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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →If a speech API says an audio upload or form field is missing, inspect the request as it actually goes over HTTP: first match the multipart boundary in the header to the body, then verify each part’s name and file bytes, and finally check the endpoint’s required fields and audio limits. Multipart syntax is general; field names and payload rules depend on the API.
1. Check the Content-Type header and boundary
A multipart/form-data body is made of parts separated by a boundary. The request’s Content-Type header must include a boundary parameter, and that value must match the delimiters in the body. If the parameter is missing or the values differ, the server may be unable to parse the parts. This relationship is specified in RFC 7578.
- Capture the outgoing request after your client or middleware has serialized it, with credentials removed.
- Compare the boundary parameter in the header with the boundary delimiters in the body.
- Check that the body uses the multipart framing required by the standard; do not build the body with one boundary token and send a different token in the header.
Inspect the wire request rather than assuming that the options in your source code produced the intended envelope.
2. Let browser FormData set its boundary
When sending a browser FormData object with fetch or XMLHttpRequest, pass the object as the request body and do not manually set Content-Type. The browser must add the boundary expression that matches its serialized body. MDN’s FormData guidance warns that setting the header yourself prevents the browser from setting the matching boundary.
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const form = new FormData();
form.append("file", audioFile);
form.append("model", "your-model");
const response = await fetch(endpointUrl, {
method: "POST",
body: form
});Do not copy this browser-specific rule blindly to command-line tools, server-side clients, or SDKs. Follow the multipart serialization rules of the client you use; the important check is whether its final header and body agree.
3. Inspect the parts, field names, and file representation
RFC 7578 requires each part to have a Content-Disposition header with disposition form-data and a name parameter. A file part commonly includes a filename when one is meaningful. Give it an appropriate Content-Type when known; application/octet-stream can be used when the type is unknown.
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Field names are part of the endpoint contract, not universal multipart rules. In OpenAI’s file transcription guide, the transcription request uses a file part and a separate model field. Its curl example uses --form file=@... to upload file contents and --form model=... for the model value.
- Check the exact spelling and capitalization of every required part name against the target API’s current reference.
- Confirm the file part contains file bytes supplied through the client’s file, stream, or blob mechanism. A local path written as an ordinary text value is not the same thing as uploading that file.
- Inspect the serialized part headers and values to catch a missing part, unexpected text value, or incorrect representation.
4. Distinguish multipart parsing errors from API validation
If the server acts as though fields are missing, investigate boundary agreement and part names first. If it recognizes the submitted fields but rejects the request, check the endpoint’s required parameters and payload constraints. A syntactically valid multipart envelope can still carry an invalid or unsupported audio upload.
For the OpenAI file transcription endpoint, documented at /v1/audio/transcriptions, the current guide lists a maximum file size of 25 MB and the formats mp3, mp4, mpeg, mpga, m4a, wav, and webm. These are OpenAI-specific requirements, not general limits for speech APIs; consult the target provider’s current documentation for its own fields, size limit, and accepted formats.
5. Reproduce the issue with a minimal request
Start with the smallest request that the target API documents: its required file and other required fields. For the OpenAI example, use the official SDK or curl pattern from its transcription guide, including the file upload and model field. Remove optional prompts, arrays, metadata, custom headers, and middleware while diagnosing the failure.
- Send the minimal request and inspect its serialized headers and parts.
- If it succeeds, add optional fields back one at a time until the failure returns.
- If it fails, compare the final request with the provider’s example, focusing on boundary agreement, field names, file bytes, and endpoint-specific audio requirements.
This isolates whether the problem comes from multipart construction, an added field, or payload validation without assuming that all speech APIs accept the same request shape.
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