Handle notifications correctly

Author: adwsingh

mcp/mcp-schemas/model/main.smithy

@@ -9,7 +9,6 @@ structure JsonRpcRequest {
     @required
     method: String
 
-    @required
     id: Document
 
     params: Document
@@ -23,7 +22,6 @@ structure JsonRpcResponse {
 
     error: JsonRpcErrorResponse
 
-    @required
     id: Document
 }
 

mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/McpServer.java

@@ -182,10 +182,19 @@ private void handleRequest(JsonRpcRequest req) {
 
     private void validate(JsonRpcRequest req) {
         Document id = req.getId();
-        if (!(id.isType(ShapeType.INTEGER) || id.isType(ShapeType.STRING))) {
-            throw ValidationException.builder()
-                    .message("Request id is of invalid type " + id.type().name())
-                    .build();
+        boolean isRequest = !req.getMethod().startsWith("notifications/");
+        if (isRequest) {
+            if (id == null) {
+                throw ValidationException.builder()
+                        .withoutStackTrace()
+                        .message("Requests are expected to have ids")
+                        .build();
+            } else if (!(id.isType(ShapeType.INTEGER) || id.isType(ShapeType.STRING))) {
+                throw ValidationException.builder()
+                        .withoutStackTrace()
+                        .message("Request id is of invalid type " + id.type().name())
+                        .build();
+            }
         }
     }
 

mcp/mcp-server/src/test/java/software/amazon/smithy/java/mcp/server/McpServerTest.java

@@ -204,6 +204,27 @@ void testInvalidIds() {
         assertTrue(response.getError().getMessage().contains("Request id is of invalid type"));
     }
 
+    @Test
+    void testRequestsRequireIds() {
+        server = McpServer.builder()
+                .input(input)
+                .output(output)
+                .addService(ProxyService.builder()
+                        .service(ShapeId.from("smithy.test#TestService"))
+                        .proxyEndpoint("http://localhost")
+                        .model(MODEL)
+                        .build())
+                .build();
+
+        server.start();
+
+        // Test regular request without ID (should fail with specific message)
+        write("tools/list", Document.of(Map.of()), null);
+        var response = read();
+        assertNotNull(response.getError());
+        assertTrue(response.getError().getMessage().contains("Requests are expected to have ids"));
+    }
+
     @Test
     void testInputAdaptation() {
         AtomicReference<StructDocument> capturedInput = new AtomicReference<>();
@@ -309,6 +330,34 @@ public void readBeforeSerialization(InputHook<?, ?> hook) {
         server.shutdown().join();
     }
 
+    @Test
+    void testNotificationsDoNotRequireRequestId() {
+        server = McpServer.builder()
+                .input(input)
+                .output(output)
+                .addService(ProxyService.builder()
+                        .service(ShapeId.from("smithy.test#TestService"))
+                        .proxyEndpoint("http://localhost")
+                        .model(MODEL)
+                        .build())
+                .build();
+
+        server.start();
+
+        // Test notifications/initialized without ID (should not produce any error response)
+        writeNotification("notifications/initialized", Document.of(Map.of()));
+        output.assertNoOutput();
+
+        // Test another notification to ensure it's consistently handled
+        writeNotification("notifications/progress", Document.of(Map.of("progressToken", Document.of("test"))));
+        output.assertNoOutput();
+
+        // Send a regular request to verify the server is still functioning
+        write("tools/list", Document.of(Map.of()));
+        var response = read();
+        assertNotNull(response.getResult());
+    }
+
     private void validateNestedStructure(Map<String, Document> properties) {
         var nestedStr = properties.get("nestedStr").asStringMap();
         assertEquals("string", nestedStr.get("type").asString());
@@ -348,6 +397,16 @@ private JsonRpcResponse read() {
         return CODEC.deserializeShape(line, JsonRpcResponse.builder());
     }
 
+    private void writeNotification(String method, Document params) {
+        var request = JsonRpcRequest.builder()
+                .method(method)
+                .params(params)
+                .jsonrpc("2.0")
+                .build();
+        input.write(CODEC.serializeToString(request));
+        input.write("\n");
+    }
+
     private static final String MODEL_STR = """
             $version: "2"
 

mcp/mcp-server/src/test/java/software/amazon/smithy/java/mcp/server/TestOutputStream.java

@@ -63,4 +63,25 @@ String read() {
             throw new RuntimeException(e);
         }
     }
+
+    boolean hasOutput() {
+        return !lines.isEmpty();
+    }
+
+    void assertNoOutput() {
+        assertNoOutput(50);
+    }
+
+    void assertNoOutput(long waitMillis) {
+        // Wait briefly to allow any potential response to be written
+        try {
+            Thread.sleep(waitMillis);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+        // Verify no output was produced
+        if (hasOutput()) {
+            throw new AssertionError("Expected no output but got : " + read());
+        }
+    }
 }