What HTTP status response code should I use if the request is missing a required parameter

Question

I am thinking 412  Precondition Failed  but there may be a better standard

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I m not sure there s a set standard  but I would have used 400 Bad Request  which the latest HTTP spec  from 2014  documents as follows      6 5 1   400 Bad Request      The 400  Bad Request  status code indicates that the server cannot or      will not process the request due to something that is perceived to be      a client error  e g   malformed request syntax  invalid request      message framing  or deceptive request routing

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The WCF API in  NET handles missing parameters by returning an HTTP 404  Endpoint Not Found  error   when using the webHttpBinding    The 404 Not Found can make sense if you consider your web service method name together with its parameter signature  That is  if you expose a web service method LoginUser string  string  and you request LoginUser string   the latter is not found   Basically this would mean that the web service method you are calling  together with the parameter signature you specified  cannot be found       10 4 5 404 Not Found      The server has not found anything matching the Request-URI  No   indication is given of whether the condition is temporary or   permanent    The 400 Bad Request  as Gert suggested  remains a valid response code  but I think it is normally used to indicate lower-level problems  It could easily be interpreted as a malformed HTTP request  maybe missing or invalid HTTP headers  or similar      10 4 1 400 Bad Request      The request could not be understood by the server due to malformed   syntax  The client SHOULD NOT repeat the request without   modifications

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I often use a 403 Forbidden error  The reasoning is that the request was understood  but I m not going to do as asked  because things are wrong   The response entity explains what is wrong  so if the response is an HTML page  the error messages are in the page  If it s a JSON or XML response  the error information is in there   From rfc2616      10 4 4 403 Forbidden      The server understood the request  but is refusing to fulfill it    Authorization will not help and the request SHOULD NOT be repeated    If the request method was not HEAD and the server wishes to make   public why the request has not been fulfilled  it SHOULD describe the    reason for the refusal in the entity   If the server does not wish to    make this information available to the client  the status code 404    Not Found  can be used instead

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In one of our API project we decide to set a 409 Status to some request  when we can t full fill it at 100  because of missing parameter        HTTP Status Code  409 Conflict  was for us a good try because it s definition   require to include enough information for the user to recognize the   source of the conflict    Reference  w3 org Protocols   So among other response like 400 or 404 we chose 409 to enforce the need for looking over some notes in the request helpful to set up a new and right request   Any way our case it was particular because we need to send out some data eve if the request was not completely correct  and we need to enforce the client to look at the message and understand what was wrong in the request   In general if we have only some missing parameter we go for a 400 and an array of missing parameter  But when we need to send some more information  like a particular case message and we want to be more sure the client will take care of it we send a 409

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I Usually go for 422  Unprocessable entity  if something in the required parameters didn t match what the API endpoint required  like a too short password  but for a missing parameter i would go for 406  Unacceptable

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It could be argued that a 404 Not Found should be used since the resource specified could not be found

User · Answer

You can send a 400 Bad Request code  It s one of the more general-purpose 4xx status codes  so you can use it to mean what you intend  the client is sending a request that s missing information parameters that your application requires in order to process it correctly

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For those interested  Spring MVC  3 x at least  returns a 400 in this case  which seems wrong to me   I tested several Google URLs  accounts google com  and removed required parameters  and they generally return a 404 in this case   I would copy Google

User · Answer

Status 422 seems most appropiate based on the spec      The 422  Unprocessable Entity  status code means the server       understands the content type of the request entity  hence a       415 Unsupported Media Type  status code is inappropriate   and the       syntax of the request entity is correct  thus a 400  Bad Request        status code is inappropriate  but was unable to process the contained       instructions   For example  this error condition may occur if an XML       request body contains well-formed  i e   syntactically correct   but       semantically erroneous  XML instructions    They state that malformed xml is an example of bad syntax  calling for a 400   A malformed query string seems analogous to this  so 400 doesn t seem appropriate for a well-formed query-string which is missing a param   UPDATE  DavidV correctly points out that this spec is for WebDAV  not core HTTP  But some popular non-WebDAV APIs are using 422 anyway  for lack of a better status code  see this

[http] What HTTP status response code should I use if the request is missing a required parameter?

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