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Avoid unnecessary UTF-8 decoding and byte[] allocations when flushing MCP structs #1028

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Merged
rhernandez35 merged 1 commit into from
Feb 3, 2026
+14 −21
Merged

Avoid unnecessary UTF-8 decoding and byte[] allocations when flushing MCP structs #1028

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35 changes: 14 additions & 21 deletions mcp/mcp-server/src/main/java/software/amazon/smithy/java/mcp/server/McpServer.java
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Original file line number Diff line number Diff line change
Expand Up @@ -11,11 +11,12 @@
import java.util.Scanner;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CountDownLatch;
import software.amazon.smithy.java.core.schema.SerializableStruct;
import software.amazon.smithy.java.io.ByteBufferUtils;
import software.amazon.smithy.java.json.JsonCodec;
import software.amazon.smithy.java.json.JsonSettings;
import software.amazon.smithy.java.logging.InternalLogger;
import software.amazon.smithy.java.mcp.model.JsonRpcRequest;
import software.amazon.smithy.java.mcp.model.JsonRpcResponse;
import software.amazon.smithy.java.server.Server;
import software.amazon.smithy.java.server.Service;
import software.amazon.smithy.utils.SmithyUnstableApi;
Expand Down Expand Up @@ -81,9 +82,9 @@ private void handleRequest(JsonRpcRequest req) {
}
}

var response = mcpService.handleRequest(req, this::writeResponse, protocolVersion);
var response = mcpService.handleRequest(req, this::writeStructToOutput, protocolVersion);
if (response != null) {
writeResponse(response);
writeStructToOutput(response);
}
}

Expand All @@ -108,43 +109,35 @@ public void addNewService(String id, Service service) {
}

public void addNewProxy(McpServerProxy mcpServerProxy) {
mcpService.addNewProxy(mcpServerProxy, this::writeResponse);
mcpService.addNewProxy(mcpServerProxy, this::writeStructToOutput);
refreshTools();
}

public boolean containsMcpServer(String id) {
return mcpService.containsMcpServer(id);
}

private void writeResponse(JsonRpcResponse response) {
private void writeStructToOutput(SerializableStruct shape) {
synchronized (os) {
var bytes = CODEC.serialize(shape);
try {
os.write(CODEC.serializeToString(response).getBytes(StandardCharsets.UTF_8));
if (bytes.hasArray()) {
os.write(bytes.array(), bytes.arrayOffset() + bytes.position(), bytes.remaining());
} else {
os.write(ByteBufferUtils.getBytes(bytes));
}
os.write('\n');
os.flush();
} catch (Exception e) {
LOG.error("Error encoding response", e);
}
}
}

private void writeNotification(JsonRpcRequest notification) {
synchronized (os) {
try {
LOG.debug("Writing notification to stdout: method={}", notification.getMethod());
os.write(CODEC.serializeToString(notification).getBytes(StandardCharsets.UTF_8));
os.write('\n');
os.flush();
} catch (Exception e) {
LOG.error("Error encoding notification", e);
LOG.error("Error writing to output", e);
}
}
}

@Override
public void start() {
// Set up notification writer for proxies
mcpService.setNotificationWriter(this::writeNotification);
mcpService.setNotificationWriter(this::writeStructToOutput);

// Initialize proxies
mcpService.startProxies();
Expand Down