Error in client request message

HTTP response status codes

HTTP response status codes indicate whether a specific HTTP request has been successfully completed. Responses are grouped in five classes:

The status codes listed below are defined by RFC 9110.

Note: If you receive a response that is not in this list, it is a non-standard response, possibly custom to the server’s software.

Information responses

This interim response indicates that the client should continue the request or ignore the response if the request is already finished.

This code is sent in response to an Upgrade request header from the client and indicates the protocol the server is switching to.

This code indicates that the server has received and is processing the request, but no response is available yet.

This status code is primarily intended to be used with the Link header, letting the user agent start preloading resources while the server prepares a response.

Successful responses

The request succeeded. The result meaning of «success» depends on the HTTP method:

  • GET : The resource has been fetched and transmitted in the message body.
  • HEAD : The representation headers are included in the response without any message body.
  • PUT or POST : The resource describing the result of the action is transmitted in the message body.
  • TRACE : The message body contains the request message as received by the server.

201 Created

The request succeeded, and a new resource was created as a result. This is typically the response sent after POST requests, or some PUT requests.

The request has been received but not yet acted upon. It is noncommittal, since there is no way in HTTP to later send an asynchronous response indicating the outcome of the request. It is intended for cases where another process or server handles the request, or for batch processing.

This response code means the returned metadata is not exactly the same as is available from the origin server, but is collected from a local or a third-party copy. This is mostly used for mirrors or backups of another resource. Except for that specific case, the 200 OK response is preferred to this status.

There is no content to send for this request, but the headers may be useful. The user agent may update its cached headers for this resource with the new ones.

Tells the user agent to reset the document which sent this request.

This response code is used when the Range header is sent from the client to request only part of a resource.

Conveys information about multiple resources, for situations where multiple status codes might be appropriate.

Used inside a response element to avoid repeatedly enumerating the internal members of multiple bindings to the same collection.

The server has fulfilled a GET request for the resource, and the response is a representation of the result of one or more instance-manipulations applied to the current instance.

Redirection messages

The request has more than one possible response. The user agent or user should choose one of them. (There is no standardized way of choosing one of the responses, but HTML links to the possibilities are recommended so the user can pick.)

The URL of the requested resource has been changed permanently. The new URL is given in the response.

This response code means that the URI of requested resource has been changed temporarily. Further changes in the URI might be made in the future. Therefore, this same URI should be used by the client in future requests.

The server sent this response to direct the client to get the requested resource at another URI with a GET request.

This is used for caching purposes. It tells the client that the response has not been modified, so the client can continue to use the same cached version of the response.

Defined in a previous version of the HTTP specification to indicate that a requested response must be accessed by a proxy. It has been deprecated due to security concerns regarding in-band configuration of a proxy.

This response code is no longer used; it is just reserved. It was used in a previous version of the HTTP/1.1 specification.

The server sends this response to direct the client to get the requested resource at another URI with same method that was used in the prior request. This has the same semantics as the 302 Found HTTP response code, with the exception that the user agent must not change the HTTP method used: if a POST was used in the first request, a POST must be used in the second request.

This means that the resource is now permanently located at another URI, specified by the Location: HTTP Response header. This has the same semantics as the 301 Moved Permanently HTTP response code, with the exception that the user agent must not change the HTTP method used: if a POST was used in the first request, a POST must be used in the second request.

Client error responses

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).

Although the HTTP standard specifies «unauthorized», semantically this response means «unauthenticated». That is, the client must authenticate itself to get the requested response.

This response code is reserved for future use. The initial aim for creating this code was using it for digital payment systems, however this status code is used very rarely and no standard convention exists.

The client does not have access rights to the content; that is, it is unauthorized, so the server is refusing to give the requested resource. Unlike 401 Unauthorized , the client’s identity is known to the server.

The server cannot find the requested resource. In the browser, this means the URL is not recognized. In an API, this can also mean that the endpoint is valid but the resource itself does not exist. Servers may also send this response instead of 403 Forbidden to hide the existence of a resource from an unauthorized client. This response code is probably the most well known due to its frequent occurrence on the web.

