No automatic completion support unless needed - Revisited yet again (#1237)

Author: cliffhall

src/server/mcp.test.ts

@@ -2669,6 +2669,41 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
             ).rejects.toThrow(/Resource test:\/\/nonexistent not found/);
         });
 
+        /***
+         * Test: Registering a resource template without a complete callback should not update server capabilities to advertise support for completion
+         */
+        test('should not advertise support for completion when a resource template without a complete callback is defined', async () => {
+            const mcpServer = new McpServer({
+                name: 'test server',
+                version: '1.0'
+            });
+            const client = new Client({
+                name: 'test client',
+                version: '1.0'
+            });
+
+            mcpServer.resource(
+                'test',
+                new ResourceTemplate('test://resource/{category}', {
+                    list: undefined
+                }),
+                async () => ({
+                    contents: [
+                        {
+                            uri: 'test://resource/test',
+                            text: 'Test content'
+                        }
+                    ]
+                })
+            );
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+            await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
+
+            expect(client.getServerCapabilities()).not.toHaveProperty('completions');
+        });
+
         /***
          * Test: Registering a resource template with a complete callback should update server capabilities to advertise support for completion
          */
@@ -3548,6 +3583,46 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
             ).rejects.toThrow(/Prompt nonexistent-prompt not found/);
         });
 
+        /***
+         * Test: Registering a prompt without a completable argument should not update server capabilities to advertise support for completion
+         */
+        test('should not advertise support for completion when a prompt without a completable argument is defined', async () => {
+            const mcpServer = new McpServer({
+                name: 'test server',
+                version: '1.0'
+            });
+            const client = new Client({
+                name: 'test client',
+                version: '1.0'
+            });
+
+            mcpServer.prompt(
+                'test-prompt',
+                {
+                    name: z.string()
+                },
+                async ({ name }) => ({
+                    messages: [
+                        {
+                            role: 'assistant',
+                            content: {
+                                type: 'text',
+                                text: `Hello ${name}`
+                            }
+                        }
+                    ]
+                })
+            );
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+            await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
+
+            const capabilities = client.getServerCapabilities() || {};
+            const keys = Object.keys(capabilities);
+            expect(keys).not.toContain('completions');
+        });
+
         /***
          * Test: Registering a prompt with a completable argument should update server capabilities to advertise support for completion
          */

src/server/mcp.ts

@@ -62,6 +62,7 @@ import { Transport } from '../shared/transport.js';
 import { validateAndWarnToolName } from '../shared/toolNameValidation.js';
 import { ExperimentalMcpServerTasks } from '../experimental/tasks/mcp-server.js';
 import type { ToolTaskHandler } from '../experimental/tasks/interfaces.js';
+import { ZodOptional } from 'zod';
 
 /**
  * High-level MCP server that provides a simpler API for working with resources, tools, and prompts.
@@ -557,8 +558,6 @@ export class McpServer {
             throw new McpError(ErrorCode.InvalidParams, `Resource ${uri} not found`);
         });
 
-        this.setCompletionRequestHandler();
-
         this._resourceHandlersInitialized = true;
     }
 
@@ -623,8 +622,6 @@ export class McpServer {
             }
         });
 
-        this.setCompletionRequestHandler();
-
         this._promptHandlersInitialized = true;
     }
 
@@ -815,6 +812,14 @@ export class McpServer {
             }
         };
         this._registeredResourceTemplates[name] = registeredResourceTemplate;
+
+        // If the resource template has any completion callbacks, enable completions capability
+        const variableNames = template.uriTemplate.variableNames;
+        const hasCompleter = Array.isArray(variableNames) && variableNames.some(v => !!template.completeCallback(v));
+        if (hasCompleter) {
+            this.setCompletionRequestHandler();
+        }
+
         return registeredResourceTemplate;
     }
 
@@ -848,6 +853,18 @@ export class McpServer {
             }
         };
         this._registeredPrompts[name] = registeredPrompt;
+
+        // If any argument uses a Completable schema, enable completions capability
+        if (argsSchema) {
+            const hasCompletable = Object.values(argsSchema).some(field => {
+                const inner: unknown = field instanceof ZodOptional ? field._def?.innerType : field;
+                return isCompletable(inner);
+            });
+            if (hasCompletable) {
+                this.setCompletionRequestHandler();
+            }
+        }
+
         return registeredPrompt;
     }