diff --git a/.github/workflows/plugin-ci-workflow.yml b/.github/workflows/plugin-ci-workflow.yml
index 3b5082b..721dc1b 100644
--- a/.github/workflows/plugin-ci-workflow.yml
+++ b/.github/workflows/plugin-ci-workflow.yml
@@ -74,6 +74,8 @@ jobs:
       uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
       with:
         path: cacti/plugins/quicktree
+        # patch-coverage.php diffs against the base branch below.
+        fetch-depth: 0
 
     - name: Install PHP ${{ matrix.php }}
       uses: shivammathur/setup-php@f3e473d116dcccaddc5834248c87452386958240 # 2.37.2
@@ -186,13 +188,23 @@ jobs:
       run: echo -n "${{ env.CACTI }}" | sudo tee ${{ github.workspace }}/cacti/plugins/quicktree/tests/.cacti-version > /dev/null
 
     - name: Run Pest Unit Tests
-      env:
-        COMPOSER_ROOT_VERSION: 1.3.0-dev
       run: |
         cd ${{ github.workspace }}/cacti
         include/vendor/bin/pest --configuration=plugins/quicktree/phpunit.xml \
           --coverage-clover=plugins/quicktree/coverage/clover.xml
 
+    # Whole-file coverage is meaningless here: most of the plugin only runs
+    # inside a live Cacti. What is enforceable is that a change covers the
+    # lines it adds.
+    - name: Enforce coverage of changed lines
+      if: github.event_name == 'pull_request'
+      env:
+        BASE_REF: ${{ github.event.pull_request.base.sha }}
+      run: |
+        cd ${{ github.workspace }}/cacti/plugins/quicktree
+        git config --global --add safe.directory ${{ github.workspace }}/cacti/plugins/quicktree
+        php tests/bin/patch-coverage.php coverage/clover.xml "$BASE_REF" 100
+
     - name: Upload coverage report
       if: always()
       uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
diff --git a/.gitignore b/.gitignore
index 7610df5..94c9014 100644
--- a/.gitignore
+++ b/.gitignore
@@ -22,3 +22,4 @@
 locales/po/*.mo
 locales/LC_MESSAGES/*.mo
 vendor/
+locales/po/*.po
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9b326a7..177ed3d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 --- develop ---
 
