From 94ffb7922c09d413ab9d12b7c563be7b90e470dd Mon Sep 17 00:00:00 2001
From: Jerome Swannack <jerome@anthropic.com>
Date: Mon, 26 Jan 2026 15:23:39 +0000
Subject: [PATCH] Log client capabilities and extensions on initialize requests

Add INFO-level logging to both the stateful SHTTP handler and the
stateless example-apps handler to show the full client capabilities
sent during MCP initialization, including any extensions like
io.modelcontextprotocol/ui.

This helps verify that clients are correctly sending UI extension
capabilities during the initialization handshake.
---
 src/modules/example-apps/index.ts | 19 +++++++++++++++++++
 src/modules/mcp/handlers/shttp.ts | 13 +++++++++----
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/modules/example-apps/index.ts b/src/modules/example-apps/index.ts
index 1ac4576..b4a2248 100644
--- a/src/modules/example-apps/index.ts
+++ b/src/modules/example-apps/index.ts
@@ -16,6 +16,7 @@ import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/
 import type { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
 import { ITokenValidator } from '../../interfaces/auth-validator.js';
 import { AuthInfo } from '@modelcontextprotocol/sdk/server/auth/types.js';
+import { isInitializeRequest } from '@modelcontextprotocol/sdk/types.js';
 
 // Import createServer from each example package (compiled JS)
 // All packages are published on the public npm registry
@@ -115,6 +116,24 @@ export class ExampleAppsModule {
       }
 
       try {
+        // Log initialization requests to inspect client capabilities/extensions
+        if (isInitializeRequest(req.body)) {
+          const initParams = req.body?.params;
+          // eslint-disable-next-line @typescript-eslint/no-explicit-any
+          const capabilities = initParams?.capabilities as Record<string, any> | undefined;
+          console.log(JSON.stringify({
+            severity: 'INFO',
+            message: `=== MCP INITIALIZE REQUEST (${slug}) ===`,
+            timestamp: new Date().toISOString(),
+            slug,
+            clientInfo: initParams?.clientInfo,
+            protocolVersion: initParams?.protocolVersion,
+            capabilities,
+            hasExtensions: !!capabilities?.extensions,
+            extensions: capabilities?.extensions,
+          }));
+        }
+
         // Create fresh server and transport for each request (stateless mode)
         const server = createServer();
         const transport = new StreamableHTTPServerTransport({
diff --git a/src/modules/mcp/handlers/shttp.ts b/src/modules/mcp/handlers/shttp.ts
index a7a58e2..27f3fba 100644
--- a/src/modules/mcp/handlers/shttp.ts
+++ b/src/modules/mcp/handlers/shttp.ts
@@ -105,11 +105,16 @@ export async function handleStreamableHTTP(req: Request, res: Response) {
       shttpTransport = await getShttpTransport(sessionId, onsessionclosed, isGetRequest);
     } else if (isInitializeRequest(req.body)) {
       // New initialization request - use JSON response mode
-      logger.debug('Processing initialize request', {
-        body: req.body,
+      const initParams = req.body?.params;
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const capabilities = initParams?.capabilities as Record<string, any> | undefined;
+      logger.info('=== MCP INITIALIZE REQUEST ===', {
         userId,
-        headerSessionId: sessionId, // This is the sessionId from header (should be undefined for init)
-        isInitializeRequest: true
+        clientInfo: initParams?.clientInfo,
+        protocolVersion: initParams?.protocolVersion,
+        capabilities,
+        hasExtensions: !!capabilities?.extensions,
+        extensions: capabilities?.extensions,
       });
       
       const onsessioninitialized = async (sessionId: string) => {