[Capability] Do not announce an externally loaded registry as changed

`Registry` suppresses its `*ListChangedEvent`s while it is loading — `dispatch()`
returns early on the `loading` guard, so the elements a loader registers are the
registry's initial contents rather than a change to them.
`testListChangedEventsAreSuppressedDuringTheDeferredLoad` pins that.

The guard only covers `Registry::load()`, which needs the loader the constructor
took. A registry the caller built cannot be given one that way, so
`Builder::resolve()` loads it from the outside instead — by calling
`$chainLoader->load($registry)` directly, which never sets the guard. Every
element then dispatches on the way in.

With a notification bus configured that is not quiet. `PublishingEventDispatcher`
turns each event into a published notification, so every `build()` puts one
`list_changed` per element on the bus for a registry that did not change. Under
PHP-FPM, where the server is built per request and `Psr16NotificationBus` is
shared and persistent, every request broadcasts its whole element list to every
open `subscriptions/listen` stream and consumes the 256-entry backlog — after
which a reader that fell behind silently skips real notifications.

Split the guarded body of `load()` into `loadFrom(LoaderInterface $loader)` and
route the custom-registry branch through it.

`loadFrom()` takes only the `loading` guard, not the `loaded` bookkeeping, which
stays with `load()`. The loader it runs belongs to the caller, so it cannot stand
in for the one the registry was constructed with: marking the registry loaded
would retire a constructor loader that never ran, and would make a second
`loadFrom()` — one registry handed to two builders — a silent no-op. Keeping
`loaded` out of it also keeps `load()`'s promise that a transient failure is
retried on the next read, including when the failing loader reads the registry
during its own run.

`RegistryInterface` declares no `load()`, so this stays on the concrete
`Registry`, and a third-party implementation keeps the path it has today.

Author: chr-hertel

src/Capability/Registry.php

@@ -88,10 +88,28 @@ public function load(): void
             return;
         }
 
+        $this->loadFrom($this->loader);
+
+        // Only on success: a failure propagates, so it is retried on the next read.
+        $this->loaded = true;
+    }
+
+    /**
+     * Runs $loader with the change events its registrations would dispatch suppressed, since they
+     * describe the registry filling up rather than changing.
+     *
+     * Re-entrant-safe, and failure propagates. Does not mark the registry loaded: $loader is the
+     * caller's, and the one the constructor took is still owed its run.
+     */
+    public function loadFrom(LoaderInterface $loader): void
+    {
+        if ($this->loading) {
+            return;
+        }
+
         $this->loading = true;
         try {
-            $this->loader->load($this);
-            $this->loaded = true;
+            $loader->load($this);
         } finally {
             $this->loading = false;
         }

src/Server/Builder.php

@@ -522,7 +522,8 @@ public function setRegistry(RegistryInterface $registry): self
      *
      * Lazy (the default) defers loading to the first registry read so a persistent runtime does not
      * freeze the registry to a source not yet ready at build time. Disable to load eagerly at build.
-     * A registry supplied via setRegistry() is always loaded eagerly.
+     * A registry supplied via setRegistry() is always loaded eagerly; its own constructor loader,
+     * if it has one, still runs on the first read.
      */
     public function setLazyLoading(bool $lazyLoading = true): self
     {
@@ -1045,8 +1046,13 @@ private function resolve(): array
 
         if ($this->hasCustomRegistry) {
             // Builder can't inject the loader into an already-constructed instance, so load it eagerly.
+            // Via loadFrom(), which suppresses the change events the load would otherwise dispatch.
             $registry = $this->registry;
-            $chainLoader->load($registry);
+            if ($registry instanceof Registry) {
+                $registry->loadFrom($chainLoader);
+            } else {
+                $chainLoader->load($registry);
+            }
             $eagerlyLoaded = true;
         } else {
             $registry = new Registry($eventDispatcher, $logger, loader: $chainLoader);

tests/Unit/Capability/RegistryTest.php

@@ -753,6 +753,74 @@ public function load(RegistryInterface $registry): void
         $this->assertTrue($registry->hasPrompts());
     }
 
