diff --git a/.github/workflows/commit-built-file-changes.yml b/.github/workflows/commit-built-file-changes.yml
index 64e734d05e8e8..a1df2bb9eb04f 100644
--- a/.github/workflows/commit-built-file-changes.yml
+++ b/.github/workflows/commit-built-file-changes.yml
@@ -1,13 +1,13 @@
 # Commits all missed changes to built files back to pull request branches.
 name: Commit Built File Changes (PRs)
 
-on:
-  workflow_run:
-    workflows:
-      - 'Check Built Files (PRs)'
-      - 'Test Default Themes & Create ZIPs'
-    types:
-      - completed
+# on:
+#   workflow_run:
+#     workflows:
+#       - 'Check Built Files (PRs)'
+#       - 'Test Default Themes & Create ZIPs'
+#     types:
+#       - completed
 
 # Cancels all previous workflow runs for pull requests that have not completed.
 concurrency:
diff --git a/composer.json b/composer.json
index 4cd14415597c7..dc3cad30a0421 100644
--- a/composer.json
+++ b/composer.json
@@ -20,17 +20,147 @@
 	},
 	"require-dev": {
 		"composer/ca-bundle": "1.5.8",
+		"james-heinrich/getid3": "1.9.23",
+		"paragonie/sodium_compat": "1.23.0",
+		"phpmailer/phpmailer": "6.11.1",
+		"rmccue/requests": "2.0.11",
+		"simplepie/simplepie": "1.9.0",
 		"squizlabs/php_codesniffer": "3.13.2",
 		"wp-coding-standards/wpcs": "~3.2.0",
 		"phpcompatibility/phpcompatibility-wp": "~2.1.3",
+		"wordpress/custom-installer-plugin": "@dev",
 		"yoast/phpunit-polyfills": "^1.1.0"
 	},
+	"repositories": [
+		{
+			"type": "path",
+			"url": "tools/composer"
+		}
+	],
 	"config": {
 		"allow-plugins": {
+			"wordpress/custom-installer-plugin": true,
 			"dealerdirect/phpcodesniffer-composer-installer": true
 		},
 		"lock": false
 	},
+	"extra": {
+		"installer-paths": {
+			"james-heinrich/getid3": {
+				"target": "src/wp-includes/ID3",
+				"source": "getid3",
+				"ignore": [
+					"extension.cache.dbm.php",
+					"extension.cache.mysql.php",
+					"extension.cache.mysqli.php",
+					"extension.cache.sqlite3.php",
+					"module.archive.7zip.php",
+					"module.archive.gzip.php",
+					"module.archive.hpk.php",
+					"module.archive.rar.php",
+					"module.archive.szip.php",
+					"module.archive.tar.php",
+					"module.archive.xz.php",
+					"module.archive.zip.php",
+					"module.audio-video.bink.php",
+					"module.audio-video.ivf.php",
+					"module.audio-video.mpeg.php",
+					"module.audio-video.nsv.php",
+					"module.audio-video.real.php",
+					"module.audio-video.swf.php",
+					"module.audio-video.ts.php",
+					"module.audio-video.wtv.php",
+					"module.audio.aa.php",
+					"module.audio.aac.php",
+					"module.audio.amr.php",
+					"module.audio.au.php",
+					"module.audio.avr.php",
+					"module.audio.bonk.php",
+					"module.audio.dsdiff.php",
+					"module.audio.dsf.php",
+					"module.audio.dss.php",
+					"module.audio.la.php",
+					"module.audio.lpac.php",
+					"module.audio.midi.php",
+					"module.audio.mod.php",
+					"module.audio.monkey.php",
+					"module.audio.mpc.php",
+					"module.audio.optimfrog.php",
+					"module.audio.rkau.php",
+					"module.audio.shorten.php",
+					"module.audio.tak.php",
+					"module.audio.tta.php",
+					"module.audio.voc.php",
+					"module.audio.vqf.php",
+					"module.audio.wavpack.php",
+					"module.graphic.bmp.php",
