Exploring RuboCop with MCP

by Koichi ITO

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Support OSS community

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The Age of Artificial Intelligence .PEFSO%FWFMPQNFOU

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Generative Pre-trained Transformer

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P(next_token ∣ context)

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P(xt+1 ∣ x1 , …, xt )

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$POUFOUT **3VCP$PQY.$1 * .$13VCZ4%, Ruby 4.0.3, RuboCop 1.86.1, MCP Ruby SDK 0.13.0

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Scaffolding...

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require 'mcp' server = MCP::Server.new( name: 'weather', version: '1.0.0', tools: [GetForecast] ) transport = MCP::Server::Transports:: StdioTransport.new(server) transport.open class GetForecast < MCP::Tool description "Get forecast for a location" input_schema( properties: { latitude: {type: "number"}, longitude: {type: "number"} }, required: [ "latitude", "longitude" ] ) def self.call(latitude:, longitude:) forecast = fetch_forecast(latitude, longitude) MCP::Tool::Response.new([{ type: "text", text: forecast }]) end 4UEJP&YBNQMF.$14FSWFS IUUQTHJUIVCDPNNPEFMDPOUFYUQSPUPDPMRVJDLTUBSUSFTPVSDFTCMPCNBJOXFBUIFSTFSWFSSVCZXFBUIFSSC weather.rb 'FUDIFTXFBUIFSEBUBGSPN BOFYUFSOBM"1* 5PPMBSHT EF fi OFEBT +40/4DIFNB GPS--. 5IF.$1TFSWFS3VCZ fi MF TQBXOFECZ BO.$1DMJFOU UPDPOOFDUXJUI UIFQBSFOU.$1DMJFOUQSPDFTTWJBTUEJP

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class StdioTransport < Transport def open @open = true @session = ServerSession.new(server: @server, transport: self) while @open && (line = $stdin.gets) response = @session.handle_json(line.strip) send_response(response) if response end rescue Interrupt warn("\nExiting..."); exit(STATUS_INTERRUPTED) end def send_response(message) json_message = message.is_a?(String) ? message : JSON.generate(message) $stdout.puts(json_message) $stdout.flush end *OTJEF4UEJP5SBOTQPSUPQFO IUUQTHJUIVCDPNNPEFMDPOUFYUQSPUPDPMSVCZTELCMPCWMJCNDQTFSWFSUSBOTQPSUTTUEJP@USBOTQPSUSC -PPQTPWFS$stdin.gets DBMMThandle_json(json) BOETFOETUIFSFTQPOTF 8SJUFUIF+40/SFTQPOTFUP TUEPVUGPSUIF.$1DMJFOU

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server = MCP::Server.new( name: 'my_server', title: 'Example Server Display Name', version: '1.0.0', instructions: 'Use the tools of this server...', tools: [SomeTool, AnotherTool] ) transport = MCP::Server::Transports:: StreamableHTTPTransport.new(server) Rails.application.routes.draw do mount transport => '/mcp' end config/routes.rb 4USFBNBCMF)551&YBNQMF.$14FSWFS 4USFBNBCMF)5515SBOTQPSUBT B3BDLBQQMJDBUJPO 4UBUFGVM FH 3BDL Mechanism of MCP Ruby SDK 0.13+

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class StreamableHTTPTransport < Transport # Rack app interface. This transport can be mounted as a Rack app. def call(env) handle_request(Rack::Request.new(env)) end 3BJMT*OUFHSBUJPO *OUFHSBUF4USFBNBCMF)5515SBOTQPSUJOUPB3BDLBQQMJDBUJPOTVDIBT3BJMT Mechanism of MCP Ruby SDK 0.13+

