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fix(kotlin): support oneOf with primitive types in kotlinx_serialization #22986

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fix(kotlin): support oneOf with primitive types in kotlinx_serialization #22986

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84 changes: 80 additions & 4 deletions modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache
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Original file line number Diff line number Diff line change
Expand Up @@ -39,22 +39,22 @@ import kotlinx.serialization.encoding.Encoder
{{/enumUnknownDefaultCase}}
{{^enumUnknownDefaultCase}}
{{#generateOneOfAnyOfWrappers}}
{{#discriminator}}
import kotlinx.serialization.KSerializer
import kotlinx.serialization.encoding.Decoder
import kotlinx.serialization.encoding.Encoder
{{/discriminator}}
{{/generateOneOfAnyOfWrappers}}
{{/enumUnknownDefaultCase}}
{{#generateOneOfAnyOfWrappers}}
{{#discriminator}}
import kotlinx.serialization.SerializationException
import kotlinx.serialization.descriptors.SerialDescriptor
import kotlinx.serialization.descriptors.buildClassSerialDescriptor
import kotlinx.serialization.json.JsonDecoder
import kotlinx.serialization.json.JsonEncoder
import kotlinx.serialization.json.JsonObject
import kotlinx.serialization.json.JsonElement
import kotlinx.serialization.json.JsonNull
import kotlinx.serialization.json.JsonPrimitive
{{#discriminator}}
import kotlinx.serialization.json.JsonObject
import kotlinx.serialization.json.jsonObject
import kotlinx.serialization.json.jsonPrimitive
{{/discriminator}}
Expand Down Expand Up @@ -100,13 +100,17 @@ import java.io.IOException
{{#discriminator}}
@Serializable(with = {{classname}}Serializer::class)
{{/discriminator}}
{{^discriminator}}
{{#serializableModel}}@KSerializable(with = {{classname}}.{{classname}}Serializer::class){{/serializableModel}}{{^serializableModel}}@Serializable(with = {{classname}}.{{classname}}Serializer::class){{/serializableModel}}
{{/discriminator}}
{{/generateOneOfAnyOfWrappers}}
{{/kotlinx_serialization}}
{{#isDeprecated}}
@Deprecated(message = "This schema is deprecated.")
{{/isDeprecated}}
{{>additionalModelTypeAnnotations}}
{{#kotlinx_serialization}}
{{#discriminator}}
{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}sealed interface {{classname}} {
{{#discriminator.mappedModels}}
@JvmInline
Expand Down Expand Up @@ -150,6 +154,78 @@ import java.io.IOException
}
}
}
{{/discriminator}}
{{^discriminator}}
{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}data class {{classname}}(var actualInstance: Any? = null) {

{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}object {{classname}}Serializer : KSerializer<{{classname}}> {
override val descriptor: SerialDescriptor = buildClassSerialDescriptor("{{classname}}") {
element("type", JsonPrimitive.serializer().descriptor)
element("actualInstance", JsonElement.serializer().descriptor)
}

override fun serialize(encoder: Encoder, value: {{classname}}) {
val jsonEncoder = encoder as? JsonEncoder ?: throw SerializationException("{{classname}} can only be serialized with Json")

when (val instance = value.actualInstance) {
{{#composedSchemas}}
{{#oneOf}}
{{#isPrimitiveType}}
{{#isString}}
is kotlin.String -> jsonEncoder.encodeString(instance)
{{/isString}}
{{#isBoolean}}
is kotlin.Boolean -> jsonEncoder.encodeBoolean(instance)
{{/isBoolean}}
{{#isInteger}}
{{^isLong}}
is kotlin.Int -> jsonEncoder.encodeInt(instance)
{{/isLong}}
{{/isInteger}}
{{#isLong}}
is kotlin.Long -> jsonEncoder.encodeLong(instance)
{{/isLong}}
{{#isNumber}}
{{#isDouble}}
is kotlin.Double -> jsonEncoder.encodeDouble(instance)
{{/isDouble}}
{{#isFloat}}
is kotlin.Float -> jsonEncoder.encodeFloat(instance)
{{/isFloat}}
{{/isNumber}}
{{/isPrimitiveType}}
{{^isPrimitiveType}}
is {{{dataType}}} -> jsonEncoder.encodeSerializableValue({{{dataType}}}.serializer(), instance)
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@cubic-dev-ai cubic-dev-ai bot Feb 19, 2026 •
edited
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P2: Generated code uses {{{dataType}}}.serializer() for non‑primitive oneOf types, which breaks for generic/array types (e.g., List<String>.serializer() is invalid Kotlin). This causes compile errors for oneOf schemas containing arrays or other generics.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache, line 198:

<comment>Generated code uses `{{{dataType}}}.serializer()` for non‑primitive oneOf types, which breaks for generic/array types (e.g., `List<String>.serializer()` is invalid Kotlin). This causes compile errors for oneOf schemas containing arrays or other generics.</comment>

