Calls to Flags API do not respond with server flags when using a custom user agent

[s6nedwards]

Bug Description

Custom user agents do not return server flags unless that user agent is mocking a known such as curl. This is annoying as it forces languages that do not have an official SDK to pretend to be a different library/client.

How to reproduce

Using curl make a request without a setting a custom user agent. Curl sets its own User Agent and thats recognized by Posthog.

curl https://us.i.posthog.com/flags/?v=2 \
    -X POST \
    -H 'Content-Type: application/json' \
    -d '{
    "api_key": "[API_KEY]",
    "distinct_id": "[user distinct id]"
}'

This will return server flags.
Now set a custom user agent

curl https://us.i.posthog.com/flags/?v=2 \
    -X POST \
    -H 'Content-Type: application/json' \
    -H 'User-Agent: this-is-not-accepted' \
    -d '{
    "api_key": "[API_KEY]",
    "distinct_id": "[user distinct id]"
}'

This will only return flags marked for all environments.

Additional context

Debug info

[abalinda]

Not a bug but yep, you are spot on. The API defaults to 'Client Mode' (hiding server-only flags) for any User Agent it doesn't explicitly recognize as a server-side library (like curl, posthog-python, etc.). This is a security default to prevent accidental leaks to unknown clients.

PostHog Runtime Detection Docs