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class StreamableHTTPTransport < Transport def handle_request(request) case request.env["REQUEST_METHOD"] when "POST" handle_post(request) when "GET" handle_get(request) when "DELETE" handle_delete(request) else method_not_allowed_response end end def handle_post(request) accept_error = validate_accept_header(request, REQUIRED_POST_ACCEPT_TYPES) return accept_error if accept_error body_string = request.body.read session_id = extract_session_id(request) *OTJEF4USFBNBCMF)5515SBOTQPSUIBOEMF@SFRVFTU 3PVUFTSFRVFTUTCZ)551NFUIPE 1045(&5%&-&5& &YUSBDUMCP-Session-IdBOE QSPDFTTFTUIFSFRVFTUBDDPSEJOHMZ def extract_session_id(request) request.env["HTTP_MCP_SESSION_ID"] end

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4FTTJPO.BOBHFNFOUJO4USFBNBCMF)551 Client A ──┐ ┌── 1 Server Client B ──┤ Streamable HTTP │ ├── tools, prompts, resources Client C ──┘ │ └── handlers, capabilities │ └── 1 StreamableHTTPTransport @sessions = { A => { stream: io_a, session_a } B => { stream: io_b, session_b } C => { stream: io_c, session_c } } ┌──────────────┬──────────────┐ ▼ ▼ ▼ session_a session_b session_c @client=A @client=B @client=C @logging=error @logging=debug @logging=info @writer=io_a @writer=io_b @writer=io_c notify_*→io_a notify_*→io_b notify_*→io_c N sessions × M concurrent requests on Puma "TFTTJPOJTDSFBUFE GPSFBDIDMJFOU )PMETPOMZTIBSFEDPO fi H MCP-Session-IdJTUIFLFZPG 5SBOTQPSUTJOTUBODFWBSJBCMF DPOOFDUTDMJFOUBOETFSWFSWJB MCP-Session-IdFTUBCMJTIFE EVSJOHUIFJOJUJBMJ[FIBOETIBLF &BDITFTTJPO IPMETQFSDMJFOU DPO fi HVSBUJPO

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44& 4FSWFS4FOU&WFOUT def DataProcessingTool.call(server_context:) server_context.report_progress(0, total: 100) data = fetch_data server_context.report_progress(50, total: 100) result = process(data) server_context.report_progress(100, total: 100) # data: {"method":"notifications/progress",...}\n\n MCP::Tool::Response.new([{type: "text", text: result}]) end def handle_request_with_sse_response(...) body = proc do |stream| @sessions[session_id][:post_request_streams][request_id] = stream response = dispatch_handle_json(body_string, server_session) send_to_stream(stream, response) # data: {"jsonrpc":"2.0","id":"1","result":{...}}\n\n stream.close end [200, SSE_HEADERS, body] end def send_to_stream(stream, data) stream.write( "data: #{data.to_json}\n\n" ) stream.flush end 5SBOTQPSU 4%,JOUFSOBM 5PPM VTFSDPEF DBMMFEJOTJEFEJTQBUDI@IBOEMF@KTPO 3BDL4USFBNJOH#PEZ 1VNBDBMMTCPEZDBMM TUSFBN # data: {"jsonrpc":"2.0","id":"1","result":{...}}\n\n # data: {"jsonrpc":"2.0","id":"1","result":{...}}\n\n # data: {"jsonrpc":"2.0","id":"1","result":{...}}\n\n # data: {"jsonrpc":"2.0","id":"1","result":{...}}\n\n

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$SPTTSFRVFTUTZODWJB2VFVF @pending_responses (shared hash on StreamableHTTPTransport) ┌──────────────────────────────────┐ │ request_id_A => { queue: QueueA }│←protected by @mutex (prevents TOCTOU) │ request_id_B => { queue: QueueB }│ └──────────────────────────────────┘ ↑ register ↑ lookup │ │ Request 1 Request 2 (tools/call) (sampling result POST with │ with request_id_A) Generate request_id_A │ Create QueueA │ Store in @pending │ │ │ ←── Send request_id_A │ to client via SSE │ │ Lookup QueueA by request_id_A │ │ QueueA.pop ←──────── QueueA.push(result) (block) (unblock) │ Receives result Resumes tool execution ←── Sends Tool::Response via SSE )BTILFZFECZ SFRVFTU@JE SFRVFTUUPPMTDBMM (FOFSBUF SFRVFTU@JEBOE 2VFVF" 3FTQPOTFUPDMJFOU GSPNSFRVFTU #MPDLIFSF SFRVFTU3FTQPOTF GSPNDMJFOUXJUI SFRVFTU@JE -PPLVQ2VFVF" GSPN !QFOEJOH@SFTQPOTFT 1VTIUP2VFVF" QPQSFRSFTVNFT 💡When a response from the client is needed inside tools/call 3FTQPOETUPDMJFOU BOEDPNQMFUFT

