lmcp/lmcp.lua

-- lmcp.lua — Lightweight MCP server in pure Lua
-- Zero external dependencies (uses built-in socket or luasocket)
-- SPDX-License-Identifier: MIT

local json = require('json')

local lmcp = {}
lmcp.__index = lmcp

-- Read auth token from config file if present
local function read_conf(path)
    local conf = {}
    local f = io.open(path, 'r')
    if not f then return conf end
    for line in f:lines() do
        local k, v = line:match('^%s*(%S+)%s*=%s*(.-)%s*$')
        if k and not k:match('^#') then conf[k] = v end
    end
    f:close()
    return conf
end

-- Protocol constants
local MCP_VERSION = "2025-06-18"
local JSONRPC = "2.0"

function lmcp.new(name, opts)
    opts = opts or {}
    local self = setmetatable({}, lmcp)
    self.name = name or "lmcp"
    self.version = opts.version or "0.1.0"
    self.host = opts.host or "0.0.0.0"
    self.port = opts.port or 8080
    self.tools = {}
    -- Resources primitive (MCP 2025-06-18 §Server/Resources). Storage is
    -- always present; capability is advertised iff `opts.resources` is
    -- truthy OR at least one resource/template has been registered by
    -- initialize time. The opt-in covers servers that register resources
    -- after :run() — strict clients cache the capability set from
    -- initialize and won't call resources/list otherwise.
    self.resources = {}
    self.resource_templates = {}
    self._force_resources_cap = opts.resources and true or false
    -- Prompts primitive (MCP 2025-06-18 §Server/Prompts). Same capability
    -- discipline as resources: advertised iff at least one is registered
    -- OR opts.prompts forces it (strict clients cache the capability set
    -- from initialize and won't call prompts/list otherwise).
    self.prompts = {}
    self._force_prompts_cap = opts.prompts and true or false
    -- Completion (MCP issue #7). Keyed by "ref_type:ref_id:arg_name".
    -- ref_type ∈ {"ref/prompt", "ref/resource"}, ref_id is the prompt
    -- name or resource-template uriTemplate, arg_name is the parameter
    -- whose value the client wants completions for.
    self.completions = {}
    self._force_completions_cap = opts.completions and true or false
    -- Logging (MCP issue #8). RFC-5424 severity levels in ascending order.
    -- Client sets minimum level via logging/setLevel; messages below are
    -- dropped. Default level "warning" until the client picks one. Capability
    -- is opt-in via opts.logging (servers that want a structured log channel
    -- must declare it at construction; we don't presume).
    self._log_level = "warning"
    self._force_logging_cap = opts.logging and true or false
    -- Client capabilities captured at initialize time (MCP issue #9).
    -- Used to guard server-initiated requests (sampling, roots) — we don't
    -- issue them unless the client claimed support during handshake.
    self._client_caps = {}
    self._client_info = {}
    -- Roots cache (MCP issue #10), keyed by session_id. Populated when the
    -- server calls `:roots(session_id, ...)`; invalidated when the client
    -- sends notifications/roots/list_changed.
    self._roots_cache = {}
    -- Notification queue: drained by Streamable HTTP transport (issue #16).
    -- Today delivery is a no-op; we still enqueue so the emission code
    -- path is exercised. Capped + deduped to keep the queue useful.
    self._notify_queue = {}
    self._notify_cap = 100
    self._session_id = nil
    -- Auth: explicit opt > conf file > LMCP_TOKEN env > nil (no auth)
    if opts.auth_token then
        self._auth_token = opts.auth_token
    elseif opts.conf then
        local conf = read_conf(opts.conf)
        self._auth_token = conf['.godparticle']
    else
        local env_token = os.getenv("LMCP_TOKEN")
        if env_token and env_token ~= "" then
            self._auth_token = env_token
        end
    end
    return self
end

-- Cursor pagination helper for list methods (MCP issue #12). Cursor is
-- an opaque base64 string per spec; we use it to encode the next offset.
-- Page size default 50 covers every plausible lmcp deployment today;
-- larger registered sets are still handled correctly.
local _PAGE_SIZE = 50

local function paginate(items, cursor)
    local n = #items
    -- Decode incoming cursor. Malformed → start from 0.
    local offset = 0
    if type(cursor) == "string" and cursor ~= "" then
        local mime_ok, mime = pcall(require, "mime")
        if mime_ok then
            local decoded = mime.unb64(cursor) or ""
            local parsed = tonumber(decoded)
            if parsed and parsed >= 0 and parsed <= n then
                offset = math.floor(parsed)
            end
        end
    end
    local page = {}
    local stop = math.min(offset + _PAGE_SIZE, n)
    for i = offset + 1, stop do page[#page + 1] = items[i] end
    local next_cursor
    if stop < n then
        local mime_ok, mime = pcall(require, "mime")
        if mime_ok then
            next_cursor = (mime.b64(tostring(stop)) or ""):gsub("[\r\n]", "")
        end
    end
    return page, next_cursor
end

-- Register a tool.
-- opts (optional, 5th arg):
--   annotations = { title?, readOnlyHint?, destructiveHint?,
--                   idempotentHint?, openWorldHint? }
--   outputSchema = <JSON Schema>     -- shape of structuredContent (issue #13)
-- Handler signature: function(args, ctx) where ctx = { _meta = … } from
-- the request. ctx is optional — existing 1-arg handlers keep working.
-- Handler return shapes:
--   string → single text content block (no structured)
--   { type = "...", ... } → typed content block (image/etc.); no structured
--   table without `type` → JSON-encoded into text content AND mirrored as
--     structuredContent (issue #13; spec-strict clients get first-class
--     structured access)
function lmcp:tool(name, description, params_schema, handler, opts)
    self.tools[name] = {
        name = name,
        description = description,
        inputSchema = params_schema or { type = "object", properties = {} },
        handler = handler,
        annotations  = opts and opts.annotations  or nil,
        outputSchema = opts and opts.outputSchema or nil,
    }
    return self
end

-- Register a resource (exact URI). opts: { name, description?, mimeType? }.
-- Handler signature: function(args) — args is always a table (empty for
-- literal resources, populated with template captures for templates).
function lmcp:resource(uri, opts, handler)
    opts = opts or {}
    if type(uri) ~= "string" or uri == "" then
        error("resource: uri required")
    end
    if type(handler) ~= "function" then
        error("resource: handler required")
    end
    self.resources[uri] = {
        uri         = uri,
        name        = opts.name or uri,
        description = opts.description,
        mimeType    = opts.mimeType,
        handler     = handler,
    }
    self:notify_resources_changed()
    return self
end

