diff --git a/LICENSE b/COPYING
similarity index 100%
rename from LICENSE
rename to COPYING
diff --git a/README.md b/README.md
index 80f87be..e31533e 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,3 @@
-# xdg-sanity
-
 The `xdg-sanity` script is built to replace your default web browser in your
 desktop/XDG settings. It intercepts http/s URIs sent to the default browser by
 `xdg-open` and sends it to the appropriate application. For example, it will
@@ -7,28 +5,73 @@ send `image/jpeg` MIME type files to your image viewer.
 
 ## Installation
 
-### Arch
+### Arch Linux
 
 I maintain a package [on the
 AUR](https://aur.archlinux.org/packages/xdg-sanity).
 
 ### From Source
 
-First, make sure you have `curl(1)` and `handlr(1)` installed. Then, clone this
-repository and move the `xdg-sanity/` folder into `/etc` and `xdg-sanity.sh` to
-`xdg-sanity` wherever your operating system stores locally-installed binaries.
-This is usually `/usr/local/bin`. Make sure the installation location is in your
-`$PATH`.
+Dependencies:
+- `curl(1)`
+- `handlr(1)`
+- [`tomcat(1)`](https://git.tebibyte.media/emma/tomcat) (optional; enables
+[extensions](#extensions))
 
-Create a `xdg-sanity.desktop` file either manually or with `gendesk(1)`,
-placing it, also, where your OS stores locally-installed `.desktop` files,
-usually `/usr/local/applications`. Set your default web browser to that
-`.desktop` file.
+Instructions:
+Clone this repository and move the `xdg-sanity` binary wherever your operating
+system stores locally-installed binaries. This is usually `/usr/local/bin/` or
+`"$HOME/.local/bin/"` for your user. Make sure the installation location is in
+your `$PATH`.
 
-Add your default web browser to `/etc/xdg-sanity/xdg-sanity.conf` so the
-script can forward links to it.
+Create an `xdg-sanity.desktop` file, placing it where your OS stores
+locally-installed `.desktop` files, which is usually
+`/usr/local/share/applications` or `$XDG_DATA_HOME/applications` for your user.
+Set your default web browser to that `.desktop` file using `handlr set`.
 
