Exploring RuboCop with MCP

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Support OSS community

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The Age of Artificial Intelligence .PEFSO%FWFMPQNFOU

Generative Pre-trained Transformer

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w "HFOUT "DSPTT"MM </&8> Exploring the AI Era

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

Scaffolding...

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1SPNQU1SPWJEFQSPNQUUFNQMBUFT It matters that Ruby remains an official MCP SDK, beyond just RuboCop

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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

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+

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+

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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44& 4FSWFS4FOU&WFOUT def DataProcessingTool.call(server_context:) server_context.report_progress(0, total: 100) data = fetch_data
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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

# 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

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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Now that the MCP foundation is in place, let's think about calling Ruby tools

Exploring...

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

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'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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QSPHSBNNBUJDBMMZ -41GPSNBU Offenses: test.rb:10:20: C: Style/RedundantLineContinuation: Redundant line continuation. "longlong-string \ ... ^ 1 file inspected, 1 offense detected { "method": "textDocument/publishDiagnostics", "params": { "uri": "file:///path/to/test.rb", "diagnostics": [{ "message": "Redundant line continuation.", "code": "Style/RedundantLineContinuation", "range": { "start": { "line": 9, "character": 19 }, "end": { "line": 9, "character": 20 } } }] } } $MBOH'PSNBU -41 +40/31$

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

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

*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

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...

Exploring RuboCop with MCP

Exploring RuboCop with MCP

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