-- Register a resource template (RFC 6570 subset). Each {name} captures
-- one greedy segment. opts: { name, description?, mimeType? }.
-- Handler signature: function(args) — args[name] = captured_string.
-- Limitation: adjacent captures ({a}{b}) bind ambiguously; register
-- separate resources if you need precision.
function lmcp:resource_template(uriTemplate, opts, handler)
    opts = opts or {}
    if type(uriTemplate) ~= "string" or uriTemplate == "" then
        error("resource_template: uriTemplate required")
    end
    if type(handler) ~= "function" then
        error("resource_template: handler required")
    end
    -- Compile template → Lua pattern + arg-name list.
    local arg_names = {}
    -- Escape every Lua-pattern magic char EXCEPT {} (handled separately).
    local escaped = uriTemplate:gsub("([%%%(%)%.%+%-%*%?%[%]%^%$])", "%%%1")
    local pattern = escaped:gsub("{([%w_]+)}", function(name)
        arg_names[#arg_names + 1] = name
        return "(.+)"
    end)
    pattern = "^" .. pattern .. "$"
    self.resource_templates[#self.resource_templates + 1] = {
        uriTemplate = uriTemplate,
        name        = opts.name or uriTemplate,
        description = opts.description,
        mimeType    = opts.mimeType,
        pattern     = pattern,
        arg_names   = arg_names,
        handler     = handler,
    }
    self:notify_resources_changed()
    return self
end

-- Internal: enqueue a parameterless list_changed notification, with tail
-- dedup (consecutive notifications of the same kind collapse — they carry
-- no state, so N → 1 "go refetch"). Cap is a backstop, not the policy.
-- params omitted on purpose (json.lua empty-table → [] gotcha, would be
-- malformed JSON-RPC; spec allows omitting params for parameterless).
local function _enqueue_list_changed(self, method)
    local tail = self._notify_queue[#self._notify_queue]
    if tail and tail.method == method then return end
    if #self._notify_queue >= self._notify_cap then
        table.remove(self._notify_queue, 1)
    end
    self._notify_queue[#self._notify_queue + 1] = {
        jsonrpc = JSONRPC,
        method  = method,
    }
end

function lmcp:notify_resources_changed()
    _enqueue_list_changed(self, "notifications/resources/list_changed")
end

function lmcp:notify_prompts_changed()
    _enqueue_list_changed(self, "notifications/prompts/list_changed")
end

-- Resolve a URI to (resource_or_template_entry, args). Literal match wins
-- over templates; templates tried in registration order.
local function _resolve_resource(self, uri)
    local lit = self.resources[uri]
    if lit then return lit, {} end
    for _, t in ipairs(self.resource_templates) do
        local captures = { string.match(uri, t.pattern) }
        if captures[1] then
            local args = {}
            for i, name in ipairs(t.arg_names) do
                args[name] = captures[i]
            end
            return t, args
        end
    end
    return nil
end

-- Run a resource handler under pcall, normalise the return into a single
-- contents item. Returns (item_table, nil) on success, (nil, err_msg)
-- on failure.
local function _read_resource(entry, args, uri)
    local ok, result = pcall(entry.handler, args)
    if not ok then
        return nil, "resource handler error: " .. tostring(result)
    end
    if result == nil then
        return nil, "resource handler returned no content"
    end
    if type(result) == "string" then
        return { uri = uri, mimeType = entry.mimeType or "text/plain", text = result }
    end
    if type(result) == "table" then
        local mt = result.mimeType or entry.mimeType
        if result.text ~= nil then
            return { uri = uri, mimeType = mt or "text/plain", text = result.text }
        end
        if result.blob ~= nil then
            return { uri = uri, mimeType = mt or "application/octet-stream",
                     blob = result.blob }
        end
        if result.blob_bytes ~= nil then
            local mime_ok, mime = pcall(require, "mime")
            if not mime_ok then
                return nil, "mime module unavailable; pre-encode blob and return { blob = … }"
            end
            local b64 = mime.b64(result.blob_bytes) or ""
            b64 = b64:gsub("[\r\n]", "")  -- some luasocket builds line-wrap
            return { uri = uri, mimeType = mt or "application/octet-stream", blob = b64 }
        end
    end
    return nil, "resource handler returned unsupported shape"
end

-- Register a prompt (MCP issue #6). opts: { description?, arguments? }
-- where arguments is a list of { name, description?, required? }.
-- Handler signature: function(args) where args[name] = supplied string.
-- Return either:
--   string → single user text message
--   { description?, messages = {...} } → full custom shape (passthrough)
function lmcp:prompt(name, opts, handler)
    opts = opts or {}
    if type(name) ~= "string" or name == "" then
        error("prompt: name required")
    end
    if type(handler) ~= "function" then
        error("prompt: handler required")
    end
    self.prompts[name] = {
        name        = name,
        description = opts.description,
        arguments   = opts.arguments,  -- list of { name, description?, required? }
        handler     = handler,
    }
    self:notify_prompts_changed()
    return self
end

-- Run a prompt handler under pcall, normalise the return to the spec
-- shape { description?, messages = [{ role, content = { type, text } }] }.
-- Returns (table, nil) on success, (nil, err_msg) on failure.
local function _get_prompt(entry, args)
    local ok, result = pcall(entry.handler, args or {})
    if not ok then
        return nil, "prompt handler error: " .. tostring(result)
    end
    if result == nil then
        return nil, "prompt handler returned no content"
    end
    if type(result) == "string" then
        return {
            description = entry.description,
            messages = {{
                role = "user",
                content = { type = "text", text = result },
            }},
        }
    end
    if type(result) == "table" and type(result.messages) == "table" then
        return result
    end
    return nil, "prompt handler returned unsupported shape (expected string or { messages = … })"
end

-- Register a completion handler for a prompt/resource-template argument
-- (MCP issue #7). ref_type ∈ {"ref/prompt", "ref/resource"}; ref_id is
-- the prompt name or the resource-template uriTemplate. fn signature:
-- fn(value, ctx) → list of candidate strings (server filters / sorts as
-- it likes; spec allows up to 100). ctx mirrors the spec context object
-- (currently { arguments = {...} } of previously-completed sibling args).
function lmcp:complete(ref_type, ref_id, arg_name, fn)
    if ref_type ~= "ref/prompt" and ref_type ~= "ref/resource" then
        error("complete: ref_type must be 'ref/prompt' or 'ref/resource'")
    end
    if type(ref_id) ~= "string" or ref_id == "" then
        error("complete: ref_id required")
    end
    if type(arg_name) ~= "string" or arg_name == "" then
        error("complete: arg_name required")
    end
    if type(fn) ~= "function" then
        error("complete: fn required")
    end
    self.completions[ref_type .. ":" .. ref_id .. ":" .. arg_name] = fn
    return self
end

