Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The generated ApiClient.kt uses response.body (OkHttp's Response.body,
which returns ResponseBody?) without a null-safety check. This fails to compile
with Kotlin 2.x, which enforces stricter null-safety on platform types.

openapi-generator version

• Generator: kotlin, library: jvm-okhttp4
• Generator version: 7.21.0 (also affects earlier versions)
• Kotlin version: 2.1.10 (any Kotlin 2.x triggers this)
• OkHttp version: 4.12.0

OpenAPI declaration file content or url

The specific spec doesn't matter — this bug is in the shared ApiClient.kt infrastructure template, not in spec-specific generated code. Any valid spec triggers it. Here's the minimal one used in the reproducer:

json
{
  "openapi": "3.0.3",
  "info": { "title": "Minimal API", "version": "1.0.0" },
  "paths": {
    "/items/{id}": {
      "get": {
        "operationId": "getItem",
        "parameters": [
          { "name": "id", "in": "path", "required": true, "schema": { "type": "integer" } }
        ],
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "application/json": {
                "schema": { "$ref": "#/components/schemas/Item" }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Item": {
        "type": "object",
        "properties": {
          "id": { "type": "integer" },
          "name": { "type": "string" }
        }
      }
    }
  }
}

Generation Details

CLI (after brew install openapi-generator on a Mac):

openapi-generator generate \
  -g kotlin \
  -i api-spec.json \
  -o generated \
  --additional-properties=library=jvm-okhttp4

Steps to reproduce

• Perform the generation step (which should complete with no errors)
• Compile the generated code with Kotlin 2.x.

./gradlew compileKotlin

Please refer to the full reproducer project: https://github.com/oymo/openapi-kotlin2-nullable-repro

git clone https://github.com/oymo/openapi-kotlin2-nullable-repro.git
cd openapi-kotlin2-nullable-repro
./gradlew compileKotlin   # fails with null-safety error on Kotlin 2.x

Related issues/PRs

I could not find any similar reported issues, neither open nor closed.

Suggest a fix

I believe the issue can be fixed by a simple edit of

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-okhttp/infrastructure/ApiClient.kt.mustache

Change the assignment from:

val body = response.body

to:

val body = response.body ?: return null

This is safe because the enclosing responseBody() function returns T?.