Merge pull request #516 from koic/counter_offer_legacy_version_on_initialize
Stop negotiating modern protocol versions through the `initialize` handshake
This commit is contained in:
Koichi ITO
GitHub
commit
a62e56ef48
2 changed files with 472 additions and 207 deletions
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@ -1630,6 +1630,52 @@ module MCP
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client.send_request(request: { jsonrpc: "2.0", id: "2", method: "tools/list" })
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end
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def test_adopts_a_counter_offered_protocol_version_from_the_handshake
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# A server may answer `initialize` with a version other than the one offered, and everything
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# after the handshake speaks the answer rather than the offer. This server counter-offers
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# a handshake version to clients asking `initialize` for a modern one, which only resolves
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# anything because clients behave this way.
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counter_offered = "2025-06-18"
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refute_equal(
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counter_offered,
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MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION,
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"the counter-offer has to differ from the offer for this to test anything",
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)
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offered = nil
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stub_request(:post, url).with do |req|
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body = JSON.parse(req.body)
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offered = body.dig("params", "protocolVersion") if body["method"] == "initialize"
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body["method"] == "initialize"
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end.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json", "Mcp-Session-Id" => "session-abc" },
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body: { result: { protocolVersion: counter_offered } }.to_json,
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)
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stub_notification
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client.connect
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assert_equal(MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION, offered)
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assert_equal(counter_offered, client.protocol_version)
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# Scoped to the body as well: `notifications/initialized` already went out under
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# the adopted version, and a header-only stub would count that too.
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header_stub = stub_request(:post, url).with(
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headers: { "MCP-Protocol-Version" => counter_offered },
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) do |req|
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JSON.parse(req.body)["method"] == "tools/list"
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end.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json" },
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body: { result: { tools: [] } }.to_json,
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)
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client.send_request(request: { jsonrpc: "2.0", id: "2", method: "tools/list" })
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assert_requested(header_stub)
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end
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def test_does_not_send_protocol_version_header_before_initialize
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stub_request(:post, url)
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.with { |req| !req.headers.keys.map(&:downcase).include?("mcp-protocol-version") }
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@ -1884,14 +1930,14 @@ module MCP
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.with do |req|
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body = JSON.parse(req.body)
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body["method"] == "initialize" &&
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body["params"]["protocolVersion"] == MCP::Configuration::LATEST_STABLE_PROTOCOL_VERSION &&
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body["params"]["protocolVersion"] == MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION &&
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body["params"]["clientInfo"] == { "name" => "mcp-ruby-client", "version" => MCP::VERSION } &&
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body["params"]["capabilities"] == {}
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end
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.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json" },
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body: { result: { protocolVersion: MCP::Configuration::LATEST_STABLE_PROTOCOL_VERSION } }.to_json,
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body: { result: { protocolVersion: MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION } }.to_json,
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)
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client.connect
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@ -1927,9 +1973,31 @@ module MCP
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assert_requested(notification_stub)
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end
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def test_connect_warns_for_deprecated_capabilities_when_negotiated_protocol_version_is_2026_07_28
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notification_stub = stub_notification
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def test_connect_offers_the_latest_handshake_version_by_default
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# The handshake negotiates legacy versions only (SEP-2575 era model); modern versions are
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# selected via `mode: :modern`/`:auto`, so the default offer is the latest handshake version.
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offered = nil
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init_stub = stub_request(:post, url).with do |req|
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body = JSON.parse(req.body)
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offered = body.dig("params", "protocolVersion") if body["method"] == "initialize"
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body["method"] == "initialize"
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end.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json" },
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body: { result: { protocolVersion: "2025-11-25" } }.to_json,
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)
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stub_notification
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client.connect
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assert_requested(init_stub)
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assert_equal(MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION, offered)
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end
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def test_connect_rejects_a_modern_protocol_version_in_the_initialize_result
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# A server answering `initialize` with a modern version is refused like an unknown one:
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# the handshake settles on a legacy version by definition, and the TypeScript and Python clients
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# refuse a modern counter-offer the same way.
