Flask pretty-prints response generated by the flask.json.jsonify function. Avoiding this on a per request basis doesn't seem to be intended: There is a configuration variable for the whole application: JSONIFY_PRETTYPRINT_REGULAR. But setting that to False minifies every JSON responses. And, in general, I enjoyed the pretty printed output. So implementing X-PrettyPrint - which seems to be a quasi-standard - also sounded like a the wrong way because it means replacing the call to jsonify by a custom implementation. This short story has a happy end though: Flask does not pretty-print the response if it receives an AJAX request. So one can just send the appropriate header:

curl -H 'X-Requested-With: XMLHttpRequest' "https://exmaple.com/api/endpoint.json";