Assign Message Policy does not work on Error responses - apigee

Question

In Apigee, the Assign Message Policy only seems to work on successful responses. If the server responds with an error code, say, 403 Forbidden, the policy does not change the response. Is there any other way to modify error responses in Apigee.
This is my policy.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="false" enabled="true" name="Assign-Message-1">
<DisplayName>Assign Message-1</DisplayName>
<Properties/>
<Remove>
<Headers>
<Header name="Server"/>
</Headers>
</Remove>
<AssignTo createNew="false" transport="http" type="request"/>
</AssignMessage>

Answer 1

You can modify error responses in Apigee using FaultRules. You can take a look at Apigee's documentation about Fault handling .
The reason your AssignMessage policy is not executed is, that the error happens before your policy is being executed, thus being skipped.
You can either move your AssignMessage policy to be executed earlier in the flow or catch the error using fault handling logic and throw the error when you have executed all the desired logic.

Answer 2

In case of different response message of Success and Error,
I usually create 2 policy of Assign Message, one for Success and other for Error response.
Example of Success:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="false" enabled="true" name="AM-ReturnSuccessResponse">
<DisplayName>AM-ReturnSuccessResponse</DisplayName>
<IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
<AssignTo createNew="false" transport="http" type="response"/>
<Set>
<Payload contentType="application/json" variablePrefix="#" variableSuffix="#">
#resp.responseMessage#
</Payload>
<StatusCode>{resp.httpStatusCode}</StatusCode>
<ReasonPhrase>{resp.reasonPhrase}</ReasonPhrase>
</Set>
</AssignMessage>
Example of Error:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <AssignMessage async="false" continueOnError="false" enabled="true" name="AM-ReturnErrorResponse">
<DisplayName>AM-ReturnErrorResponse"</DisplayName>
<IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
<AssignTo createNew="false" transport="http" type="response"/>
<Properties/>
<Set>
<Payload contentType="application/json" variablePrefix="#" variableSuffix="#">
{
"error" : "#resp.error#",
"timeStamp" : "#resp.responseDateTime#"
}
</Payload>
<StatusCode>{resp.httpStatusCode}</StatusCode>
<ReasonPhrase>{resp.reasonPhrase}</ReasonPhrase>
</Set> </AssignMessage>
And then in your flow you have to set condition for routing the policy like this
<Flow name="test">
<Description/>
<Request/>
<Response>
<Step>
<Name>AM-ReturnSuccessResponse</Name>
<Condition>resp.httpStatusCode == 200</Condition>
</Step>
<Step>
<Name>AM-ReturnErrorResponse</Name>
<Condition>resp.httpStatusCode != "200"</Condition>
</Step>
</Response>
</Flow>
Remark: Because normally http status code for success is 200, So I have set the condition like code above. But this is for example, you can design the condition up to your scenario.