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The request method is known by the server but is not supported by the target resource. For example, an API may not allow calling DELETE to remove a resource.

This response is sent when the web server, after performing server-driven content negotiation, doesn’t find any content that conforms to the criteria given by the user agent.

This is similar to 401 Unauthorized but authentication is needed to be done by a proxy.

This response is sent on an idle connection by some servers, even without any previous request by the client. It means that the server would like to shut down this unused connection. This response is used much more since some browsers, like Chrome, Firefox 27+, or IE9, use HTTP pre-connection mechanisms to speed up surfing. Also note that some servers merely shut down the connection without sending this message.

This response is sent when a request conflicts with the current state of the server.

This response is sent when the requested content has been permanently deleted from server, with no forwarding address. Clients are expected to remove their caches and links to the resource. The HTTP specification intends this status code to be used for «limited-time, promotional services». APIs should not feel compelled to indicate resources that have been deleted with this status code.

Server rejected the request because the Content-Length header field is not defined and the server requires it.

The client has indicated preconditions in its headers which the server does not meet.

Request entity is larger than limits defined by server. The server might close the connection or return an Retry-After header field.

The URI requested by the client is longer than the server is willing to interpret.

The media format of the requested data is not supported by the server, so the server is rejecting the request.

The range specified by the Range header field in the request cannot be fulfilled. It’s possible that the range is outside the size of the target URI’s data.

This response code means the expectation indicated by the Expect request header field cannot be met by the server.

The server refuses the attempt to brew coffee with a teapot.

The request was directed at a server that is not able to produce a response. This can be sent by a server that is not configured to produce responses for the combination of scheme and authority that are included in the request URI.

The request was well-formed but was unable to be followed due to semantic errors.

The resource that is being accessed is locked.

The request failed due to failure of a previous request.

Indicates that the server is unwilling to risk processing a request that might be replayed.

The server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades to a different protocol. The server sends an Upgrade header in a 426 response to indicate the required protocol(s).

The origin server requires the request to be conditional. This response is intended to prevent the ‘lost update’ problem, where a client GET s a resource’s state, modifies it and PUT s it back to the server, when meanwhile a third party has modified the state on the server, leading to a conflict.

The user has sent too many requests in a given amount of time («rate limiting»).

The server is unwilling to process the request because its header fields are too large. The request may be resubmitted after reducing the size of the request header fields.

The user agent requested a resource that cannot legally be provided, such as a web page censored by a government.

Server error responses

The server has encountered a situation it does not know how to handle.

The request method is not supported by the server and cannot be handled. The only methods that servers are required to support (and therefore that must not return this code) are GET and HEAD .

This error response means that the server, while working as a gateway to get a response needed to handle the request, got an invalid response.

The server is not ready to handle the request. Common causes are a server that is down for maintenance or that is overloaded. Note that together with this response, a user-friendly page explaining the problem should be sent. This response should be used for temporary conditions and the Retry-After HTTP header should, if possible, contain the estimated time before the recovery of the service. The webmaster must also take care about the caching-related headers that are sent along with this response, as these temporary condition responses should usually not be cached.

This error response is given when the server is acting as a gateway and cannot get a response in time.

The HTTP version used in the request is not supported by the server.

The server has an internal configuration error: the chosen variant resource is configured to engage in transparent content negotiation itself, and is therefore not a proper end point in the negotiation process.

The method could not be performed on the resource because the server is unable to store the representation needed to successfully complete the request.

The server detected an infinite loop while processing the request.

Further extensions to the request are required for the server to fulfill it.

Indicates that the client needs to authenticate to gain network access.


REST API Error Codes 101

Some Background

REST APIs use the Status-Line part of an HTTP response message to inform clients of their request’s overarching result.
RFC 2616 defines the Status-Line syntax as shown below:

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Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

A great amount of applications are using Restful APIs that are based on the HTTP protocol for connecting their clients. In all the calls, the server and the endpoint at the client both return a call status to the client which can be in the form of:

  • The success of API call.
  • Failure of API call.

