On JSON requests, the JSON response should have Content-Type: application/json and the body of the response should be a JSON object.

flask/wrappers.py

@@ -10,7 +10,7 @@
 """
 
 from werkzeug.wrappers import Request as RequestBase, Response as ResponseBase
-from werkzeug.exceptions import BadRequest
+from werkzeug.exceptions import BadRequest, HTTPException
 from werkzeug.utils import cached_property
 
 from .debughelpers import attach_enctype_error_multidict
@@ -18,6 +18,43 @@ from .helpers import json, _assert_have_json
 from .globals import _request_ctx_stack
 
 
+class JSONHTTPException(HTTPException):
+    """A base class for HTTP exceptions with ``Content-Type:
+    application/json``.
+
+    The ``description`` attribute of this class must set to a string (*not* an
+    HTML string) which describes the error.
+
+    """
+
+    def get_body(self, environ):
+        """Overrides :meth:`werkzeug.exceptions.HTTPException.get_body` to
+        return the description of this error in JSON format instead of HTML.
+
+        """
+        return json.dumps(dict(description=self.get_description(environ)))
+
+    def get_headers(self, environ):
+        """Returns a list of headers including ``Content-Type:
+        application/json``.
+
+        """
+        return [('Content-Type', 'application/json')]
+
+
+class JSONBadRequest(JSONHTTPException, BadRequest):
+    """Represents an HTTP ``400 Bad Request`` error whose body contains an
+    error message in JSON format instead of HTML format (as in the superclass).
+
+    """
+
+    #: The description of the error which occurred as a string.
+    description = (
+        'The browser (or proxy) sent a request that this server could not '
+        'understand.'
+    )
+
+
 class Request(RequestBase):
     """The request object used by default in Flask.  Remembers the
     matched endpoint and view arguments.
@@ -108,12 +145,23 @@ class Request(RequestBase):
 
     def on_json_loading_failed(self, e):
         """Called if decoding of the JSON data failed.  The return value of
-        this method is used by :attr:`json` when an error ocurred.  The
-        default implementation raises a :class:`~werkzeug.exceptions.BadRequest`.
+        this method is used by :attr:`json` when an error ocurred.  The default
+        implementation raises a :class:`JSONBadRequest`, which is a subclass of
+        :class:`~werkzeug.exceptions.BadRequest` which sets the
+        ``Content-Type`` to ``application/json`` and provides a JSON-formatted
+        error description::
+
+            {"description": "The browser (or proxy) sent a request that \
+                             this server could not understand."}
+
+        .. versionchanged:: 0.9
+
+           Return a :class:`JSONBadRequest` instead of a
+           :class:`~werkzeug.exceptions.BadRequest` by default.
 
         .. versionadded:: 0.8
         """
-        raise BadRequest()
+        raise JSONBadRequest()
 
     def _load_form_data(self):
         RequestBase._load_form_data(self)