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# Request 2 # client response POST def handle_response(body, ...) pending = @pending_responses[body[:id]] pending[:queue].push(body[:result]) end # Request 1 # tools/call def send_request(method, ...) request_id = generate_request_id # SecureRandom.uuid queue = Queue.new @pending_responses[request_id] = {queue:} request = {jsonrpc: "2.0", id: request_id, method:} send_to_stream(stream, request) response = queue.pop # returns to caller, resumes tool execution. response end $SPTTSFRVFTUTZODWJB2VFVF IUUQTEPDTSVCZMBOHPSHFO5ISFBE2VFVFIUNM 2VFVF JOJWBS $SFBUFBRVFVF 8BJUUIFRVFVF &ORVFVFUIF DMJFOUSFTQPOTF %FRVFVF UIFSFTQPOTF

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class McpController < ActionController::API def create # POST server = MCP::Server.new( name: 'weather', version: '1.0.0', tools: [GetAlert, GetForecast] ) transport = MCP::Server::Transports:: StreamableHTTPTransport.new(server, stateless: true) server.transport = transport status, headers, body = transport.handle_request(request) render(json: body.first, status:, headers:) end 4USFBNBCMF)551&YBNQMF.$14FSWFS 💡Stateless Mode: available on Puma (workers >= 2) or Unicorn 4UBUFMFTT FH "DUJPO.FUIPET TUBUFMFTTUSVFCFDBVTFB OFXUSBOTQPSUJOTUBODFJT DSFBUFEQFSSFRVFTU TP MCP-Session-IdDBOOPU CFTIBSFE .$1QSPUPDPMSFRVFTUIBOEMJOH

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Exploring...

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rubocop/rubocop#14911 **3VCP$PQY.$1

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w "VUPDPSSFDU--.HFOFSBUFE3VCZDPEF OFFETMJOUJOHGPSNBUUJOH w *OTQFDUJPO-41+40/31$TUZMF OPU DMBOHTUZMFEJBHOPTUJDT XJUIP ff FOTFTVNNBSZ 'FBUVSFTJO3VCP$PQ ❯ /mcp ─────────────────────────────────────────────────────────────────────────────── Tools for rubocop 2 tools ❯ 1. RuboCop's inspection read-only 2. RuboCop's autocorrection destructive $MBVEF$PEF

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Built-in LSP 3VCP$PQ 3VOOFS SVO DBMM SFRVFTU EFMFHBUF 3VCP$PQ$-* $PNNBOE -41 rubocop --lsp 3VCP$PQ-41 4FSWFS 1MVHJO &EJUPS*%& 3VCP$PQ-41 3PVUFT 3VCP$PQ-41 3VOUJNF P ff FOTFT GPSNBU XSJUF TUBSU IUUQTSVCZLBJHJPSHQSFTFOUBUJPOTLPJDIUNMEBZ 3VCP$PQTCVJMUJO-41 Request / response (JSON-RPC) 💡This design was first introduced at RubyKaigi 2023

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Built-in LSP Request / response (JSON-RPC) 3VCP$PQ 3VOOFS SVO DBMM SFRVFTU EFMFHBUF 3VCP$PQ$-* $PNNBOE -41 rubocop --lsp 3VCP$PQ-41 4FSWFS 1MVHJO &EJUPS*%& 3VCP$PQ-41 3PVUFT 3VCP$PQ-41 3VOUJNF P ff FOTFT GPSNBU XSJUF TUBSU SVCPDPQNDQ "HFOU Built-in MCP 3VC$PQ$-* $PNNBOE .$1 3VCP$PQ.$1 4FSWFS 3VCP$PQTCVJMUJO.$1 --. .$1$MJFOU Request / Response (JSON-RPC) "HFOU TUBSU 💡Based on RuboCop's built-in LSP 6TFTTUEJPUSBOTQPSU BOEUPPMTJOUFSOBMMZ

