Merge commit from fork

Bound stdio frame reads with max_line_bytes

README.md

@@ -102,6 +102,8 @@ transport = MCP::Server::Transports::StdioTransport.new(server)
 transport.open
 ```
 
+`StdioTransport.new` accepts an optional `max_line_bytes:` keyword that caps the byte length of a single newline-delimited request frame. A frame that reaches this limit without a newline is rejected and the connection is closed, preventing unbounded memory growth from a peer that never emits a newline. It defaults to `4 * 1024 * 1024` (4 MiB).
+
 You can run this script and then type in requests to the server at the command line.
 
 ```console
@@ -1958,6 +1960,7 @@ Use the `MCP::Client::Stdio` transport to interact with MCP servers running as s
 | `args:` | No | An array of arguments passed to the command. Defaults to `[]`. |
 | `env:` | No | A hash of environment variables to set for the server process. Defaults to `nil`. |
 | `read_timeout:` | No | Timeout in seconds for waiting for a server response. Defaults to `nil` (no timeout). |
+| `max_line_bytes:` | No | Maximum byte length of a single newline-delimited response frame. A frame that reaches this limit without a newline is rejected as a transport error, preventing unbounded memory growth from a server that never emits a newline. Defaults to `4 * 1024 * 1024` (4 MiB). |
 
 Example usage:
 

docs/building-clients.md

@@ -65,6 +65,7 @@ stdio_transport.close
 | `args:` | No | An array of arguments passed to the command. Defaults to `[]`. |
 | `env:` | No | A hash of environment variables for the server process. Defaults to `nil`. |
 | `read_timeout:` | No | Timeout in seconds for waiting for a server response. Defaults to `nil`. |
+| `max_line_bytes:` | No | Maximum byte length of a single newline-delimited response frame. A frame that reaches this limit without a newline is rejected as a transport error, preventing unbounded memory growth from a server that never emits a newline. Defaults to `4 * 1024 * 1024` (4 MiB). |
 
