From e38a66383453e7367e68ddc8fea8ca68613eb3c0 Mon Sep 17 00:00:00 2001
From: David Kaya <david@kaya.sk>
Date: Sat, 28 Mar 2026 19:39:23 +0100
Subject: [PATCH] fix: fall back to SSE transport when Streamable HTTP probe
 fails

Many MCP servers only support legacy SSE despite being configured as
generic HTTP endpoints. Follow the MCP SDK's recommended fallback
pattern: try Streamable HTTP first, then retry with SSE on failure.
This matches VS Code's behavior for these servers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 src/main/services/mcpToolProber.ts | 34 +++++++++++++++++++++---------
 1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/src/main/services/mcpToolProber.ts b/src/main/services/mcpToolProber.ts
index 14e6eab..ab80063 100644
--- a/src/main/services/mcpToolProber.ts
+++ b/src/main/services/mcpToolProber.ts
@@ -89,7 +89,29 @@ async function probeServerCore(
   server: McpServerDefinition,
   tokenLookup?: (serverUrl: string) => string | undefined,
 ): Promise<McpProbedTool[]> {
-  const transport = createTransport(server, tokenLookup);
+  if (server.transport === 'local' || server.transport === 'sse') {
+    return probeWithTransport(createTransport(server, tokenLookup));
+  }
+
+  // For HTTP servers, try Streamable HTTP first, then fall back to SSE.
+  // Many MCP servers only support SSE despite being configured as generic HTTP.
+  const headers = buildHeaders(server.url, server.headers, tokenLookup);
+  const headerOpts = headers ? { requestInit: { headers } } : undefined;
+
+  try {
+    return await probeWithTransport(
+      new StreamableHTTPClientTransport(new URL(server.url), headerOpts),
+    );
+  } catch {
+    return probeWithTransport(
+      new SSEClientTransport(new URL(server.url), headerOpts),
+    );
+  }
+}
+
+async function probeWithTransport(
+  transport: InstanceType<typeof StdioClientTransport> | InstanceType<typeof SSEClientTransport> | InstanceType<typeof StreamableHTTPClientTransport>,
+): Promise<McpProbedTool[]> {
   const client = new Client(CLIENT_INFO, { capabilities: {} });
 
   try {
@@ -133,15 +155,7 @@ function createTransport(
   }
 
   const headers = buildHeaders(server.url, server.headers, tokenLookup);
-
-  if (server.transport === 'sse') {
-    return new SSEClientTransport(
-      new URL(server.url),
-      headers ? { requestInit: { headers } } : undefined,
-    );
-  }
-
-  return new StreamableHTTPClientTransport(
+  return new SSEClientTransport(
     new URL(server.url),
     headers ? { requestInit: { headers } } : undefined,
   );