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*OTJEF3VCP$PQ.$14FSWFS class RuboCop::MCP::Server def initialize(config_store) = (@runtime = RuboCop::LSP::Runtime.new(config_store)) def start server = ::MCP::Server.new( name: 'rubocop_mcp_server', version: RuboCop::Version::STRING, tools: [inspection_tool] ) ::MCP::Server::Transports::StdioTransport.new(server).open end private def inspection_tool ::MCP::Tool.define( name: 'rubocop_inspection', description: 'Inspect Ruby code for offenses.', input_schema: { properties: {path: {type: 'string'}, source_code: {type: 'string'}} } ) do |path: nil, source_code: nil| offenses = @runtime.offenses(path || 'example.rb', source_code) ::MCP::Tool::Response.new([{type: 'text', text: offenses.to_json}]) end end end

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{ "hooks": { "PostToolUse": [{ "matcher": "Edit|Write", "hooks": [{ "type": "command", "command": "file_path=$(cat - | jq -r '.tool_input.file_path // empty') && [ -n \"$file_path\" ] && bundle exec rubocop -a --force-exclusion \"$file_path\" 2>/dev/null || true" }] }] } } 1PTU5PPM6TFXJUI3VCP$PQ 3VOBVUPDPSSFDUPOUIFHFOFSBUFE fi MFTVTJOHrubocop -a .claude/settings.json ⚠Hooks always execute, so there is a trade-off with token consumption

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More to Explore...

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Introducing the probabilistic into deterministic tools

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class SummarizeTool < MCP::Tool description "Summarize text using LLM" input_schema( properties: {text: {type: "string"}}, required: ["text"] ) def self.call(text:, server_context:) result = server_context.create_sampling_message( messages: [ {role: "user", content: {type: "text", text: "Please summarize: #{text}"}} ], max_tokens: 500 ) MCP::Tool::Response.new([{ type: "text", text: result[:content][:text] }]) end end 4BNQMJOH&YBNQMF.$14FSWFS create_sampling_message TZODISPOPVTMZTFOETBO--. SFRVFTUUPUIFDMJFOUWJB44&BOE CMPDLTVOUJMUIFSFTQPOTFJTSFUVSOFE 3FUVSOTUIFSFTQPOTFUPUIFDMJFOU JODPSQPSBUJOHUIF4BNQMJOHSFTVMU

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class ConfirmDeleteTool < MCP::Tool description "Delete a record with user confirmation" input_schema(properties: {id: {type: "string"}}, required: ["id"]) def self.call(id:, server_context:) result = server_context.create_form_elicitation( message: "Are you sure you want to delete record #{id}?", requested_schema: { type: "object", properties: {confirm: {type: "boolean", description: "Confirm deletion"}}, required: ["confirm"] } ) if result[:content][:confirm] delete_record(id) text = "Record #{id} deleted." else text = "Deletion cancelled." end MCP::Tool::Response.new([{type: "text", text: text}]) end end &MJDJUBUJPO&YBNQMF.$14FSWFS 3FUVSOTUIFSFTQPOTFUPUIFDMJFOU JODPSQPSBUJOHUIF&MJDJUBUJPOSFTVMU create_elicitation TFOETBGPSNSFRVFTU UPUIFDMJFOUWJB44& BOECMPDLTVOUJMUIF VTFSTVCNJUTUIFJOQVU requested_schemaEF fi OFTUIFGPSN fi FMET UIFVTFSNVTU fi MMJO +40/4DIFNB

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The exploration continues... Attention(Q, K, V) = softmax ( QKT dk ) V Thank you for your...