+					"module.graphic.efax.php",
+					"module.graphic.gif.php",
+					"module.graphic.jpg.php",
+					"module.graphic.pcd.php",
+					"module.graphic.png.php",
+					"module.graphic.svg.php",
+					"module.graphic.tiff.php",
+					"module.misc.cue.php",
+					"module.misc.exe.php",
+					"module.misc.iso.php",
+					"module.misc.msoffice.php",
+					"module.misc.par2.php",
+					"module.misc.pdf.php",
+					"module.misc.torrent.php",
+					"module.tag.nikon-nctg.php",
+					"module.tag.xmp.php",
+					"write.apetag.php",
+					"write.id3v1.php",
+					"write.id3v2.php",
+					"write.lyrics3.php",
+					"write.metaflac.php",
+					"write.php",
+					"write.real.php",
+					"write.vorbiscomment.php"
+				]
+			},
+			"paragonie/sodium_compat": {
+				"target": "src/wp-includes/sodium_compat",
+				"ignore": [
+					"README.md",
+					"composer-php52.json"
+				]
+			},
+			"phpmailer/phpmailer": {
+				"target": "src/wp-includes/PHPMailer",
+				"source": "src"
+			},
+			"rmccue/requests": {
+				"target": "src/wp-includes/Requests",
+				"ignore": [
+					".editorconfig",
+					"certificates",
+					"CHANGELOG.md",
+					"composer.json",
+					"library/Deprecated.php",
+					"library/README.md",
+					"LICENSE",
+					"README.md"
+				],
+				"replace": {
+					"library/Requests.php": "Requests.php"
+				}
+			},
+			"simplepie/simplepie": {
+				"target": "src/wp-includes/SimplePie",
+				"ignore": [
+					"CHANGELOG.md",
+					"composer.json",
+					"db.sql",
+					"LICENSES",
+					"phpstan.dist.neon",
+					"README.markdown",
+					"utils/PHPStan"
+				]
+			}
+		}
+	},
 	"scripts": {
 		"compat": "@php ./vendor/squizlabs/php_codesniffer/bin/phpcs --standard=phpcompat.xml.dist --report=summary,source",
 		"format": "@php ./vendor/squizlabs/php_codesniffer/bin/phpcbf --report=summary,source",
diff --git a/tools/composer/composer.json b/tools/composer/composer.json
new file mode 100644
index 0000000000000..e2d05de9c96ed
--- /dev/null
+++ b/tools/composer/composer.json
@@ -0,0 +1,17 @@
+{
+	"name": "wordpress/custom-installer-plugin",
+	"description": "Custom Composer installer plugin for WordPress vendor dependencies",
+	"type": "composer-plugin",
+	"license": "GPL-2.0-or-later",
+	"require": {
+		"composer-plugin-api": "^2.0"
+	},
+	"autoload": {
+		"psr-4": {
+			"WordPress\\Composer\\": "src/"
+		}
+	},
+	"extra": {
+		"class": "WordPress\\Composer\\CustomInstallerPlugin"
+	}
+}
diff --git a/tools/composer/replacements/Requests.php b/tools/composer/replacements/Requests.php
new file mode 100644
index 0000000000000..9e42db596b8c9
--- /dev/null
+++ b/tools/composer/replacements/Requests.php
@@ -0,0 +1,12 @@
+<?php
+/**
+ * Loads the old Requests class file when the autoloader
+ * references the original PSR-0 Requests class.
+ *
+ * @deprecated 6.2.0
+ * @package WordPress
+ * @subpackage Requests
+ * @since 6.2.0
+ */
+
+include_once ABSPATH . WPINC . '/class-requests.php';
diff --git a/tools/composer/src/CustomInstaller.php b/tools/composer/src/CustomInstaller.php
new file mode 100644
index 0000000000000..782e29258fee6
--- /dev/null
+++ b/tools/composer/src/CustomInstaller.php
@@ -0,0 +1,199 @@
+<?php
+/**
+ * Installer for handling custom installation paths.
