[Server] Complete the resource template a completion reference names

CHANGELOG.md

@@ -79,6 +79,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
   * Fix a resource whose text is not valid UTF-8 failing `resources/read`: `ResourceResultFormatter` sends it as a blob with the same MIME type.
   * Encode arrays and objects in results and `TextContent` as compact JSON instead of pretty-printed JSON.
   * Fix `EnumCompletionProvider` and `ListCompletionProvider` offering all values for the prefix `"0"`.
+  * Fix `completion/complete` for a resource template completing the arguments of another template whose pattern matches the referenced URI template, e.g. `weather://{city}` for `weather://{city},{country}`: the template named by the reference is used.
   * Log the name of a called tool at info level; its arguments stay at debug level.
 * Discovery and registration
   * [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.

src/Server/Handler/Request/CompletionCompleteHandler.php

@@ -105,6 +105,15 @@ public function handle(Request $request, SessionInterface $session): Response|Er
      */
     private function resourceCompletionProviders(string $uri): array
     {
+        // Clients name a template by its URI template, which another template's pattern may match too
+        if (str_contains($uri, '{')) {
+            try {
+                return $this->registry->getResourceTemplate($uri)->completionProviders;
+            } catch (ResourceNotFoundException) {
+                // Not a registered template: matched below like any other URI.
+            }
+        }
+
         $reference = $this->registry->getResource($uri);
 
         return $reference instanceof ResourceTemplateReference ? $reference->completionProviders : [];

tests/Unit/Server/Handler/Request/CompletionCompleteHandlerTest.php

@@ -16,7 +16,9 @@
 use Mcp\Schema\Request\CompletionCompleteRequest;
 use Mcp\Schema\ResourceTemplate;
 use Mcp\Schema\Result\CompletionCompleteResult;
+use Mcp\Server\Capability\Completion\ListCompletionProvider;
 use Mcp\Server\Capability\Completion\ProviderInterface;
+use Mcp\Server\Capability\Registry;
 use Mcp\Server\Capability\Registry\ResourceReference;
 use Mcp\Server\Capability\Registry\ResourceTemplateReference;
 use Mcp\Server\Capability\RegistryInterface;
@@ -82,6 +84,26 @@ public function testReturnsCompletionsForResourceTemplate(): void
         $this->assertEquals(new CompletionCompleteResult(['alpha'], 1, false), $response->result);
     }
 
+    public function testCompletesTheTemplateTheReferenceNames(): void
+    {
+        $registry = new Registry();
+        $registry->registerResourceTemplate(new ResourceTemplate('weather://{city}', 'city'), static fn () => null, [
+            'city' => new ListCompletionProvider(['Paris', 'Madrid']),
+        ]);
+        $registry->registerResourceTemplate(new ResourceTemplate('weather://{city},{country}', 'city_in_country'), static fn () => null, [
+            'city' => new ListCompletionProvider(['Paris', 'Madrid']),
+            'country' => new ListCompletionProvider(['France', 'Spain']),
+        ]);
+
+        $handler = new CompletionCompleteHandler($registry);
+        $request = $this->createCompletionRequest('weather://{city},{country}', ['name' => 'country', 'value' => '']);
+
+        $response = $handler->handle($request, $this->session);
+
+        $this->assertInstanceOf(Response::class, $response);
+        $this->assertEquals(new CompletionCompleteResult(['France', 'Spain'], 2, false), $response->result);
+    }
+
     public function testLogsProviderFailureBeforeReturningInternalError(): void
     {
         $uri = 'file://users/alice';