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Warn on implicit stdio initialization #338

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4 changes: 4 additions & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

## [Unreleased]

### Deprecated

- Warn when stdio clients rely on implicit initialization instead of `MCP::Client#connect` (#334)

## [0.15.0] - 2026-05-05

### Added
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5 changes: 4 additions & 1 deletion lib/mcp/client/stdio.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -134,7 +134,10 @@ def connected?

def send_request(request:)
start unless @started
connect unless @initialized
unless @initialized
warn("Calling `MCP::Client::Stdio#send_request` without calling `MCP::Client#connect` is deprecated. Use `MCP::Client#connect` before sending requests instead.", uplevel: 1)
connect
end

write_message(request)
read_response(request)
Expand Down
130 changes: 112 additions & 18 deletions test/mcp/client/stdio_test.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,9 @@
module MCP
class Client
class StdioTest < Minitest::Test
IMPLICIT_CONNECT_DEPRECATION_WARNING =
/Calling `MCP::Client::Stdio#send_request` without calling `MCP::Client#connect` is deprecated\. Use `MCP::Client#connect` before sending requests instead\./.freeze

def test_send_request_starts_process_and_returns_response
stdin_read, stdin_write = IO.pipe
stdout_read, stdout_write = IO.pipe
Expand Down Expand Up @@ -56,7 +59,10 @@ def test_send_request_starts_process_and_returns_response
stdout_write.flush
end

response = transport.send_request(request: request)
response = nil
assert_implicit_connect_deprecation_warning do
response = transport.send_request(request: request)
end

assert_equal("test-id", response["id"])
assert_equal(1, response.dig("result", "tools").size)
Expand Down Expand Up @@ -123,7 +129,9 @@ def test_send_request_initializes_session_on_first_call
stdout_write.flush
end

transport.send_request(request: request)
assert_implicit_connect_deprecation_warning do
transport.send_request(request: request)
end

assert_equal(["initialize", "notifications/initialized", "tools/list"], received_methods)
ensure
Expand Down Expand Up @@ -184,10 +192,13 @@ def test_send_request_skips_notifications
stdout_write.flush
end

response = transport.send_request(request: request)
response = nil
assert_implicit_connect_deprecation_warning do
response = transport.send_request(request: request)
end

assert_equal("test-id", response["id"])
assert_equal([], response.dig("result", "tools"))
assert_empty(response.dig("result", "tools"))
ensure
server_thread.join
stdin_read.close
Expand Down Expand Up @@ -217,8 +228,11 @@ def test_send_request_raises_error_when_process_exits
method: "tools/list",
}

error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
error = nil
assert_implicit_connect_deprecation_warning do
error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
end
end

assert_equal("Server process has exited", error.message)
Expand Down Expand Up @@ -268,8 +282,11 @@ def test_send_request_raises_error_on_closed_stdout
stdout_write.close
end

error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
error = nil
assert_implicit_connect_deprecation_warning do
error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
end
end

assert_equal("Server process closed stdout unexpectedly", error.message)
Expand Down Expand Up @@ -380,7 +397,9 @@ def test_send_request_skips_initialization_on_second_call
stdout_write.flush
end

transport.send_request(request: { jsonrpc: "2.0", id: "first", method: "tools/list" })
assert_implicit_connect_deprecation_warning do
transport.send_request(request: { jsonrpc: "2.0", id: "first", method: "tools/list" })
end
transport.send_request(request: { jsonrpc: "2.0", id: "second", method: "tools/list" })

assert_equal(
Expand Down Expand Up @@ -444,8 +463,11 @@ def test_send_request_raises_error_on_invalid_json
stdout_write.flush
end

error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
error = nil
assert_implicit_connect_deprecation_warning do
error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
end
end

assert_equal("Failed to parse server response", error.message)
Expand Down Expand Up @@ -486,8 +508,11 @@ def test_send_request_raises_error_when_initialization_fails
stdout_write.flush
end

error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
error = nil
assert_implicit_connect_deprecation_warning do
error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
end
end

assert_equal("Server initialization failed: Invalid Request", error.message)
Expand Down Expand Up @@ -563,8 +588,11 @@ def test_read_response_raises_error_on_timeout
stdin_read.gets
end

error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
error = nil
assert_implicit_connect_deprecation_warning do
error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
end
end

assert_equal("Timed out waiting for server response", error.message)
Expand Down Expand Up @@ -616,7 +644,9 @@ def test_send_request_raises_error_when_stdin_is_closed
end

# Complete handshake with a successful request
transport.send_request(request: { jsonrpc: "2.0", id: "setup", method: "ping" })
assert_implicit_connect_deprecation_warning do
transport.send_request(request: { jsonrpc: "2.0", id: "setup", method: "ping" })
end
server_thread.join

# Now close stdin to simulate broken pipe
Expand Down Expand Up @@ -708,8 +738,11 @@ def test_send_request_raises_error_for_missing_result
stdout_write.flush
end

error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
error = nil
assert_implicit_connect_deprecation_warning do
error = assert_raises(RequestHandlerError) do
transport.send_request(request: request)
end
end

assert_equal("Server initialization failed: missing result in response", error.message)
Expand Down Expand Up @@ -768,6 +801,59 @@ def test_connect_performs_initialize_handshake_explicitly
stdout_write.close
end

def test_send_request_does_not_warn_after_explicit_connect
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"])

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

ping_line = stdin_read.gets
ping_request = JSON.parse(ping_line)
stdout_write.puts(JSON.generate(
jsonrpc: "2.0",
id: ping_request["id"],
result: {},
))
stdout_write.flush
end

assert_silent do
transport.connect
end

response = nil
assert_silent do
response = transport.send_request(request: { jsonrpc: "2.0", id: "ping-id", method: "ping" })
end

assert_equal("ping-id", response["id"])
ensure
server_thread.join
stdin_read.close
stdin_write.close
stdout_read.close
stdout_write.close
end

def test_connect_caches_server_info
transport, server_thread, pipes = stub_successful_connect

Expand Down Expand Up @@ -1141,6 +1227,14 @@ def test_server_info_is_cleared_after_close

private

def assert_implicit_connect_deprecation_warning(&block)
original_verbose = $VERBOSE
$VERBOSE = false
assert_output(nil, IMPLICIT_CONNECT_DEPRECATION_WARNING, &block)
ensure
$VERBOSE = original_verbose
end

def stub_successful_connect
stdin_read, stdin_write = IO.pipe
stdout_read, stdout_write = IO.pipe
Expand Down