<file context>
@@ -150,6 +154,78 @@ import java.io.IOException
+                {{/isNumber}}
+                {{/isPrimitiveType}}
+                {{^isPrimitiveType}}
+                is {{{dataType}}} -> jsonEncoder.encodeSerializableValue({{{dataType}}}.serializer(), instance)
+                {{/isPrimitiveType}}
+                {{/oneOf}}
</file context>
Fix with Cubic

{{/isPrimitiveType}}
{{/oneOf}}
{{/composedSchemas}}
null -> jsonEncoder.encodeJsonElement(JsonNull)
else -> throw SerializationException("Unknown type in actualInstance: ${instance::class}")
}
}

override fun deserialize(decoder: Decoder): {{classname}} {
val jsonDecoder = decoder as? JsonDecoder ?: throw SerializationException("{{classname}} can only be deserialized with Json")
val jsonElement = jsonDecoder.decodeJsonElement()

val errorMessages = mutableListOf<String>()

{{#composedSchemas}}
{{#oneOf}}
try {
val instance = jsonDecoder.json.decodeFromJsonElement<{{{dataType}}}>(jsonElement)
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@cubic-dev-ai cubic-dev-ai bot Feb 19, 2026 •
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P2: Missing import for kotlinx.serialization.json.decodeFromJsonElement causes generated code to fail to compile.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache, line 216:

<comment>Missing import for kotlinx.serialization.json.decodeFromJsonElement causes generated code to fail to compile.</comment>

<file context>
@@ -150,6 +154,78 @@ import java.io.IOException
+            {{#composedSchemas}}
+            {{#oneOf}}
+            try {
+                val instance = jsonDecoder.json.decodeFromJsonElement<{{{dataType}}}>(jsonElement)
+                return {{classname}}(actualInstance = instance)
+            } catch (e: Exception) {
</file context>
Fix with Cubic

return {{classname}}(actualInstance = instance)
} catch (e: Exception) {
errorMessages.add("Failed to deserialize as {{{dataType}}}: ${e.message}")
}
{{/oneOf}}
{{/composedSchemas}}

throw SerializationException("Cannot deserialize {{classname}}. Tried: ${errorMessages.joinToString(", ")}")
}
}
}
{{/discriminator}}
{{/kotlinx_serialization}}
{{^kotlinx_serialization}}
{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}data class {{classname}}(var actualInstance: Any? = null) {
Expand Down
34 changes: 34 additions & 0 deletions ...generator/src/test/java/org/openapitools/codegen/kotlin/KotlinClientCodegenModelTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -614,6 +614,40 @@ public void polymorphicKotlinxSerialization() throws IOException {
TestUtils.assertFileContains(birdKt, "@SerialName(value = \"BIRD\")");
}

@Test(description = "generate oneOf wrapper with primitive types using kotlinx_serialization")
public void oneOfPrimitiveKotlinxSerialization() throws IOException {
File output = Files.createTempDirectory("test").toFile();
output.deleteOnExit();

final CodegenConfigurator configurator = new CodegenConfigurator()
.setGeneratorName("kotlin")
.setLibrary("jvm-retrofit2")
.setAdditionalProperties(new HashMap<>() {{
put(CodegenConstants.SERIALIZATION_LIBRARY, "kotlinx_serialization");
put("generateOneOfAnyOfWrappers", true);
}})
.setInputSpec("src/test/resources/3_0/issue_19942.json")
.setOutputDir(output.getAbsolutePath().replace("\\", "/"));

final ClientOptInput clientOptInput = configurator.toClientOptInput();
DefaultGenerator generator = new DefaultGenerator();
generator.opts(clientOptInput).generate();

final Path oneOfModelKt = Paths.get(output + "/src/main/kotlin/org/openapitools/client/models/ObjectWithComplexOneOfId.kt");
// generates data class with actualInstance (not empty sealed interface)
TestUtils.assertFileContains(oneOfModelKt, "data class ObjectWithComplexOneOfId");
TestUtils.assertFileContains(oneOfModelKt, "var actualInstance: Any?");
// has a custom KSerializer
TestUtils.assertFileContains(oneOfModelKt, "object ObjectWithComplexOneOfIdSerializer : KSerializer<ObjectWithComplexOneOfId>");
// serializer handles primitive types
TestUtils.assertFileContains(oneOfModelKt, "is kotlin.String -> jsonEncoder.encodeString(instance)");
// serializer handles deserialization via try-each
TestUtils.assertFileContains(oneOfModelKt, "decodeFromJsonElement<kotlin.String>(jsonElement)");
// parent model references the oneOf wrapper type
final Path parentModelKt = Paths.get(output + "/src/main/kotlin/org/openapitools/client/models/ObjectWithComplexOneOf.kt");
TestUtils.assertFileContains(parentModelKt, "val id: ObjectWithComplexOneOfId?");
}

@Test(description = "generate polymorphic jackson model")
public void polymorphicJacksonSerialization() throws IOException {
File output = Files.createTempDirectory("test").toFile();
Expand Down
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