apex REST WEB SERVICE : Unexpected parameter encountered during deserialization: startdate at

Question

I have a few json's that I'm posting with the access-token as header, when I send two parameters I get a status message from my rest api successfully, but when I send more than 2 parameters I get the following error:

{
  "message" : "Unexpected parameter encountered during deserialization: <keyName> at [line:3, column:16]",
  "errorCode" : "JSON_PARSER_ERROR"
}

I enter the same json I give as input in the workbench and there seems to be no error, I get a custom status message atleast even if there are no values retrived(given below):

{
"statusReason":"No records were found with submitted criteria.",
"Accountid":"XXXXXXXXXXXXXXXX",
"SOACount":"0",
"SOAs":null,
"status":"Failure"
}

any one who knows the answer to this bizarre problem...let me know why. WHY ONLY 2?????? my api works, else it would not retrieve the values on work bench! I send the same JSON in both cases, I assure you that! is there some permission or something? then it shouldn't run for any case right? why run where there are 2 parameters??

INPUT JSON:

{
 "AccountId":"UxooAgHab001gUm",
 "Key":"okay",
 "startdate":"2016-12-17"
}

REST API CODE:

@HttpPost
Global static APIService.Response OutREST(String AccountId, String Key, String startdate)
{
    APIService.Response response = APIHandler.getData('REST', accountId, Key, startdate, null , null, false);
    return response;
}

Answer 1

Notice that you have Invalid JSON for input ,The correct JSON can be validated at JSONlint.com

Here is the corrected JSON try this

{
"AccountId": "UxooAgHab001gUm",
"Key": "okay",
"startdate": "2016-12-17"
}

Carefully notice the comma you have after third parameter .