Add configurable session garbage collection (gcProbability/gcDivisor)

Make session GC probability configurable via gcProbability and gcDivisor
parameters, mirroring PHP's session.gc_probability/session.gc_divisor.
Exposed through both SessionManager constructor and Builder::setSession().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: chr-hertel

CHANGELOG.md

@@ -9,6 +9,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * Add client component for building MCP clients
 * Add `Builder::setReferenceHandler()` to allow custom `ReferenceHandlerInterface` implementations (e.g. authorization decorators)
 * Add elicitation enum schema types per SEP-1330: `TitledEnumSchemaDefinition`, `MultiSelectEnumSchemaDefinition`, `TitledMultiSelectEnumSchemaDefinition`
+* Add configurable session garbage collection (`gcProbability`/`gcDivisor`)
 
 0.4.0
 -----

docs/server-builder.md

@@ -154,11 +154,6 @@ use Mcp\Server\Session\Psr16SessionStore;
 use Symfony\Component\Cache\Psr16Cache;
 use Symfony\Component\Cache\Adapter\RedisAdapter;
 
-// Use default in-memory sessions with custom TTL
-$server = Server::builder()
-    ->setSession(ttl: 7200) // 2 hours
-    ->build();
-
 // Override with file-based storage
 $server = Server::builder()
     ->setSession(new FileSessionStore(__DIR__ . '/sessions'))
@@ -186,6 +181,45 @@ $server = Server::builder()
     ->build();
 ```
 
+### Garbage Collection Configuration
+
+The SDK periodically runs garbage collection to clean up expired sessions, similar to PHP's native
+`session.gc_probability` and `session.gc_divisor` settings. The probability that GC runs on any given
+request is `gcProbability / gcDivisor`.
+
+```php
+// Default: 1/100 (1% chance per request)
+$server = Server::builder()
+    ->setSession(new FileSessionStore(__DIR__ . '/sessions'))
+    ->build();
+
+// Higher frequency: 1/10 (10% chance per request)
+$server = Server::builder()
+    ->setSession(
+        new FileSessionStore(__DIR__ . '/sessions'),
+        gcProbability: 1,
+        gcDivisor: 10,
+    )
+    ->build();
+
+// Run GC on every request
+$server = Server::builder()
+    ->setSession(gcProbability: 1, gcDivisor: 1)
+    ->build();
+
+// Disable GC entirely (e.g. when using an external cleanup process)
+$server = Server::builder()
+    ->setSession(gcProbability: 0)
+    ->build();
+```
+
+**Parameters:**
+- `$gcProbability` (int): The numerator of the GC probability fraction (default: `1`). Set to `0` to disable GC.
+- `$gcDivisor` (int): The denominator of the GC probability fraction (default: `100`). Must be >= 1.
+
+> **Note**: When providing a custom `SessionManagerInterface` via the `$sessionManager` parameter,
+> the `gcProbability` and `gcDivisor` settings are ignored — you control GC behavior in your own implementation.
+
 **Available Session Stores:**
 - `InMemorySessionStore`: Fast in-memory storage (default)
 - `FileSessionStore`: Persistent file-based storage
@@ -569,7 +603,7 @@ $server = Server::builder()
 | `setPaginationLimit()` | limit | Set max items per page |
 | `setInstructions()` | instructions | Set usage instructions |
 | `setDiscovery()` | basePath, scanDirs?, excludeDirs?, cache? | Configure attribute discovery |
-| `setSession()` | store?, factory?, ttl? | Configure session management |
+| `setSession()` | sessionStore?, sessionManager?, gcProbability?, gcDivisor? | Configure session management |
 | `setLogger()` | logger | Set PSR-3 logger |
 | `setContainer()` | container | Set PSR-11 container |
 | `setEventDispatcher()` | dispatcher | Set PSR-14 event dispatcher |

src/Server/Builder.php

@@ -78,6 +78,10 @@ final class Builder
 
     private ?SessionStoreInterface $sessionStore = null;
 
+    private int $gcProbability = 1;
+
+    private int $gcDivisor = 100;
+
     private int $paginationLimit = 50;
 
     private ?string $instructions = null;
@@ -329,12 +333,22 @@ public function setResourceSubscriptionManager(SubscriptionManagerInterface $sub
         return $this;
     }
 
+    /**
+     * Configures the session layer.
+     *
+     * @param int $gcProbability The numerator of the GC probability fraction (like PHP's session.gc_probability). Set to 0 to disable GC.
+     * @param int $gcDivisor     The denominator of the GC probability fraction (like PHP's session.gc_divisor). Probability = gcProbability/gcDivisor.
+     */
     public function setSession(
         ?SessionStoreInterface $sessionStore = null,
         ?SessionManagerInterface $sessionManager = null,
+        int $gcProbability = 1,
+        int $gcDivisor = 100,
     ): self {
         $this->sessionStore = $sessionStore;
         $this->sessionManager = $sessionManager;
