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Merged
adwsingh merged 1 commit into from
Feb 25, 2026
Merged

Send back a notification after successful initialize #1040

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28 changes: 3 additions & 25 deletions mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/HttpMcpProxy.java
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Original file line number Diff line number Diff line change
Expand Up @@ -25,7 +25,6 @@
import software.amazon.smithy.java.mcp.model.JsonRpcErrorResponse;
import software.amazon.smithy.java.mcp.model.JsonRpcRequest;
import software.amazon.smithy.java.mcp.model.JsonRpcResponse;
import software.amazon.smithy.model.shapes.ShapeType;
import software.amazon.smithy.utils.SmithyUnstableApi;

@SmithyUnstableApi
Expand Down Expand Up @@ -208,7 +207,7 @@ private JsonRpcResponse parseSseResponse(HttpResponse response, JsonRpcRequest r
// This is a notification - convert Document to JsonRpcRequest and forward
JsonRpcRequest notification = jsonDocument.asShape(JsonRpcRequest.builder());
LOG.debug("Received notification from SSE stream: method={}", notification.getMethod());
notifyRequest(notification);
notify(notification);
} else {
// This is a response - convert Document to JsonRpcResponse
finalResponse = jsonDocument.asShape(JsonRpcResponse.builder());
Expand Down Expand Up @@ -236,14 +235,14 @@ private JsonRpcResponse parseSseResponse(HttpResponse response, JsonRpcRequest r
.build();
LOG.debug("Received notification from remaining SSE buffer: method={}",
notification.getMethod());
notifyRequest(notification);
notify(notification);
} else {
JsonRpcResponse message = JsonRpcResponse.builder()
.deserialize(jsonDocument.createDeserializer())
.build();

if (message.getId() == null) {
notifyRequest(JsonRpcRequest.builder()
notify(JsonRpcRequest.builder()
.jsonrpc("2.0")
.method("notifications/unknown")
.build());
Expand Down Expand Up @@ -282,27 +281,6 @@ private JsonRpcResponse parseSseResponse(HttpResponse response, JsonRpcRequest r
}
}

/**
* Determines if a Document represents a notification (has "method" but no "id")
* rather than a response (has "id").
*
* - Responses have an "id" field at the top level
* - Notifications have a "method" field but no "id" field at the top level
*/
private boolean isNotification(Document doc) {
try {
if (!doc.isType(ShapeType.STRUCTURE) && !doc.isType(ShapeType.MAP)) {
return false;
}

// If it has a "method" field but no "id", it's a notification
return doc.getMember("id") == null && doc.getMember("method") != null;
} catch (Exception e) {
LOG.warn("Failed to determine if notification from Document", e);
return false;
}
}

private JsonRpcResponse handleErrorResponse(HttpResponse response) {
long contentLength = response.body().contentLength();
String errorMessage = "HTTP " + response.statusCode();
Expand Down
32 changes: 31 additions & 1 deletion mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/McpServerProxy.java
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,7 @@
import software.amazon.smithy.java.mcp.model.ListToolsResult;
import software.amazon.smithy.java.mcp.model.PromptInfo;
import software.amazon.smithy.java.mcp.model.ToolInfo;
import software.amazon.smithy.model.shapes.ShapeType;

public abstract class McpServerProxy {

Expand Down Expand Up @@ -81,6 +82,14 @@ public void initialize(
if (result.getError() != null) {
throw new RuntimeException("Error during initialization: " + result.getError().getMessage());
}

// Send the initialized notification per MCP protocol spec
JsonRpcRequest initializedNotification = JsonRpcRequest.builder()
.method("notifications/initialized")
.jsonrpc("2.0")
.build();
rpc(initializedNotification);

this.notificationConsumer.set(notificationConsumer);
this.requestNotificationConsumer.set(requestNotificationConsumer);
this.protocolVersion.set(protocolVersion);
Expand Down Expand Up @@ -127,7 +136,7 @@ protected void notify(JsonRpcResponse response) {
* Forwards a notification request by converting it to a response format.
* Notifications have a method field but no id.
*/
protected void notifyRequest(JsonRpcRequest notification) {
protected void notify(JsonRpcRequest notification) {
var rnc = requestNotificationConsumer.get();
if (rnc != null) {
LOG.debug("Forwarding notification to consumer: method={}", notification.getMethod());
Expand All @@ -138,5 +147,26 @@ protected void notifyRequest(JsonRpcRequest notification) {
}
}

/**
* Determines if a Document represents a notification (has "method" but no "id")
* rather than a response (has "id").
*
* - Responses have an "id" field at the top level
* - Notifications have a "method" field but no "id" field at the top level
*/
protected static boolean isNotification(Document doc) {
try {
if (!doc.isType(ShapeType.STRUCTURE) && !doc.isType(ShapeType.MAP)) {
return false;
}