-- Log severity ordering (RFC 5424). Lower index = more severe.
local LOG_LEVELS = {
    emergency = 1, alert = 2, critical = 3, error = 4,
    warning = 5, notice = 6, info = 7, debug = 8,
}

-- Emit a structured log record. Below the client-set level → drop.
-- Today's delivery channel is stderr; once issue #16 lands the
-- bidirectional transport, this also enqueues notifications/message
-- for the client. data is free-form (string, table, etc.).
function lmcp:log(level, logger, data)
    local lvl = LOG_LEVELS[level]
    local thr = LOG_LEVELS[self._log_level] or LOG_LEVELS.warning
    if not lvl or lvl > thr then return end  -- below threshold; drop
    -- stderr fallback: human-readable. The structured form goes on the
    -- notifications queue for the future Streamable-HTTP delivery path
    -- (issue #16). Cap + drop-oldest like list_changed.
    io.stderr:write(string.format("lmcp[%s/%s]: %s\n",
        level, tostring(logger or "-"),
        type(data) == "string" and data or json.encode(data)))
    if #self._notify_queue >= self._notify_cap then
        table.remove(self._notify_queue, 1)
    end
    self._notify_queue[#self._notify_queue + 1] = {
        jsonrpc = JSONRPC,
        method  = "notifications/message",
        params  = { level = level, logger = logger, data = data },
    }
end

-- JSON-RPC response helpers
local function jsonrpc_result(id, result)
    return json.encode({ jsonrpc = JSONRPC, id = id, result = result })
end

local function jsonrpc_error(id, code, message)
    return json.encode({
        jsonrpc = JSONRPC,
        id = id,
        error = { code = code, message = message },
    })
end

-- Handle a single JSON-RPC request
function lmcp:handle_request(req)
    local method = req.method
    local id = req.id  -- nil for notifications

    -- JSON-RPC 2.0: notifications (no id) MUST NOT receive a response.
    -- Some notifications carry server-side side effects (cache invalidation,
    -- progress signals); handle those before the early return. Anything
    -- not recognised silently drops — clients expect no response either way.
    if id == nil then
        if method == "notifications/roots/list_changed" then
            -- Invalidate cached roots for the session that sent this.
            if req._session_id then self._roots_cache[req._session_id] = nil end
        end
        -- (Other client→server notifications: cancelled, message — no
        -- action today; add side-effects here as needed.)
        return nil
    end

    if method == "initialize" then
        self._session_id = self._session_id or tostring(os.time())
        -- Capture client capabilities (MCP issue #9 — sampling needs to
        -- know if the client supports it before issuing a request).
        local p = req.params or {}
        self._client_caps = p.capabilities or {}
        self._client_info = p.clientInfo or {}
        local caps = { tools = { listChanged = false } }
        if self._force_resources_cap
           or next(self.resources)
           or self.resource_templates[1] then
            caps.resources = { listChanged = true, subscribe = false }
        end
        if self._force_prompts_cap or next(self.prompts) then
            caps.prompts = { listChanged = true }
        end
        if self._force_completions_cap or next(self.completions) then
            -- Spec uses an empty object as the "supported" marker.
            -- json.empty_object → {} (not [] from the empty-table gotcha).
            caps.completions = json.empty_object
        end
        if self._force_logging_cap then
            caps.logging = json.empty_object
        end
        return jsonrpc_result(id, {
            protocolVersion = MCP_VERSION,
            capabilities = caps,
            serverInfo = {
                name = self.name,
                version = self.version,
            },
        })

    elseif method == "ping" then
        return jsonrpc_result(id, json.empty_object)

    elseif method == "tools/list" then
        local tool_list = {}
        for _, t in pairs(self.tools) do
            local entry = {
                name = t.name,
                description = t.description,
                inputSchema = t.inputSchema,
            }
            -- Emit annotations / outputSchema only when registered. Empty
            -- Lua tables would JSON-encode as [] (see
            -- project_json_empty_table_gotcha memory) and break
            -- spec-strict clients.
            if t.annotations  then entry.annotations  = t.annotations  end
            if t.outputSchema then entry.outputSchema = t.outputSchema end
            tool_list[#tool_list + 1] = entry
        end
        local page, next_cursor = paginate(tool_list, (req.params or {}).cursor)
        local result = { tools = page }
        if next_cursor then result.nextCursor = next_cursor end
        return jsonrpc_result(id, result)

    elseif method == "tools/call" then
        local params = req.params or {}
        local tool_name = params.name
        local arguments = params.arguments or {}
        local tool = self.tools[tool_name]
        if not tool then
            return jsonrpc_error(id, -32601, "Tool not found: " .. tostring(tool_name))
        end
        -- ctx exposes the request's _meta (issue #13) and the session_id
        -- (issue #9 — so handlers can call self:sample(ctx.session_id, …)).
        -- Handlers that don't declare a second parameter ignore it (Lua
        -- call discards extras).
        local ctx = {
            _meta = params._meta,
            request_id = id,
            session_id = req._session_id,
        }
        local ok, result = pcall(tool.handler, arguments, ctx)
        if ok then
            local resp = { isError = false }
            local meta_out
            if type(result) == "string" then
                resp.content = {{ type = "text", text = result }}
            elseif type(result) == "table" and result.type then
                -- Typed content block (e.g. image). No structured emission.
                resp.content = { result }
            elseif type(result) == "table" then
                -- Issue #13: extract response _meta before mirroring as
                -- structuredContent, so server metadata doesn't leak into
                -- the structured payload.
                meta_out = result._meta
                local clean = result
                if meta_out ~= nil then
                    clean = {}
                    for k, v in pairs(result) do
                        if k ~= "_meta" then clean[k] = v end
                    end
                end
                resp.content = {{ type = "text", text = json.encode(clean) }}
                resp.structuredContent = clean
            else
                resp.content = {{ type = "text", text = tostring(result) }}
            end
            if meta_out ~= nil then resp._meta = meta_out end
            return jsonrpc_result(id, resp)
        else
            return jsonrpc_result(id, {
                content = {{ type = "text", text = "Error: " .. tostring(result) }},
                isError = true,
            })
        end