+ */
+
+namespace WordPress\Composer;
+
+use Composer\PartialComposer;
+use Composer\IO\IOInterface;
+use Composer\Installer\LibraryInstaller;
+use Composer\Installer\BinaryInstaller;
+use Composer\Package\PackageInterface;
+use Composer\Repository\InstalledRepositoryInterface;
+use Composer\Util\Filesystem;
+use React\Promise\PromiseInterface;
+
+/**
+ * Installer class.
+ */
+final class CustomInstaller extends LibraryInstaller {
+	/**
+	 * Custom installer paths configuration.
+	 */
+	private array $installerPaths = array();
+
+	/**
+	 * Initializes library installer.
+	 */
+	public function __construct(
+		IOInterface $io,
+		PartialComposer $composer,
+		?string $type = 'library',
+		?Filesystem $filesystem = null,
+		?BinaryInstaller $binaryInstaller = null
+	) {
+		parent::__construct( $io, $composer, $type, $filesystem, $binaryInstaller );
+
+		$this->installerPaths = $this->composer->getPackage()->getExtra()['installer-paths'];
+	}
+
+	/**
+	 * Check if this installer supports the given package type.
+	 *
+	 * @param string $packageType The package type.
+	 * @return bool
+	 */
+	public function supports( $packageType ) {
+		return true;
+	}
+
+	/**
+	 * Get the installation path for a package.
+	 *
+	 * @param PackageInterface $package The package.
+	 * @return string The installation path.
+	 */
+	public function getInstallPath( PackageInterface $package ) {
+		$packageName = $package->getName();
+
+		if ( ! isset( $this->installerPaths[ $packageName ] ) ) {
+			return parent::getInstallPath( $package );
+		}
+
+		return realpath( getcwd() ) . '/' . $this->installerPaths[ $packageName ]['target'];
+	}
+
+	/**
+	 * Install a package.
+	 *
+	 * @param InstalledRepositoryInterface $repo    The installed repository.
+	 * @param PackageInterface             $package The package to install.
+	 * @return PromiseInterface|null
+	 */
+	public function install( InstalledRepositoryInterface $repo, PackageInterface $package ) {
+		$installer = parent::install( $repo, $package );
+
+		if ( $installer instanceof PromiseInterface ) {
+			return $installer->then( fn() => $this->modifyPaths( $package ) );
+		}
+
+		$this->modifyPaths( $package );
+		return null;
+	}
+
+	/**
+	 * Update a package.
+	 *
+	 * @param InstalledRepositoryInterface $repo    The installed repository.
+	 * @param PackageInterface             $initial The initial package.
+	 * @param PackageInterface             $target  The target package.
+	 * @return PromiseInterface|null
+	 */
+	public function update( InstalledRepositoryInterface $repo, PackageInterface $initial, PackageInterface $target ) {
+		$updater = parent::update( $repo, $initial, $target );
+
+		if ( $updater instanceof PromiseInterface ) {
+			return $updater->then( fn() => $this->modifyPaths( $target ) );
+		}
+
+		$this->modifyPaths( $target );
+		return null;
+	}
+
+	/**
+	 * Modify installation paths based on source subdirectory and ignore patterns.
+	 *
+	 * @param PackageInterface $package The package.
+	 */
+	private function modifyPaths( PackageInterface $package ): void {
+		$installPath = $this->getInstallPath( $package );
+
+		$this->applySourceSubdirectory( $package, $installPath );
+		$this->applyReplacements( $package, $installPath );
+		$this->applyIgnorePatterns( $package, $installPath );
+	}
+
+	/**
+	 * Apply file replacements from custom replacement files.
+	 *
+	 * @param PackageInterface $package     The package.
+	 * @param string           $installPath The installation path.
+	 */
+	private function applyReplacements( PackageInterface $package, string $installPath ): void {
+		$packageName = $package->getName();
+
+		if ( ! isset( $this->installerPaths[ $packageName ]['replace'] ) ) {
+			return;
+		}
+
+		$filesystem      = new Filesystem();
+		$replacementsDir = realpath( getcwd() ) . '/tools/composer/replacements';
+
+		foreach ( $this->installerPaths[ $packageName ]['replace'] as $target => $source ) {
+			$sourcePath = $replacementsDir . '/' . $source;
+
+			if ( ! file_exists( $sourcePath ) ) {
+				throw new \RuntimeException( "Replacement file 'tools/composer/replacements/{$source}' does not exist for package '{$packageName}'." );
+			}
+
+			$filesystem->copy( $sourcePath, $installPath . '/' . $target );
+		}
+	}
+
+	/**
+	 * Apply ignore patterns to remove unwanted files after installation.
