diff --git a/CONTRIBUTING-mods.md b/CONTRIBUTING-mods.md
index 295934f9..c2a8f462 100644
--- a/CONTRIBUTING-mods.md
+++ b/CONTRIBUTING-mods.md
@@ -246,17 +246,37 @@ real Gold boot.
 
 Your code runs in a sandbox (`src/mods/Sandbox.lua`), not against the
 engine's globals. Every chunk you author gets it: `main.lua`, your
-`options_schema`, and anything you `load()` yourself. What is absent:
+`options_schema`, and anything you `load()` yourself.
 
-| Absent | Use instead |
+The globals the sandbox took away are still *reachable*, as compat
+stand-ins (`src/mods/LegacyCompat.lua`) that answer with the new API
+underneath. A mod written before the sandbox keeps working; it logs one
+warning per call it should migrate, and the mod manager lists them. What
+each stand-in actually does:
+
+| Pre-sandbox call | What it does now | Migrate to |
+| --- | --- | --- |
+| `io.open`, `io.lines`, `love.filesystem.read`/`lines`/`newFile` | reads your own shipped files, then your overlay, then `mod.storage` | `mod:read`, `mod.storage` |
+| `love.filesystem.write`/`append`, `io.open(…, "w")`, `os.remove`, `os.rename` | writes to a private per-mod overlay under `mod_compat/<your id>/` | `mod.storage` |
+| `love.filesystem.getDirectoryItems`/`getInfo` | your own directory plus your overlay | `mod:list`, `mod:info` |
+| `love.filesystem.getSaveDirectory` and friends | a virtual root; anything joined to it lands in your overlay | `mod.storage` |
+| `os.getenv` | `nil`, except home-like names, which answer with that same virtual root | nothing |
+| `love.filesystem.load`, `dofile`, `loadfile` | compiles the chunk into your sandbox | `require`, `mod:read` plus `load` |
+| `love.system` | `getOS`/`getPowerInfo`/`getProcessorCount` read through; clipboard and `openURL` do nothing | `mod.device:powerInfo()`, `mod.steps` |
+| `love.event` | passes through, except `quit`, which does nothing | `mod.events`, `mod.hooks` |
+| `love.mousemoved = fn` and the other callbacks | installs on the real `love` table, the way it always did | `mod.hooks`, `mod.events` |
+| `package` | an inert stub, so `package.path = …` does not crash | `require` |
+
+What has no stand-in, because there is nothing honest to reroute it to:
+
+| Still refused | Why |
 | --- | --- |
-| `io`, and `require("io")` | `mod:read` for your own files, `mod.storage` to persist |
-| `os.getenv`, `os.execute`, `os.remove`, `os.rename`, `os.exit` | nothing; `os.time`/`os.date`/`os.clock` still work |
-| `package`, `dofile`, `loadfile`, `debug`, `getfenv`, `setfenv` | `require` for the supported engine modules |
-| `require("ffi")`, `require("love.*")` | the `love` table you are given |
-| `love.filesystem` | `mod.storage` (per-mod, per-playthrough), `mod:read` for a known file, `mod:list` / `mod:info` to iterate your own directory |
-| `love.thread`, `love.event` | `mod.events`, `mod.hooks` |
-| `love.system` | `mod.device:powerInfo()` for battery information; `mod.steps` (with the `steps` permission) for the step bridge |
+| `love.thread` | a LÖVE thread is a fresh Lua state with the full standard library, which no environment-based sandbox in this state can reach |
+| `require("ffi")` | arbitrary C |
+| `debug`, `getfenv`, `setfenv` | each one undoes the sandbox from inside |
+| `io.popen`, `os.execute` | spawning a process |
+| `love.run`, `love.errorhandler` | the engine's own loop and its crash path |
+| replacing a `love` module table (`love.filesystem = {}`) | the engine reads those tables too |
 
 The rest of `love` passes through unchanged, so graphics, audio, timers and
 input work as they always have.
@@ -283,8 +303,12 @@ permission that grants raw filesystem access, because no mod needs one:
 everything a mod legitimately writes is already scoped by
 `mod.storage` or the asset-transform derived root.
 
-If your mod used one of the absent globals, the fix is almost always
-`mod.storage`. Open an issue if you have a case it does not cover.
+If your mod used one of the rerouted globals, the fix is almost always
+`mod.storage`. The overlay is a compatibility floor, not a second storage
+system: it is not scoped per playthrough, it does not migrate, and it is
+the first thing that will be dropped once the mods on the index have
+moved off it. Open an issue if you have a case `mod.storage` does not
+cover.
 
