diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6ad40ba..a8d48a5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -109,3 +109,20 @@ jobs:
         run: composer install --no-progress --no-interaction --prefer-dist
       - name: Run PHPUnit (Brain Monkey — no WP install required)
         run: vendor/bin/phpunit --testdox
+
+  check-constants:
+    name: Check PRAUTOBLOGGER_* constants
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup PHP 8.1
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.1'
+          coverage: none
+      - name: Check PRAUTOBLOGGER_* constants
+        # Fails if any PRAUTOBLOGGER_* constant referenced in the shipped
+        # plugin PHP is neither defined in prautoblogger.php nor guarded
+        # by defined() at the reference site. Prevents the v0.19.0 incident
+        # class (constant deleted from bootstrap, test fallback masks the gap).
+        run: php scripts/check-constants.php
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 776ddee..2dfa494 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,11 @@ All notable changes to PRAutoBlogger will be documented in this file.
 Format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project uses [Semantic Versioning](https://semver.org/).
 
+## [Unreleased - Internal]
+
+### Internal
+- CI constants guard (`scripts/check-constants.php`): new required CI step (`Check PRAUTOBLOGGER_* constants`) fails the build when any `PRAUTOBLOGGER_*` constant referenced in shipped plugin PHP is neither defined in `prautoblogger.php` nor `defined()`-guarded at the reference site. Prevents a recurrence of the v0.19.0 admin-500 regression class.
+
 ## [0.20.1] - 2026-06-12
 
 ### Fixed
diff --git a/scripts/check-constants.php b/scripts/check-constants.php
new file mode 100644
index 0000000..6b0b9b6
--- /dev/null
+++ b/scripts/check-constants.php
@@ -0,0 +1,212 @@
+<?php
+/**
+ * CI guard: every PRAUTOBLOGGER_* constant referenced in the shipped plugin PHP
+ * must be either (a) defined in prautoblogger.php, or (b) protected by a
+ * defined() guard at the reference site.
+ *
+ * Purpose: prevent the class of regression that caused the v0.19.0 admin-500
+ * incident — a constant deleted from prautoblogger.php while tests/bootstrap.php
+ * kept a fallback copy, masking the gap until prod boot.
+ *
+ * Usage (from repo root):
+ *   php scripts/check-constants.php [--verbose]
+ *
+ * Exit 0 = all clear. Exit 1 = unguarded undefined constants found.
+ *
+ * Include/exclude summary (relative to repo root):
+ *   Defined-set source  prautoblogger.php only
+ *   Scanned for refs    prautoblogger.php, includes/, templates/, uninstall.php
+ *   Excluded from refs  tests/ vendor/ .github/ .git/ node_modules/
+ *   Excluded from defs  everything except prautoblogger.php
+ *
+ * Guard window: a bare constant use is considered guarded if a defined() call
+ * for the same constant appears within GUARD_WINDOW lines above it in the same
+ * file (covers if/elseif blocks and multi-line && chains).
+ *
+ * @package PRAutoBlogger
+ */
+
+declare(strict_types=1);
+
+// How many lines above the bare use to look for a defined() guard.
+const CONSTANTS_GUARD_WINDOW = 4;
+
+// Source of truth for define()d constants -- test-bootstrap fallbacks excluded.
+const CONSTANTS_BOOTSTRAP_FILE = 'prautoblogger.php';
+
+// Directories skipped when walking the shipped tree for references.
+const CONSTANTS_EXCLUDE_DIRS = ['tests', 'vendor', '.github', '.git', 'node_modules'];
+
+// Entry points for the reference scan (files and dirs relative to repo root).
+const CONSTANTS_SCAN_ROOTS = [
+	'prautoblogger.php',
+	'uninstall.php',
+	'includes',
+	'templates',
+];
+
+$verbose = in_array( '--verbose', $argv ?? [], true );
+
+// -------------------------------------------------------------------------
+// 1. Build the defined set from prautoblogger.php only.
+// -------------------------------------------------------------------------
+
+if ( ! file_exists( CONSTANTS_BOOTSTRAP_FILE ) ) {
+	fwrite( STDERR, "ERROR: bootstrap file not found -- run from repo root.\n" );
+	fwrite( STDERR, '  Expected: ' . CONSTANTS_BOOTSTRAP_FILE . "\n" );
+	exit( 1 );
+}
+
+$bootstrap_src = (string) file_get_contents( CONSTANTS_BOOTSTRAP_FILE );
+
+preg_match_all(
+	"/define\s*\(\s*'(PRAUTOBLOGGER_[A-Z0-9_]+)'/",
+	$bootstrap_src,
+	$def_matches
+);
+
+/** @var array<string, int> $defined_set constant name => 1 */
+$defined_set = array_flip( $def_matches[1] ?? [] );
+
+if ( $verbose ) {
+	echo 'Defined in ' . CONSTANTS_BOOTSTRAP_FILE . ' (' . count( $defined_set ) . "):\n";
+	foreach ( array_keys( $defined_set ) as $constant_name ) {
+		echo "  {$constant_name}\n";
+	}
+	echo "\n";
+}
+
+// -------------------------------------------------------------------------
+// 2. Collect shipped PHP files to scan.
