[$http:badreq] The request is not properly formatted. $http transformRequest default in AngularJS 1.6

Direct Answers

1. Conditions that trigger the error: A 2xx (success) HTTP response whose body cannot be parsed as JSON — empty body, HTML error page, plain text, XML, or any non-JSON content. The status must be in the 200–299 range; non-2xx responses with non-JSON bodies reject the promise without this specific message.
2. Configurability of the default transformRequest: Yes. Use the built-in $httpParamSerializerJQLike service via $httpProvider.defaults.transformRequest (global) or per-request config. No custom serialization function is required.
3. Error origin: transformResponse parsing failure, not request formatting validation. The message is thrown inside defaultTransformResponse when JSON.parse fails on a 2xx response.

Why the Error Is Misleading

The string [$http:badreq] The request is not properly formatted. suggests a request problem, but AngularJS 1.6 source (src/ng/http.js) shows it is raised in the default transformResponse function:

function defaultTransformResponse(data, headersGetter) {  if (isString(data)) {    try {      data = JSON.parse(data);    } catch (e) {      throw minErr('$http')('badreq', 'The request is not properly formatted.');    }  }  return data;}

It catches JSON.parse failures only for 2xx responses. A backend that returns a 400/500 HTML page with a 2xx status, or an empty 200 body, triggers this exact error.

Restoring Form-Encoded Request Serialization

AngularJS 1.6 changed the default transformRequest to JSON-stringify plain objects. The documented opt-out is the built-in $httpParamSerializerJQLike service, which reproduces the pre-1.6 application/x-www-form-urlencoded behavior.

Global configuration (affects all $http calls)

angular.module('app').config(['$httpProvider', function($httpProvider) {  $httpProvider.defaults.transformRequest = [$httpParamSerializerJQLike];}]);

Per-request override (safer for mixed backends)

$http({  method: 'POST',  url: '/api/endpoint',  data: { key: 'value' },  transformRequest: $httpParamSerializerJQLike,  headers: { 'Content-Type': 'application/x-www-form-urlencoded' }});

Note: setting the Content-Type header explicitly is recommended when overriding transformRequest so the backend knows how to parse the payload.

Verification Checklist

• Open the Network tab: confirm the failing response has a 2xx status code and a non-JSON body (HTML, empty, text, etc.).
• Temporarily add responseType: 'text' to the request config — this bypasses JSON parsing and lets you inspect the raw response body.
• If the backend is under your control, ensure it returns valid JSON (even {}) on success, or returns a proper 4xx/5xx status for errors.

One Diagnostic Detail

What is the exact HTTP status code and Content-Type header of the response that produces this error? (This determines whether the fix is backend-side, frontend transformRequest configuration, or adding responseType: 'text' for a known non-JSON endpoint.)