After thinking about it again (I only had the diff in mind in my previous comment, sorry), do we want the interpolated --title parameter value in the JSON output, or the individual stream metadata properties?

The --title parameter is just a utility for formatting the window title of the player launched by Streamlink, which is not useful for JSON data, where individual metadata properties make much more sense.

After thinking about it again (I only had the diff in mind in my previous comment, sorry), do we want the interpolated --title parameter value in the JSON output, or the individual stream metadata properties?

both

The --title parameter is just a utility for formatting the window title of the player launched by Streamlink, which is not useful for JSON data, where individual metadata properties make much more sense.

i have a script that use streamlink as mpv backend instead of youtube-dl for some specific sites like twitch... $
just by typing "mpv %url%"
it run 'streamlink -j url quality' then extract streamurl from the response and passe it to mpv
this is needed so they are no need to call streamlink two times to get the title

What I am saying is that the JSON data should not include the custom --title stuff but rather the individual values of the plugin metadata, so one who uses the JSON response can read metadata separately, which is impossible with a single string based on the user's --title value. In your case, these single metadata properties can be used and combined for formatting the player window title.

{
  "type": "hls",
  "url": "https://foo",
  "master": "https://bar",
  "headers": {},
  "metadata": {
    "author": ...,
    "category": ...,
    "title": ...,
  }
}

i am not sure if this is worth adding right now ;
if user wanted only a specific metadata like the author name he can alwayse call streamlink -j --title "{author}" %url% best
if he wanted multiple metadata properties, he probably gonna combine them; so instead of just calling streamlink with the formate he wanted, he gonna need to combine them manually

also this way respect the formate in streamlink config file if i called streamlink -j %url% best