    elseif method == "resources/list" then
        local out = {}
        for _, r in pairs(self.resources) do
            out[#out + 1] = {
                uri         = r.uri,
                name        = r.name,
                description = r.description,
                mimeType    = r.mimeType,
            }
        end
        local page, next_cursor = paginate(out, (req.params or {}).cursor)
        local result = { resources = page }
        if next_cursor then result.nextCursor = next_cursor end
        return jsonrpc_result(id, result)

    elseif method == "resources/templates/list" then
        local out = {}
        for _, t in ipairs(self.resource_templates) do
            out[#out + 1] = {
                uriTemplate = t.uriTemplate,
                name        = t.name,
                description = t.description,
                mimeType    = t.mimeType,
            }
        end
        local page, next_cursor = paginate(out, (req.params or {}).cursor)
        local result = { resourceTemplates = page }
        if next_cursor then result.nextCursor = next_cursor end
        return jsonrpc_result(id, result)

    elseif method == "resources/read" then
        local params = req.params or {}
        local uri = params.uri
        if type(uri) ~= "string" or uri == "" then
            return jsonrpc_error(id, -32602, "uri required (string)")
        end
        local entry, args = _resolve_resource(self, uri)
        if not entry then
            -- -32002 is the MCP-conventional "Resource not found" code;
            -- distinct from -32602 "Invalid params" so retry/UX logic
            -- can tell a malformed URI from a missing one.
            return jsonrpc_error(id, -32002, "Resource not found: " .. uri)
        end
        local item, err = _read_resource(entry, args, uri)
        if not item then
            return jsonrpc_error(id, -32603, err)
        end
        return jsonrpc_result(id, { contents = { item } })

    elseif method == "prompts/list" then
        local out = {}
        for _, p in pairs(self.prompts) do
            local entry = { name = p.name, description = p.description }
            if p.arguments then entry.arguments = p.arguments end
            out[#out + 1] = entry
        end
        local page, next_cursor = paginate(out, (req.params or {}).cursor)
        local result = { prompts = page }
        if next_cursor then result.nextCursor = next_cursor end
        return jsonrpc_result(id, result)

    elseif method == "prompts/get" then
        local params = req.params or {}
        local name = params.name
        if type(name) ~= "string" or name == "" then
            return jsonrpc_error(id, -32602, "name required (string)")
        end
        local entry = self.prompts[name]
        if not entry then
            return jsonrpc_error(id, -32002, "Prompt not found: " .. name)
        end
        local result, err = _get_prompt(entry, params.arguments)
        if not result then
            return jsonrpc_error(id, -32603, err)
        end
        return jsonrpc_result(id, result)

    elseif method == "logging/setLevel" then
        local lvl = (req.params or {}).level
        if type(lvl) ~= "string" or not LOG_LEVELS[lvl] then
            return jsonrpc_error(id, -32602,
                "level must be one of: debug, info, notice, warning, error, critical, alert, emergency")
        end
        self._log_level = lvl
        return jsonrpc_result(id, json.empty_object)

    elseif method == "completion/complete" then
        local params = req.params or {}
        local ref = params.ref or {}
        local arg = params.argument or {}
        local ref_id = ref.name or ref.uri or ref.uriTemplate or ""
        if type(ref.type) ~= "string" or ref_id == ""
           or type(arg.name) ~= "string" then
            return jsonrpc_error(id, -32602,
                "ref.type, ref.name/uri/uriTemplate, and argument.name required")
        end
        local fn = self.completions[ref.type .. ":" .. ref_id .. ":" .. arg.name]
        if not fn then
            -- No completer registered → return empty values (spec-allowed;
            -- clients typically render no suggestions and let the user type).
            return jsonrpc_result(id, {
                completion = { values = {}, hasMore = false },
            })
        end
        local value = arg.value or ""
        local ok, values = pcall(fn, value, params.context or {})
        if not ok then
            return jsonrpc_error(id, -32603,
                "completion handler error: " .. tostring(values))
        end
        if type(values) ~= "table" then
            return jsonrpc_error(id, -32603,
                "completion handler must return a table of strings")
        end
        -- Spec cap: at most 100 values per response. If more, truncate
        -- and set hasMore=true so the client knows there's more.
        local total = #values
        local has_more = false
        if total > 100 then
            local out = {}
            for i = 1, 100 do out[i] = values[i] end
            values = out
            has_more = true
        end
        return jsonrpc_result(id, {
            completion = { values = values, total = total, hasMore = has_more },
        })

    else
        return jsonrpc_error(id, -32601, "Method not found: " .. tostring(method))
    end
end

-- ---- HTTP Server (raw sockets) ----

local function parse_http_request(client)
    -- Read request line
    local line, err = client:receive('*l')
    if not line then return nil, err end

    local method, path, version = line:match('^(%S+)%s+(%S+)%s+(%S+)')
    if not method then return nil, 'bad request line' end

    -- Read headers
    local headers = {}
    while true do
        line, err = client:receive('*l')
        if not line or line == '' then break end
        local k, v = line:match('^(%S+):%s*(.*)')
        if k then headers[k:lower()] = v end
    end

    -- Read body
    local body = ''
    local content_length = tonumber(headers['content-length'] or 0)
    if content_length > 0 then
        body, err = client:receive(content_length)
        if not body then return nil, err end
    end

    return {
        method = method,
        path = path,
        version = version,
        headers = headers,
        body = body,
    }
end

local function send_response(client, status, headers, body)
    local parts = { string.format('HTTP/1.1 %s', status) }
    headers['Content-Length'] = tostring(#body)
    headers['Connection'] = 'close'
    for k, v in pairs(headers) do
        parts[#parts + 1] = k .. ': ' .. v
    end
    parts[#parts + 1] = ''
    parts[#parts + 1] = body
    client:send(table.concat(parts, '\r\n'))
end

local function send_sse_event(client, data)
    client:send('event: message\r\ndata: ' .. data .. '\r\n\r\n')
end

-- ---- Streamable HTTP transport (MCP issue #16) ----
--
-- select()-based single-thread event loop. All sockets non-blocking.
-- Per-connection FSM: reading_head → reading_body → dispatching → writing | sse_open.
--
-- Session model: each session has a Mcp-Session-Id; at most one open
-- SSE stream (the GET /mcp connection). Server-initiated requests
-- (sampling, roots — issues #9/#10) ride on the SSE stream and await
-- client responses via subsequent POSTs.
--
-- Queue routing:
--   self._notify_queue (global): list_changed, log messages → fans out
--     to ALL open sse_conn (broadcast).
--   sess.notify_q (per-session): server-initiated requests → only that
--     session's sse_conn.
--
-- write_buf discipline: append-only via `..`; consume via :sub(offset+1)
-- after partial-send. NEVER reorder or rewrite past bytes.

