OpenFeign/feign · error · HttpMessageConversionException

Content-Type missing boundary information.

Error message

Content-Type missing boundary information.

What it means

The reader derives the multipart boundary from the boundary parameter of the Content-Type header. If the parameter is absent or empty it cannot delimit parts, so it throws HttpMessageConversionException.

Solutions

1. Ensure the client sends Content-Type like multipart/form-data; boundary=<token> and does not override the encoder-generated header
2. Let feign-form's encoder set the Content-Type instead of setting it manually in the interface/interceptors
3. Check interceptors, filters, or gateways that replace the Content-Type header

Example fix

// before
@Headers("Content-Type: multipart/form-data")
@Post("/upload") Response upload(MultipartFile file);
// after
@Headers("Content-Type: multipart/form-data")
// and use SpringFormEncoder, which appends the boundary automatically
Feign.builder().encoder(new SpringFormEncoder())...

String ct = headers.getFirst("Content-Type");
boolean ok = ct != null && ct.matches("multipart/[\w-]+;.*boundary=.+");
if (!ok) throw new IllegalArgumentException("Content-Type missing boundary parameter");

When it happens

Trigger: readInternal/boundaryBytes is invoked on a request whose Content-Type is multipart/* but lacks the boundary parameter (or it is empty after unquoting).

Common situations: A client or intermediate proxy rewrites the Content-Type and strips the boundary; hand-built multipart requests forget the boundary parameter; code sets Content-Type manually instead of letting the encoder do it.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Content-Type is missing.
• Multipart body could not be read.
• SpringManyMultipartFilesReader does not support writing to…
• Content-Disposition is not of type form-data.
• Unable to decode response ( ) ...

AI-assisted analysis of OpenFeign/feign@e2a1e27560 (2026-09-10). Data as JSON: /api/errors/5bf982879b08779e. Report an issue: GitHub.

Appendix: source

Thrown at form-spring/src/main/java/feign/form/spring/converter/SpringManyMultipartFilesReader.java:127

        throw new HttpMessageNotReadableException(
            "Multipart body could not be read.", e, inputMessage);
      }
      multiparts.add(multiPart);
    }
    return multiparts.toArray(new ByteArrayMultipartFile[0]);
  }

  @Override
  protected void writeInternal(
      MultipartFile[] byteArrayMultipartFiles, HttpOutputMessage outputMessage) {
    throw new UnsupportedOperationException(
        getClass().getSimpleName() + " does not support writing to HTTP body.");
  }

  private byte[] getMultiPartBoundary(MediaType contentType) {
    val boundaryString = unquote(contentType.getParameter("boundary"));
    if (StringUtils.hasLength(boundaryString) == false) {
      throw new HttpMessageConversionException("Content-Type missing boundary information.");
    }
    return boundaryString.getBytes(UTF_8);
  }

  private ByteArrayMultipartFile readMultiPart(MultipartStream multipartStream) throws IOException {
    val multiPartHeaders =
        splitIntoKeyValuePairs(
            multipartStream.readHeaders(), NEWLINES_PATTERN, COLON_PATTERN, false);

    val contentDisposition =
        splitIntoKeyValuePairs(
            multiPartHeaders.get(CONTENT_DISPOSITION),
            SEMICOLON_PATTERN,
            EQUALITY_SIGN_PATTERN,
            true);

    if (!contentDisposition.containsKey("form-data")) {
      throw new HttpMessageConversionException("Content-Disposition is not of type form-data.");

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