// If it has a "method" field but no "id", it's a notification
return doc.getMember("id") == null && doc.getMember("method") != null;
} catch (Exception e) {
LOG.warn("Failed to determine if notification from Document", e);
return false;
}
}

public abstract String name();
}
48 changes: 35 additions & 13 deletions mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/StdioProxy.java
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Original file line number Diff line number Diff line change
Expand Up @@ -116,6 +116,25 @@ public CompletableFuture<JsonRpcResponse> rpc(JsonRpcRequest request) {
return future;
}

// Notifications don't have an ID and don't expect a response
if (request.getId() == null) {
try {
writeLock.lock();
String serializedRequest = JSON_CODEC.serializeToString(request);
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@rhernandez35 rhernandez35 Feb 25, 2026

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I would do serialization outside of the lock

LOG.debug("Sending notification: {}", serializedRequest);
writer.write(serializedRequest);
writer.newLine();
writer.flush();
} catch (IOException e) {
LOG.error("Error sending notification to MCP server", e);
return CompletableFuture.failedFuture(
new RuntimeException("Failed to send notification to MCP server: " + e.getMessage(), e));
} finally {
writeLock.unlock();
}
return CompletableFuture.completedFuture(null);
}

String requestId = getStringRequestId(request.getId());
CompletableFuture<JsonRpcResponse> responseFuture = new CompletableFuture<>();
pendingRequests.put(requestId, responseFuture);
Expand Down Expand Up @@ -185,20 +204,23 @@ public synchronized void start() {
}

LOG.debug("Received response: {}", responseLine);
JsonRpcResponse response = JsonRpcResponse.builder()
.deserialize(
JSON_CODEC.createDeserializer(
responseLine.getBytes(StandardCharsets.UTF_8)))
.build();

String responseId = getStringRequestId(response.getId());
LOG.debug("Processing response ID: {}", responseId);

CompletableFuture<JsonRpcResponse> future = pendingRequests.remove(responseId);
if (future != null) {
future.complete(response);
var output =
JSON_CODEC.createDeserializer(responseLine.getBytes(StandardCharsets.UTF_8))
.readDocument();
if (isNotification(output)) {
notify(output.asShape(JsonRpcRequest.builder()));
} else {
notify(response);
JsonRpcResponse response = output.asShape(JsonRpcResponse.builder());

String responseId = getStringRequestId(response.getId());
LOG.debug("Processing response ID: {}", responseId);

CompletableFuture<JsonRpcResponse> future = pendingRequests.remove(responseId);
if (future != null) {
future.complete(response);
} else {
notify(response);
}
}
} catch (IOException e) {
if (running) {
Expand Down
20 changes: 19 additions & 1 deletion mcp/mcp-server/src/test/java/software/amazon/smithy/java/mcp/server/McpServerTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -1431,6 +1431,10 @@ void testToolsListChangedNotificationInvalidatesCache() {
.build();
service.handleRequest(initRequest, r -> {}, ProtocolVersion.defaultVersion());

// Verify notifications/initialized was sent during initialization
assertTrue(mockProxy.getSentNotifications().contains("notifications/initialized"),
"notifications/initialized should be sent during initialization");

// First tools/list - fetches from proxy
var toolsRequest = JsonRpcRequest.builder()
.method("tools/list")
Expand Down Expand Up @@ -1488,6 +1492,10 @@ void testOtherNotificationsDoNotInvalidateCache() {
.build();
service.handleRequest(initRequest, r -> {}, ProtocolVersion.defaultVersion());

// Verify notifications/initialized was sent during initialization
assertTrue(mockProxy.getSentNotifications().contains("notifications/initialized"),
"notifications/initialized should be sent during initialization");

// First tools/list
var toolsRequest = JsonRpcRequest.builder()
.method("tools/list")
Expand Down Expand Up @@ -1516,6 +1524,7 @@ void testOtherNotificationsDoNotInvalidateCache() {

private static class CacheTestProxy extends McpServerProxy {
private final AtomicInteger callCounter;
private final List<String> sentNotifications = new ArrayList<>();

CacheTestProxy(AtomicInteger callCounter) {
this.callCounter = callCounter;
Expand All @@ -1539,6 +1548,11 @@ public List<software.amazon.smithy.java.mcp.model.PromptInfo> listPrompts() {

@Override
CompletableFuture<JsonRpcResponse> rpc(JsonRpcRequest request) {
// Notifications have no ID
if (request.getId() == null) {
sentNotifications.add(request.getMethod());
return CompletableFuture.completedFuture(null);
}
return CompletableFuture.completedFuture(
JsonRpcResponse.builder()
.id(request.getId())
Expand All @@ -1547,6 +1561,10 @@ CompletableFuture<JsonRpcResponse> rpc(JsonRpcRequest request) {
.build());
}

List<String> getSentNotifications() {
return sentNotifications;
}

@Override
void start() {}

Expand All @@ -1561,7 +1579,7 @@ public String name() {
}

void sendNotification(JsonRpcRequest notification) {
notifyRequest(notification);
notify(notification);
}
}
}