Bad state: Cannot set the body fields of a Request with content-type "application/json"

You need to wrap the body in jsonEncode.

import 'package:http/http.dart' as http;import 'dart:convert';Map<String,String> headers = {'Content-Type':'application/json','authorization':'Basic c3R1ZHlkb3RlOnN0dWR5ZG90ZTEyMw=='};final msg = jsonEncode({"grant_type":"password","username":"******","password":"*****","scope":"offline_access"});var response = await post(Urls.getToken,               headers: headers,               body: msg,            );

Use jsonEncode to wrap your body object.

import 'package:http/http.dart' as http;import 'dart:convert';var headers = {    'Content-Type':'application/json',    'authorization':'Basic c3R1ZHlkb3RlOnN0dWR5ZG90ZTEyMw=='};final body = {    'username':'foo',    'password':'pass123'}var response = await post(    Urls.getToken,    headers: headers,    body: jsonEncode(body), // use jsonEncode());

Why jsonEncode?

body: It can be a [String], a [List] or a [Map<String, String>].It's encoded using encoding and used as the body of the request. The content-type of the request will default to "text/plain".

but because you set the content-type to json you must have to pass a JSON as the body.

but you are passing Map<String, String> as the body, which obviously throws an error.

So to solve this issue you have to change or encode your Map<String, String> to json (JSON string) via using jsonEncode.

had similar issue with http library...changed for dio 2.1.0 and the problem with headers is gone.

jsonEncode(body) didn't do the trick, because the docs say:

Sends an HTTP POST request with the given headers and body to the givenURL, which can be a [Uri] or a [String].

[body] sets the body of the request. It can be a [String], a [List]or a [Map<String, String>]. If it's a String, it's encoded using[encoding] and used as the body of the request. The content-type of therequest will default to "text/plain".

If [body] is a List, it's used as a list of bytes for the body of therequest.

If [body] is a Map, it's encoded as form fields using [encoding]. Thecontent-type of the request will be set to"application/x-www-form-urlencoded"; this cannot be overridden.[encoding] defaults to [utf8].

For more fine-grained control over the request, use [send] instead.

Future<Response> post(url, {Map<String, String> headers, body, Encoding encoding});