diff --git a/main.lua b/main.lua
index 8d3a368c..bbe44f54 100644
--- a/main.lua
+++ b/main.lua
@@ -10,6 +10,8 @@
 
 local editorMode = os.getenv("POKEPORT_EDITOR") == "1" or POKEPORT_EDITOR_MODE == true
 
+local SwitchDiagnostics = require("src.debug.SwitchDiagnostics")
+
 local Game, EditorApp, Importer, TouchEditor
 
 local autopilot -- optional scripted-input dev tool (tests/autopilot.lua)
@@ -268,6 +270,7 @@ function love.load(args)
 end
 
 function love.update(dt)
+  SwitchDiagnostics.maybeFlush(false)
   if editorMode then return EditorApp.update(dt) end
   if TouchEditor then return TouchEditor.update(dt) end
   if Importer then return Importer:update(dt) end
@@ -342,48 +345,56 @@ function love.keyreleased(key)
 end
 
 function love.gamepadpressed(joystick, button)
+  SwitchDiagnostics.onJoystickEvent("gamepadpressed", joystick, button)
   if editorMode or TouchEditor then return end
   if Importer then return Importer:gamepadpressed(joystick, button) end
   Game:gamepadpressed(joystick, button)
 end
 
 function love.gamepadreleased(joystick, button)
+  SwitchDiagnostics.onJoystickEvent("gamepadreleased", joystick, button)
   if editorMode or TouchEditor then return end
   if Importer then return Importer:gamepadreleased(joystick, button) end
   Game:gamepadreleased(joystick, button)
 end
 
 function love.gamepadaxis(joystick, axis, value)
+  SwitchDiagnostics.onJoystickEvent("gamepadaxis", joystick, axis, { value = value })
   if editorMode or TouchEditor then return end
   if Importer then return Importer:gamepadaxis(joystick, axis, value) end
   Game:gamepadaxis(joystick, axis, value)
 end
 
 function love.joystickpressed(joystick, button)
+  SwitchDiagnostics.onJoystickEvent("joystickpressed", joystick, button)
   if editorMode or TouchEditor then return end
   if Importer then return Importer:joystickpressed(joystick, button) end
   Game:joystickpressed(joystick, button)
 end
 
 function love.joystickreleased(joystick, button)
+  SwitchDiagnostics.onJoystickEvent("joystickreleased", joystick, button)
   if editorMode or TouchEditor then return end
   if Importer then return Importer:joystickreleased(joystick, button) end
   Game:joystickreleased(joystick, button)
 end
 
 function love.joystickaxis(joystick, axis, value)
+  SwitchDiagnostics.onJoystickEvent("joystickaxis", joystick, axis, { value = value })
   if editorMode or TouchEditor then return end
   if Importer then return Importer:joystickaxis(joystick, axis, value) end
   Game:joystickaxis(joystick, axis, value)
 end
 
 function love.joystickhat(joystick, hat, direction)
+  SwitchDiagnostics.onJoystickEvent("joystickhat", joystick, hat, { direction = direction })
   if editorMode or TouchEditor then return end
   if Importer then return Importer:joystickhat(joystick, hat, direction) end
   Game:joystickhat(joystick, hat, direction)
 end
 