+* dev: Enforce patch coverage of changed lines in CI and remove the inert COMPOSER_ROOT_VERSION env from the Pest step
 * security: Add a version-safe CSP nonce (`plugin_quicktree_csp_nonce()`) to every inline `<script>` tag so pages stay compatible with Cacti's Content-Security-Policy nonce enforcement, while falling back cleanly on older Cacti releases that lack the `CactiSecureHeaders` class
 - issue: Quicktree continually registers a realm
 - issue: Fix quicktree_show_tab() building an invalid `?location=tabtab`/`?location=tabconsole` URL, an undefined-variable text-domain typo in the QuickTree page's "no graph specified" message, and a misspelled `qucktree` text domain
diff --git a/tests/bin/patch-coverage.php b/tests/bin/patch-coverage.php
new file mode 100644
index 0000000..e65b53d
--- /dev/null
+++ b/tests/bin/patch-coverage.php
@@ -0,0 +1,198 @@
+<?php
+/*
+ +-------------------------------------------------------------------------+
+ | Copyright (C) 2004-2026 The Cacti Group                                 |
+ |                                                                         |
+ | This program is free software; you can redistribute it and/or           |
+ | modify it under the terms of the GNU General Public License             |
+ | as published by the Free Software Foundation; either version 2          |
+ | of the License, or (at your option) any later version.                  |
+ +-------------------------------------------------------------------------+
+ | Cacti: The Complete RRDtool-based Graphing Solution                     |
+ +-------------------------------------------------------------------------+
+ | http://www.cacti.net/                                                   |
+ +-------------------------------------------------------------------------+
+*/
+
+/*
+ * Report line coverage for the lines a branch changes.
+ *
+ * Whole-file coverage is not a useful gate here: most of the plugin only runs
+ * inside a live Cacti, so the repository figure would sit near zero no matter
+ * how well a change is tested.  What a reviewer wants to know is whether the
+ * lines this branch adds are exercised, which is what this measures.
+ *
+ * Usage: php tests/bin/patch-coverage.php <clover.xml> <base-ref> [min-percent]
+ *
+ * Exits 1 if coverage is below the threshold, 2 on bad input.
+ */
+
+if ($argc < 3) {
+	fwrite(STDERR, "usage: patch-coverage.php <clover.xml> <base-ref> [min-percent]\n");
+
+	exit(2);
+}
+
+$clover_path = $argv[1];
+$base_ref    = $argv[2];
+$minimum     = isset($argv[3]) ? (float) $argv[3] : 100.0;
+
+if (!is_readable($clover_path)) {
+	fwrite(STDERR, "cannot read coverage report: $clover_path\n");
+
+	exit(2);
+}
+
+/**
+ * Line numbers each measured file changed, keyed by repository-relative path.
+ *
+ * Only added and modified lines count. Deletions have nothing left to cover,
+ * and context lines were not part of this change.
+ *
+ * Paths stay repository-relative so the report can be produced in a container
+ * and evaluated on the host, where the absolute paths differ.
+ *
+ * @param string $base_ref Git ref to diff against.
+ *
+ * @return array<string, array<int, bool>>
+ */
+function changed_lines($base_ref) {
+	$command = 'git diff --no-ext-diff --unified=0 --no-color --diff-filter=AM ' . escapeshellarg($base_ref) . '...HEAD -- "*.php"';
+	$output  = [];
+	$status  = 0;
+
+	$last_line = exec($command, $output, $status);
+
+	if ($last_line === false || $status !== 0) {
+		fwrite(STDERR, "git diff failed\n");
+
+		exit(2);
+	}
+
+	$diff = implode("\n", $output);
+
+	$changed = [];
+	$file    = null;
+
+	foreach (explode("\n", $diff) as $line) {
+		if (strncmp($line, '+++ b/', 6) === 0) {
+			$file           = substr($line, 6);
+
+			if (strncmp($file, 'tests/', 6) === 0) {
+				$file = null;
+
+				continue;
+			}
+
+			$changed[$file] = [];
+		} elseif (strncmp($line, '@@', 2) === 0 && $file !== null) {
+			if (preg_match('/\+(\d+)(?:,(\d+))?/', $line, $match)) {
+				$start = (int) $match[1];
+				$count = isset($match[2]) ? (int) $match[2] : 1;
+
+				for ($i = 0; $i < $count; $i++) {
+					$changed[$file][$start + $i] = true;
+				}
+			}
+		}
+	}
+
+	return $changed;
+}
+
+$changed = changed_lines($base_ref);
+$clover  = simplexml_load_file($clover_path);
+
+if ($clover === false) {
+	fwrite(STDERR, "cannot parse coverage report: $clover_path\n");
+
+	exit(2);
+}
+
+$covered  = 0;
+$total    = 0;
+$missing  = [];
+$measured = [];
+
+foreach ($clover->xpath('//file') as $file) {
+	$path     = (string) $file['name'];
+	$relative = null;
+
+	foreach (array_keys($changed) as $candidate) {
+		if ($path === $candidate || substr($path, -strlen('/' . $candidate)) === '/' . $candidate) {
+			$relative = $candidate;
+
+			break;
+		}
+	}
+
+	if ($relative === null) {
+		continue;
+	}
+
+	$measured[$relative] = true;
+
+	foreach ($file->line as $line) {
+		$number = (int) $line['num'];
+
+		// Only statement lines are measurable; method markers double-count.
+		if ((string) $line['type'] !== 'stmt' || !isset($changed[$relative][$number])) {
+			continue;
+		}
+
+		$total++;
+
+		if ((int) $line['count'] > 0) {
+			$covered++;
+		} else {
+			$missing[] = $relative . ':' . $number;
+		}
+	}
+}
+
+/*
+ * Production PHP entry points that cannot safely be loaded into the isolated
+ * unit process (CLI/daemon/web entry points that chdir + include auth.php,
+ * do pcntl signal handling, or execute at the top level) belong here, each
+ * with a one-line justification. Keep the exception explicit: any newly
+ * changed production PHP file must either appear in Clover or be added here.
+ *
+ * Empty by default; add entries per repository as the need arises.
+ */
+$unmeasured_allowlist = [
+];
+$unmeasured            = array_values(array_diff(array_keys($changed), array_keys($measured)));
+$unexpected_unmeasured = array_values(array_diff($unmeasured, $unmeasured_allowlist));
+
+if ($unmeasured !== []) {
+	print "Changed production PHP files absent from Clover:\n  " . implode("\n  ", $unmeasured) . "\n";
+}
+
+if ($unexpected_unmeasured !== []) {
+	print "FAIL: changed production PHP files are not measured or allowlisted:\n  "
+		. implode("\n  ", $unexpected_unmeasured) . "\n";
+
+	exit(1);
+}
+
+if ($total === 0) {
+	print "Patch coverage: no measured lines changed.\n";
+
+	exit(0);
+}
+
+$percent = ($covered / $total) * 100;
+
+printf("Patch coverage: %.2f%% (%d/%d lines)\n", $percent, $covered, $total);
+
+if ($missing !== []) {
+	print "Uncovered changed lines:\n  " . implode("\n  ", $missing) . "\n";
+}
+
+if ($percent + 0.005 < $minimum) {
+	printf("FAIL: below the %.2f%% minimum.\n", $minimum);
+
+	exit(1);
+}
+
+exit(0);