 ## HTTP Transport
 

docs/building-servers.md

@@ -59,6 +59,8 @@ transport = MCP::Server::Transports::StdioTransport.new(server)
 transport.open
 ```
 
+`StdioTransport.new` accepts an optional `max_line_bytes:` keyword that caps the byte length of a single newline-delimited request frame. A frame that reaches this limit without a newline is rejected and the connection is closed, preventing unbounded memory growth from a peer that never emits a newline. It defaults to `4 * 1024 * 1024` (4 MiB).
+
 ## Streamable HTTP Transport
 
 `MCP::Server::Transports::StreamableHTTPTransport` is a standard Rack app, so it can be mounted in any Rack-compatible framework.

lib/mcp/client/stdio.rb

@@ -19,13 +19,28 @@ module MCP
       CLOSE_TIMEOUT = 2
       STDERR_READ_SIZE = 4096
 
+      # Default upper bound on a single newline-delimited frame read from the
+      # server's stdout. CRuby's `IO#gets` without a limit accumulates bytes until a
+      # newline arrives, so a spawned server that never emits one can grow a single
+      # String until the host process is OOM-killed. 4 MiB is large enough for any
+      # realistic JSON-RPC frame, including base64-embedded images.
+      MAX_LINE_BYTES = 4 * 1024 * 1024
+
       attr_reader :command, :args, :env, :server_info
 
-      def initialize(command:, args: [], env: nil, read_timeout: nil)
+      def initialize(command:, args: [], env: nil, read_timeout: nil, max_line_bytes: MAX_LINE_BYTES)
+        # Reject `nil` or non-positive values: `IO#gets("\n", nil)` and a negative
+        # limit read without an upper bound, which would silently disable the
+        # protection this option exists to provide.
+        unless max_line_bytes.is_a?(Integer) && max_line_bytes > 0
+          raise ArgumentError, "max_line_bytes must be a positive Integer"
+        end
+
         @command = command
         @args = args
         @env = env
         @read_timeout = read_timeout
+        @max_line_bytes = max_line_bytes
         @stdin = nil
         @stdout = nil
         @stderr = nil
@@ -239,7 +254,7 @@ module MCP
         loop do
           ensure_running!
           wait_for_readable!(method, params) if @read_timeout
-          line = @stdout.gets
+          line = read_line(method, params)
           raise_connection_error!(method, params) if line.nil?
 
           parsed = JSON.parse(line.strip)
@@ -278,6 +293,22 @@ module MCP
         )
       end
 
+      # Reads one newline-delimited frame from the server's stdout, bounded by `@max_line_bytes`.
+      # Returns the line (including its trailing newline) or `nil` at EOF. Raises when the limit
+      # is reached before a newline arrives, which signals a server streaming an unbounded frame.
+      # A short final frame without a trailing newline (EOF) is still returned, since its length
+      # stays under the limit.
+      def read_line(method, params)
+        line = @stdout.gets("\n", @max_line_bytes)
+        return line unless line && !line.end_with?("\n") && line.bytesize >= @max_line_bytes
+
+        raise RequestHandlerError.new(
+          "Server response frame exceeds #{@max_line_bytes} bytes without a newline",
+          { method: method, params: params },
+          error_type: :internal_error,
+        )
+      end
+
       def raise_connection_error!(method, params)
         raise RequestHandlerError.new(
           "Server process closed stdout unexpectedly",

lib/mcp/server/transports/stdio_transport.rb

@@ -9,10 +9,25 @@ module MCP
       class StdioTransport < Transport
         STATUS_INTERRUPTED = Signal.list["INT"] + 128
 
-        def initialize(server)
+        # Default upper bound on a single newline-delimited frame. CRuby's `IO#gets`
+        # without a limit accumulates bytes until a newline arrives, so a peer that
+        # never emits one can grow a single String until the process is OOM-killed.
+        # 4 MiB is large enough for any realistic JSON-RPC frame, including
+        # base64-embedded images.
+        MAX_LINE_BYTES = 4 * 1024 * 1024
+
+        def initialize(server, max_line_bytes: MAX_LINE_BYTES)
           super(server)
           @open = false
           @session = nil
+          # Reject `nil` or non-positive values: `IO#gets("\n", nil)` and a negative
+          # limit read without an upper bound, which would silently disable the
+          # protection this option exists to provide.
+          unless max_line_bytes.is_a?(Integer) && max_line_bytes > 0
+            raise ArgumentError, "max_line_bytes must be a positive Integer"
+          end
+
+          @max_line_bytes = max_line_bytes
           $stdin.set_encoding("UTF-8")
           $stdout.set_encoding("UTF-8")
         end
@@ -20,7 +35,20 @@ module MCP
         def open
           @open = true
           @session = ServerSession.new(server: @server, transport: self)
-          while @open && (line = $stdin.gets)
+          while @open
+            begin
+              line = read_line($stdin)
+            rescue RequestHandlerError => e
+              # Stop accumulating and end the connection gracefully rather than
+              # letting an unbounded read exhaust memory or escape as an uncaught
+              # backtrace. Scoped to the read so genuine request errors raised while
+              # handling a frame are not swallowed here.
+              @open = false
+              MCP.configuration.exception_reporter.call(e, { error: "stdio frame exceeds limit" })
+              break
+            end
+            break if line.nil?
+
             response = @session.handle_json(line.strip)
             send_response(response) if response
           end
@@ -73,7 +101,7 @@ module MCP
             raise
           end
 
-          while @open && (line = $stdin.gets)
+          while @open && (line = read_line($stdin))
             begin
               parsed = JSON.parse(line.strip, symbolize_names: true)
             rescue JSON::ParserError => e
@@ -95,6 +123,26 @@ module MCP
 
           raise "Transport closed while waiting for response to #{method} request."
         end
+
+        private
+
+        # Reads one newline-delimited frame, bounded by `@max_line_bytes`. Returns
