From e757931a233e8ea238ce4d2f819ce96ce4e797f0 Mon Sep 17 00:00:00 2001
From: Koichi ITO <koic.ito@gmail.com>
Date: Wed, 21 Oct 2026 22:05:08 +0700
Subject: [PATCH] Strip stdio client frames as bytes

## Motivation and Context

`MCP::Client::Stdio` reads the server's stdout as UTF-8 and strips each frame before parsing it. `String#strip`
raises for a UTF-8 string holding an invalid byte at either end on every supported Ruby, and anywhere in it from
Ruby 3.2 on, and the read loop rescued only `JSON::ParserError`. A server writing such a frame therefore failed
the awaited request with `ArgumentError` or `Encoding::CompatibilityError` instead of the transport's own
`RequestHandlerError`, which callers rescue. The server transport strips its frames the same way and has the same
change separately.

Frames are now stripped as bytes. A frame that is not JSON fails the request with the transport's parse failure,
as any other unparsable frame does, and a response holding such a byte in a value is returned with the byte kept.

## How Has This Been Tested?

`test/mcp/client/stdio_test.rb`: a frame starting with an invalid byte and a response holding one in a value.
Against the previous library the first raises `ArgumentError` and the second `Encoding::CompatibilityError`.

## Breaking Changes

None.
---
 lib/mcp/client/stdio.rb       | 10 +++++++++-
 test/mcp/client/stdio_test.rb | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/lib/mcp/client/stdio.rb b/lib/mcp/client/stdio.rb
index ba4558f8..a2156c0a 100644
--- a/lib/mcp/client/stdio.rb
+++ b/lib/mcp/client/stdio.rb
@@ -434,7 +434,7 @@ def read_response(request)
           line = read_line(method, params)
           raise_connection_error!(method, params) if line.nil?
 
-          parsed = JSON.parse(line.strip)
+          parsed = JSON.parse(strip_frame(line))
 
           # A frame carrying `method` is a request or notification, never the awaited response,
           # whatever its id says: JSON-RPC id spaces are per sender, so a server-chosen ping id
@@ -534,6 +534,14 @@ def read_line(method, params)
         )
       end
 
+      # Strips a frame as bytes. The server's stdout is read as UTF-8, and `String#strip` raises for a UTF-8 string
+      # holding an invalid byte at either end on every supported Ruby, and anywhere in it from Ruby 3.2 on, which would
+      # surface as that exception instead of the client's own error. As bytes, the frame reaches the parser, which
+      # refuses what is not JSON with the client's parse failure.
+      def strip_frame(line)
+        line.b.strip.force_encoding(Encoding::UTF_8)
+      end
+
       def raise_connection_error!(method, params)
         raise RequestHandlerError.new(
           "Server process closed stdout unexpectedly",
diff --git a/test/mcp/client/stdio_test.rb b/test/mcp/client/stdio_test.rb
index fd5d3823..76f5c862 100644
--- a/test/mcp/client/stdio_test.rb
+++ b/test/mcp/client/stdio_test.rb
@@ -709,6 +709,41 @@ def test_send_request_raises_error_on_invalid_json
         stdout_write.close
       end
 
+      def test_send_request_raises_its_parse_error_on_a_frame_starting_with_an_invalid_byte
+        server = lambda do |requests, frames|
+          answer_handshake(requests, frames)
+          request = JSON.parse(requests.gets)
+          frames.write("\xFF".b + JSON.generate({ jsonrpc: "2.0", id: request["id"], result: {} }) + "\n")
+          frames.flush
+        end
+
+        with_scripted_server(server) do |transport|
+          transport.connect
+          error = assert_raises(RequestHandlerError) do
+            transport.send_request(request: { jsonrpc: "2.0", id: "test-id", method: "tools/list" })
+          end
+
+          assert_equal("Failed to parse server response", error.message)
+          assert_instance_of(JSON::ParserError, error.original_error)
+        end
+      end
+
+      def test_send_request_returns_a_response_holding_an_invalid_byte_in_a_value
+        server = lambda do |requests, frames|
+          answer_handshake(requests, frames)
+          request = JSON.parse(requests.gets)
+          frames.write("{\"jsonrpc\":\"2.0\",\"id\":#{JSON.generate(request["id"])},\"result\":{\"x\":\"a\xFFb\"}}\n".b)
+          frames.flush
+        end
+
+        with_scripted_server(server) do |transport|
+          transport.connect
+          response = transport.send_request(request: { jsonrpc: "2.0", id: "test-id", method: "tools/list" })
+
+          assert_equal("a\xFFb".b, response.dig("result", "x").b)
+        end
+      end
+
       def test_send_request_raises_error_on_a_response_that_is_not_json_rpc_2_0
         answers = [
           { result: { tools: [] } },