In both the cases, it is necessary to let the client know so that they can proceed to the next step. In the case of a successful API call they can proceed to the next call or whatever their intent was in the first place but in the case of latter they will be forced to modify their call so that the failed call can be recovered.


To enable the best user experience for your customer, it is necessary on the part of the developers to make excellent error messages that can help their client to know what they want to do with the information they get. An excellent error message is precise and lets the user know about the nature of the error so that they can figure their way out of it.

A good error message also allows the developers to get their way out of the failed call.

Next step is to know what error messages to integrate into your framework so that the clients on the end point and the developers at the server are constantly made aware of the situation which they are in. in order to do so, the rule of thumb is to keep the error messages to a minimum and only incorporate those error messages which are helpful.

HTTP defines over 40 standard status codes that can be used to convey the results of a client’s request. The status codes are divided into the five categories presented here:

  • 1xx: Informational — Communicates transfer protocol-level information
  • 2xx: Success -Indicates that the client’s request was accepted successfully.
  • 3xx: Redirection — Indicates that the client must take some additional action in order to complete their request.
  • 4xx: Client Error — This category of error status codes points the finger at clients.
  • 5xx: Server Error — The server takes responsibility for these error status codes.

If you would ask me 5 years ago about HTTP Status codes I would guess that the talk is about web sites, status 404 meaning that some page was not found and etc. But today when someone asks me about HTTP Status codes, it is 99.9% refers to REST API web services development. I have lots of experience in both areas (Website development, REST API web services development) and it is sometimes hard to come to a conclusion about what and how use the errors in REST APIs.

There are some cases where this status code is always returned, even if there was an error that occurred. Some believe that returning status codes other than 200 is not good as the client did reach your REST API and got response.

Proper use of the status codes will help with your REST API management and REST API workflow management.

If for example the user asked for “account” and that account was not found there are 2 options to use for returning an error to the user:

Return 200 OK Status and in the body return a json containing explanation that the account was not found.

Return 404 not found status. The first solution opens up a question whether the user should work a bit harder to parse the json received and to see whether that json contains error or not.

There is also a third solution: Return 400 Error — Client Error. I will explain a bit later why this is my favorite solution.

It is understandable that for the user it is easier to check the status code of 404 without any parsing work to do.

I my opinion this solution is actually miss-use of the HTTP protocol

We did reach the REST API, we did got response from the REST API, what happens if the users misspells the URL of the REST API – he will get the 404 status but that is returned not by the REST API itself.

I think that these solutions should be interesting to explore and to see the benefits of one versus the other.

There is also one more solution that is basically my favorite – this one is a combination of the first two solutions, he is also gives better Restful API services automatic testing support because only several status codes are returned, I will try to explain about it.

Error handling Overview

Error responses should include a common HTTP status code, message for the developer, message for the end-user (when appropriate), internal error code (corresponding to some specific internally determined ID), links where developers can find more info. For example:

‘ < "status" : 400,
«developerMessage» : «Verbose, plain language description of the problem. Provide developers suggestions about how to solve their problems here», «userMessage» : «This is a message that can be passed along to end-users, if needed.», «errorCode» : «444444», «moreInfo» : «,», >’

How to think about errors in a pragmatic way with REST?
Apigee’s blog post that talks about this issue compares 3 top API providers.


No matter what happens on a Facebook request, you get back the 200 status code — everything is OK. Many error messages also push down into the HTTP response. Here they also throw an #803 error but with no information about what #803 is or how to react to it.


Twilio does a great job aligning errors with HTTP status codes. Like Facebook, they provide a more granular error message but with a link that takes you to the documentation. Community commenting and discussion on the documentation helps to build a body of information and adds context for developers experiencing these errors.

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Provides error codes but with no additional value in the payload.

Error Handling — Best Practises

First of all: Use HTTP status codes! but don’t overuse them.
Use HTTP status codes and try to map them cleanly to relevant standard-based codes.
There are over 70 HTTP status codes. However, most developers don’t have all 70 memorized. So if you choose status codes that are not very common you will force application developers away from building their apps and over to wikipedia to figure out what you’re trying to tell them.

