Swagger TypeError: Failed to execute 'fetch' on 'Window': Request with GET/HEAD method cannot have body

The error message actually says what the problem is. You post data with curl using the -d option while trying to use GET.

If you use the -d option curl will do POST.
If you use -X GET option curl will do GET.

The HTTP GET method is for requesting a representation of the specified resource. Requests using GET should only retrieve data and hence cannot have body.

More info on GET vs POST

I ran into this issue. Here is how I resolved it:

I had a method like this:

[HttpGet]
public IEnumerable<MyObject> Get(MyObject dto)
{
      ...
}

and I was getting the error. I believe swagger UI is interpreting the Get parameters as FromBody, so it uses the curl -d flag. I added the [FromQuery] decorator and the problem was resolved:

[HttpGet]
public IEnumerable<MyObject> Get([FromQuery]MyObject dto)
{
      ...
}

FYI this also changes the UI experience for that method. instead of supplying json, you will have a form field for each property of the parameter object.

I had same problem with my .net core 2.0 solution and GET method that takes element id as header key or search for it by parameters in body. That is not the best way to implement but it's kind of special case.

As mentioned in this discussion. The HTTP spec does not forbid using body on a GET, but swagger is not implementing it like this. Even if there are APIs that work fine with body in GET Requests.

What more, the swagger frontend adds this body object into request even if it is null/undefined/empty object. It is -d "body_content_here" parameter. So in my case when i only search by id and body is empty, it still sends empty object (-d "{}") and throws mentioned error.

Possible solutions:

• Start using postman app for this request - It will work fine. Tested.

• Consider moving more advanced GET request (like search with criteria) to the independent POST Method

• Use swagger generated CURL request request without -d parameter