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logging/setLevel from one session changes the log level of other sessions #658
Description
Server#configure_logging_level sets the level on the calling session, then always sets the server-wide @logging_message_notification too (server.rb:1386).
Legacy sessions that did not set their own level fall back to that server-wide value (server_session.rb:310-314), and Server#notify_log_message uses it for broadcasts (server.rb:448-450). So when one client calls logging/setLevel, every other legacy session without its own level starts to get notifications/message at that level. This includes sessions created later.
The spec treats the log level as a preference of the connection that sets it. The modern lifecycle is not affected, because it uses the per-request logLevel from the envelope.
Repro
Run from the repository root with bundle exec ruby -I lib repro.rb:
require "mcp"
class RecordingTransport
attr_reader :log_messages
def initialize
@log_messages = []
end
def send_notification(method, params = nil, session_id: nil, **)
@log_messages << [session_id, params["data"]] if method == "notifications/message"
true
end
end
log_tool = MCP::Tool.define(name: "log_tool") do |server_context:|
server_context.notify_log_message(data: "debug detail", level: "debug")
MCP::Tool::Response.new([{ type: "text", text: "ok" }])
end
transport = RecordingTransport.new
server = MCP::Server.new(name: "example", tools: [log_tool])
server.transport = transport
initialize_request = {
jsonrpc: "2.0", id: 1, method: "initialize",
params: { protocolVersion: "2025-11-25", capabilities: {}, clientInfo: { name: "client", version: "1" } },
}
session_a = MCP::ServerSession.new(server: server, transport: transport, session_id: "A")
session_b = MCP::ServerSession.new(server: server, transport: transport, session_id: "B")
session_a.handle(initialize_request)
session_b.handle(initialize_request)
session_a.handle({ jsonrpc: "2.0", id: 2, method: "logging/setLevel", params: { level: "debug" } })
session_b.handle({ jsonrpc: "2.0", id: 3, method: "tools/call", params: { name: "log_tool", arguments: {} } })
p transport.log_messagesExpected: []. Session B never called logging/setLevel.
Actual: [["B", "debug detail"]]
Suggested fix
Set the server-wide value only on the session-less path (session is nil). With a session, change only that session's level. Applications can still set a server-wide default with Server#logging_message_notification=.
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