OpenAPITools/openapi-generator · error · RuntimeException

Multiplatform only supports string and kotlinx-datetime. Try

Error message

Multiplatform only supports string and kotlinx-datetime. Try adding '--additional-properties dateLibrary=kotlinx-datetime' to your command.

What it means

The Kotlin client generator's 'multiplatform' library produces common Kotlin code shared across JVM/JS/native targets, and common code cannot use JVM-only date types. processMultiplatformLibrary() therefore accepts only the 'string' and 'kotlinx-datetime' date libraries and throws a RuntimeException for anything else. The Kotlin client's default dateLibrary is 'java8', so the error fires during option processing unless dateLibrary is changed explicitly.

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java:905

        additionalProperties.put(JVM_SPRING_WEBCLIENT, true);
    }

    private void processJvmSpringRestClientLibrary(final String infrastructureFolder) {
        if (additionalProperties.getOrDefault(USE_SPRING_BOOT3, false).equals(false)
                && additionalProperties.getOrDefault(USE_SPRING_BOOT4, false).equals(false)) {
            throw new RuntimeException("This library requires Spring Boot 3 or 4. Try adding '--additional-properties useSpringBoot3=true' or '--additional-properties useSpringBoot4=true' to your command.");
        }

        processJvmSpring(infrastructureFolder);
        additionalProperties.put(JVM_SPRING_RESTCLIENT, true);
    }

    private void processMultiplatformLibrary(final String infrastructureFolder) {
        commonJvmMultiplatformSupportingFiles(infrastructureFolder);
        additionalProperties.put(MULTIPLATFORM, true);

        if (!DateLibrary.STRING.value.equals(dateLibrary) && !DateLibrary.KOTLINX_DATETIME.value.equals(dateLibrary)) {
            throw new RuntimeException("Multiplatform only supports string and kotlinx-datetime. Try adding '--additional-properties dateLibrary=kotlinx-datetime' to your command.");
        }

        setRequestDateConverter(RequestDateConverter.TO_STRING.value);

        // multiplatform default includes
        defaultIncludes.add("io.ktor.client.request.forms.InputProvider");
        defaultIncludes.add(packageName + ".infrastructure.Base64ByteArray");
        defaultIncludes.add(packageName + ".infrastructure.OctetByteArray");

        // multiplatform type mapping
        typeMapping.put("number", "kotlin.Double");
        typeMapping.put("file", "OctetByteArray");
        typeMapping.put("binary", "OctetByteArray");
        typeMapping.put("ByteArray", "Base64ByteArray");
        typeMapping.put("object", "kotlin.String");  // kotlin.Any not serializable

        // multiplatform import mapping
        importMapping.put("BigDecimal", "kotlin.Double");

View on GitHub (pinned to fcec517be3)

Solutions

Add `--additional-properties dateLibrary=kotlinx-datetime` to the same command that sets `library=multiplatform` (this is exactly what the message suggests).
Alternatively use `--additional-properties dateLibrary=string` if you want to serialize dates as plain strings with no datetime dependency.
If you actually need java8/threetenbp dates, drop `library=multiplatform` and use the default JVM client (e.g. library=jvm or the default kotlin client).

Example fix

# before
openapi-generator-cli generate -g kotlin -i api.yaml \
  --additional-properties library=multiplatform
# after
openapi-generator-cli generate -g kotlin -i api.yaml \
  --additional-properties library=multiplatform,dateLibrary=kotlinx-datetime

Defensive patterns

Strategy: validation

Validate before calling

// before invoking the generator programmatically
String library = (String) opts.get("library");
String dateLibrary = (String) opts.getOrDefault("dateLibrary", "java8"); // kotlin default
if ("multiplatform".equals(library)
        && !("string".equals(dateLibrary) || "kotlinx-datetime".equals(dateLibrary))) {
    throw new IllegalArgumentException(
        "library=multiplatform requires dateLibrary=string or kotlinx-datetime");
}

Try / catch

try {
    new DefaultGenerator().opts(clientOptInput).generate();
} catch (RuntimeException e) {
    // message contains the exact --additional-properties fix to suggest to the user
    throw new BuildException("openapi-generator config rejected: " + e.getMessage(), e);
}

Prevention

Treat library= and dateLibrary= as one coupled choice for kotlin; change them together.
Keep generator invocations in a single scripted location so option pairs cannot drift.
After upgrading openapi-generator, re-run generation in CI to catch newly-validated option constraints.

When it happens

Trigger: Running `openapi-generator-cli generate -g kotlin` with `--additional-properties library=multiplatform` while dateLibrary is left at its default (java8) or explicitly set to java8/threetenbp/rx (any value other than string/kotlinx-datetime). The check at KotlinClientCodegen.java:905 runs during processMultiplatformLibrary() after `library=multiplatform` is selected.

Common situations: Switching an existing kotlin client configuration to Kotlin Multiplatform (e.g. for a Kotlin/JS or KMP app) without touching the date options; reusing CI generator flags from a JVM-only kotlin project after a version upgrade; copy-pasting examples that pair library=multiplatform with the default java8 dates.

Related errors

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/1c58f8432e53736c. Report an issue: GitHub.