}

Layer 10 — Analysis Models (final boolean)

File: compiler/.../ir/analysis/AnalysisModels.kt

Add resolved boolean to both InterfaceAnalysis and ClassAnalysis:

val generate{OptionName}: Boolean = {defaultValue},

Layer 11 is where the option actually affects generated code — generators read the resolved boolean and conditionally emit different Kotlin source.

Layer 11 — Code Generators (consume the boolean)

Modify generators based on what the option controls:

• ImplementationGenerator.kt — pass analysis.generate{OptionName} to generateCompleteFake()
• ConfigurationDslGenerator.kt — conditionally generate DSL elements
• FakeGenerator.kt / extensions — conditional code generation

3. Validate Implementation

Checklist:

• Enum file created in annotations/ with DEFAULT + two values
• @Fake annotation has new parameter with DEFAULT default
• Gradle extension has Property<Boolean> with convention
• Subplugin passes option via SubpluginOption
• CLI processor has KEY, OPTION, pluginOptions entry, processOption branch
• FaktOptions has field + load() reads it
• FIR mirror enum + fields in both ValidatedFakeInterface and ValidatedFakeClass
• Both checkers have extract{OptionName}Mode() + call in analyzeMetadata()
• IR metadata has config field, accessor, resolver function
• Both toInterfaceAnalysis() and toClassAnalysis() pass resolved boolean
• InterfaceAnalysis and ClassAnalysis have generate{OptionName}: Boolean
• Generators consume the boolean
• make publish-local && make test-sample passes
• make format applied

4. Three-Tier Resolution Pattern

Every option follows this resolution:

Annotation value (per-interface)
    ↓ if DEFAULT
Plugin setting (build.gradle.kts)
    ↓ if not set
Hardcoded default (in FaktOptions)

This allows: global default via Gradle, per-interface override via annotation, sensible fallback.

Related Skills

• workflow — full development cycle for implementing the feature
• compiler-architecture-validator — validate FIR/IR separation
• codegen — understand code generation pipeline for Layer 11