From b29d522d272a756df2052082ababf3a89a020e18 Mon Sep 17 00:00:00 2001
From: Koichi ITO <koic.ito@gmail.com>
Date: Sun, 7 Dec 2025 10:46:34 +0900
Subject: [PATCH] Show exception messages for tool call errors

Claude Code sometimes only shows `Error: MCP error -32603: Internal error`
when an unexpected exception occurs during a tool call.

This change broadens the exception handling scope so the server can surface
the exception message more often, making failures easier to diagnose.
---
 lib/mcp/server.rb       | 10 ++++------
 test/mcp/server_test.rb | 30 ++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/lib/mcp/server.rb b/lib/mcp/server.rb
index 5e34e0a..75d1155 100644
--- a/lib/mcp/server.rb
+++ b/lib/mcp/server.rb
@@ -338,13 +338,11 @@ def call_tool(request)
         end
       end
 
-      begin
-        call_tool_with_args(tool, arguments)
-      rescue => e
-        report_exception(e, { request: request })
+      call_tool_with_args(tool, arguments)
+    rescue => e
+      report_exception(e, request: request)
 
-        error_tool_response("Internal error calling tool #{tool_name}: #{e.message}")
-      end
+      error_tool_response("Internal error calling tool #{tool_name}: #{e.message}")
     end
 
     def list_prompts(request)
diff --git a/test/mcp/server_test.rb b/test/mcp/server_test.rb
index 6859da6..0fcef28 100644
--- a/test/mcp/server_test.rb
+++ b/test/mcp/server_test.rb
@@ -431,6 +431,36 @@ class Example < Tool
       assert_instrumentation_data({ method: "tools/call", tool_name: "tool_that_raises" })
     end
 
+    test "#handle tools/call returns error response with isError true if input_schema raises an error during validation" do
+      tool = Tool.define(
+        name: "tool_with_faulty_schema",
+        title: "Tool with faulty schema",
+        description: "A tool with a faulty schema",
+        input_schema: { type: "object", properties: { message: { type: "string" } }, required: ["message"] },
+      ) { Tool::Response.new("success") }
+
+      tool.input_schema.expects(:missing_required_arguments?).raises(RuntimeError, "Unexpected schema error")
+
+      server = Server.new(name: "test_server", tools: [tool])
+
+      request = {
+        jsonrpc: "2.0",
+        method: "tools/call",
+        params: {
+          name: "tool_with_faulty_schema",
+          arguments: { message: "test" },
+        },
+        id: 1,
+      }
+
+      response = server.handle(request)
+
+      assert_nil response[:error], "Expected no JSON-RPC error"
+      assert response[:result][:isError]
+      assert_equal "text", response[:result][:content][0][:type]
+      assert_match(/Internal error calling tool tool_with_faulty_schema: Unexpected schema error/, response[:result][:content][0][:text])
+    end
+
     test "#handle tools/call returns error response with isError true for unknown tool" do
       request = {
         jsonrpc: "2.0",