 function love.joystickremoved(joystick)
+  SwitchDiagnostics.onJoystickEvent("joystickremoved", joystick)
   if editorMode or TouchEditor then return end
   if Importer then return end
   Game:joystickremoved(joystick)
diff --git a/src/debug/SwitchDiagnostics.lua b/src/debug/SwitchDiagnostics.lua
new file mode 100644
index 00000000..c256475f
--- /dev/null
+++ b/src/debug/SwitchDiagnostics.lua
@@ -0,0 +1,143 @@
+-- Opt-in Switch diagnostics: ring buffer + ≤1 Hz flush when switch-debug.txt exists.
+-- Never logs ROM/save bytes — see spec SWNX-13/28.
+
+local SwitchDiagnostics = {}
+
+local MARKER = "switch-debug.txt"
+local LOG_FILE = "switch.log"
+local FLUSH_INTERVAL = 1.0
+local RING_SIZE = 64
+
+local enabled = nil
+local buffer = {}
+local bufCount = 0
+local lastFlushAt = -math.huge
+local identityLine = nil
+
+local function fs()
+  return love and love.filesystem
+end
+
+local function redactString(s)
+  if type(s) ~= "string" then return s end
+  for i = 1, #s do
+    local b = s:byte(i)
+    if b < 32 or b > 126 then return "<redacted>" end
+  end
+  return s
+end
+
+local function sanitize(value, depth)
+  depth = depth or 0
+  if depth > 4 then return "<deep>" end
+  local t = type(value)
+  if t == "string" then return redactString(value) end
+  if t == "number" or t == "boolean" or value == nil then return value end
+  if t == "table" then
+    local out = {}
+    for k, v in pairs(value) do
+      local key = type(k) == "string" and k or tostring(k)
+      if key:lower():find("rom") or key:lower():find("save") then
+        out[key] = "<redacted>"
+      else
+        out[key] = sanitize(v, depth + 1)
+      end
+    end
+    return out
+  end
+  return tostring(value)
+end
+
+local function encodePayload(payload)
+  if payload == nil then return "" end
+  if type(payload) == "string" then return redactString(payload) end
+  local parts = {}
+  for k, v in pairs(sanitize(payload)) do
+    parts[#parts + 1] = tostring(k) .. "=" .. tostring(v)
+  end
+  table.sort(parts)
+  return table.concat(parts, " ")
+end
+
+function SwitchDiagnostics._resetForTests()
+  enabled = nil
+  buffer = {}
+  bufCount = 0
+  lastFlushAt = -math.huge
+  identityLine = nil
+end
+
+function SwitchDiagnostics.isEnabled()
+  if enabled ~= nil then return enabled end
+  local filesystem = fs()
+  if not filesystem then
+    enabled = false
+    return false
+  end
+  enabled = filesystem.getInfo(MARKER) ~= nil
+  return enabled
+end
+
+function SwitchDiagnostics.identityOverlay()
+  if identityLine then return identityLine end
+  local gitCommit = os.getenv("POKEPORT_GIT_COMMIT") or "unknown"
+  local loveNxTag = "11.5-nx1"
+  local buildVersion = "dev"
+  local filesystem = fs()
+  if filesystem then
+    local raw = filesystem.read("build-info.json")
+    if raw and raw ~= "" then
+      local ver = raw:match('"version"%s*:%s*"([^"]+)"')
+      if ver then buildVersion = ver end
+      local tag = raw:match('"loveNxTag"%s*:%s*"([^"]+)"')
+      if tag then loveNxTag = tag end
+      local commit = raw:match('"gitCommit"%s*:%s*"([^"]+)"')
+      if commit then gitCommit = commit end
+    end
+  end
+  identityLine = ("gitCommit=%s loveNxTag=%s buildVersion=%s os=%s"):format(
+    gitCommit, loveNxTag, buildVersion,
+    love and love.system and love.system.getOS() or "unknown")
+  return identityLine
+end
+
+function SwitchDiagnostics.onEvent(kind, payload)
+  if not SwitchDiagnostics.isEnabled() then return end
+  bufCount = bufCount + 1
+  local slot = ((bufCount - 1) % RING_SIZE) + 1
+  buffer[slot] = ("%s %s"):format(tostring(kind), encodePayload(payload))
+end
+