local READ_BUF_CAP    = 64 * 1024       -- 64 KiB for header section
local BODY_CAP        = 8 * 1024 * 1024 -- 8 MiB for request body
local WRITE_BUF_CAP   = 1 * 1024 * 1024 -- 1 MiB per-conn write buffer
local HEARTBEAT_SEC   = 30
local SESSION_IDLE_SEC = 60
local SELECT_TIMEOUT  = 0.1

local function _new_session_id()
    return string.format("%d-%09d", os.time(), math.random(0, 999999999))
end

local function _http_status_line(status)
    return "HTTP/1.1 " .. status .. "\r\n"
end

local function _http_header_block(headers)
    local parts = {}
    for k, v in pairs(headers) do
        parts[#parts + 1] = k .. ": " .. v
    end
    parts[#parts + 1] = ""  -- blank line
    parts[#parts + 1] = ""  -- trailing CRLF
    return table.concat(parts, "\r\n")
end

local function _queue_write(conn, s)
    -- Append-only. If cap exceeded, evict the connection.
    if #conn.write_buf + #s > WRITE_BUF_CAP then
        conn.state = "closing"
        return false
    end
    conn.write_buf = conn.write_buf .. s
    return true
end

local function _build_http_response(status, headers, body, session_id)
    headers = headers or {}
    headers["Content-Length"] = tostring(#body)
    headers["Connection"]     = "close"
    if session_id then headers["Mcp-Session-Id"] = session_id end
    return _http_status_line(status) .. _http_header_block(headers) .. body
end

local function _build_sse_headers(session_id)
    local h = {
        ["Content-Type"]                = "text/event-stream",
        ["Cache-Control"]               = "no-cache",
        ["Connection"]                  = "keep-alive",
        ["Access-Control-Allow-Origin"] = "*",
    }
    if session_id then h["Mcp-Session-Id"] = session_id end
    return _http_status_line("200 OK") .. _http_header_block(h)
end

-- Format a JSON-RPC payload as one SSE message event.
local function _sse_event(payload_str)
    return "event: message\r\ndata: " .. payload_str .. "\r\n\r\n"
end

-- Format a server-initiated JSON-RPC request from the notification queue
-- entry table { jsonrpc, id?, method, params? }.
local function _encode_notify(entry)
    return json.encode(entry)
end

-- ---- Per-connection FSM helpers ----

local function _parse_request_head(conn)
    -- Look for \r\n\r\n. If found, parse request line + headers.
    local sep = conn.buf:find("\r\n\r\n", 1, true)
    if not sep then return false end  -- not complete yet
    local head = conn.buf:sub(1, sep - 1)
    conn.buf = conn.buf:sub(sep + 4)  -- preserve any body bytes already buffered

    local lines = {}
    for line in head:gmatch("[^\r\n]+") do lines[#lines + 1] = line end
    if #lines == 0 then return nil, "empty head" end

    local method, path, version = lines[1]:match("^(%S+)%s+(%S+)%s+(%S+)")
    if not method then return nil, "bad request line" end
    conn.method, conn.path, conn.version = method, path, version

    local headers = {}
    for i = 2, #lines do
        local k, v = lines[i]:match("^(%S+):%s*(.*)")
        if k then headers[k:lower()] = v end
    end
    conn.headers = headers
    conn.body_remain = tonumber(headers["content-length"] or 0) or 0
    if conn.body_remain > BODY_CAP then
        return nil, "body too large"
    end
    return true
end

local function _check_auth(self, conn)
    if not self._auth_token then return true end
    if conn.method == "OPTIONS" then return true end
    local auth = conn.headers["authorization"] or ""
    local token = auth:match("^Bearer%s+(.+)$")
    return token == self._auth_token
end

-- ---- Session lookup / create ----

-- Resolve session by id. Returns the session table on success, or
-- (nil, "unknown") if `sid` is non-nil but no such session exists
-- (spec: 400/404 — caller decides). With nil `sid` (sessionless POST,
-- backwards compat), auto-issues a fresh session.
local function _resolve_session(self, sid)
    if sid then
        local sess = self._sessions[sid]
        if not sess then return nil, "unknown" end
        sess.last_activity = os.time()
        return sess
    end
    local new_id = _new_session_id()
    self._sessions[new_id] = {
        id            = new_id,
        sse_conn      = nil,
        pending       = {},     -- req_id → on_response
        notify_q      = {},     -- per-session, server-initiated requests
        created       = os.time(),
        last_activity = os.time(),
    }
    return self._sessions[new_id]
end

-- For `initialize` specifically, always mint a new session id regardless
-- of any client-provided header (the spec lets the server choose).
local function _create_session(self)
    local new_id = _new_session_id()
    self._sessions[new_id] = {
        id            = new_id,
        sse_conn      = nil,
        pending       = {},
        notify_q      = {},
        created       = os.time(),
        last_activity = os.time(),
    }
    return self._sessions[new_id]
end

-- Scan all sessions for a pending server-initiated request matching id.
-- Returns (session, callback) or nil.
local function _find_pending(self, req_id)
    for _, sess in pairs(self._sessions) do
        local cb = sess.pending[req_id]
        if cb then return sess, cb end
    end
    return nil
end

-- ---- Dispatch a fully-parsed POST body ----

local function _dispatch_post(self, conn)
    local body = conn.body
    if body == "" then
        return _build_http_response("400 Bad Request",
            { ["Content-Type"] = "application/json",
              ["Access-Control-Allow-Origin"] = "*" },
            jsonrpc_error(nil, -32700, "Empty body"), nil)
    end
    local ok, rpc_req = pcall(json.decode, body)
    if not ok then
        return _build_http_response("400 Bad Request",
            { ["Content-Type"] = "application/json",
              ["Access-Control-Allow-Origin"] = "*" },
            jsonrpc_error(nil, -32700, "Parse error"), nil)
    end

    -- Server-initiated response routing: if id matches a pending
    -- server-initiated request in ANY session, this POST is a response.
    if rpc_req.id and (rpc_req.result ~= nil or rpc_req.error ~= nil)
       and not rpc_req.method then
        local sess, cb = _find_pending(self, rpc_req.id)
        if sess then
            sess.pending[rpc_req.id] = nil
            pcall(cb, rpc_req)
            return _build_http_response("202 Accepted",
                { ["Content-Type"] = "application/json",
                  ["Access-Control-Allow-Origin"] = "*" },
                "", sess.id)
        end
    end