+        $this->gcProbability = $gcProbability;
+        $this->gcDivisor = $gcDivisor;
 
         if (null !== $sessionManager && null !== $sessionStore) {
             throw new InvalidArgumentException('Cannot set both SessionStore and SessionManager. Set only one or the other.');
@@ -520,6 +534,8 @@ public function build(): Server
         $sessionManager = $this->sessionManager ?? new SessionManager(
             $this->sessionStore ?? new InMemorySessionStore(),
             $logger,
+            $this->gcProbability,
+            $this->gcDivisor,
         );
 
         if (null !== $this->discoveryBasePath) {

src/Server/Session/SessionManager.php

@@ -11,6 +11,7 @@
 
 namespace Mcp\Server\Session;
 
+use Mcp\Exception\InvalidArgumentException;
 use Psr\Log\LoggerInterface;
 use Psr\Log\NullLogger;
 use Symfony\Component\Uid\Uuid;
@@ -22,10 +23,22 @@
  */
 class SessionManager implements SessionManagerInterface
 {
+    /**
+     * @param int $gcProbability The probability (numerator) that GC will run on any given request. Combined with $gcDivisor to calculate the actual probability. Set to 0 to disable GC. Similar to PHP's session.gc_probability.
+     * @param int $gcDivisor     The divisor used with $gcProbability to calculate GC probability. The probability is gcProbability/gcDivisor (e.g. 1/100 = 1%). Similar to PHP's session.gc_divisor.
+     */
     public function __construct(
         private readonly SessionStoreInterface $store,
         private readonly LoggerInterface $logger = new NullLogger(),
+        private readonly int $gcProbability = 1,
+        private readonly int $gcDivisor = 100,
     ) {
+        if ($gcProbability < 0) {
+            throw new InvalidArgumentException('gcProbability must be greater than or equal to 0.');
+        }
+        if ($gcDivisor < 1) {
+            throw new InvalidArgumentException('gcDivisor must be greater than or equal to 1.');
+        }
     }
 
     public function create(): SessionInterface
@@ -54,7 +67,11 @@ public function destroy(Uuid $id): bool
      */
     public function gc(): void
     {
-        if (random_int(0, 100) > 1) {
+        if (0 === $this->gcProbability) {
+            return;
+        }
+
+        if (random_int(1, $this->gcDivisor) > $this->gcProbability) {
             return;
         }
 

tests/Unit/Server/Session/SessionManagerTest.php

@@ -0,0 +1,80 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Tests\Unit\Server\Session;
+
+use Mcp\Exception\InvalidArgumentException;
+use Mcp\Server\Session\InMemorySessionStore;
+use Mcp\Server\Session\SessionManager;
+use PHPUnit\Framework\TestCase;
+
+class SessionManagerTest extends TestCase
+{
+    public function testGcDisabledWhenProbabilityIsZero(): void
+    {
+        $store = $this->createMock(InMemorySessionStore::class);
+        $store->expects($this->never())->method('gc');
+
+        $manager = new SessionManager($store, gcProbability: 0);
+
+        // Call gc many times — it should never trigger
+        for ($i = 0; $i < 100; ++$i) {
+            $manager->gc();
+        }
+    }
+
+    public function testGcAlwaysRunsWhenProbabilityEqualsDivisor(): void
+    {
+        $store = $this->createMock(InMemorySessionStore::class);
+        $store->expects($this->exactly(10))->method('gc')->willReturn([]);
+
+        $manager = new SessionManager($store, gcProbability: 1, gcDivisor: 1);
+
+        for ($i = 0; $i < 10; ++$i) {
+            $manager->gc();
+        }
+    }
+
+    public function testGcAlwaysRunsWhenProbabilityExceedsDivisor(): void
+    {
+        $store = $this->createMock(InMemorySessionStore::class);
+        $store->expects($this->exactly(5))->method('gc')->willReturn([]);
+
+        $manager = new SessionManager($store, gcProbability: 100, gcDivisor: 1);
+
+        for ($i = 0; $i < 5; ++$i) {
+            $manager->gc();
+        }
+    }
+
+    public function testGcProbabilityMustBeNonNegative(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('gcProbability must be greater than or equal to 0.');
+
+        new SessionManager(new InMemorySessionStore(), gcProbability: -1);
+    }
+
+    public function testGcDivisorMustBePositive(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('gcDivisor must be greater than or equal to 1.');
+
+        new SessionManager(new InMemorySessionStore(), gcDivisor: 0);
+    }
+
+    public function testDefaultGcConfiguration(): void
+    {
+        // Default should be 1/100 — just verify construction works
+        $manager = new SessionManager(new InMemorySessionStore());
+        $this->assertInstanceOf(SessionManager::class, $manager);
+    }
+}