If your upload request gets a 400 response and the server reports a missing or invalid multipart boundary, check the request’s Content-Type and body together. For browser fetch or XMLHttpRequest requests that send a FormData object, remove any manually set Content-Type header and pass the object directly as the request body. The browser must generate a boundary and use the same value in the header and body. A 400 response alone does not prove that multipart formatting is the problem.
Why a multipart boundary matters
A multipart/form-data request separates its fields and files into parts. The Content-Type header must include a boundary parameter, and the body must use that exact value to mark where the parts begin and end. RFC 7578 describes this format in RFC 7578; MDN illustrates the header form multipart/form-data; boundary=ExampleBoundaryString and matching body delimiters in its Content-Type reference.
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In the body, a delimiter starts with a carriage return and line feed, followed by two hyphens and the boundary value. The final delimiter adds two trailing hyphens. If the header omits the boundary, or its value does not match the body delimiters, the receiver cannot reliably identify the parts. Each part also needs the appropriate Content-Disposition information, including a name parameter for its form field, as specified by RFC 7578.
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Fix browser Fetch and XMLHttpRequest uploads
When the body is a browser FormData object, do not manually set Content-Type. MDN’s FormData guidance explains that setting the header yourself can prevent the browser from adding the boundary expression it uses in the body.
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const formData = new FormData();
formData.append("file", fileInput.files[0]);
const response = await fetch("/upload", {
method: "POST",
body: formData
});For XMLHttpRequest, the equivalent is to call send(formData) without setting a multipart Content-Type header:
const request = new XMLHttpRequest();
request.open("POST", "/upload");
request.send(formData);Do not copy a boundary from another request or construct the header by hand when sending a FormData object. Let the browser create the header and matching body delimiters.
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Apply Axios guidance for the actual runtime
Axios behavior depends on the environment, adapter, and FormData implementation. Its multipart documentation says browser, web worker, and React Native FormData should be sent without explicitly setting Content-Type, so the runtime can add the boundary.
Do these 3 things before closing this tab:
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A report opened September 27, 2025, describes Axios 1.12.0 with the fetch adapter omitting a boundary after a manual Content-Type: multipart/form-data override; the server rejected that request. See Axios issue 7054. This is a report about that stated configuration, not evidence that every Axios version or adapter behaves the same way. Reproduce the issue with your installed setup and inspect the outgoing request.
Reproduce the request in Postman
- Open the request and choose Body > form-data.
- Add each field using the name the endpoint expects. For file data, choose a File field and select the file.
- Inspect the request headers. Postman automatically adds
Content-Typefor form-data, but a manually selectedContent-Typeheader takes precedence and can override the generated value. Remove a stale or conflicting manual header. - Send the request and inspect its outgoing headers and body if the server still rejects it.
Postman’s request-body documentation explains form-data handling. A successful Postman request helps isolate request construction, but it does not establish that your production client sends the same headers and body.
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Verify the outgoing request before changing server code
Inspect the request as transmitted, using browser developer tools, a proxy, or the API client’s console. Source-code configuration alone is not conclusive because clients can transform headers and bodies.
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- Record the HTTP method and endpoint, the complete
Content-Typeheader including parameters, and whether the transmitted body is actually multipart. - If the header says
multipart/form-data, confirm it includesboundary=.... - Compare the boundary parameter character for character with the delimiters in the body. A missing or mismatched value is a formatting defect to address.
- For browser
FormData, remove the manual header and let the browser supply the boundary. For Axios, identify the runtime, adapter, version, and FormData implementation. In Postman, check for a manual header overriding the generated one. - Confirm the endpoint expects
multipart/form-data, not JSON or URL-encoded input, and that your part names—including the file field name—match its requirements.
If the boundary and body match but the request still gets a 400, inspect the response body and server logs for endpoint-specific validation failures, required fields, file size or type limits, authentication, or other application requirements. Those possibilities depend on the particular endpoint; a 400 status does not identify any one of them.
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