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init_stub = stub_request(:post, url)
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.with { |req| JSON.parse(req.body)["method"] == "initialize" }
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.to_return(
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@ -1938,12 +2006,20 @@ module MCP
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body: { result: { protocolVersion: "2026-07-28" } }.to_json,
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)
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assert_deprecation_warning(/MCP Roots .*2026-07-28.*MCP Sampling .*2026-07-28/m) do
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client.connect(capabilities: { roots: { listChanged: true }, sampling: {} })
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error = assert_raises(RequestHandlerError) do
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client.connect
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end
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assert_includes(error.message, "2026-07-28")
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assert_requested(init_stub)
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assert_requested(notification_stub)
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end
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def test_connect_raises_argument_error_for_an_explicit_modern_protocol_version_on_the_legacy_handshake
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error = assert_raises(ArgumentError) do
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client.connect(protocol_version: "2026-07-28", mode: :legacy)
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end
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assert_includes(error.message, "cannot be negotiated through the legacy `initialize` handshake")
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end
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def test_connect_does_not_warn_for_deprecated_capabilities_when_negotiated_protocol_version_is_older
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@ -2167,6 +2243,37 @@ module MCP
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assert_raises(ArgumentError) { client.connect(mode: :modern, protocol_version: "2025-11-25") }
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end
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def test_connect_auto_propagates_the_discovery_failure_for_an_explicitly_modern_version
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# An explicitly requested modern version is never downgraded by the fallback:
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# the legacy handshake cannot negotiate it, so the probe's failure is the real answer.
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stub_request(:post, url).with do |req|
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JSON.parse(req.body)["method"] == "server/discover"
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end.to_return(
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status: 200,
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headers: { "Content-Type" => "application/json" },
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body: { result: { supportedVersions: ["2025-11-25"] } }.to_json,
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)
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error = assert_raises(RequestHandlerError) do
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client.connect(mode: :auto, protocol_version: "2026-07-28")
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end
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assert_includes(error.message, "no mutually supported modern protocol version")
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refute_predicate(client, :connected?)
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end
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def test_connect_modern_warns_for_deprecated_capabilities
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# SEP-2577 deprecates roots and sampling at 2026-07-28, the revision every
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# modern connection speaks.
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stub_discover
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assert_deprecation_warning(/MCP Roots .*2026-07-28.*MCP Sampling .*2026-07-28/m) do
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client.connect(mode: :modern, capabilities: { roots: { listChanged: true }, sampling: {} })
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end
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assert_predicate(client, :modern?)
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end
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def test_reconnect_after_close
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stub_initialize
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stub_notification
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@ -818,7 +818,56 @@ module MCP
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stdout_write.close
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end
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def test_connect_warns_for_deprecated_capabilities_when_negotiated_protocol_version_is_2026_07_28
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def test_connect_adopts_a_counter_offered_protocol_version
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# A server may answer `initialize` with a version other than the one offered, and the connection
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# then speaks the answer rather than the offer. This server counter-offers a handshake version
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# to clients asking `initialize` for a modern one, which only resolves anything because
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# clients behave this way.
