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Default missing MCP-Protocol-Version to 2025-03-26 in StreamableHTTPTransport #392

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Default missing MCP-Protocol-Version to 2025-03-26 in StreamableHTTPTransport #392

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1 change: 1 addition & 0 deletions lib/mcp/configuration.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ class Configuration
SUPPORTED_STABLE_PROTOCOL_VERSIONS = [
LATEST_STABLE_PROTOCOL_VERSION, "2025-06-18", "2025-03-26", "2024-11-05",
]
DEFAULT_NEGOTIATED_PROTOCOL_VERSION = "2025-03-26"

attr_writer :exception_reporter, :around_request

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3 changes: 1 addition & 2 deletions lib/mcp/server/transports/streamable_http_transport.rb
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Expand Up @@ -543,8 +543,7 @@ def initialize_request?(body)
end

def validate_protocol_version_header(request)
header_value = request.env["HTTP_MCP_PROTOCOL_VERSION"]
return if header_value.nil?
header_value = request.env["HTTP_MCP_PROTOCOL_VERSION"] || MCP::Configuration::DEFAULT_NEGOTIATED_PROTOCOL_VERSION
return if MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.include?(header_value)

supported = MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.join(", ")
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44 changes: 44 additions & 0 deletions test/mcp/server/transports/streamable_http_transport_test.rb
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Expand Up @@ -1778,6 +1778,50 @@ def string
assert_equal 200, response[0]
end

test "missing MCP-Protocol-Version header falls back to default for validation" do
MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.stubs(:include?).returns(false)

request = Rack::Request.new(
"REQUEST_METHOD" => "POST",
"PATH_INFO" => "/",
"rack.input" => StringIO.new(""),
)

response = @transport.send(:validate_protocol_version_header, request)
assert_equal 400, response[0]

body = JSON.parse(response[2][0])
assert_includes body["error"]["message"], MCP::Configuration::DEFAULT_NEGOTIATED_PROTOCOL_VERSION
end

test "POST request with empty MCP-Protocol-Version header returns 400" do
init_request = create_rack_request(
"POST",
"/",
{ "CONTENT_TYPE" => "application/json" },
{ jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
)
init_response = @transport.handle_request(init_request)
session_id = init_response[1]["Mcp-Session-Id"]

request = create_rack_request(
"POST",
"/",
{
"CONTENT_TYPE" => "application/json",
"HTTP_MCP_SESSION_ID" => session_id,
"HTTP_MCP_PROTOCOL_VERSION" => "",
},
{ jsonrpc: "2.0", method: "tools/list", id: "list" }.to_json,
)

response = @transport.handle_request(request)
assert_equal 400, response[0]

body = JSON.parse(response[2][0])
assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, body["error"]["code"]
end

test "POST request with array body and unsupported MCP-Protocol-Version returns 400" do
init_request = create_rack_request(
"POST",
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