    -- Session resolution (deferred from header-parse time so we can detect
    -- `initialize`). Rules:
    --   - `initialize`: always mint a fresh session, ignoring any client sid
    --   - other methods, sid absent: auto-issue (backwards compat)
    --   - other methods, sid known: use it
    --   - other methods, sid unknown: 404
    local sess
    if rpc_req.method == "initialize" then
        sess = _create_session(self)
    else
        local s, serr = _resolve_session(self, conn.requested_sid)
        if not s then
            return _build_http_response("404 Not Found",
                { ["Content-Type"] = "text/plain",
                  ["Access-Control-Allow-Origin"] = "*" },
                "Session not found: " .. tostring(conn.requested_sid), nil)
        end
        sess = s
    end
    conn.session_id = sess.id
    -- Stash session id on the request so handle_request → tools/call can
    -- expose it to handler ctx (issue #9 — sampling needs to know which
    -- session to push the request onto).
    rpc_req._session_id = sess.id

    -- Normal client request / notification. Dispatch via handle_request.
    local response = self:handle_request(rpc_req)
    if not response then
        -- Notification → 202 Accepted, no body.
        return _build_http_response("202 Accepted",
            { ["Content-Type"] = "application/json",
              ["Access-Control-Allow-Origin"] = "*" },
            "", conn.session_id)
    end

    -- If client accepts SSE, respond as a single-event SSE stream.
    -- Otherwise plain JSON body.
    local accept = conn.headers["accept"] or ""
    if accept:find("text/event%-stream") then
        local hdrs = _build_sse_headers(conn.session_id)
        return hdrs .. _sse_event(response)
    end
    return _build_http_response("200 OK",
        { ["Content-Type"] = "application/json",
          ["Access-Control-Allow-Origin"] = "*" },
        response, conn.session_id)
end

local function _dispatch_options(conn)
    local acrh = conn.headers["access-control-request-headers"]
    return _build_http_response("204 No Content", {
        ["Access-Control-Allow-Origin"]   = "*",
        ["Access-Control-Allow-Methods"]  = "GET, POST, DELETE, OPTIONS",
        -- '*' does NOT cover Authorization per CORS spec; list explicitly.
        ["Access-Control-Allow-Headers"]  = acrh and (acrh .. ", Authorization")
            or "Content-Type, Accept, Authorization, Mcp-Session-Id, Mcp-Protocol-Version",
        ["Access-Control-Max-Age"]        = "86400",
    }, "", conn.session_id)
end

local function _dispatch_delete(self, conn)
    if not conn.session_id or not self._sessions[conn.session_id] then
        return _build_http_response("404 Not Found",
            { ["Content-Type"] = "text/plain",
              ["Access-Control-Allow-Origin"] = "*" },
            "Session not found", nil)
    end
    local sess = self._sessions[conn.session_id]
    if sess.sse_conn then
        sess.sse_conn.state = "closing"
        sess.sse_conn = nil
    end
    self._sessions[conn.session_id] = nil
    return _build_http_response("204 No Content",
        { ["Access-Control-Allow-Origin"] = "*" }, "", nil)
end

-- ---- Main loop helpers ----

local function _conn_read(self, conn)
    local chunk, err, partial = conn.sock:receive(8192)
    local data = chunk or partial or ""
    if data ~= "" then
        if #conn.buf + #data > READ_BUF_CAP and conn.state == "reading_head" then
            -- Header section too large.
            conn.write_buf = _build_http_response("431 Request Header Fields Too Large",
                { ["Content-Type"] = "text/plain" }, "Headers too large", nil)
            conn.state = "writing"
            return
        end
        conn.buf = conn.buf .. data
    end
    if err == "closed" then
        conn.state = "closing"
        return
    end
    -- Advance FSM.
    if conn.state == "reading_head" then
        local ok, perr = _parse_request_head(conn)
        if perr then
            conn.write_buf = _build_http_response("400 Bad Request",
                { ["Content-Type"] = "text/plain" }, tostring(perr), nil)
            conn.state = "writing"
            return
        end
        if not ok then return end  -- still waiting for full head

        -- Headers parsed: auth check, session resolve.
        if not _check_auth(self, conn) then
            conn.write_buf = _build_http_response("401 Unauthorized",
                { ["Content-Type"] = "application/json",
                  ["WWW-Authenticate"] = "Bearer" },
                '{"error":"unauthorized"}', nil)
            conn.state = "writing"
            return
        end
        -- Session resolution happens at dispatch time (after body is read),
        -- because `initialize` always mints a fresh session and we need to
        -- know the method to distinguish 404 (unknown id, non-initialize)
        -- from "auto-issue on initialize". Just stash the requested id.
        conn.requested_sid = conn.headers["mcp-session-id"]