-### Usage
+#### Extensions
 
-Open links from applications outside your web browser as normal. Alternatively,
-you can call `xdg-sanity` directly with the only argument accepted being a URI.
\ No newline at end of file
+Extensions are written using TOML and are stored in either
+`/usr/share/xdg-sanity` when installed from a package manager,
+`/usr/local/share/xdg-sanity` when installed locally, or
+`$XDG_DATA_HOME/xdg-sanity` for your user.
+
+Extension support requires the installation of the `tomcat(1)` tool, which
+parses TOML for the command line.
+
+There are three kinds of extensions: MIME, replace, and scheme. MIME extensions
+are parsed first and replace the MIME type of the content being fetched. Replace
+extensions change the URI passed to the command to another. Scheme extensions
+replace the protocol by which the URI hostname is being accessed.
+
+The type of the extension depends on the file name. MIME extensions should have
+a name ending in `.mime.toml`, replace extensions should have 
+`.replace.toml`, and scheme extensions should end with `.scheme.toml`.
+
+Here's what a MIME extension looks like:
+```
+$ cat $XDG_DATA_HOME/xdg-sanity/youtube.mime.toml
+[replace]
+urls = [ "youtube.com", "youtu.be" ]
+
+[with]
+mime = "video/vnd.youtube.yt"
+```
+what a replace extension looks like:
+```
+$ cat $XDG_DATA_HOME/xdg-sanity/youtube.replace.toml
+[replace]
+urls = [ "youtube.com", "youtu.be" ]
+
+[with] 
+url = "https://piped.mint.lgbt/"
+```
+and what a scheme extension looks like:
+```
+$ cat $XDG_DATA_HOME/xdg-sanity/spotify.scheme.toml
+[replace]
+urls = [ "spotify.com" ]
+
+[with]
+scheme = "spotify://"
+```
diff --git a/xdg-sanity b/xdg-sanity
new file mode 100755
index 0000000..8d4724f
--- /dev/null
+++ b/xdg-sanity
@@ -0,0 +1,133 @@
+#!/bin/sh
+
+# Copyright (c) 2022–2023 Emma Tebibyte <emma@tebibyte.media>
+# SPDX-License-Identifier: AGPL-3.0-or-later
+#
+# This program is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option) any
+# later version.
+# 
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see https://www.gnu.org/licenses/.
+
+set -e
+
+argv0="$0"
+
+get_urls() {
+  i="0"
+  until ! tomcat replace.urls["$i"] "$file" >/dev/null 2>&1
+  do
+    URLS="$(printf "%s\n%s" \
+      "$URLS" \
+      "$(tomcat replace.urls["$i"] "$file")"
+    )"
+
+    i="$(printf "%s+1\n" "$i" | bc)"
+  done
+}
+
+# check if usage is valid
+if test -z "$1"; then
+  printf "Usage: %s [resource...]\n" "$argv0" 1>&2
+  exit 64 # sysexits(3) EX_USAGE
+fi
+
+for dep in \
+  curl \
+  handlr
+do
+  if ! command -v "$dep" >/dev/null 2>&1; then
+    printf "%s: Missing dependency: %s(1)\n" "$argv0" "$dep" 1>&2
+    exit 69 # sysexits(3) EX_UNAVAILABLE
+  fi
+done
+
+# check if we have a BROWSER
+if test -z "$BROWSER"; then
+  printf "%s: \$BROWSER not filled.\n" "$argv0" 1>&2
+  exit 71 # sysexits(3) EX_OSERR
+fi
+
+while test -n "$1"; do
+  URL="$1"
+
+  # use curl(1) to write out the request header's content_type, strip everything
+  # after the first semicolon, chop off any weird whitespace remnants
+  MIME="$(curl -Ls -o /dev/null -w '%{content_type}' "$1" | sed 's/\;.*//' \
+    | xargs echo)"
+
+  # get the URL with no scheme
+  NO_SCHEME="$(printf "%s\n" "$URL" | sed -n 's/^h.\+\/\///p')"
+
+  # get the pattern for the extensions to MATCH
+  MATCH="$(printf "%s\n" "$NO_SCHEME" | sed 's/\/.*\+//g')"
+
+  # get the non-domain of the URI for tacking onto the end of replaced URIs
+  URI="$(printf "%s\n" "$NO_SCHEME" | sed -n 's/^[^/]*\///p')"
+  
+  # only check for extensions if tomcat(1) is installed
+  if command -v tomcat >/dev/null 2>&1; then
+    # run through MIME extensions
+    for file in \
+      "$XDG_DATA_HOME"/xdg-sanity/*.mime.toml \
+      /usr/share/xdg-sanity/*.mime.toml \
+      /usr/local/share/xdg-sanity/*.mime.toml
+    do
+      if test -e "$file"; then
+        get_urls
+        for url in $URLS; do
+          if [ "$MATCH" = "$url" ]; then
+            MIME="$(tomcat with.mime "$file")"
+          fi
