From ae00f009e64c40510aaa783f1246658c75b8af40 Mon Sep 17 00:00:00 2001
From: Sameh Khamis <sameh@halfpixel.ai>
Date: Fri, 2 Oct 2026 18:16:05 -0700
Subject: [PATCH] Log why a tool call failed in the stdio server's log

A failed call's reason went only to the client, so client logs showed a
failure without its cause.
---
 server/mcp/stdio.js            |  6 +++++-
 src/tools/localServer.test.mjs | 27 +++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/server/mcp/stdio.js b/server/mcp/stdio.js
index 563ef30ce..c89407be3 100644
--- a/server/mcp/stdio.js
+++ b/server/mcp/stdio.js
@@ -54,7 +54,11 @@ async function respond(server, line, log) {
   }
   const described = describeRequest(message);
   if (described) log(described);
-  return server.handle(message);
+  const response = await server.handle(message);
+  // A failed tool call answers the client, not the log; say why here too.
+  if (response?.result?.isError)
+    log(`   ${described} failed: ${response.result.content?.[0]?.text}`);
+  return response;
 }
 
 /** Read `--api-base <url>` from command-line arguments. */
diff --git a/src/tools/localServer.test.mjs b/src/tools/localServer.test.mjs
index 61aa5c024..c9b901bf0 100644
--- a/src/tools/localServer.test.mjs
+++ b/src/tools/localServer.test.mjs
@@ -104,3 +104,30 @@ test('the stdio log names methods, tools and resources but never arguments', asy
   );
   assert.equal(describeRequest({ result: {} }), null);
 });
+
+test('a failed tool call is logged with its reason', async () => {
+  const server = {
+    handle: async (message) => ({
+      jsonrpc: '2.0',
+      id: message.id,
+      result: { isError: true, content: [{ type: 'text', text: 'bad area' }] },
+    }),
+  };
+  const input = new PassThrough();
+  const output = new PassThrough();
+  output.resume();
+  const logged = [];
+  const served = serveStdio(server, {
+    input,
+    output,
+    log: (line) => logged.push(line),
+  });
+  input.end(
+    '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"show_in_gods_eye_view"}}\n',
+  );
+  await served;
+  assert.deepEqual(logged, [
+    'tools/call show_in_gods_eye_view',
+    '   tools/call show_in_gods_eye_view failed: bad area',
+  ]);
+});