fix: allow dynamic handler registration after connect with pre-supplied capabilities

.changeset/fix-capabilities-reregistration-after-connect.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/sdk': patch
+---
+
+Allow dynamic tool/resource/prompt registration after `connect()` when capabilities were pre-supplied at construction, by making `registerCapabilities` idempotent for already-present capability keys.

src/server/index.ts

@@ -207,7 +207,14 @@ export class Server<
      */
     public registerCapabilities(capabilities: ServerCapabilities): void {
         if (this.transport) {
-            throw new Error('Cannot register capabilities after connecting to transport');
+            // After connecting, allow if all requested capability keys are already present
+            // (supports dynamic handler registration when capabilities were pre-supplied at construction).
+            for (const key in capabilities) {
+                if (!(key in this._capabilities)) {
+                    throw new Error('Cannot register capabilities after connecting to transport');
+                }
+            }
+            return;
         }
         this._capabilities = mergeCapabilities(this._capabilities, capabilities);
     }

test/server/mcp.test.ts

@@ -321,6 +321,27 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
             ]);
         });
 
+        /***
+         * Test: Dynamic tool registration after connect with pre-supplied capabilities
+         */
+        test('should allow registering tools after connect when capabilities were pre-supplied', async () => {
+            const mcpServer = new McpServer({ name: 'test server', version: '1.0' }, { capabilities: { tools: { listChanged: true } } });
+            const client = new Client({ name: 'test client', version: '1.0' });
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
+
+            // This should NOT throw "Cannot register capabilities after connecting to transport"
+            mcpServer.tool('dynamic-tool', async () => ({
+                content: [{ type: 'text', text: 'Dynamic response' }]
+            }));
+
+            const result = await client.request({ method: 'tools/list' }, ListToolsResultSchema);
+
+            expect(result.tools).toHaveLength(1);
+            expect(result.tools[0].name).toBe('dynamic-tool');
+        });
+
         /***
          * Test: Updating Existing Tool
          */
@@ -2091,6 +2112,29 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
             expect(result.resources[0].uri).toBe('test://resource');
         });
 
+        /***
+         * Test: Dynamic resource registration after connect with pre-supplied capabilities
+         */
+        test('should allow registering resources after connect when capabilities were pre-supplied', async () => {
+            const mcpServer = new McpServer(
+                { name: 'test server', version: '1.0' },
+                { capabilities: { resources: { listChanged: true } } }
+            );
+            const client = new Client({ name: 'test client', version: '1.0' });
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+            await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
+
+            mcpServer.resource('dynamic-resource', 'test://dynamic', async () => ({
+                contents: [{ uri: 'test://dynamic', text: 'Dynamic content' }]
+            }));
+
+            const result = await client.request({ method: 'resources/list' }, ListResourcesResultSchema);
+
+            expect(result.resources).toHaveLength(1);
+            expect(result.resources[0].name).toBe('dynamic-resource');
+        });
+
         /***
          * Test: Update Resource with URI
          */