+function SwitchDiagnostics.onJoystickEvent(kind, joystick, button, extra)
+  if not SwitchDiagnostics.isEnabled() then return end
+  local payload = { button = button }
+  if joystick then
+    if joystick.getGUID then payload.guid = joystick:getGUID() end
+    if joystick.isGamepad then payload.isGamepad = joystick:isGamepad() end
+    if joystick.getName then payload.name = joystick:getName() end
+  end
+  if extra then
+    for k, v in pairs(extra) do payload[k] = v end
+  end
+  SwitchDiagnostics.onEvent(kind, payload)
+end
+
+function SwitchDiagnostics.maybeFlush(force, now)
+  if not SwitchDiagnostics.isEnabled() then return end
+  now = now or (love and love.timer and love.timer.getTime() or 0)
+  if not force and (now - lastFlushAt) < FLUSH_INTERVAL then return end
+  lastFlushAt = now
+
+  local filesystem = fs()
+  if not filesystem then return end
+
+  local lines = { SwitchDiagnostics.identityOverlay(), "---" }
+  local start = math.max(1, bufCount - RING_SIZE + 1)
+  for i = start, bufCount do
+    local slot = ((i - 1) % RING_SIZE) + 1
+    if buffer[slot] then lines[#lines + 1] = buffer[slot] end
+  end
+  filesystem.write(LOG_FILE, table.concat(lines, "\n") .. "\n")
+end
+
+return SwitchDiagnostics
diff --git a/tests/engine/switch_diagnostics_test.lua b/tests/engine/switch_diagnostics_test.lua
new file mode 100644
index 00000000..3e83a2dc
--- /dev/null
+++ b/tests/engine/switch_diagnostics_test.lua
@@ -0,0 +1,53 @@
+-- Opt-in Switch input diagnostics (SWNX-13/28): marker file, ring buffer, flush cap.
+package.path = "./?.lua;./?/init.lua;" .. package.path
+if not _G.love then _G.love = require("tests.love_stub") end
+
+local T = require("tests.harness")
+local check = T.check
+local eq = T.eq
+
+local SwitchDiagnostics = require("src.debug.SwitchDiagnostics")
+
+local function reset()
+  SwitchDiagnostics._resetForTests()
+  love.filesystem.remove("switch-debug.txt")
+  love.filesystem.remove("switch.log")
+end
+
+reset()
+check(not SwitchDiagnostics.isEnabled(), "disabled without marker file")
+
+love.filesystem.write("switch-debug.txt", "")
+reset()
+love.filesystem.write("switch-debug.txt", "")
+check(SwitchDiagnostics.isEnabled(), "enabled when marker exists")
+
+SwitchDiagnostics.onEvent("probe", { kind = "gamepadpressed", button = "a" })
+SwitchDiagnostics.maybeFlush(true, 0)
+local log = love.filesystem.read("switch.log") or ""
+check(log:find("gamepadpressed", 1, true) ~= nil, "flush writes buffered events")
+check(log:find("gitCommit=", 1, true) ~= nil, "identity includes gitCommit field")
+check(log:find("loveNxTag=", 1, true) ~= nil, "identity includes loveNxTag field")
+
+-- ROM-like byte sequences must never appear in diagnostics output.
+local romSnippet = string.char(0xEA, 0x9B, 0xCA, 0xE6)
+SwitchDiagnostics.onEvent("probe", { sample = romSnippet, note = "redacted" })
+SwitchDiagnostics.maybeFlush(true, 1)
+log = love.filesystem.read("switch.log") or ""
+check(not log:find(romSnippet, 1, true),
+  "ROM bytes are stripped from diagnostic payloads")
+
+-- Flush rate capped at 1 Hz unless forced.
+reset()
+love.filesystem.write("switch-debug.txt", "")
+SwitchDiagnostics.onEvent("tick", { n = 1 })
+SwitchDiagnostics.maybeFlush(true, 0.0)
+SwitchDiagnostics.onEvent("tick", { n = 2 })
+SwitchDiagnostics.maybeFlush(false, 0.5)
+local logMid = love.filesystem.read("switch.log") or ""
+SwitchDiagnostics.maybeFlush(false, 1.0)
+local logLate = love.filesystem.read("switch.log") or ""
+check(not logMid:find("n=2", 1, true), "flush waits until 1s elapsed")
+check(logLate:find("n=2", 1, true) ~= nil, "flush includes events after 1s")
+
+T.finish()