 ### 6. `mod.card`
 
diff --git a/docs/modding.md b/docs/modding.md
index 36f997c9..2335bc81 100644
--- a/docs/modding.md
+++ b/docs/modding.md
@@ -802,3 +802,37 @@ the native side's pending file itself, each permissioned mod receives its
 own copy of a delivery, and steps are anchored natively so the same walk
 is never delivered twice. Without the permission, `sync` and `poll` raise
 an error naming it.
+
+## Pre-sandbox globals (compat)
+
+A mod written before the sandbox landed does not have to be updated to
+load. `io`, `package`, `dofile`, `loadfile`, `os.getenv`, `love.filesystem`,
+`love.system` and `love.event` are all present again as compat stand-ins
+(`src/mods/LegacyCompat.lua`), and assigning a LÖVE callback
+(`love.mousemoved = fn`) installs on the real table the way it always did.
+Every stand-in call logs one warning naming its replacement, and
+`loader:legacyReport(modId)` returns the same list with call counts, which
+is what a "needs updating" badge should read.
+
+The stand-ins are not the old globals. Paths are classified rather than
+passed through:
+
+- A path inside your own mod directory reads the file you shipped.
+- Anything else, including an absolute path, resolves into a private
+  per-mod overlay at `mod_compat/<your id>/` under the save directory.
+  Two mods naming the same path never see each other's bytes, and nothing
+  is written outside the game tree.
+- A read misses through the overlay to your shipped file, then to
+  `mod.storage`, so a half-migrated mod sees both.
+- A write over a path you shipped shadows it; the packaged file is never
+  modified, and `mod:read` still returns the packaged bytes.
+- `love.filesystem.getSaveDirectory()` and `os.getenv("HOME")` answer with
+  a virtual root, so a legacy mod that joins its own paths lands back in
+  the same overlay.
+
+`love.thread` stays refused. A LÖVE thread runs in a separate Lua state
+with the full standard library, which the sandbox in this state cannot
+reach, so a stand-in would be a hole rather than a reroute. The same goes
+for `ffi`, `debug`, `setfenv`, `os.execute`, `io.popen`, `love.run` and
+`love.errorhandler`. A mod that needs real background work needs an
+engine-owned facility, not a compat shim.
diff --git a/docs/new-features.md b/docs/new-features.md
index 80c47172..c59ccd44 100644
--- a/docs/new-features.md
+++ b/docs/new-features.md
@@ -40,3 +40,4 @@ A fourth game the launcher can import and play, built from pret/pokegold the sam
 * **On-screen touch pad** and controller SELECT for registered items
 