        if conn.body_remain > 0 then
            -- Any body bytes already in conn.buf land here.
            conn.body = conn.buf:sub(1, conn.body_remain)
            conn.buf = conn.buf:sub(#conn.body + 1)
            conn.body_remain = conn.body_remain - #conn.body
            conn.state = (conn.body_remain == 0) and "dispatching" or "reading_body"
        else
            conn.state = "dispatching"
        end
    end
    if conn.state == "reading_body" then
        if #conn.buf > 0 then
            local take = math.min(conn.body_remain, #conn.buf)
            conn.body = conn.body .. conn.buf:sub(1, take)
            conn.buf = conn.buf:sub(take + 1)
            conn.body_remain = conn.body_remain - take
        end
        if conn.body_remain == 0 then conn.state = "dispatching" end
    end
    if conn.state == "dispatching" then
        local path = conn.path or ""
        if not path:match("^/mcp") then
            conn.write_buf = _build_http_response("404 Not Found",
                { ["Content-Type"] = "text/plain",
                  ["Access-Control-Allow-Origin"] = "*" },
                "Not Found", nil)
            conn.state = "writing"
            return
        end
        if conn.method == "OPTIONS" then
            conn.write_buf = _dispatch_options(conn)
            conn.state = "writing"
        elseif conn.method == "DELETE" then
            -- Resolve session: no auto-issue for DELETE; unknown sid → 404.
            if not conn.requested_sid then
                conn.write_buf = _build_http_response("400 Bad Request",
                    { ["Content-Type"] = "text/plain",
                      ["Access-Control-Allow-Origin"] = "*" },
                    "Mcp-Session-Id required for DELETE", nil)
                conn.state = "writing"
                return
            end
            conn.session_id = conn.requested_sid
            conn.write_buf = _dispatch_delete(self, conn)
            conn.state = "writing"
        elseif conn.method == "GET" then
            -- Resolve session (auto-issue if missing, 404 if unknown).
            local sess, serr = _resolve_session(self, conn.requested_sid)
            if not sess then
                conn.write_buf = _build_http_response("404 Not Found",
                    { ["Content-Type"] = "text/plain",
                      ["Access-Control-Allow-Origin"] = "*" },
                    "Session not found: " .. tostring(conn.requested_sid), nil)
                conn.state = "writing"
                return
            end
            conn.session_id = sess.id
            -- Persistent SSE stream. Enforce one per session.
            if sess.sse_conn and sess.sse_conn ~= conn then
                conn.write_buf = _build_http_response("409 Conflict",
                    { ["Content-Type"] = "text/plain",
                      ["Access-Control-Allow-Origin"] = "*" },
                    "Session already has an open SSE stream", nil)
                conn.state = "writing"
                return
            end
            sess.sse_conn = conn
            local hdrs = _build_sse_headers(conn.session_id)
            -- For backwards compat with the old SDK probe shape, emit a one-shot
            -- 'endpoint' event so clients can self-discover; modern clients
            -- ignore it and just await message events.
            local endpoint_data = json.encode({
                endpoint = "/mcp", sessionId = conn.session_id,
            })
            conn.write_buf = hdrs ..
                "event: endpoint\r\ndata: " .. endpoint_data .. "\r\n\r\n"
            conn.state = "sse_open"
            conn.last_heart = os.time()
        elseif conn.method == "POST" then
            conn.write_buf = _dispatch_post(self, conn)
            conn.state = "writing"
        else
            conn.write_buf = _build_http_response("405 Method Not Allowed",
                { ["Content-Type"] = "text/plain",
                  ["Allow"] = "GET, POST, DELETE, OPTIONS" },
                "Method Not Allowed", nil)
            conn.state = "writing"
        end
    end
end

local function _conn_write(conn)
    if conn.write_buf == "" then
        if conn.state == "writing" then conn.state = "closing" end
        return
    end
    local sent, err, sent_partial = conn.sock:send(conn.write_buf)
    if err == "closed" then
        conn.state = "closing"
        return
    end
    -- luasocket: on success returns last_byte_index_sent; on partial/timeout
    -- returns (nil, "timeout"|"closed", last_byte_index_sent_so_far). The
    -- second-return numeric is an absolute index into the original string.
    local idx = sent or sent_partial or 0
    if idx > 0 then
        conn.write_buf = conn.write_buf:sub(idx + 1)
    end
    if conn.write_buf == "" and conn.state == "writing" then
        conn.state = "closing"
    end
end

local function _drain_notifications(self)
    -- Global broadcast queue: fan out to every open sse_conn.
    while #self._notify_queue > 0 do
        local entry = table.remove(self._notify_queue, 1)
        local payload = _sse_event(_encode_notify(entry))
        for _, sess in pairs(self._sessions) do
            if sess.sse_conn and sess.sse_conn.state == "sse_open" then
                _queue_write(sess.sse_conn, payload)
            end
        end
    end
    -- Per-session queues: route to that session only.
    for _, sess in pairs(self._sessions) do
        while #sess.notify_q > 0 do
            if not (sess.sse_conn and sess.sse_conn.state == "sse_open") then
                break  -- no live SSE; leave queued (or expire policy could drop)
            end
            local entry = table.remove(sess.notify_q, 1)
            _queue_write(sess.sse_conn, _sse_event(_encode_notify(entry)))
        end
    end
end

local function _heartbeat_tick(self)
    local now = os.time()
    -- Heartbeats on open SSE conns.
    for _, sess in pairs(self._sessions) do
        local conn = sess.sse_conn
        if conn and conn.state == "sse_open"
           and now - conn.last_heart >= HEARTBEAT_SEC then
            _queue_write(conn, ": heartbeat\r\n\r\n")
            conn.last_heart = now
        end
    end
    -- Idle session expiry: no SSE + no activity for SESSION_IDLE_SEC.
    for sid, sess in pairs(self._sessions) do
        if sess.sse_conn == nil and now - sess.last_activity > SESSION_IDLE_SEC then
            self._sessions[sid] = nil
        end
    end
end

-- ---- Public: server-initiated request (for sampling/roots/etc.) ----
-- Enqueues a JSON-RPC request on the session's SSE stream. The callback
-- fires when the client POSTs back the response (matched by id).
function lmcp:server_request(session_id, method, params, on_response)
    local sess = self._sessions[session_id]
    if not sess or not sess.sse_conn then
        return false, "no live SSE stream for session " .. tostring(session_id)
    end
    self._server_req_id = (self._server_req_id or 0) + 1
    local id = "srv-" .. self._server_req_id
    sess.pending[id] = on_response
    local msg = { jsonrpc = JSONRPC, id = id, method = method }
    -- Omit params if nil OR an empty Lua table (would JSON-encode as []
    -- per project_json_empty_table_gotcha memory). Real params with at
    -- least one key encode correctly as an object.
    if params ~= nil and (type(params) ~= "table" or next(params)) then
        msg.params = params
    end
    sess.notify_q[#sess.notify_q + 1] = msg
    return true, id
end

-- Sampling (MCP issue #9): ask the client's LLM to generate text. Returns
-- (true, request_id) if dispatched, (false, err) otherwise. `on_response`
-- is called with the client's JSON-RPC response shape:
--   { result = { role, content = { type = "text", text = "..." }, model, stopReason? } }
-- or { error = { code, message } }.
--
-- Today this is fire-and-forget — tool handlers cannot block waiting for
-- the response in the single-threaded event loop (see follow-up #20). A
-- tool may kick off sampling and return immediately; the callback fires
-- when the client posts the response back.
--
-- opts shape (matches MCP spec):
--   messages = { { role = "user"|"assistant", content = {type, text} }, ... }
--   modelPreferences? = { hints?, intelligencePriority?, ... }
--   systemPrompt? = string
--   includeContext? = "none"|"thisServer"|"allServers"
--   temperature? = number
--   maxTokens = integer (required)
--   stopSequences? = list of strings
function lmcp:sample(session_id, opts, on_response)
    if not (self._client_caps.sampling) then
        return false, "client did not advertise sampling capability"
    end
    if type(opts) ~= "table" or type(opts.messages) ~= "table"
       or type(opts.maxTokens) ~= "number" then
        return false, "sample: opts.messages (table) and opts.maxTokens (number) required"
    end
    return self:server_request(session_id, "sampling/createMessage", opts, on_response)
end