+	 *
+	 * @param PackageInterface $package The package.
+	 * @param string           $installPath The installation path.
+	 */
+	private function applyIgnorePatterns( PackageInterface $package, string $installPath ): void {
+		$packageName = $package->getName();
+
+		if ( ! isset( $this->installerPaths[ $packageName ]['ignore'] ) ) {
+			return;
+		}
+
+		$filesystem = new Filesystem();
+
+		foreach ( $this->installerPaths[ $packageName ]['ignore'] as $pattern ) {
+			$matches = glob( $installPath . '/' . $pattern );
+
+			if ( empty( $matches ) ) {
+				throw new \RuntimeException( "Failed to glob pattern '{$pattern}' in package '{$package->getName()}'." );
+			}
+
+			foreach ( $matches as $path ) {
+				$filesystem->remove( $path );
+			}
+		}
+	}
+
+	/**
+	 * Apply source subdirectory to flatten directory structure.
+	 *
+	 * @param PackageInterface $package     The package.
+	 * @param string           $installPath The installation path.
+	 */
+	private function applySourceSubdirectory( PackageInterface $package, string $installPath ): void {
+		$packageName = $package->getName();
+
+		if ( ! isset( $this->installerPaths[ $packageName ]['source'] ) ) {
+			return;
+		}
+
+		$sourceSubdir = $this->installerPaths[ $packageName ]['source'];
+		$sourceDir    = $installPath . '/' . $sourceSubdir;
+
+		if ( ! is_dir( $sourceDir ) ) {
+			throw new \RuntimeException( "Source directory '{$sourceSubdir}' does not exist in package '{$packageName}'." );
+		}
+
+		$filesystem = new Filesystem();
+		$tempDir    = $installPath . '_temp';
+
+		$filesystem->rename( $sourceDir, $tempDir );
+		$filesystem->removeDirectory( $installPath );
+		$filesystem->rename( $tempDir, $installPath );
+	}
+}
diff --git a/tools/composer/src/CustomInstallerPlugin.php b/tools/composer/src/CustomInstallerPlugin.php
new file mode 100644
index 0000000000000..afeba682f3407
--- /dev/null
+++ b/tools/composer/src/CustomInstallerPlugin.php
@@ -0,0 +1,46 @@
+<?php
+/**
+ * Custom Composer installer plugin for WordPress.
+ */
+
+namespace WordPress\Composer;
+
+use Composer\Composer;
+use Composer\IO\IOInterface;
+use Composer\Plugin\PluginInterface;
+
+/**
+ * Custom installer plugin class.
+ */
+final class CustomInstallerPlugin implements PluginInterface {
+	/**
+	 * Apply plugin.
+	 *
+	 * @param Composer    $composer The Composer instance.
+	 * @param IOInterface $io       The IO interface.
+	 */
+	public function activate( Composer $composer, IOInterface $io ) {
+		$installer = new CustomInstaller( $io, $composer );
+		$composer->getInstallationManager()->addInstaller( $installer );
+	}
+
+	/**
+	 * Remove any hooks from Composer.
+	 *
+	 * @param Composer    $composer The Composer instance.
+	 * @param IOInterface $io       The IO interface.
+	 */
+	public function deactivate( Composer $composer, IOInterface $io ) {
+		// Nothing to do here.
+	}
+
+	/**
+	 * Prepare the plugin to be uninstalled.
+	 *
+	 * @param Composer    $composer The Composer instance.
+	 * @param IOInterface $io       The IO interface.
+	 */
+	public function uninstall( Composer $composer, IOInterface $io ) {
+		// Nothing to do here.
+	}
+}