"Send empty value" checkbox sends empty string instead of empty array for array type parameters in multipart/form-data

Q&A (please complete the following information)

• OS: macOS 26.3 (arm64)
• Browser: Brave 1.88.136 (Chromium 146.0.7680.164)
• Method of installation: Used via FastAPI's built-in /docs endpoint (loads swagger-ui from CDN)
• Swagger-UI version: 5.x (latest, loaded from https://cdn.jsdelivr.net/npm/swagger-ui-dist@5/)
• Swagger/OpenAPI version: OpenAPI 3.1.0

Content & configuration

Example Swagger/OpenAPI definition:

paths:
  /api/test/import-file:
    post:
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                files:
                  type: array
                  items:
                    type: string

Swagger-UI configuration options:

// Default FastAPI /docs configuration (no custom options)

Describe the bug you're encountering

When the "Send empty value" checkbox is checked for an array<string> parameter in a multipart/form-data request, Swagger UI sends an empty string (-F 'files=') instead of either omitting the field entirely or sending an empty array.

This causes the server to respond with 422 Unprocessable Entity, because an empty string is not a valid value for the expected array type.

The generated curl command:

curl -X 'POST' \
  'http://localhost:8088/api/test/import-file' \
  -H 'accept: application/json' \
  -H 'Content-Type: multipart/form-data' \
  -F 'files='

Server response:

{
  "detail": [
    {
      "loc": [
        "body",
        "files"
      ]
    }
  ]
}

To reproduce...

Steps to reproduce the behavior:

1. Define a POST endpoint that accepts multipart/form-data with an array<string> parameter (e.g., files).
2. Open Swagger UI and navigate to the endpoint.
3. Do not add any string items to the files field.
4. Check the "Send empty value" checkbox for the files parameter.
5. Click Execute.
6. Observe the generated curl: -F 'files=' sends an empty string.
7. Server responds with 422 Unprocessable Entity.

Expected behavior

When "Send empty value" is checked for an array type parameter, Swagger UI should either:

• Omit the field entirely from the request body, or
• Send it as an empty array (no value entries)

It should not send an empty string, as that is a type mismatch (string vs array).

Screenshots

Start from the Swagger UI /docs flow and reproduce the multipart/form-data request using the provided OpenAPI schema and the "Send empty value" checkbox. Trace how the files array is serialized into the generated curl command, then verify that the executed request no longer sends an empty string and that the documented reproduction avoids the 422 response.