Therefore, most API providers use a small subset.
For example, the Google GData API uses only 10 status codes, Netflix uses 9, and Digg, only 8.

How many status codes should you use for your API?

When you boil it down, there are really only 3 outcomes in the interaction between an app and an API:

  • Everything worked
  • The application did something wrong
  • The API did something wrong

Start by using the following 3 codes. If you need more, add them. But you shouldn’t go beyond 8.

  • 200 — OK
  • 400 — Bad Request
  • 500 — Internal Server Error

Please keep in mind the following rules when using these status codes:

200 (OK) must not be used to communicate errors in the response body

Always make proper use of the HTTP response status codes as specified by the rules in this section. In particular, a REST API must not be compromised in an effort to accommodate less sophisticated HTTP clients.

400 (Bad Request) may be used to indicate nonspecific failure

400 is the generic client-side error status, used when no other 4xx error code is appropriate. For errors in the 4xx category, the response body may contain a document describing the client’s error (unless the request method was HEAD).

500 (Internal Server Error) should be used to indicate API malfunction 500 is the generic REST API error response.

Most web frameworks automatically respond with this response status code whenever they execute some request handler code that raises an exception. A 500 error is never the client’s fault and therefore it is reasonable for the client to retry the exact same request that triggered this response, and hope to get a different response.

If you’re not comfortable reducing all your error conditions to these 3, try adding some more but do not go beyond 8:

  • 401 — Unauthorized
  • 403 — Forbidden
  • 404 — Not Found

Please keep in mind the following rules when using these status codes:

401 (Unauthorized) must be used when there is a problem with the client’s credentials

A 401 error response indicates that the client tried to operate on a protected resource without providing the proper authorization. It may have provided the wrong credentials or none at all.

403 (Forbidden) should be used to forbid access regardless of authorization state

A 403 error response indicates that the client’s request is formed correctly, but the REST API refuses to honor it. A 403 response is not a case of insufficient client credentials; that would be 401 (“Unauthorized”). REST APIs use 403 to enforce application-level permissions. For example, a client may be authorized to interact with some, but not all of a REST API’s resources. If the client attempts a resource interaction that is outside of its permitted scope, the REST API should respond with 403.

404 (Not Found) must be used when a client’s URI cannot be mapped to a resource

The 404 error status code indicates that the REST API can’t map the client’s URI to a resource.



I believe that the best solution to handle errors in a REST API web services is the third option, in short:
Use three simple, common response codes indicating (1) success, (2) failure due to client-side problem, (3) failure due to server-side problem:

  • 200 — OK
  • 400 — Bad Request (Client Error) — A json with error \ more details should return to the client.
  • 401 — Unauthorized
  • 500 — Internal Server Error — A json with an error should return to the client only when there is no security risk by doing that.

I think that this solution can also ease the client to handle only these 4 status codes and when getting either 400 or 500 code he should take the response message and parse it in order to see what is the problem exactly and on the other hand the REST API service is simple enough.

The decision of choosing which error messages to incorporate and which to leave is based on sheer insight and intuition. For example: if an app and API only has three outcomes which are; everything worked, the application did not work properly and API did not respond properly then you are only concerned with three error codes. By putting in unnecessary codes, you will only distract the users and force them to consult Google, Wikipedia and other websites.

Most important thing in the case of an error code is that it should be descriptive and it should offer two outputs:

  • A plain descriptive sentence explaining the situation in the most precise manner.
  • An ‘if-then’ situation where the user knows what to do with the error message once it is returned in an API call.

The error message returned in the result of the API call should be very descriptive and verbal. A code is preferred by the client who is well versed in the programming and web language but in the case of most clients they find it hard to get the code.

As I stated before, 404 is a bit problematic status when talking about Restful APIs. Does this status means that the resource was not found? or that there is not mapping to the requested resource? Everyone can decide what to use and where 🙂


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