+// -------------------------------------------------------------------------
+
+/**
+ * Recursively collect *.php paths under a directory, skipping excluded dirs.
+ *
+ * @param string   $dir   Directory to walk.
+ * @param string[] $files Accumulator (modified by reference).
+ * @return void
+ */
+function constants_collect_php_files( string $dir, array &$files ): void {
+	$dir_iter = new DirectoryIterator( $dir );
+	foreach ( $dir_iter as $entry ) {
+		if ( $entry->isDot() ) {
+			continue;
+		}
+		$basename = $entry->getBasename();
+		if ( $entry->isDir() ) {
+			if ( ! in_array( $basename, CONSTANTS_EXCLUDE_DIRS, true ) ) {
+				constants_collect_php_files( $entry->getPathname(), $files );
+			}
+			continue;
+		}
+		if ( $entry->isFile() && $entry->getExtension() === 'php' ) {
+			$files[] = $entry->getPathname();
+		}
+	}
+}
+
+/** @var string[] $php_files */
+$php_files = [];
+
+foreach ( CONSTANTS_SCAN_ROOTS as $root ) {
+	if ( ! file_exists( $root ) ) {
+		// Template dir may not exist in very early checkouts -- skip silently.
+		continue;
+	}
+	if ( is_file( $root ) ) {
+		$php_files[] = $root;
+	} else {
+		constants_collect_php_files( $root, $php_files );
+	}
+}
+
+sort( $php_files );
+
+if ( $verbose ) {
+	echo 'Scanning ' . count( $php_files ) . " PHP files\n\n";
+}
+
+// -------------------------------------------------------------------------
+// 3. Scan each file for bare PRAUTOBLOGGER_* references.
+// -------------------------------------------------------------------------
+
+/** @var array<int, array{string, int, string, string}> $errors */
+$errors = [];
+
+foreach ( $php_files as $php_path ) {
+	$raw = file_get_contents( $php_path );
+	if ( $raw === false ) {
+		fwrite( STDERR, "WARNING: cannot read {$php_path} -- skipping\n" );
+		continue;
+	}
+
+	$lines = explode( "\n", $raw );
+
+	foreach ( $lines as $idx => $raw_line ) {
+		// Fast pre-check: skip lines with no PRAUTOBLOGGER_ token.
+		if ( strpos( $raw_line, 'PRAUTOBLOGGER_' ) === false ) {
+			continue;
+		}
+
+		// Skip pure comment lines (docblock * lines, // comments, # lines).
+		$trimmed = ltrim( $raw_line );
+		if (
+			str_starts_with( $trimmed, '*' ) ||
+			str_starts_with( $trimmed, '//' ) ||
+			str_starts_with( $trimmed, '#' )
+		) {
+			continue;
+		}
+
+		// Extract all constant names from this line.
+		if ( ! preg_match_all( '/PRAUTOBLOGGER_[A-Z0-9_]+/', $raw_line, $match ) ) {
+			continue;
+		}
+
+		foreach ( $match[0] as $name ) {
+			// Skip if this line is itself a define() call for this name.
+			if ( preg_match( "/define\s*\(\s*'" . preg_quote( $name, '/' ) . "'/", $raw_line ) ) {
+				continue;
+			}
+
+			// Skip if the constant is production-defined in prautoblogger.php.
+			if ( isset( $defined_set[ $name ] ) ) {
+				continue;
+			}
+
+			// Check for a defined() guard in the preceding GUARD_WINDOW lines
+			// (inclusive of the current line to catch same-line chained &&).
+			$window_start = max( 0, $idx - CONSTANTS_GUARD_WINDOW );
+			$guarded      = false;
+			for ( $w = $window_start; $w <= $idx; $w++ ) {
+				if ( preg_match( "/defined\s*\(\s*'" . preg_quote( $name, '/' ) . "'/", $lines[ $w ] ) ) {
+					$guarded = true;
+					break;
+				}
+			}
+
+			if ( ! $guarded ) {
+				$errors[] = [ $php_path, $idx + 1, $name, $raw_line ];
+			}
+		}
+	}
+}
+
+// -------------------------------------------------------------------------
+// 4. Report results.
+// -------------------------------------------------------------------------
+
+if ( $errors !== [] ) {
+	fwrite( STDERR, "\nFAIL -- Unguarded undefined PRAUTOBLOGGER_* constants:\n\n" );
+	foreach ( $errors as [ $php_path, $lineno, $name, $line_text ] ) {
+		fwrite( STDERR, "  {$php_path}:{$lineno}: {$name}\n" );
+		fwrite( STDERR, '    ' . trim( $line_text ) . "\n" );
+	}
+	fwrite( STDERR, "\nTo fix: add  define( '" . $errors[0][2] . "', ... )  to prautoblogger.php,\n" );
+	fwrite( STDERR, "or guard the reference with  if ( defined( '" . $errors[0][2] . "' ) ) { ... }\n\n" );
+	exit( 1 );
+}
+
+echo "OK -- All PRAUTOBLOGGER_* constants are production-defined or defined()-guarded.\n";
+exit( 0 );