Mitmproxy tampering with GET and POST request/ response in one script

The script you posted and Python's JSON module should get you pretty far:

def response(context, flow):
    if flow.request.url == "...": # optionally filter based on some criteria...
        with decoded(flow.response):  # automatically decode gzipped responses.
            data = json.loads(flow.response.content)
            data["foo"] = "bar"
            flow.response.content = json.dumps(data)