+        # the line (including its trailing newline) or `nil` at EOF. Raises when the
+        # limit is reached before a newline arrives, which signals a peer streaming
+        # an unbounded frame. A short final frame without a trailing newline (EOF) is
+        # still returned, since its length stays under the limit.
+        def read_line(io)
+          line = io.gets("\n", @max_line_bytes)
+          if line && !line.end_with?("\n") && line.bytesize >= @max_line_bytes
+            raise RequestHandlerError.new(
+              "stdio frame exceeds #{@max_line_bytes} bytes without a newline",
+              nil,
+              error_type: :internal_error,
+            )
+          end
+
+          line
+        end
       end
     end
   end

test/mcp/client/stdio_test.rb

@@ -1060,6 +1060,116 @@ module MCP
         end
       end
 
+      def test_new_raises_argument_error_when_max_line_bytes_is_not_a_positive_integer
+        [nil, 0, -1, 1.5, "1024"].each do |invalid|
+          error = assert_raises(ArgumentError) do
+            Stdio.new(command: "ruby", args: ["server.rb"], max_line_bytes: invalid)
+          end
+          assert_equal("max_line_bytes must be a positive Integer", error.message)
+        end
+      end
+
+      def test_send_request_raises_when_response_frame_exceeds_max_line_bytes
+        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"], max_line_bytes: 1024)
+
+        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, then stream an unbounded frame with no newline.
+          stdin_read.gets
+          stdout_write.write("A" * 2000)
+          stdout_write.flush
+        end
+
+        transport.connect
+
+        error = assert_raises(RequestHandlerError) do
+          transport.send_request(request: request)
+        end
+
+        assert_match(/exceeds 1024 bytes without a newline/, error.message)
+        assert_equal(:internal_error, error.error_type)
+      ensure
+        server_thread.join
+        stdin_read.close
+        stdin_write.close
+        stdout_read.close
+        stdout_write.close
+      end
+
+      def test_send_request_accepts_response_within_custom_max_line_bytes
+        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"], max_line_bytes: 1024)
+
+        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
+
+          stdin_read.gets
+
+          tools_line = stdin_read.gets
+          tools_request = JSON.parse(tools_line)
+          stdout_write.puts(JSON.generate({
+            jsonrpc: "2.0",
+            id: tools_request["id"],
+            result: { tools: [] },
+          }))
+          stdout_write.flush
+        end
+
+        transport.connect
+
+        response = transport.send_request(request: request)
+
+        assert_equal("test-id", response["id"])
+        assert_equal({ "tools" => [] }, response["result"])
+      ensure
+        server_thread.join
+        stdin_read.close
+        stdin_write.close
+        stdout_read.close
+        stdout_write.close
+      end
+
       def test_send_notification_writes_json_line_without_waiting_for_response
         stdin_read, stdin_write = IO.pipe
         stdout_read, stdout_write = IO.pipe

test/mcp/server/transports/stdio_transport_test.rb

@@ -441,6 +441,91 @@ module MCP
             end
           end
         end
+
+        test "raises ArgumentError when max_line_bytes is not a positive Integer" do
+          [nil, 0, -1, 1.5, "1024"].each do |invalid|
+            error = assert_raises(ArgumentError) do
+              StdioTransport.new(@server, max_line_bytes: invalid)
+            end
+            assert_equal("max_line_bytes must be a positive Integer", error.message)
+          end
+        end
+
+        test "open stops and reports when a stdin frame exceeds max_line_bytes without a newline" do
+          transport = StdioTransport.new(@server, max_line_bytes: 64)
+          input = StringIO.new("A" * 200) # No newline: an unbounded frame.
+          output = StringIO.new
+
+          reported = []
+          original_stdin = $stdin
+          original_stdout = $stdout
+          original_reporter = MCP.configuration.exception_reporter
+
+          begin
+            $stdin = input
+            $stdout = output
+            MCP.configuration.exception_reporter = ->(e, ctx) { reported << [e, ctx] }
+
+            transport.open
+
+            refute(transport.instance_variable_get(:@open))
+            assert_equal(1, reported.size)
+            error, _ctx = reported.first
+            assert_instance_of(RequestHandlerError, error)
+            assert_match(/exceeds 64 bytes without a newline/, error.message)
+          ensure
+            $stdin = original_stdin
+            $stdout = original_stdout
+            MCP.configuration.exception_reporter = original_reporter
+          end
+        end
+
+        test "open processes a valid final frame that ends at EOF without a trailing newline" do
+          request = { jsonrpc: "2.0", method: "ping", id: "eof" }
+          input = StringIO.new(JSON.generate(request)) # No trailing newline.
+          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_equal("eof", response[:id])
+            assert_equal({}, response[:result])
+          ensure
+            $stdin = original_stdin
+            $stdout = original_stdout
+          end
+        end
+
+        test "send_request raises when a response frame exceeds max_line_bytes without a newline" do
+          transport = StdioTransport.new(@server, max_line_bytes: 64)
+          input = StringIO.new("A" * 200) # No newline: an unbounded frame.
+          output = StringIO.new
+
+          original_stdin = $stdin
+          original_stdout = $stdout
+
+          begin
+            $stdin = input
+            $stdout = output
+            transport.instance_variable_set(:@open, true)
+
+            error = assert_raises(RequestHandlerError) do
+              transport.send_request("sampling/createMessage", { messages: [] })
+            end
+
+            assert_match(/exceeds 64 bytes without a newline/, error.message)
+          ensure
+            $stdin = original_stdin
+            $stdout = original_stdout
+          end
+        end
       end
     end
   end