What causes "Only 'binary' and 'serializable' are supported as variable type." in flowable/flowable-engine?

Set variable_type to exactly 'binary' or 'serializable', or omit the field entirely to default to binary

flowable/flowable-engine · warning · FlowableIllegalArgumentException

Only 'binary' and 'serializable' are supported as variable…

Error message

Only 'binary' and 'serializable' are supported as variable type.

What it means

FlowableIllegalArgumentException thrown when the optional variable_type field is provided but is neither 'binary' nor 'serializable'. The binary-variable upload endpoint only supports these two types; any other declared type is rejected, while omitting the field defaults to 'binary'.

Solutions

Set variable_type to exactly 'binary' or 'serializable', or omit the field entirely to default to binary
If the variable should hold a primitive/string value, use the JSON variable PUT endpoint instead of the binary upload
Catch FlowableIllegalArgumentException and return 400 listing allowed values

Example fix

// before
-F "variable_type=string"
// after
-F "variable_type=binary"

Defensive patterns

Strategy: validation

Validate before calling

if (variableType != null && !Set.of("binary","serializable").contains(variableType)) {
    throw new IllegalArgumentException("variable_type must be 'binary' or 'serializable' (or omitted)");
}

Try / catch

try {
    client.uploadBinaryVariable(executionId, name, file, type);
} catch (HttpClientErrorException.BadRequest e) {
    // correct variable_type and retry
}

Prevention

Omit variable_type to accept the 'binary' default
Use only the exact lowercase literals 'binary' and 'serializable'
For non-binary values use the JSON variable endpoint instead

When it happens

Trigger: Multipart variable upload including variable_type=string/integer/long/etc., or a typo like 'byte-array' instead of 'binary'.

Common situations: Clients copying the type names from general variable APIs where other types exist; case-sensitivity mistakes ('Binary' vs 'binary'); assuming type matching the actual content type.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/19be229e46b437ce. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-rest/src/main/java/org/flowable/rest/service/api/runtime/process/BaseExecutionVariableResource.java:146

                } else if ("name".equalsIgnoreCase(parameterName)) {
                    variableName = paramMap.get(parameterName)[0];

                } else if ("type".equalsIgnoreCase(parameterName)) {
                    variableType = paramMap.get(parameterName)[0];
                }
            }
        }

        try {

            // Validate input and set defaults
            if (variableName == null) {
                throw new FlowableIllegalArgumentException("No variable name was found in request body.");
            }

            if (variableType != null) {
                if (!RestResponseFactory.BYTE_ARRAY_VARIABLE_TYPE.equals(variableType) && !RestResponseFactory.SERIALIZABLE_VARIABLE_TYPE.equals(variableType)) {
                    throw new FlowableIllegalArgumentException("Only 'binary' and 'serializable' are supported as variable type.");
                }
            } else {
                variableType = RestResponseFactory.BYTE_ARRAY_VARIABLE_TYPE;
            }

            RestVariableScope scope = RestVariableScope.LOCAL;
            if (variableScope != null) {
                scope = RestVariable.getScopeFromString(variableScope);
            }

            if (variableType.equals(RestResponseFactory.BYTE_ARRAY_VARIABLE_TYPE)) {
                // Use raw bytes as variable value
                byte[] variableBytes = IOUtils.toByteArray(file.getInputStream());
                setVariable(execution, variableName, variableBytes, scope, isNew, async);

            } else if (isSerializableVariableAllowed) {
                // Try deserializing the object
                ObjectInputStream stream = new ObjectInputStream(file.getInputStream());

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