For Mastodon Caddy Setting file. (Based on official Nginx configuration example) [Last Update: Mar 07, 2021]

Caddyfile.caddy1

https://yourdomain.example.com/ {
    log / /var/log/caddy/mastodon.log "{combined}" {
        rotate_age 90
    }
    root /home/mastodon/live/public
    gzip

    header / {
        Strict-Transport-Security "max-age=31536000"
    }

    header /sw.js Cache-Control "public, max-age=0"
    header /emoji Cache-Control "public, max-age=31536000, immutable"
    header /packs Cache-Control "public, max-age=31536000, immutable"
    header /system/accounts/avatars Cache-Control "public, max-age=31536000, immutable"
    header /system/media_attachments/files Cache-Control "public, max-age=31536000, immutable"

    errors {
        * 500.html
    }

    rewrite {
        if {path} is /
        to /proxy{path}
    }

    rewrite {
        if {path} not_has /api/v1/streaming
        to {path} /proxy{path}
    }

    proxy /proxy localhost:3000 {
        without /proxy

        transparent
        websocket
    }

    proxy /api/v1/streaming localhost:4000 {
        transparent
        websocket
    }

    tls [email protected]
}

Caddyfile.caddy2

{
  email [email protected]
}

https://yourdomain.example.com {
  @local {
    file
    not path /
  }
  @local_media {
    path_regexp /system/(.*)
  }
  @streaming {
    path /api/v1/streaming/*
  }
  @cache_control {
    path_regexp ^/(emoji|packs|/system/accounts/avatars|/system/media_attachments/files)
  }

  root * /home/mastodon/live/public
  log {
    output file /var/log/caddy/mastodon.log
  }

  encode zstd gzip

  handle_errors {
    rewrite 500.html
    file_server
  }

  header {
    Strict-Transport-Security "max-age=31536000"
  }
  header /sw.js Cache-Control "public, max-age=0"
  header @cache_control Cache-Control "public, max-age=31536000, immutable"

  handle @local {
    file_server
  }

  ## If you've been migrated media from local to object storage, this navigate old URL to new one.
  # redir @local_media https://yourobjectstorage.example.com/{http.regexp.1} permanent

  reverse_proxy @streaming {
    to http://localhost:4000

    transport http {
      keepalive 5s
      keepalive_idle_conns 10
    }
  }

  reverse_proxy  {
    to http://localhost:3000

    header_up X-Forwarded-Port 443
    header_up X-Forwarded-Proto https

    transport http {
      keepalive 5s
      keepalive_idle_conns 10
    }
  }
}

FYI, line 14 on v2, seems incorrect. Its working for most clients, but certain libraries seem to be throwing strange errors (as they sometimes lack the last / on the end of the URL).

Needs changing from path /api/v1/streaming/* to path /api/v1/streaming*