Handle non-object JSON-RPC messages without raising

## Motivation and Context

A JSON-RPC message is an object, but two stdio-facing paths assumed that and raised
on a non-object value parsed from an incoming frame:

- `JsonRpcHandler.handle` maps each element of a batch array through `process_request`,
  which reads `request[:id]` assuming a Hash. A non-object batch element (for example `[[]]`
  or `[5]`) makes `[][:id]` raise `TypeError`. `handle_json` rescued only `JSON::ParserError`,
  so the error escaped; over stdio it escaped the `StdioTransport#open` read loop.
  The HTTP transport already rejects array bodies before dispatch, so this surfaced only on stdio.
- `MCP::Client::Stdio#read_response` parses each frame from the server and calls `parsed.key?("id")`,
  which raises `NoMethodError` when the server emits a non-object frame.
  Only `JSON::ParserError` was rescued, so one malformed line from the server escaped.

Both are robustness gaps: a malformed frame should be reported or skipped, not raise
an unhandled exception out of the transport.

- `process_request` now returns an `INVALID_REQUEST` error response for a non-object request
  before touching `request[:id]`, so a malformed batch element becomes a normal per-element error.

- `handle_json` gains a last-resort `StandardError` rescue that returns an `INTERNAL_ERROR` response,
  so an unexpected handling error still produces a JSON-RPC reply.
- `read_response` skips a non-object frame the same way it skips a frame without an id.

## How Has This Been Tested?

New tests in `test/json_rpc_handler_test.rb` cover non-object batch elements
(`[[]]`, `[5]`, `["x"]`, `[nil]`, `[true]`), a valid request followed by one non-object element,
and `handle_json("[[]]")` returning an error instead of raising. A test in
`test/mcp/server/transports/stdio_transport_test.rb` drives `open` with a `[[]]` line and
asserts it emits an error response without raising. A test in `test/mcp/client/stdio_test.rb`
has the server emit non-object frames before the response and asserts the client skips them
and still returns the response.

## Breaking Changes

None. A well-formed request, batch, or response is handled exactly as before;
only a non-object message now yields a JSON-RPC error on the server or is skipped by the client
instead of raising.

Author: koic

lib/json_rpc_handler.rb

@@ -63,12 +63,29 @@ def handle_json(request_json, id_validation_pattern: DEFAULT_ALLOWED_ID_CHARACTE
         message: "Parse error",
         data: "Invalid JSON",
       })
+    rescue StandardError
+      # Last-resort guard so an unexpected handling error returns a JSON-RPC error
+      # instead of escaping to the transport loop.
+      response = error_response(id: :unknown_id, id_validation_pattern: id_validation_pattern, error: {
+        code: ErrorCode::INTERNAL_ERROR,
+        message: "Internal error",
+      })
     end
 
     response&.to_json
   end
 
   def process_request(request, id_validation_pattern:, &method_finder)
+    # A batch element can be any JSON value; a non-object element cannot carry an id or a method,
+    # so reject it before the `request[:id]` access below.
+    unless request.is_a?(Hash)
+      return error_response(id: :unknown_id, id_validation_pattern: id_validation_pattern, error: {
+        code: ErrorCode::INVALID_REQUEST,
+        message: "Invalid Request",
+        data: "Request must be an object",
+      })
+    end
+
     id = request[:id]
 
     error = if !valid_version?(request[:jsonrpc])

lib/mcp/client/stdio.rb

@@ -259,7 +259,9 @@ def read_response(request)
 
           parsed = JSON.parse(line.strip)
 
