AWS REST API Gateway Integration Response Mapping - Lambda Error Regex issue - regex

Question

I would like to return a response code of 400 to the client after throwing an error in my Lambda function. (via REST API - GET)
Here is the response from the Lambda function:
{
"errorMessage": "{\"status\": 400, \"result\": \"Invalid Input\", \"errorDescription\": \"Incorrect data format, should be YYYY-MM-DD. Refer to our documentation for further instructions.\", \"documentation\": \"link\"}",
"errorType": "ValueError",
"stackTrace": [
" File \"/var/task/lambda_function.py\", line 69, in lambda_handler\n raise ValueError(date_error_json)\n"
]
}
In the settings of the Integration Response of the API I set up the Lambda Error Regex as .*"status":400.*
For the mapping template I have:
#set ($errorMessageObj = $util.parseJson($input.path('$.errorMessage')))
{
"status" : "$errorMessageObj.status"
}
When calling the API I mostly receive this result:
{"errorMessage": "{\"status\": 400, \"result\": \"Invalid Input\", \"errorDescription\": \"Incorrect data format, should be YYYY-MM-DD. Refer to our documentation for further instructions.\", \"documentation\": \"link\"}", "errorType": "ValueError", "stackTrace": [" File \"/var/task/lambda_function.py\", line 69, in lambda_handler\n raise ValueError(date_error_json)\n"]}
And the status of the HTTP request tracked in Chromes Developer Tools shows as 200.
My assumption is that the Regex is incorrect since it should at least return a 400 HTTP response, even if my mapping template was incorrect.
However, surprisingly a few times I was able to get the desired result of a 400 HTTP response, as well as the desired format displayed as:
{
"status" : "400"
}
The way I got to this result was the following: When I changed the Regex around to .*400.* (which did not help) and then back to .*"status":400.*. However, as soon as I send another request to the API it was back to the situation before. (No other changes were made, only a second API request)
And this is actually repeatable, every time I switch back and forth between those Regex values I get my desired result. For 1 API request and then it switches back.
I know it sounds weird. Here is a quick screenshare of it: https://www.loom.com/share/365a7c48119944658e157c5e0a5178e8
I would like to know if there is an issue with my code or if this might be a bug on Amazon's end.

Answer 1

For anyone coming across the same problem in the future. I have found out the following 2 things:
After trying again to set .*400.* a little later it worked fine. Of course, this may not be a good idea, because if you return any results, that contain 400 anywhere in their response, they could incorrectly trigger a 400 HTTP response.
The workaround I found was to use .*Invalid Input.*. Since it is not expected that regular results returned from the API would contain this phrase, this should work reliably.