diff --git a/src/InMemoryKvOps.php b/src/InMemoryKvOps.php
index 80b6504..3a0d0d4 100644
--- a/src/InMemoryKvOps.php
+++ b/src/InMemoryKvOps.php
@@ -96,6 +96,13 @@ public function pttl(string $key): int
         return \max(0, $this->deadlines[$key] - $this->nowMs());
     }
 
+    public function flush(): bool
+    {
+        $this->values = [];
+        $this->deadlines = [];
+        return true;
+    }
+
     /**
      * Resolve a key's value, expiring it lazily if its deadline has passed.
      */
diff --git a/src/KvOpsInterface.php b/src/KvOpsInterface.php
index 6624c6b..c0a3cc0 100644
--- a/src/KvOpsInterface.php
+++ b/src/KvOpsInterface.php
@@ -73,4 +73,15 @@ public function ttl(string $key): int;
      * -2 if the key does not exist, otherwise the ms remaining.
      */
     public function pttl(string $key): int;
+
+    /**
+     * Remove every key from the effective store. Backed by ephpm_kv_flush_all()
+     * (ePHPm v0.1.2+); a no-op returning false on older runtimes.
+     *
+     * Note: this is a *global* flush — it drops more than just session
+     * keys. KvSessionHandler deliberately does not expose this; the
+     * recommended way to invalidate every session is to bump the handler's
+     * prefix in config and let TTLs age the old entries out.
+     */
+    public function flush(): bool;
 }
diff --git a/src/SapiKvOps.php b/src/SapiKvOps.php
index 7907bf8..473b047 100644
--- a/src/SapiKvOps.php
+++ b/src/SapiKvOps.php
@@ -63,4 +63,12 @@ public function pttl(string $key): int
     {
         return (int) \ephpm_kv_pttl($key);
     }
+
+    public function flush(): bool
+    {
+        if (!\function_exists('ephpm_kv_flush_all')) {
+            return false;
+        }
+        return (bool) \ephpm_kv_flush_all();
+    }
 }
diff --git a/tests/InMemoryKvOpsTest.php b/tests/InMemoryKvOpsTest.php
index 3a5b225..fbb2f59 100644
--- a/tests/InMemoryKvOpsTest.php
+++ b/tests/InMemoryKvOpsTest.php
@@ -108,4 +108,15 @@ public function test_expire_with_zero_or_negative_is_noop(): void
         // Original TTL unchanged.
         self::assertGreaterThan(0, $ops->pttl('k'));
     }
+
+    public function test_flush_clears_values_and_deadlines(): void
+    {
+        $ops = new InMemoryKvOps();
+        $ops->set('a', 'one');
+        $ops->set('b', 'two', 60);
+        self::assertTrue($ops->flush());
+        self::assertNull($ops->get('a'));
+        self::assertNull($ops->get('b'));
+        self::assertSame(-2, $ops->pttl('b'));
+    }
 }