diff --git a/StarResponseController.php b/StarResponseController.php
index af989f2..5df06ed 100644
--- a/StarResponseController.php
+++ b/StarResponseController.php
@@ -218,8 +218,14 @@ public static function reset(): void
      */
     private static function sendPublicCacheHeaders(): void
     {
-        $maxAge = (int) apply_filters('starcache_max_age', self::DEFAULT_MAX_AGE);
-        $swr    = (int) apply_filters('starcache_stale_while_revalidate', self::DEFAULT_STALE_WHILE_REVALIDATE);
+        $maxAge = self::sanitizeDirectiveSeconds(
+            apply_filters('starcache_max_age', self::DEFAULT_MAX_AGE),
+            self::DEFAULT_MAX_AGE
+        );
+        $swr = self::sanitizeDirectiveSeconds(
+            apply_filters('starcache_stale_while_revalidate', self::DEFAULT_STALE_WHILE_REVALIDATE),
+            self::DEFAULT_STALE_WHILE_REVALIDATE
+        );
 
         header('Cache-Control: public, max-age=' . $maxAge . ', stale-while-revalidate=' . $swr);
 
@@ -231,6 +237,30 @@ private static function sendPublicCacheHeaders(): void
         header('X-StarCache-Context: ' . StarCacheContext::hash());
     }
 
+    /**
+     * Normalize cache directive durations to a safe, non-negative integer.
+     *
+     * Negative or non-sensical values from third-party filters can result in
+     * invalid Cache-Control headers. This guard keeps header output valid and
+     * predictable while still allowing plugin-level TTL customization.
+     *
+     * @param mixed $value Requested TTL value from filter callbacks.
+     * @param int $fallback Default value used when the request is invalid.
+     */
+    private static function sanitizeDirectiveSeconds(mixed $value, int $fallback): int
+    {
+        if (!is_numeric($value)) {
+            return max(0, $fallback);
+        }
+
+        $seconds = (int) $value;
+        if ($seconds < 0) {
+            return max(0, $fallback);
+        }
+
+        return $seconds;
+    }
+
     /**
      * Emit no-cache headers.
      */
diff --git a/tests/UnitTests.php b/tests/UnitTests.php
index dfe0e81..c10d79c 100644
--- a/tests/UnitTests.php
+++ b/tests/UnitTests.php
@@ -484,6 +484,28 @@ public function testResponseControllerNotEligibleWhenAuthenticated(): void
         $this->assertFalse(StarResponseController::isEligible());
     }
 
+    public function testResponseControllerSanitizeDirectiveSecondsFallsBackForNegativeValue(): void
+    {
+        $method = new \ReflectionMethod(StarResponseController::class, 'sanitizeDirectiveSeconds');
+        $method->setAccessible(true);
+
+        $maxAge = $method->invoke(null, -15, StarResponseController::DEFAULT_MAX_AGE);
+        $swr    = $method->invoke(null, -3, StarResponseController::DEFAULT_STALE_WHILE_REVALIDATE);
+
+        $this->assertSame(StarResponseController::DEFAULT_MAX_AGE, $maxAge);
+        $this->assertSame(StarResponseController::DEFAULT_STALE_WHILE_REVALIDATE, $swr);
+    }
+
+    public function testResponseControllerSanitizeDirectiveSecondsFallsBackForInvalidType(): void
+    {
+        $method = new \ReflectionMethod(StarResponseController::class, 'sanitizeDirectiveSeconds');
+        $method->setAccessible(true);
+
+        $this->assertSame(120, $method->invoke(null, null, 120));
+        $this->assertSame(90, $method->invoke(null, 'not-a-number', 90));
+        $this->assertSame(0, $method->invoke(null, false, -10));
+    }
+
     // =========================================================================
     // StarVersionStore — renamed groups
     // =========================================================================