+    public function testListChangedEventsAreSuppressedWhenTheLoaderIsSuppliedFromOutside(): void
+    {
+        // As quiet as the deferred load above: these are the initial contents, not a change.
+        $eventDispatcher = $this->createMock(EventDispatcherInterface::class);
+        $eventDispatcher->expects($this->never())->method('dispatch');
+
+        $registry = new Registry($eventDispatcher, $this->logger);
+        $registry->loadFrom($this->toolLoader($this->createValidTool('loaded')));
+
+        $this->assertTrue($registry->hasTool('loaded'));
+    }
+
+    public function testARuntimeRegistrationAfterAnExternalLoadIsStillDispatched(): void
+    {
+        $eventDispatcher = $this->createMock(EventDispatcherInterface::class);
+        $eventDispatcher->expects($this->once())
+            ->method('dispatch')
+            ->with($this->isInstanceOf(ToolListChangedEvent::class))
+            ->willReturnArgument(0);
+
+        $registry = new Registry($eventDispatcher, $this->logger);
+        $registry->loadFrom($this->toolLoader($this->createValidTool('loaded')));
+
+        $registry->registerTool($this->createValidTool('runtime'), 'handler');
+    }
+
+    public function testAnExternalLoadDoesNotConsumeTheRegistrysOwnLoader(): void
+    {
+        $registry = new Registry($this->createMock(EventDispatcherInterface::class), $this->logger, loader: $this->toolLoader($this->createValidTool('own')));
+        $registry->loadFrom($this->toolLoader($this->createValidTool('external')));
+
+        $this->assertTrue($registry->hasTool('external'));
+        $this->assertTrue($registry->hasTool('own'));
+    }
+
+    public function testAnExternalLoadCanRunMoreThanOnce(): void
+    {
+        $registry = new Registry($this->createMock(EventDispatcherInterface::class), $this->logger);
+        $registry->loadFrom($this->toolLoader($this->createValidTool('first')));
+        $registry->loadFrom($this->toolLoader($this->createValidTool('second')));
+
+        $this->assertTrue($registry->hasTool('first'));
+        $this->assertTrue($registry->hasTool('second'));
+    }
+
+    public function testAFailedExternalLoadLeavesTheRegistryRetryable(): void
+    {
+        $registry = new Registry($this->createMock(EventDispatcherInterface::class), $this->logger);
+        $failing = new class implements LoaderInterface {
+            public function load(RegistryInterface $registry): void
+            {
+                // Reads during its own run, as discovery's identity check does.
+                $registry->hasTool('anything');
+
+                throw new \RuntimeException('data source not ready');
+            }
+        };
+
+        foreach ([1, 2] as $attempt) {
+            try {
+                $registry->loadFrom($failing);
+                $this->fail('The loader was expected to fail.');
+            } catch (\RuntimeException $e) {
+                $this->assertSame('data source not ready', $e->getMessage());
+            }
+        }
+    }
+
     public function testListChangedEventsAreStillDispatchedForRuntimeRegistrations(): void
     {
         $eventDispatcher = $this->createMock(EventDispatcherInterface::class);

tests/Unit/Server/BuilderTest.php

@@ -15,6 +15,7 @@
 use Mcp\Capability\Registry\ElementReference;
 use Mcp\Capability\Registry\Loader\LoaderInterface;
 use Mcp\Capability\Registry\ReferenceHandlerInterface;
+use Mcp\Capability\RegistryInterface;
 use Mcp\Exception\InvalidArgumentException;
 use Mcp\Exception\LogicException;
 use Mcp\Schema\Content\TextContent;
@@ -32,6 +33,8 @@
 use Mcp\Server\Protocol;
 use Mcp\Server\Session\SessionInterface;
 use Mcp\Server\Stateless\StatelessProtocol;
+use Mcp\Server\Subscription\InMemoryNotificationBus;
+use Mcp\Server\Subscription\PublishingEventDispatcher;
 use Mcp\Tests\Unit\Server\Extension\ThingExtension;
 use Mcp\Tests\Unit\Server\Extension\ThingListHandler;
 use Mcp\Tests\Unit\Server\Extension\ThingListRequest;
@@ -404,6 +407,42 @@ private function callTool(Server $server, string $toolName): mixed
 
         $this->fail('CallToolHandler not found in request handlers');
     }
+
+    public function testBuildingWithASuppliedRegistryDoesNotAnnounceTheLoadAsAChange(): void
+    {
+        // Otherwise every build publishes one list_changed per element, for no change.
+        $bus = new InMemoryNotificationBus();
+        $registry = new Registry(new PublishingEventDispatcher($bus));
+
+        Server::builder()
+            ->setRegistry($registry)
+            ->setNotificationBus($bus)
+            ->addTool(static fn (): string => 'ok', 'alpha')
+            ->addTool(static fn (): string => 'ok', 'beta')
+            ->build();
+
+        $this->assertSame(0, $bus->cursor());
+        $this->assertTrue($registry->hasTool('alpha'));
+        $this->assertTrue($registry->hasTool('beta'));
+
+        // A real change after the build is still published.
+        $registry->unregisterTool('alpha');
+
+        $this->assertSame(1, $bus->cursor());
+    }
+
+    public function testAThirdPartyRegistryIsStillLoadedThroughThePlainLoader(): void
+    {
+        $registry = $this->createMock(RegistryInterface::class);
+        $registry->expects($this->once())
+            ->method('registerTool')
+            ->with($this->callback(static fn (Tool $tool): bool => 'alpha' === $tool->name));
+
+        Server::builder()
+            ->setRegistry($registry)
+            ->addTool(static fn (): string => 'ok', 'alpha')
+            ->build();
+    }
 }
 
 /**