I already had elm/json 1.0.0 in the elm.json (and elm/core 1.0.0):
“elm/core”: “1.0.0 <= v < 2.0.0”,
“elm/json”: “1.0.0 <= v < 2.0.0”
I think the bug is that when elm-test generates its application style elm.json it fails to run a version solver on all the dependencies - does it even do this? or it just takes lower bounds direct or something? The solution was simply to manually edit core to 1.0.2 and json to 1.1.2 in my elm.json.
I’ll paste my failing elm.json into a github issue against elm-test.