diff --git a/CHANGELOG.md b/CHANGELOG.md
index 96f6000c..22ad2b37 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 0.9.0
 -----
 
+* [BC Break] Remove the `providerClass` argument of `#[CompletionProvider]`. Use `provider:`, which takes the same class-string and is now the first positional argument.
 * Add `HttpTransport::getSessionId()` to read the server-minted `Mcp-Session-Id`: a request-scoped caller can persist it and pass it back through the constructor's `$headers` on a later transport. Always `null` on `2026-07-28`, which removed protocol-level sessions.
 
 0.8.0
diff --git a/src/Capability/Attribute/CompletionProvider.php b/src/Capability/Attribute/CompletionProvider.php
index 9e8dc802..a21963c9 100644
--- a/src/Capability/Attribute/CompletionProvider.php
+++ b/src/Capability/Attribute/CompletionProvider.php
@@ -24,9 +24,9 @@ class CompletionProvider
      * @param class-string<ProviderInterface>|ProviderInterface|null $provider if a class-string, it will be resolved
      *                                                                         from the container at the point of use
      * @param ?array<int, int|float|string>                          $values   a list of values to use for completion
+     * @param class-string|null                                      $enum     an enum whose cases are the completions
      */
     public function __construct(
-        public ?string $providerClass = null,
         public string|ProviderInterface|null $provider = null,
         public ?array $values = null,
         public ?string $enum = null,
diff --git a/src/Capability/Discovery/Discoverer.php b/src/Capability/Discovery/Discoverer.php
index 5b7e4765..c70559fc 100644
--- a/src/Capability/Discovery/Discoverer.php
+++ b/src/Capability/Discovery/Discoverer.php
@@ -325,8 +325,6 @@ private function getCompletionProviders(\ReflectionMethod $reflectionMethod): ar
 
                 if ($attributeInstance->provider) {
                     $completionProviders[$param->getName()] = $attributeInstance->provider;
-                } elseif ($attributeInstance->providerClass) {
-                    $completionProviders[$param->getName()] = $attributeInstance->provider;
                 } elseif ($attributeInstance->values) {
                     $completionProviders[$param->getName()] = new ListCompletionProvider($attributeInstance->values);
                 } elseif ($attributeInstance->enum) {
diff --git a/src/Capability/Registry/Loader/ReflectedElementLoader.php b/src/Capability/Registry/Loader/ReflectedElementLoader.php
index d1e849da..209644c7 100644
--- a/src/Capability/Registry/Loader/ReflectedElementLoader.php
+++ b/src/Capability/Registry/Loader/ReflectedElementLoader.php
@@ -287,7 +287,7 @@ private function getHandlerDescription(\Closure|array|string $handler): string
     }
 
     /**
-     * @return array<string, ProviderInterface>
+     * @return array<string, class-string<ProviderInterface>|ProviderInterface>
      */
     private function getCompletionProviders(\ReflectionMethod|\ReflectionFunction $reflection): array
     {
@@ -307,8 +307,6 @@ private function getCompletionProviders(\ReflectionMethod|\ReflectionFunction $r
 
