fix(kotlin): support oneOf with primitive types in kotlinx_serialization by pvcresin · Pull Request #22986 · OpenAPITools/openapi-generator

pvcresin · 2026-02-16T14:48:50Z

Description

The kotlin-client generator with jvm-retrofit2 library and kotlinx_serialization failed to generate working code for oneOf schemas containing primitive types (e.g. string, integer) without a discriminator. The existing template only handled discriminator-based oneOf via sealed interfaces, producing an empty class for primitive oneOf.

company_id:
  oneOf:
    - type: string
    - type: integer
      format: int64

Changes:

Add a non-discriminator fallback in oneof_class.mustache that generates a data class with actualInstance and a custom KSerializer, ported from the multiplatform template
Add isLong (int64) handling to the serialize branch, with an isInteger guard to avoid generating a dead branch for int64 types
Add test for oneOf primitive types with kotlinx_serialization

Before

@Serializable
class TestModelCompanyId () {
}

After

@Serializable(with = TestModelCompanyId.TestModelCompanyIdSerializer::class)
data class TestModelCompanyId(var actualInstance: Any? = null) {

    object TestModelCompanyIdSerializer : KSerializer<TestModelCompanyId> {
        override fun serialize(encoder: Encoder, value: TestModelCompanyId) {
            when (val instance = value.actualInstance) {
                is kotlin.String -> jsonEncoder.encodeString(instance)
                is kotlin.Long -> jsonEncoder.encodeLong(instance)
                null -> jsonEncoder.encodeJsonElement(JsonNull)
                else -> throw SerializationException(...)
            }
        }

        override fun deserialize(decoder: Decoder): TestModelCompanyId {
            // try each type in schema order
            try { decodeFromJsonElement<kotlin.String>(...) } catch { ... }
            try { decodeFromJsonElement<kotlin.Long>(...) } catch { ... }
        }
    }
}

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Summary by cubic

Fixes Kotlin client generation for jvm-retrofit2 + kotlinx_serialization when oneOf includes primitive types without a discriminator. Generates a data-class wrapper with a nested KSerializer that handles primitives and null, and tries each type on decode.

Bug Fixes
- Add non-discriminator fallback in oneof_class.mustache for kotlinx_serialization: data-class wrapper + nested KSerializer; handles String, Boolean, Int, Long (int64 guarded), Float, Double, and null; delegates non-primitives.
- Add test validating wrapper generation, primitive try-each deserialization, and that the parent model references the wrapper type.

^{Written for commit 7296d21. Summary will update on new commits.}

cubic-dev-ai

1 issue found across 2 files

Prompt for AI agents (all issues)


Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache:217">
P2: oneOf deserialization returns on the first successful decode without checking for multiple matches, so ambiguous primitives (e.g., Int/Long/Double) will silently select the first schema instead of enforcing exactly one match.</violation>
</file>

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modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache

pvcresin · 2026-02-17T01:08:27Z

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cubic-dev-ai · 2026-02-17T01:08:35Z

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1 issue found across 2 files

Prompt for AI agents (all issues)


Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache:198">
P2: `{{{dataType}}}.serializer()` will not compile for parameterized/collection oneOf types (e.g., `List<Foo>`/`Map<String,Bar>`), so the generated Kotlin for oneOf with array/map schemas will fail to compile. Use a collection serializer or the top‑level `serializer<T>()` instead.</violation>
</file>

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modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache

pvcresin · 2026-02-17T01:37:54Z

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cubic-dev-ai · 2026-02-17T01:38:01Z

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cubic-dev-ai

1 issue found across 2 files

Prompt for AI agents (all issues)


Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache:217">
P2: Non-discriminator oneOf deserialization returns on the first successful decode, so overlapping primitive schemas can match multiple types but the first one wins, violating oneOf’s “exactly one” semantics.</violation>
</file>

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modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache

pvcresin · 2026-02-17T05:45:37Z

This is a PR regarding Kotlin. Could you please review it?

@karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier @e5l @dennisameling

e5l

Hey @pvcresin, thank you for the PR!

lgtm

pvcresin · 2026-02-18T01:54:28Z

This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
These must match the expectations made by your contribution.

Rebased on the latest master. The code has not been changed since the PR was created.

naokiwakata · 2026-02-19T05:35:55Z

@pvcresin @e5l
Great work identifying this issue! Adding kotlinx_serialization support for non-discriminator oneOf with primitive types is a much-needed improvement.

I've opened #23007 which tackles the same problem with a different approach — using sealed interface with @JvmInline value class wrappers instead of data class with actualInstance: Any?. It also extends coverage to anyOf and array types. Since both PRs touch the same template files, #23007 is intended as an alternative approach rather than a follow-up.

Would appreciate your review on #23007 as well. Thanks!

The kotlin-client generator with `jvm-retrofit2` library and `kotlinx_serialization` failed to generate working code for oneOf schemas containing primitive types (e.g. string, integer) without a discriminator. The existing template only handled discriminator-based oneOf via sealed interfaces, producing an empty class for primitive oneOf. Changes: - Add a non-discriminator fallback in oneof_class.mustache that generates a data class with actualInstance and a custom KSerializer, ported from the multiplatform template - Add isLong (int64) handling to the serialize branch, with an isInteger guard to avoid generating a dead branch for int64 types - Add test for oneOf primitive types with kotlinx_serialization

pvcresin · 2026-02-19T14:06:36Z

Rebased on the latest master. The code has not been changed since the PR was created.

pvcresin · 2026-02-19T15:48:09Z

@cubic-dev-ai rerun a review

cubic-dev-ai · 2026-02-19T15:48:20Z

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@pvcresin I have started the AI code review. It will take a few minutes to complete.

cubic-dev-ai

2 issues found across 2 files

Prompt for AI agents (all issues)


Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache:198">
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.</violation>

<violation number="2" location="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache:216">
P2: Missing import for kotlinx.serialization.json.decodeFromJsonElement causes generated code to fail to compile.</violation>
</file>

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cubic-dev-ai · 2026-02-19T16:02:13Z

modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache

+            {{#composedSchemas}}
+            {{#oneOf}}
+            try {
+                val instance = jsonDecoder.json.decodeFromJsonElement<{{{dataType}}}>(jsonElement)


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>

cubic-dev-ai · 2026-02-19T16:02:13Z

modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache

+                {{/isNumber}}
+                {{/isPrimitiveType}}
+                {{^isPrimitiveType}}
+                is {{{dataType}}} -> jsonEncoder.encodeSerializableValue({{{dataType}}}.serializer(), instance)


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>

wing328 · 2026-02-23T23:58:36Z

please follow step 3 to update the samples

let us know if you need help on that

4brunu · 2026-02-24T09:06:50Z

Hi @pvcresin thanks for contributing to openapi generator.
Since #23007 was merged, should we remove close this PR?
Thanks

pvcresin · 2026-02-24T09:21:21Z

@4brunu No problem. I'll close this!

I also feel that #23007 is a more robust implementation.

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e5l approved these changes Feb 17, 2026

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pvcresin force-pushed the fix/kotlin-kotlinx-oneof-primitive-types branch from d673376 to cff7fbe Compare February 18, 2026 01:52

naokiwakata mentioned this pull request Feb 19, 2026

fix(kotlin): support non-discriminator oneOf/anyOf model and array types with kotlinx_serialization #23007

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pvcresin force-pushed the fix/kotlin-kotlinx-oneof-primitive-types branch from a84ec77 to 7296d21 Compare February 19, 2026 14:05

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wing328 added Client: Kotlin Enhancement: Feature Feature: OAS 3.0 spec support labels Feb 23, 2026

wing328 added this to the 7.21.0 milestone Feb 23, 2026

pvcresin closed this Feb 24, 2026

pvcresin deleted the fix/kotlin-kotlinx-oneof-primitive-types branch February 24, 2026 09:21

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