diff --git a/CHANGELOG.md b/CHANGELOG.md
index 69c8beb0..8fd07df6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 
 * [BC Break] `DocBlockParser::parseDocBlock()` no longer accepts `null`; pass `ReflectionMethod::getDocComment()` as is
 * [BC Break] Fix a `DELETE` of an unknown or ended session being answered `200` instead of the `404` the spec requires: `Protocol::destroySession()` and `BaseTransport::handleSessionEnd()` return `false` for such a session, as does the listener of `TransportInterface::onSessionEnd()`. A listener that returns nothing keeps the `200`.
+* [BC Break] `Builder::build()` and `Builder::buildStateless()` throw a `RuntimeException` when `setDiscovery()` is used without `symfony/finder` and without a custom discoverer, instead of logging a warning and discovering nothing
 * Fix tools with an array shape parameter whose keys are nullable or unions (e.g. `array{name: string, nickname: string|null}`) rejecting valid arguments with `-32602`: a `null` inside the shape no longer turns the parameter into `array|null`, and such keys get their own types instead of `object`, including lists and nested shapes (`string[]|null`, `array{x: int}|null`).
 * Fix `array<string|null>` and other generics with a union element getting `items` of type `object`. A `null` inside a generic no longer makes the parameter nullable, and neither does a DocBlock class whose name contains `null`.
 * Fix `InMemorySessionStore::exists()` reporting a session past its TTL as existing, while `read()` already treated it as gone.
diff --git a/docs/index.md b/docs/index.md
index da009ef9..1eb28a3a 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, building the server throws a `RuntimeException`.
 
 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 11d2a361..16e581a7 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()` and `buildStateless()` throw a `RuntimeException`.
+You don't need the package if 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 9bb28c81..12c62003 100644
--- a/src/Server/Builder.php
+++ b/src/Server/Builder.php
@@ -73,7 +73,6 @@
 use Psr\Log\LoggerInterface;
 use Psr\Log\NullLogger;
 use Psr\SimpleCache\CacheInterface;
-use Symfony\Component\Finder\Finder;
 
 /**
  * @phpstan-import-type Handler from ElementReference
@@ -1073,12 +1072,8 @@ 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..55abe99e 100644
--- a/tests/Unit/Server/BuilderTest.php
+++ b/tests/Unit/Server/BuilderTest.php
@@ -11,6 +11,8 @@
 
 namespace Mcp\Tests\Unit\Server;
 
+use Composer\Autoload\ClassLoader;
+use Mcp\Capability\Discovery\DiscovererInterface;
 use Mcp\Capability\Registry;
 use Mcp\Capability\Registry\ElementReference;
 use Mcp\Capability\Registry\Loader\LoaderInterface;
@@ -18,6 +20,7 @@
 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;
@@ -41,9 +44,11 @@
 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 Symfony\Component\Finder\Finder;
 
 final class BuilderTest extends TestCase
 {
@@ -81,6 +86,28 @@ 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
+    {
+        foreach (ClassLoader::getRegisteredLoaders() as $loader) {
+            $loader->setPsr4('Symfony\\Component\\Finder\\', []);
+        }
+        $this->assertFalse(class_exists(Finder::class));
+
+        $this->assertInstanceOf(Server::class, Server::builder()->setServerInfo('test', '1.0.0')->build());
+        $this->assertInstanceOf(Server::class, Server::builder()
+            ->setServerInfo('test', '1.0.0')
+            ->setDiscovery(__DIR__)
+            ->setDiscoverer($this->createStub(DiscovererInterface::class))
+            ->build());
+
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('File-based discovery requires symfony/finder. Run: composer require symfony/finder');
+
+        Server::builder()->setServerInfo('test', '1.0.0')->setDiscovery(__DIR__)->build();
+    }
+
     #[TestDox('Custom ReferenceHandler is used when calling a tool')]
     public function testCustomReferenceHandlerIsUsedForToolCalls(): void
     {