                 if ($attributeInstance->provider) {
                     $completionProviders[$param->getName()] = $attributeInstance->provider;
-                } elseif ($attributeInstance->providerClass) {
-                    $completionProviders[$param->getName()] = $attributeInstance->providerClass;
                 } elseif ($attributeInstance->values) {
                     $completionProviders[$param->getName()] = new ListCompletionProvider($attributeInstance->values);
                 } elseif ($attributeInstance->enum) {
diff --git a/tests/Integration/CompletionTest.php b/tests/Integration/CompletionTest.php
new file mode 100644
index 00000000..0da021a4
--- /dev/null
+++ b/tests/Integration/CompletionTest.php
@@ -0,0 +1,57 @@
+<?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\Integration;
+
+use Mcp\Schema\PromptReference;
+use PHPUnit\Framework\Attributes\TestDox;
+
+/**
+ * Argument completion driven by a `#[CompletionProvider(provider: …)]` class-string.
+ *
+ * The fixture's provider only exists in the container, so completions coming
+ * back at all is what proves the attribute reached the registry and the
+ * container was asked to build it.
+ *
+ * @see Fixture/completion.php for the server under test
+ */
+final class CompletionTest extends IntegrationTestCase
+{
+    #[TestDox('a class-string provider completes from the container-built instance')]
+    public function testClassStringProviderCompletesFromTheContainer(): void
+    {
+        $client = $this->connect('completion');
+
+        $result = $client->complete(new PromptReference('book_seat'), ['name' => 'seat', 'value' => '12']);
+
+        $this->assertSame(['12A', '12B'], $result->values);
+    }
+
+    #[TestDox('an empty value offers every completion the provider knows')]
+    public function testEmptyValueOffersEveryCompletion(): void
+    {
+        $client = $this->connect('completion');
+
+        $result = $client->complete(new PromptReference('book_seat'), ['name' => 'seat', 'value' => '']);
+
+        $this->assertSame(['12A', '12B', '14C'], $result->values);
+    }
+
+    #[TestDox('an argument the prompt does not declare completes to nothing')]
+    public function testUnknownArgumentCompletesToNothing(): void
+    {
+        $client = $this->connect('completion');
+
+        $result = $client->complete(new PromptReference('book_seat'), ['name' => 'unknown', 'value' => '1']);
+
+        $this->assertSame([], $result->values);
+    }
+}
diff --git a/tests/Integration/Fixture/Completion/BookingElements.php b/tests/Integration/Fixture/Completion/BookingElements.php
new file mode 100644
index 00000000..2a64e5d4
--- /dev/null
+++ b/tests/Integration/Fixture/Completion/BookingElements.php
@@ -0,0 +1,38 @@
+<?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\Integration\Fixture\Completion;
+
+use Mcp\Capability\Attribute\CompletionProvider;
+use Mcp\Capability\Attribute\McpPrompt;
+
+/**
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class BookingElements
+{
+    /**
+     * Confirms a seat booking.
+     *
+     * @param string $seat the seat to book
+     *
+     * @return array the prompt messages
+     */
+    #[McpPrompt(name: 'book_seat')]
+    public function bookSeat(
+        #[CompletionProvider(provider: SeatCompletionProvider::class)]
+        string $seat,
+    ): array {
+        return [
+            ['role' => 'user', 'content' => \sprintf('Book seat %s for me.', $seat)],
+        ];
+    }
+}
diff --git a/tests/Integration/Fixture/Completion/SeatCompletionProvider.php b/tests/Integration/Fixture/Completion/SeatCompletionProvider.php
new file mode 100644
index 00000000..a749fcf7
--- /dev/null
+++ b/tests/Integration/Fixture/Completion/SeatCompletionProvider.php
@@ -0,0 +1,42 @@
+<?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\Integration\Fixture\Completion;
+
+use Mcp\Capability\Completion\ProviderInterface;
+
+/**
+ * A provider that cannot be built without its seat map.
+ *
+ * The constructor takes a scalar the auto-wiring container cannot supply, so a
+ * completion that comes back with seats in it proves the provider was taken
+ * from the container rather than instantiated on the spot.
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class SeatCompletionProvider implements ProviderInterface
+{
+    /**
+     * @param list<string> $seats
+     */
+    public function __construct(
+        private readonly array $seats,
+    ) {
+    }
+
+    public function getCompletions(string $currentValue): array
+    {
+        return array_values(array_filter(
+            $this->seats,
+            static fn (string $seat): bool => str_starts_with($seat, $currentValue),
+        ));
+    }
+}
diff --git a/tests/Integration/Fixture/completion.php b/tests/Integration/Fixture/completion.php
new file mode 100644
index 00000000..f33e9511
--- /dev/null
+++ b/tests/Integration/Fixture/completion.php
@@ -0,0 +1,31 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+/*
+ * Server for {@see \Mcp\Tests\Integration\CompletionTest}.
+ */
+
+use Mcp\Capability\Registry\Container;
+use Mcp\Server;
+use Mcp\Server\Transport\StdioTransport;
+use Mcp\Tests\Integration\Fixture\Completion\SeatCompletionProvider;
+
+require_once dirname(__DIR__, 3).'/vendor/autoload.php';
+
+$container = new Container();
+$container->set(SeatCompletionProvider::class, new SeatCompletionProvider(['12A', '12B', '14C']));
+
+Server::builder()
+    ->setServerInfo('integration-server', '1.0.0')
+    ->setContainer($container)
+    ->setDiscovery(__DIR__, ['Completion'])
+    ->build()
+    ->run(new StdioTransport());
diff --git a/tests/Unit/Capability/Attribute/CompletionProviderTest.php b/tests/Unit/Capability/Attribute/CompletionProviderTest.php
index 19a78750..4b5269d3 100644
--- a/tests/Unit/Capability/Attribute/CompletionProviderTest.php
+++ b/tests/Unit/Capability/Attribute/CompletionProviderTest.php
@@ -26,6 +26,15 @@ public function testCanBeConstructedWithProviderClass(): void
         $this->assertNull($attribute->enum);
     }
 