+        done
+      fi
+    done
+
+    # then replace extensions
+    for file in \
+      "$XDG_DATA_HOME"/xdg-sanity/*.replace.toml \
+      /usr/share/xdg-sanity/*.replace.toml \
+      /usr/local/share/xdg-sanity/*.replace.toml
+    do
+      if test -e "$file"; then
+        get_urls
+        for url in $URLS; do
+          if [ "$MATCH" = "$url" ]; then
+            URL="$(tomcat with.url "$file")$URI"
+          fi
+        done
+      fi
+    done
+
+    # and the scheme extensions
+    for file in \
+      "$XDG_DATA_HOME"/xdg-sanity/*.scheme.toml \
+      /usr/share/xdg-sanity/*.scheme.toml \
+      /usr/local/share/xdg-sanity/*.scheme.toml
+    do
+      get_urls
+      for url in $URLS; do
+        if [ "$URL" = "$url" ]; then
+          URL="$(tomcat with.scheme "$file")$MATCH"
+        fi
+      done
+    done
+  fi
+
+  # these commands may fail; this is intentional
+  if [ "$MIME" = "text/html" ]; then
+    "$BROWSER" "$URL"
+  else
+    handlr launch "$MIME" -- "$URL"
+  fi
+  shift
+done
+
+exit 0
diff --git a/xdg-sanity.sh b/xdg-sanity.sh
deleted file mode 100755
index 3ee015c..0000000
--- a/xdg-sanity.sh
+++ /dev/null
@@ -1,66 +0,0 @@
-#!/bin/sh
-
-INPUT=$(echo $1)
-echo "Loading configuration..."
-CONFIG=$(cat /etc/xdg-sanity/xdg-sanity.conf)
-BROWSER=$(echo $CONFIG | sed -ne 's/^browser *= *//p')
-
-if [ "$BROWSER" = "" ]
-then
-echo "Please place the path to your default browser's executable in /etc/xdg-sanity/xdg-sanity.conf"
-exit
-else
-echo "Found default browser $BROWSER"
-fi
-
-echo "Loading extensions..."
-for EXT in /etc/xdg-sanity/extensions/*.sh
-do
-
-if [ "$EXT" = "/etc/xdg-sanity/extensions/*.sh" ]
-then
-echo "No extensions to load"
-else
-
-for EXT in /etc/xdg-sanity/extensions/*.sh
-do
-TYPE=$(cat $EXT | sed -ne 's/^# EXT-TYPE=//p' | tr -d '\n')
-echo "Found $TYPE extension $EXT"
-
-if [ "$TYPE" = "replace" ]
-then
-echo "Modifying $INPUT..."
-INPUT=$($EXT "$INPUT")
-echo "Got $INPUT"
-
-else
-if [ "$TYPE" = "mime" ]
-then
-echo "Modifying MIME type..."
-MIME=$($EXT "$INPUT")
-echo "Got $MIME"
-
-fi
-fi
-
-done
-fi
-
-done
-
-if [ "$MIME" = "" ] || [ "$MIME" = "$1" ] || [ "$MIME" = "$INPUT" ]
-then
-echo "Determining MIME type of $INPUT:"
-# Determines HTTP code, might use for something else?
-# CODE=$(curl -fLIs "$INPUT" | sed -ne 's/ [[:space:]]*$//p' | sed -ne 's|^HTTP/.\+ ||p')
-MIME=$(curl -fLIs "$INPUT" | sed -ne 's/^[cC]ontent-[tT]ype: //p' | sed -e 's/;.\+//p' | tail -n1 | tr -d '\r')
-echo $MIME
-fi
-
-if [ "$MIME" = "text/html" ]
-then
-$BROWSER $INPUT
-
-else
-handlr launch "$MIME" -- "$INPUT"
-fi
\ No newline at end of file
diff --git a/xdg-sanity/extensions/teddit-mime.sample b/xdg-sanity/extensions/teddit-mime.sample
deleted file mode 100755
index 74908ee..0000000
--- a/xdg-sanity/extensions/teddit-mime.sample
+++ /dev/null
@@ -1,12 +0,0 @@
-#!/bin/sh
-
-# EXT-TYPE=mime
-
-TEST=$(echo $1 | sed -ne 's/^h.\+\/\///p' | sed -e 's/\/.*\+//g')
-
-if [ "$TEST" = "teddit.net" ]
-then
-echo "text/html"
-else
-echo $1
-fi
\ No newline at end of file
diff --git a/xdg-sanity/extensions/youtube-replace b/xdg-sanity/extensions/youtube-replace
deleted file mode 100755
index e0276d2..0000000
--- a/xdg-sanity/extensions/youtube-replace
+++ /dev/null
@@ -1,3 +0,0 @@
-# EXT-TYPE=replace
-
-replace youtube.com or youtu.be with piped.mint.lgbt
\ No newline at end of file
diff --git a/xdg-sanity/extensions/youtube-replace.sample b/xdg-sanity/extensions/youtube-replace.sample
deleted file mode 100755
index a1fd473..0000000
--- a/xdg-sanity/extensions/youtube-replace.sample
+++ /dev/null
@@ -1,12 +0,0 @@
-#!/bin/sh
-
-# EXT-TYPE=replace
-
-TEST=$(echo $1 | sed -ne 's/^h.\+\/\///p' | sed -e 's/\/.*\+//g')
-
-if [ "$TEST" = "youtube.com" ]
-then
-echo $1 | sed -ne 's/youtube.com/piped.mint.lgbt/p'
-else
-echo $1
-fi
diff --git a/xdg-sanity/xdg-sanity.conf b/xdg-sanity/xdg-sanity.conf
deleted file mode 100644
index 1acbb9a..0000000
--- a/xdg-sanity/xdg-sanity.conf
+++ /dev/null
@@ -1 +0,0 @@
-browser =