From swagger docs:
https://swagger.io/docs/specification/describing-responses/
You can use the default response to describe these errors collectively, not individually. “Default” means this response is used for all HTTP codes that are not covered individually for this operation.
responses:
'200':
description: Success
content:
application/json:
schema:
$ref: '#/components/schemas/User'
# Definition of all error statuses
default:
description: Unexpected error
content:
application/json:
schema:
$ref: '#/components/schemas/Error'
But ApiResponse only allows number

Add option for 'default' or expose ApiDefaultResponse
Added in the next version (to install run npm i @nestjs/swagger@next). Note: remember to update @nestjs/common, @nestjs/core and typescript as well to ensure that you're using the latest versions.
Steps to migrate: https://github.com/nestjs/swagger/pull/355#issuecomment-547925879
4.0.0 has been published
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Most helpful comment
Added in the
nextversion (to install runnpm i @nestjs/swagger@next). Note: remember to update@nestjs/common,@nestjs/coreandtypescriptas well to ensure that you're using the latest versions.Steps to migrate: https://github.com/nestjs/swagger/pull/355#issuecomment-547925879