-          next unless parsed.key?("id")
+          # A JSON-RPC message is an object; skip a non-object frame (array or scalar)
+          # the same way as a frame without an id.
+          next unless parsed.is_a?(Hash) && parsed.key?("id")
 
           return parsed if parsed["id"] == request_id
         end

test/json_rpc_handler_test.rb

@@ -682,6 +682,37 @@
         assert_equal 3, @response.first[:result]
         assert_equal(-32600, @response.last.dig(:error, :code))
       end
+
+      it "returns an invalid request error for a non-object batch element instead of raising" do
+        [[], [1], ["x"], [nil], [true]].each do |batch|
+          handle batch
+
+          assert_equal(-32600, @response.dig(:error, :code))
+          assert_equal "Invalid Request", @response.dig(:error, :message)
+          assert_nil @response[:id]
+        end
+      end
+
+      it "handles a valid request followed by a non-object element without raising" do
+        register("add") { |params| params[:a] + params[:b] }
+
+        handle [
+          { jsonrpc: "2.0", id: 100, method: "add", params: { a: 1, b: 2 } },
+          [],
+        ]
+
+        assert @response.is_a?(Array)
+        assert_equal 3, @response.first[:result]
+        assert_equal(-32600, @response.last.dig(:error, :code))
+        assert_nil @response.last[:id]
+      end
+
+      it "does not raise from handle_json when a batch element is not an object" do
+        result = handle_json("[[]]")
+
+        assert_equal(-32600, @response.dig(:error, :code))
+        refute_nil result
+      end
     end
 
     # 7 Examples

test/mcp/client/stdio_test.rb

@@ -146,6 +146,68 @@ def test_send_request_skips_notifications
         stdout_write.close
       end
 
+      def test_send_request_skips_non_object_frames
+        stdin_read, stdin_write = IO.pipe
+        stdout_read, stdout_write = IO.pipe
+        stderr_read, _ = IO.pipe
+
+        Open3.stubs(:popen3).returns([stdin_write, stdout_read, stderr_read, mock_wait_thread])
+
+        transport = Stdio.new(command: "ruby", args: ["server.rb"])
+
+        request = {
+          jsonrpc: "2.0",
+          id: "test-id",
+          method: "tools/list",
+        }
+
+        server_thread = Thread.new do
+          init_line = stdin_read.gets
+          init_request = JSON.parse(init_line)
+          stdout_write.puts(JSON.generate({
+            jsonrpc: "2.0",
+            id: init_request["id"],
+            result: {
+              protocolVersion: "2025-11-25",
+              capabilities: {},
+              serverInfo: { name: "test-server", version: "1.0.0" },
+            },
+          }))
+          stdout_write.flush
+
+          # Read initialized notification
+          stdin_read.gets
+
+          # Read tools/list request
+          stdin_read.gets
+
+          # Non-object frames are skipped like any other non-matching frame.
+          stdout_write.puts("[[]]")
+          stdout_write.puts("null")
+          stdout_write.flush
+
+          # Then send the actual response
+          stdout_write.puts(JSON.generate({
+            jsonrpc: "2.0",
+            id: "test-id",
+            result: { tools: [] },
+          }))
+          stdout_write.flush
+        end
+
+        transport.connect
+        response = transport.send_request(request: request)
+
+        assert_equal("test-id", response["id"])
+        assert_empty(response.dig("result", "tools"))
+      ensure
+        server_thread.join
+        stdin_read.close
+        stdin_write.close
+        stdout_read.close
+        stdout_write.close
+      end
+
       def test_send_request_raises_error_when_process_exits
         stdin_read, stdin_write = IO.pipe
         stdout_read, stdout_write = IO.pipe

test/mcp/server/transports/stdio_transport_test.rb

@@ -133,6 +133,27 @@ class StdioTransportTest < ActiveSupport::TestCase
           end
         end
 
+        test "open handles a malformed JSON-RPC batch element and emits an error response" do
+          input = StringIO.new("[[]]\n")
+          output = StringIO.new
+          original_stdin = $stdin
+          original_stdout = $stdout
+
+          begin
+            $stdin = input
+            $stdout = output
+            @transport.open
+
+            response = JSON.parse(output.string, symbolize_names: true)
+            assert_nil(response[:id])
+            assert_equal(-32600, response[:error][:code])
+            assert_equal("Invalid Request", response[:error][:message])
+          ensure
+            $stdin = original_stdin
+            $stdout = original_stdout
+          end
+        end
+
         test "rejects duplicate initialize on the same stdio session with -32600" do
           first = {
             jsonrpc: "2.0",