Trying to compile my schema using this command:
oapi-codegen -package generated openapi/api.yaml > api.gen.go
I'm on version v1.13.0
openapi: 3.0.0

This error is given:

error generating code: error formatting Go code
: generated.go:263:16: '_' must separate successive digits (and 1 more errors)

I've tried to create a minimal example where this issue occurs:

This is a simple schema with cats, dogs and rats.
api.yaml.txt

This is what's written to stderr:
api.gen.go.txt

You can see the error at the top and bottom of the go file.
At line 263 and 320, invalid Go is generated.

This works up until version 1.12.1, but after that, it is broken.