[Bug]: String in json format as input for [FromForm] is not recognized in UI

[KillerBoogie]

Describe the bug

For an endpoint that accepts a file, the title, and a description in multiple languages I use this method

public async Task<ActionResult<GuidIdResponse>> UploadImage([FromForm] string title, [FromForm] string? description, IFormFile file)

The title is entered in JSON format like:

{de":"Bildschirmfoto","en":"screen shot"}

When I enter the above string in the title field of the generated Swashbuckle UI I'm getting error 400:

{
  "type": "https://tools.ietf.org/html/rfc9110#section-15.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "errors": {
    "title": [
      "The title field is required."
    ]
  },
  "traceId": "00-259db306db8ce9b0015192084dfc72f2-cd711e4da768cce5-00"
}

The code of my endpoint method is not hit. The error is produced by the framework. When I send the same request with Postman it works fine!

This only happens if the JSON string is well formed and not escaped.

This is sent correctly:
{\"de\":\"Bildschirmfoto\",\"en\":\"screen shot\"}

Those also pass through:

"de":"Bildschirmfoto","en":"screen shot"}
{"de":"Bildschirmfoto",x"en":"screen shot"}
{"de":"Bildschirmfoto",,"en":"screen shot"}

As soon as the JSON is correctly formed, but not escaped the string is not passed and treated as not entered.

Since it works fine with Postman t must be a bug in Swashbuckle.,

Expected behavior

Pass the entered string into the variable.

Actual behavior

A string in valid JSON format is treated as not provided:

{
"type": "https://tools.ietf.org/html/rfc9110#section-15.5.1",
"title": "One or more validation errors occurred.",
"status": 400,
"errors": {
"title": [
"The title field is required."
]
},
"traceId": "00-259db306db8ce9b0015192084dfc72f2-cd711e4da768cce5-00"
}

Steps to reproduce

No response

Exception(s) (if any)

No response

Swashbuckle.AspNetCore version

6.7.0

.NET Version

8.0.303

Anything else?

[martincostello]

Can you share your OpenAPI document and/or a runnable code sample in the form of a GitHub repo?

This could be either an issue with Swashbuckle (the OpenAPI document being incorrectly generated) or SwaggerUI (incorrectly interpreting the OpenAPI document).

[martincostello]

Sounds very similar to #2930, which is an issue in SwaggerUI.

[KillerBoogie]

This is exactly the same issue as in #2930. Do you still need an OpenAPI document from me?

[martincostello]

If it's because of SwaggerUI, then no I don't think so.

If there isn't already an issue for this, I suggest opening one over in their repo. We can then ingest the fix when it's available.