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counter_offered = "2025-06-18"
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refute_equal(
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counter_offered,
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MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION,
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"the counter-offer has to differ from the offer for this to test anything",
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)
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stdin_read, stdin_write = IO.pipe
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stdout_read, stdout_write = IO.pipe
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stderr_read, _ = IO.pipe
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Open3.stubs(:popen3).returns([stdin_write, stdout_read, stderr_read, mock_wait_thread])
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transport = Stdio.new(command: "ruby", args: ["server.rb"])
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offered = nil
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server_thread = Thread.new do
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init_request = JSON.parse(stdin_read.gets)
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offered = init_request.dig("params", "protocolVersion")
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stdout_write.puts(JSON.generate(
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jsonrpc: "2.0",
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id: init_request["id"],
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result: { protocolVersion: counter_offered },
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))
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stdout_write.flush
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stdin_read.gets
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end
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transport.connect
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assert_equal(MCP::Configuration::LATEST_HANDSHAKE_PROTOCOL_VERSION, offered)
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assert_equal(counter_offered, transport.protocol_version)
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ensure
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server_thread.join
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stdin_read.close
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stdin_write.close
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stdout_read.close
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stdout_write.close
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end
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def test_connect_rejects_a_modern_protocol_version_in_the_initialize_result
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# A server answering `initialize` with a modern version is refused like an unknown one:
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# the handshake settles on a legacy version by definition (SEP-2575 era model),
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# and the TypeScript and Python clients refuse a modern counter-offer the same way.
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stdin_read, stdin_write = IO.pipe
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stdout_read, stdout_write = IO.pipe
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stderr_read, _ = IO.pipe
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@ -836,12 +885,14 @@ module MCP
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result: { protocolVersion: "2026-07-28" },
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))
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stdout_write.flush
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stdin_read.gets
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end
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assert_deprecation_warning(/MCP Roots .*2026-07-28.*MCP Sampling .*2026-07-28/m) do
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error = assert_raises(RequestHandlerError) do
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transport.connect(capabilities: { roots: { listChanged: true }, sampling: {} })
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end
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assert_includes(error.message, "2026-07-28")
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refute_predicate(transport, :connected?)
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ensure
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server_thread.join
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stdin_read.close
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@ -850,6 +901,19 @@ module MCP
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stdout_write.close
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end
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def test_connect_raises_argument_error_for_an_explicit_modern_protocol_version_on_the_legacy_handshake
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# Validated before the child process is spawned: a pure argument error must not leave
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# an orphaned server process behind.
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Open3.expects(:popen3).never
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transport = Stdio.new(command: "ruby", args: ["server.rb"])
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error = assert_raises(ArgumentError) do
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transport.connect(protocol_version: "2026-07-28", mode: :legacy)
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end
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assert_includes(error.message, "cannot be negotiated through the legacy `initialize` handshake")
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end
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def test_connect_raises_on_jsonrpc_error_response
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stdin_read, stdin_write = IO.pipe
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stdout_read, stdout_write = IO.pipe
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@ -1498,6 +1562,43 @@ module MCP
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end
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end
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def test_connect_modern_warns_for_deprecated_capabilities
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# SEP-2577 deprecates roots and sampling at 2026-07-28, the revision every modern connection speaks.
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stdin_read, stdin_write = IO.pipe
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stdout_read, stdout_write = IO.pipe
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stderr_read, _ = IO.pipe
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Open3.stubs(:popen3).returns([stdin_write, stdout_read, stderr_read, mock_wait_thread])
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transport = Stdio.new(command: "ruby", args: ["server.rb"])
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server_thread = Thread.new do
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discover_request = JSON.parse(stdin_read.gets)
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stdout_write.puts(JSON.generate({
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jsonrpc: "2.0",
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id: discover_request["id"],
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result: {
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supportedVersions: ["2026-07-28"],
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capabilities: {},
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serverInfo: { name: "test-server", version: "1.0" },
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ttlMs: 0,
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cacheScope: "private",
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},
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}))
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stdout_write.flush
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end
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assert_deprecation_warning(/MCP Roots .*2026-07-28.*MCP Sampling .*2026-07-28/m) do
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transport.connect(mode: :modern, capabilities: { roots: { listChanged: true }, sampling: {} })
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end
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assert_predicate(transport, :modern?)
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ensure
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server_thread&.join
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[stdin_read, stdin_write, stdout_read, stdout_write, stderr_read].each do |io|
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io.close unless io.closed?
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end
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end
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def test_connect_auto_falls_back_to_the_legacy_handshake
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stdin_read, stdin_write = IO.pipe
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stdout_read, stdout_write = IO.pipe
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