diff --git a/CHANGELOG.md b/CHANGELOG.md
index 468c1e9d..69f504f2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 All notable changes to `mcp/sdk` will be documented in this file.
 
+0.10.0
+------
+
+* [BC Break] `Builder::build()` throws a `RuntimeException` when `setDiscovery()` is used without `symfony/finder` and without a custom discoverer, instead of logging a warning and discovering nothing
+
 0.9.0
 -----
 
diff --git a/docs/index.md b/docs/index.md
index da009ef9..7b947287 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -74,7 +74,7 @@ That's a complete MCP server. It exposes one **tool**, `add`, and one **resource
 `config://calculator/settings`.
 
 Attribute discovery needs `symfony/finder` (`composer require symfony/finder`). Without
-it the server still starts, but discovers nothing and only logs a warning.
+it, `build()` throws a `RuntimeException` instead of starting a server with no tools.
 
 Look at what you did *not* write: no JSON Schema — `int $a, int $b` *is* the schema —
 no request parsing, no serialization, no protocol handling. You wrote a PHP class with
diff --git a/docs/run/server-builder.md b/docs/run/server-builder.md
index b1fb6a16..ab079ef9 100644
--- a/docs/run/server-builder.md
+++ b/docs/run/server-builder.md
@@ -160,7 +160,8 @@ $server = Server::builder()
     );
 ```
 
-Discovery needs the `symfony/finder` package. Without it, the server skips discovery and logs a warning.
+Discovery needs the `symfony/finder` package. Without it, `build()` throws a `RuntimeException`, unless you set your own
+discoverer with `setDiscoverer()`.
 See [First server](../get-started/first-server.md) for the install command.
 
 **Parameters:**
diff --git a/src/Server/Builder.php b/src/Server/Builder.php
index 13f1eb59..1b8cbb07 100644
--- a/src/Server/Builder.php
+++ b/src/Server/Builder.php
@@ -71,7 +71,6 @@
 use Psr\Log\LoggerInterface;
 use Psr\Log\NullLogger;
 use Psr\SimpleCache\CacheInterface;
-use Symfony\Component\Finder\Finder;
 
 /**
  * @phpstan-import-type Handler from ElementReference
@@ -1070,12 +1069,8 @@ private function resolve(): array
         ];
 
         if (null !== $this->discoveryBasePath) {
-            if (null !== $this->discoverer || class_exists(Finder::class)) {
-                $discoverer = $this->discoverer ?? $this->createDiscoverer($logger);
-                $loaders[] = new DiscoveryLoader($this->discoveryBasePath, $this->discoveryScanDirs, $this->discoveryExcludeDirs, $discoverer, $this->discoveryNamePatterns, $logger);
-            } else {
-                $logger->warning('File-based discovery requires symfony/finder. Skipping automatic discovery. Run: composer require symfony/finder');
-            }
+            $discoverer = $this->discoverer ?? $this->createDiscoverer($logger);
+            $loaders[] = new DiscoveryLoader($this->discoveryBasePath, $this->discoveryScanDirs, $this->discoveryExcludeDirs, $discoverer, $this->discoveryNamePatterns, $logger);
         }
 
         $chainLoader = new ChainLoader($loaders);
diff --git a/tests/Unit/Server/BuilderTest.php b/tests/Unit/Server/BuilderTest.php
index 53c793f0..40299117 100644
--- a/tests/Unit/Server/BuilderTest.php
+++ b/tests/Unit/Server/BuilderTest.php
@@ -11,6 +11,9 @@
 
 namespace Mcp\Tests\Unit\Server;
 
+use Composer\Autoload\ClassLoader;
+use Mcp\Capability\Discovery\Discoverer;
+use Mcp\Capability\Discovery\DiscovererInterface;
 use Mcp\Capability\Registry;
 use Mcp\Capability\Registry\ElementReference;
 use Mcp\Capability\Registry\Loader\LoaderInterface;
@@ -18,6 +21,7 @@
 use Mcp\Capability\RegistryInterface;
 use Mcp\Exception\InvalidArgumentException;
 use Mcp\Exception\LogicException;
+use Mcp\Exception\RuntimeException;
 use Mcp\Schema\Content\TextContent;
 use Mcp\Schema\Enum\ProtocolVersion;
 use Mcp\Schema\Extension\Apps\McpApps;
@@ -41,12 +45,20 @@
 use Mcp\Tests\Unit\Server\Extension\ThingListRequest;
 use Mcp\Tests\Unit\Server\Session\Fixture\RecordingSessionLock;
 use Mcp\Tests\Unit\Server\Session\Fixture\RecordingSessionStore;
+use PHPUnit\Framework\Attributes\RunInSeparateProcess;
 use PHPUnit\Framework\Attributes\TestDox;
 use PHPUnit\Framework\TestCase;
 use Psr\EventDispatcher\EventDispatcherInterface;
+use Psr\Log\LogLevel;
+use Symfony\Component\Finder\Finder;
 
 final class BuilderTest extends TestCase
 {
+    private const DISCOVERY_DEPENDENCY_ERROR = 'File-based discovery requires symfony/finder. Run: composer require symfony/finder';
+    private const FINDER_LOADED_ERROR = 'Symfony Finder was loaded after Composer autoloaders were disabled.';
+    private const TEST_SERVER_NAME = 'test';
+    private const TEST_SERVER_VERSION = '1.0.0';
+
     #[TestDox('setReferenceHandler() returns the builder for fluent chaining')]
     public function testSetReferenceHandlerReturnsSelf(): void
     {
@@ -81,6 +93,49 @@ public function testBuildWithoutCustomReferenceHandler(): void
         $this->assertInstanceOf(Server::class, $server);
     }
 
+    #[RunInSeparateProcess]
+    #[TestDox('build() fails when file-based discovery is configured without Symfony Finder')]
+    public function testBuildFailsWhenDiscoveryDependencyIsMissing(): void
+    {
+        // Load every class the failing path needs before the Composer autoloaders are unregistered.
+        Server::builder()->setServerInfo(self::TEST_SERVER_NAME, self::TEST_SERVER_VERSION)->build();
+        Server::builder()
+            ->setServerInfo(self::TEST_SERVER_NAME, self::TEST_SERVER_VERSION)
+            ->setDiscovery(__DIR__)
+            ->setDiscoverer($this->createStub(DiscovererInterface::class))
+            ->build();
+        class_exists(Discoverer::class);
+        class_exists(RuntimeException::class);
+        class_exists(LogLevel::class);
+
+        $autoloaders = array_values(array_filter(
+            spl_autoload_functions(),
+            static fn (mixed $autoloader): bool => \is_array($autoloader) && $autoloader[0] instanceof ClassLoader,
+        ));
+        $this->assertNotEmpty($autoloaders);
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage(self::DISCOVERY_DEPENDENCY_ERROR);
+
+        foreach ($autoloaders as $autoloader) {
+            $autoloader[0]->unregister();
+        }
+
+        try {
+            if (class_exists(Finder::class)) {
+                throw new LogicException(self::FINDER_LOADED_ERROR);
+            }
+
+            Server::builder()
+                ->setServerInfo(self::TEST_SERVER_NAME, self::TEST_SERVER_VERSION)
+                ->setDiscovery(__DIR__)
+                ->build();
+        } finally {
+            foreach ($autoloaders as $autoloader) {
+                $autoloader[0]->register(true);
+            }
+        }
+    }
+
     #[TestDox('Custom ReferenceHandler is used when calling a tool')]
     public function testCustomReferenceHandlerIsUsedForToolCalls(): void
     {