All responses are, by default, returned in a REST-friendly XML format unless a format parameter is sent and its value is not rest.

Successful Response

<rsp stat="ok">
  <api_key>9bb013121e8f3650f350b622a1735442</api_key>
  <foo>bar</foo>
  <method>rtm.test.echo</method>
</rsp>

Failure Response

<rsp stat="fail">
  <err code="112" msg="Method "rtm.test.ech" not found"/>
</rsp>