fix: keep HTTP client handler exception details local

docs/_client/transports.md

@@ -184,6 +184,9 @@ since servers deliver requests that are not tied to a client request on that str
 a JSON-RPC `-32601` (method not found) error. To handle methods other than `elicitation/create`, register directly on the transport with
 `http_transport.on_server_request("method/name") { |params| ... }`.
 
+If a handler raises an unexpected exception, the HTTP transport answers with JSON-RPC error `-32603`, `Internal error`.
+The exception class and message are written to local stderr, not sent to the server.
+
 On a [modern](/client/lifecycle/) connection servers cannot send requests at all; the same
 registered handlers instead drive the requests embedded in `input_required` results,
 as documented on [Multi Round-Trip Requests](/client/mrtr/).

lib/mcp/client/http.rb

@@ -1196,12 +1196,13 @@ def dispatch_server_request(message)
               error: { code: e.code, message: e.message },
             }
           rescue StandardError => e
+            warn("MCP::Client::HTTP: error handling #{message["method"]} request: #{e.class}: #{e.message.dump}")
             {
               jsonrpc: JsonRpcHandler::Version::V2_0,
               id: message["id"],
               error: {
                 code: JsonRpcHandler::ErrorCode::INTERNAL_ERROR,
-                message: "Internal error handling #{message["method"]} request: #{e.message}",
+                message: "Internal error",
               },
             }
           end

test/mcp/client/http_test.rb

@@ -1660,6 +1660,9 @@ def test_send_request_answers_unregistered_server_request_with_method_not_found
       end
 
       def test_send_request_answers_raising_handler_with_internal_error
+        original_verbose = $VERBOSE
+        $VERBOSE = false
+
         request = {
           jsonrpc: "2.0",
           id: "test_id",
@@ -1671,7 +1674,7 @@ def test_send_request_answers_raising_handler_with_internal_error
           jsonrpc: "2.0",
           id: 9,
           method: "elicitation/create",
-          params: {},
+          params: { message: "boom\r\n\e[31m" },
         }
         tool_result = { jsonrpc: "2.0", id: "test_id", result: { content: [] } }
         sse_body = "event: message\ndata: #{server_request.to_json}\n\n" \
@@ -1688,20 +1691,24 @@ def test_send_request_answers_raising_handler_with_internal_error
         expected_error_response = {
           jsonrpc: "2.0",
           id: 9,
-          error: { code: -32603, message: "Internal error handling elicitation/create request: boom" },
+          error: { code: -32603, message: "Internal error" },
         }
         error_stub = stub_request(:post, url).with(
           body: expected_error_response.to_json,
         ).to_return(
           status: 202, body: "",
         )
 
-        client.on_server_request("elicitation/create") { raise "boom" }
+        client.on_server_request("elicitation/create") { |params| raise params["message"] }
 
-        response = client.send_request(request: request)
+        assert_output("", "MCP::Client::HTTP: error handling elicitation/create request: RuntimeError: \"boom\\r\\n\\e[31m\"\n") do
+          response = client.send_request(request: request)
 
-        assert_equal({ "content" => [] }, response["result"])
-        assert_requested(error_stub)
+          assert_equal({ "content" => [] }, response["result"])
+          assert_requested(error_stub)
+        end
+      ensure
+        $VERBOSE = original_verbose
       end
 
       def test_send_request_answers_server_request_error_with_its_code