What causes "Mixed Multipart found in a previous Mixed Multipart" in quarkusio/quarkus?

Flatten the payload on the server so mixed multipart is not nested inside another mixed multipart.

quarkusio/quarkus · error · ErrorDataDecoderException

Mixed Multipart found in a previous Mixed Multipart

Error message

Mixed Multipart found in a previous Mixed Multipart

What it means

The decoder encountered a nested 'multipart/mixed' Content-Type inside a part that is already being parsed as a mixed-multipart child. Nested mixed multiparts are not supported, so parsing fails with ErrorDataDecoderException. This protects the state machine from unsupported recursive multipart structures.

Source

Thrown at independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/impl/multipart/QuarkusMultipartResponseDecoder.java:774

                Attribute attribute;
                try {
                    attribute = factory.createAttribute(response, HttpHeaderNames.CONTENT_LENGTH.toString(),
                            cleanString(contents[1]));
                } catch (NullPointerException | IllegalArgumentException e) {
                    throw new ErrorDataDecoderException(e);
                }

                currentFieldAttributes.put(HttpHeaderNames.CONTENT_LENGTH, attribute);
            } else if (HttpHeaderNames.CONTENT_TYPE.contentEqualsIgnoreCase(contents[0])) {
                // Take care of possible "multipart/mixed"
                if (HttpHeaderValues.MULTIPART_MIXED.contentEqualsIgnoreCase(contents[1])) {
                    if (currentStatus == MultiPartStatus.DISPOSITION) {
                        String values = StringUtil.substringAfter(contents[2], '=');
                        multipartMixedBoundary = "--" + values;
                        currentStatus = MultiPartStatus.MIXEDDELIMITER;
                        return decodeMultipart(MultiPartStatus.MIXEDDELIMITER);
                    } else {
                        throw new ErrorDataDecoderException("Mixed Multipart found in a previous Mixed Multipart");
                    }
                } else {
                    for (int i = 1; i < contents.length; i++) {
                        final String charsetHeader = HttpHeaderValues.CHARSET.toString();
                        if (contents[i].regionMatches(true, 0, charsetHeader, 0, charsetHeader.length())) {
                            String values = StringUtil.substringAfter(contents[i], '=');
                            Attribute attribute;
                            try {
                                attribute = factory.createAttribute(response, charsetHeader, cleanString(values));
                            } catch (NullPointerException | IllegalArgumentException e) {
                                throw new ErrorDataDecoderException(e);
                            }
                            currentFieldAttributes.put(HttpHeaderValues.CHARSET, attribute);
                        } else {
                            Attribute attribute;
                            try {
                                attribute = factory.createAttribute(response,
                                        cleanString(contents[0]), contents[i]);

View on GitHub (pinned to e1c734241f)

Solutions

Flatten the payload on the server so mixed multipart is not nested inside another mixed multipart.
If nested mixed content is required, pre-process the raw body yourself (e.g. parse the outer layer and feed inner parts separately) instead of relying on the decoder.
Switch the parts to a simple multipart/form-data structure without the mixed wrapper.
File/track a feature request if your protocol genuinely needs nested mixed support.

Example fix

// before (nested, unsupported)
Content-Type: multipart/mixed; boundary=outer
  Content-Type: multipart/mixed; boundary=inner
// after (flattened)
Content-Type: multipart/mixed; boundary=outer
  Content-Type: application/octet-stream

Defensive patterns

Strategy: validation

Validate before calling

// Detect nested multipart/mixed before sending the response
if (isMixedMultipart(partContentType) && parentStatus == MIXED) {
    throw new IllegalStateException("Nested multipart/mixed is not supported by the client decoder");
}

Try / catch

try {
    response = client.multipartCall().await();
} catch (BadRequestException e) {
    if (e.getCause() instanceof ErrorDataDecoderException
            && e.getCause().getMessage().contains("Mixed Multipart")) {
        // fall back to raw-body handling / custom parser
        response = client.rawCall().await();
    } else {
        throw e;
    }
}

Prevention

Never nest multipart/mixed inside multipart/mixed in server responses.
Flatten attachments into a single-level multipart structure.
Document the decoder limitation for API producers targeting Quarkus clients.
Add a server-side test asserting single-level mixed parts.

When it happens

Trigger: A part within a multipart/mixed section itself declares Content-Type: multipart/mixed (currentStatus is DISPOSITION while already in mixed mode), so the state machine cannot transition to MIXEDDELIMITER twice.

Common situations: Servers implementing RFC-allowed but rarely-used nested multipart/mixed payloads (e.g. some email-like or SOAP-with-attachments stacks); middleware aggregating multipart payloads recursively.

Related errors

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/77140b63ae47b43f. Report an issue: GitHub.