+    public function testCanBeConstructedWithAPositionalProviderClass(): void
+    {
+        $attribute = new CompletionProvider(CompletionProviderFixture::class);
+
+        $this->assertSame(CompletionProviderFixture::class, $attribute->provider);
+        $this->assertNull($attribute->values);
+        $this->assertNull($attribute->enum);
+    }
+
     public function testCanBeConstructedWithProviderInstance(): void
     {
         $instance = new CompletionProviderFixture();
diff --git a/tests/Unit/Capability/Discovery/DiscoveryTest.php b/tests/Unit/Capability/Discovery/DiscoveryTest.php
index 5c716d7d..628aaa6a 100644
--- a/tests/Unit/Capability/Discovery/DiscoveryTest.php
+++ b/tests/Unit/Capability/Discovery/DiscoveryTest.php
@@ -94,7 +94,7 @@ public function testDiscoversAllElementTypesCorrectlyFromFixtureFiles(): void
         $this->assertEquals([InvocablePromptFixture::class, '__invoke'], $prompts['InvokableGreeterPrompt']->handler);
 
         $this->assertArrayHasKey('content_creator', $prompts);
-        $this->assertCount(3, $prompts['content_creator']->completionProviders);
+        $this->assertCount(4, $prompts['content_creator']->completionProviders);
 
         $templates = $discovery->getResourceTemplates();
         $this->assertCount(4, $templates);
@@ -165,7 +165,7 @@ public function testDiscoversEnhancedCompletionProvidersWithValuesAndEnumAttribu
         $discovery = $this->discoverer->discover(__DIR__, ['Fixtures']);
 
         $this->assertArrayHasKey('content_creator', $prompts = $discovery->getPrompts());
-        $this->assertCount(3, $prompts['content_creator']->completionProviders);
+        $this->assertCount(4, $prompts['content_creator']->completionProviders);
 
         $typeProvider = $prompts['content_creator']->completionProviders['type'];
         $this->assertInstanceOf(ListCompletionProvider::class, $typeProvider);
@@ -182,4 +182,14 @@ public function testDiscoversEnhancedCompletionProvidersWithValuesAndEnumAttribu
         $categoryProvider = $templates['content://{category}/{slug}']->completionProviders['category'];
         $this->assertInstanceOf(ListCompletionProvider::class, $categoryProvider);
     }
+
+    public function testDiscoversPositionalCompletionProviderAsClassString(): void
+    {
+        $discovery = $this->discoverer->discover(__DIR__, ['Fixtures']);
+
+        $this->assertArrayHasKey('content_creator', $prompts = $discovery->getPrompts());
+
+        // Kept as a class-string so the container resolves it at the point of use.
+        $this->assertEquals(CompletionProviderFixture::class, $prompts['content_creator']->completionProviders['author']);
+    }
 }
diff --git a/tests/Unit/Capability/Discovery/Fixtures/EnhancedCompletionHandler.php b/tests/Unit/Capability/Discovery/Fixtures/EnhancedCompletionHandler.php
index 5ac67ddb..9975f9a6 100644
--- a/tests/Unit/Capability/Discovery/Fixtures/EnhancedCompletionHandler.php
+++ b/tests/Unit/Capability/Discovery/Fixtures/EnhancedCompletionHandler.php
@@ -14,13 +14,14 @@
 use Mcp\Capability\Attribute\CompletionProvider;
 use Mcp\Capability\Attribute\McpPrompt;
 use Mcp\Capability\Attribute\McpResourceTemplate;
+use Mcp\Tests\Unit\Capability\Attribute\CompletionProviderFixture;
 use Mcp\Tests\Unit\Fixtures\Enum\PriorityEnum;
 use Mcp\Tests\Unit\Fixtures\Enum\StatusEnum;
 
 class EnhancedCompletionHandler
 {
     /**
-     * Create content with list and enum completion providers.
+     * Create content with list, enum and positional provider completion providers.
      */
     #[McpPrompt(name: 'content_creator')]
     public function createContent(
@@ -30,9 +31,11 @@ public function createContent(
         string $status,
         #[CompletionProvider(enum: PriorityEnum::class)]
         string $priority,
+        #[CompletionProvider(CompletionProviderFixture::class)]
+        string $author,
     ): array {
         return [
-            ['role' => 'user', 'content' => "Create a {$type} with status {$status} and priority {$priority}"],
+            ['role' => 'user', 'content' => "Create a {$type} with status {$status} and priority {$priority} for {$author}"],
         ];
     }