-- Roots (MCP issue #10): ask the client which filesystem/URL roots are
-- in scope for this session. Async like sample(); on_fetched(roots_list,
-- err) fires when the client responds. Result is also cached on
-- self._roots_cache[session_id] for later sync lookups.
--
-- Client→server `notifications/roots/list_changed` invalidates the cache;
-- next call to :roots() re-fetches.
function lmcp:roots(session_id, on_fetched)
    if not (self._client_caps.roots) then
        return false, "client did not advertise roots capability"
    end
    return self:server_request(session_id, "roots/list", {}, function(resp)
        if resp.error then
            if on_fetched then on_fetched(nil, resp.error.message or "rpc error") end
            return
        end
        local list = resp.result and resp.result.roots or {}
        self._roots_cache[session_id] = {
            roots = list, fetched = os.time(),
        }
        if on_fetched then on_fetched(list, nil) end
    end)
end

-- Synchronous lookup of the cached roots. Returns the list (possibly
-- empty) if previously fetched, or nil if no :roots() call has completed
-- for this session yet.
function lmcp:roots_cached(session_id)
    local entry = self._roots_cache[session_id]
    return entry and entry.roots or nil
end

-- Synchronous helper: is `path` (a file:// URI or absolute filesystem
-- path) within any cached root for this session? Returns:
--   true  → matches at least one root
--   false → cache is populated but path is outside all roots
--   nil   → no roots cached yet; caller should :roots() first
function lmcp:path_in_roots(session_id, path)
    local roots = self:roots_cached(session_id)
    if not roots then return nil end
    -- Normalise: treat file:// URIs and bare paths uniformly.
    local norm = path:gsub("^file://", "")
    for _, r in ipairs(roots) do
        local root_path = (r.uri or ""):gsub("^file://", "")
        if root_path ~= "" and norm:sub(1, #root_path) == root_path then
            return true
        end
    end
    return false
end

function lmcp:run()
    local socket = require("socket")
    local server_sock = assert(socket.bind(self.host, self.port))
    server_sock:settimeout(0)
    self._conns = {}
    self._sessions = self._sessions or {}

    local addr, port = server_sock:getsockname()
    io.stderr:write(string.format("lmcp: %s v%s listening on %s:%d/mcp\n",
        self.name, self.version, addr, port))

    while true do
        -- Build select watch lists.
        local reads, writes = { server_sock }, {}
        for sock, conn in pairs(self._conns) do
            if conn.state == "reading_head" or conn.state == "reading_body"
               or conn.state == "sse_open" then
                reads[#reads + 1] = sock
            end
            if conn.write_buf ~= "" then
                writes[#writes + 1] = sock
            end
        end

        local ready_r, ready_w = socket.select(reads, writes, SELECT_TIMEOUT)

        for _, sock in ipairs(ready_r or {}) do
            if sock == server_sock then
                local new_sock, aerr = server_sock:accept()
                if new_sock then
                    new_sock:settimeout(0)
                    self._conns[new_sock] = {
                        sock = new_sock, state = "reading_head",
                        buf = "", body = "", headers = {},
                        method = nil, path = nil,
                        body_remain = 0, write_buf = "",
                        session_id = nil, last_heart = os.time(),
                    }
                elseif aerr and aerr ~= "timeout" then
                    io.stderr:write("lmcp: accept error: " .. tostring(aerr) .. "\n")
                end
            else
                local conn = self._conns[sock]
                if conn then
                    local ok, rerr = pcall(_conn_read, self, conn)
                    if not ok then
                        io.stderr:write("lmcp: read error: " .. tostring(rerr) .. "\n")
                        conn.state = "closing"
                    end
                end
            end
        end

        for _, sock in ipairs(ready_w or {}) do
            local conn = self._conns[sock]
            if conn then
                local ok, werr = pcall(_conn_write, conn)
                if not ok then
                    io.stderr:write("lmcp: write error: " .. tostring(werr) .. "\n")
                    conn.state = "closing"
                end
            end
        end

        -- Per-tick maintenance.
        _drain_notifications(self)
        _heartbeat_tick(self)

        -- After draining, attempt immediate writes on conns whose write_buf
        -- just got bytes (so list_changed / heartbeat appears within one tick).
        for sock, conn in pairs(self._conns) do
            if conn.write_buf ~= "" and conn.state ~= "closing" then
                pcall(_conn_write, conn)
            end
        end

        -- Sweep closing conns.
        for sock, conn in pairs(self._conns) do
            if conn.state == "closing" then
                -- Detach from session if it was the sse_conn.
                if conn.session_id then
                    local sess = self._sessions[conn.session_id]
                    if sess and sess.sse_conn == conn then
                        sess.sse_conn = nil
                        sess.last_activity = os.time()
                    end
                end
                pcall(sock.close, sock)
                self._conns[sock] = nil
            end
        end
    end
end

-- ---- stdio transport (MCP issue #15) ----
-- Line-delimited JSON-RPC: one message per line on stdin, one response
-- line per request on stdout, diagnostics on stderr. EOF closes cleanly.
-- Does NOT require luasocket — handle_request is transport-agnostic.
-- Bearer auth bypassed: stdio means the parent process is the trust
-- boundary.
function lmcp:run_stdio()
    -- Default stdout buffering on a pipe is "full" — a response would
    -- sit in the buffer until it fills, deadlocking the MCP client.
    -- Set "no" once + per-write flush belt-and-braces.
    io.stdout:setvbuf("no")
    io.stderr:write(string.format(
        "lmcp: %s v%s serving stdio\n", self.name, self.version))

    for line in io.stdin:lines() do
        if line ~= "" then
            -- pcall the whole body so a transient error (malformed JSON,
            -- handler bug, exotic pipe state) doesn't crash the loop.
            local body_ok, body_err = pcall(function()
                local parse_ok, req = pcall(json.decode, line)
                local response
                if not parse_ok then
                    response = jsonrpc_error(nil, -32700, "Parse error")
                else
                    response = self:handle_request(req)
                end
                if type(response) == "string" then
                    io.stdout:write(response, "\n")
                    io.stdout:flush()
                elseif response ~= nil then
                    io.stderr:write(
                        "lmcp: handler returned non-string ("
                        .. type(response) .. "); dropped\n")
                end
            end)
            if not body_ok then
                io.stderr:write("lmcp: stdio loop error: "
                    .. tostring(body_err) .. "\n")
            end
        end
    end
end

return lmcp