 
+* **Older mods keep loading** after the sandbox change, through per-mod compat stand-ins for the pre-sandbox globals
diff --git a/src/mods/LegacyCompat.lua b/src/mods/LegacyCompat.lua
new file mode 100644
index 00000000..0f8a1cf2
--- /dev/null
+++ b/src/mods/LegacyCompat.lua
@@ -0,0 +1,943 @@
+local Logger = require("src.core.Logger")
+local SafePath = require("src.mods.SafePath")
+
+local okSave, SaveData = pcall(require, "src.core.SaveData")
+if not okSave then SaveData = nil end
+local okStorage, Storage = pcall(require, "src.mods.Storage")
+if not okStorage then Storage = nil end
+
+local LegacyCompat = {}
+
+local ROOT = "mod_compat"
+local OWN = "_own"
+local MAX_SEGMENTS = 24
+
+LegacyCompat.reports = {}
+
+function LegacyCompat.reset()
+  LegacyCompat.reports = {}
+end
+
+function LegacyCompat.report(modId)
+  if modId then
+    local entry = LegacyCompat.reports[modId]
+    return entry and entry.order or {}
+  end
+  local out = {}
+  for id, entry in pairs(LegacyCompat.reports) do
+    out[id] = entry.order
+  end
+  return out
+end
+
+local function note(ctx, call, advice)
+  local entry = LegacyCompat.reports[ctx.modId]
+  if not entry then
+    entry = { calls = {}, order = {} }
+    LegacyCompat.reports[ctx.modId] = entry
+  end
+  local row = entry.calls[call]
+  if row then
+    row.count = row.count + 1
+    return
+  end
+  row = { call = call, advice = advice, count = 1 }
+  entry.calls[call] = row
+  entry.order[#entry.order + 1] = row
+  Logger.warn("[%s] %s was removed from the mod sandbox; %s", ctx.modId, call,
+    advice)
+end
+
+local function refuse(ctx, call, advice)
+  note(ctx, call, advice)
+  return nil, ("%s is not available to mods; %s"):format(call, advice)
+end
+
+-- ------- path routing
+
+local function normalize(path)
+  if type(path) ~= "string" or path == "" then return nil end
+  path = path:gsub("\\", "/")
+  while path:find("//", 1, true) do path = (path:gsub("//", "/")) end
+  if path ~= "/" then path = (path:gsub("/$", "")) end
+  return path
+end
+
+local function flatten(path)
+  local parts = {}
+  for segment in path:gmatch("[^/]+") do
+    if segment ~= "." and segment ~= ".." then
+      parts[#parts + 1] = (segment:gsub("[^%w_%.%-]", "_"))
+    end
+  end
+  if #parts == 0 then return nil end
+  while #parts > MAX_SEGMENTS do table.remove(parts, 1) end
+  return table.concat(parts, "/")
+end
+
+local function storageKey(key)
+  local parts = {}
+  for segment in key:gmatch("[^/]+") do parts[#parts + 1] = segment end
+  if #parts == 0 then return nil end
+  parts[#parts] = (parts[#parts]:gsub("%.[^%.]*$", ""))
+  for i = 1, #parts do
+    local cleaned = (parts[i]:gsub("[^%w_%-]", "_"))
+    if cleaned == "" then return nil end
+    parts[i] = cleaned
+  end
+  return table.concat(parts, "/")
+end
+
+local function ownRelative(ctx, path)
+  if not ctx.modPath then return nil end
+  if path == ctx.modPath then return "" end
+  if path:sub(1, #ctx.modPath + 1) == ctx.modPath .. "/" then
+    return path:sub(#ctx.modPath + 2)
+  end
+  return nil
+end
+
+local function ownFull(ctx, rel)
+  if rel == "" then return ctx.modPath end
+  local safe = SafePath.safe(rel)
+  if not safe then return nil end
+  return ctx.modPath .. "/" .. safe
+end
+
+local function ownExists(ctx, rel)
+  local full = ownFull(ctx, rel)
+  local fs = ctx.fs
+  if not (full and fs and fs.getInfo) then return nil end
+  return fs.getInfo(full)
+end
+
+local function classify(ctx, path)
+  path = normalize(path)
+  if not path then return nil end
+
+  local root = ctx.virtualRoot
+  if path == root then return { key = "", dir = true } end
+  if path:sub(1, #root + 1) == root .. "/" then
+    return { key = flatten(path:sub(#root + 2)) }
+  end
+
+  local rel = ownRelative(ctx, path)
+  if not rel and path:sub(1, 1) ~= "/" and not path:match("^%a:")
+      and ownExists(ctx, path) then
+    rel = path
+  end
+  if rel then
+    if rel == "" then return { key = OWN, rel = "", dir = true } end
+    local flat = flatten(rel)
+    return { key = flat and (OWN .. "/" .. flat) or nil, rel = rel }
+  end
+
+  return { key = flatten(path) }
+end
+
+-- ------- the overlay
+
+local function persistFs(ctx)
+  if SaveData and SaveData.persistenceFs then
+    return SaveData.persistenceFs(ctx.fs)
+  end
+  return ctx.fs
+end
+
+local function overlayPath(ctx, key)
+  if key == nil or key == "" then return ROOT .. "/" .. ctx.modId end
+  return ROOT .. "/" .. ctx.modId .. "/" .. key
+end
+
+local function ensureParent(fs, path)
+  if not fs.createDirectory then return end
+  local dir = path:match("^(.*)/[^/]+$")
+  if not dir then return end
+  local built = nil
+  for segment in dir:gmatch("[^/]+") do
+    built = built and (built .. "/" .. segment) or segment
+    fs.createDirectory(built)
+  end
+end
+
+local function overlayRead(ctx, key)
+  local fs = persistFs(ctx)
+  if not (fs and fs.read and fs.getInfo) then return nil end
+  local path = overlayPath(ctx, key)
+  local info = fs.getInfo(path)
+  if not info or info.type == "directory" then return nil end
+  local body = fs.read(path)
+  if type(body) ~= "string" then return nil end
+  return body
+end
+
+local function overlayWrite(ctx, key, data)
+  local fs = persistFs(ctx)
+  if not (fs and fs.write) then
+    return false, "no writable filesystem is available"
+  end
+  local path = overlayPath(ctx, key)
+  ensureParent(fs, path)
+  local ok, err = fs.write(path, data)
+  if ok == false then return false, err or "write failed" end
+  return true
+end
+
+local function overlayRemove(ctx, key)
+  local fs = persistFs(ctx)
+  if not (fs and fs.remove and fs.getInfo) then return false end
+  local path = overlayPath(ctx, key)
+  if not fs.getInfo(path) then return false end
+  fs.remove(path)
+  return true
+end
+
+local function overlayInfo(ctx, key)
+  local fs = persistFs(ctx)
+  if not (fs and fs.getInfo) then return nil end
+  return fs.getInfo(overlayPath(ctx, key))
+end
+
+local function storageRead(ctx, key)
+  if not (Storage and key and key ~= "") then return nil end
+  if key:sub(1, #OWN + 1) == OWN .. "/" then return nil end
+  local game = ctx.game and ctx.game()
+  if not game then return nil end
+  local sk = storageKey(key)
+  if not sk then return nil end
+  ctx.storage = ctx.storage or Storage.new(ctx.modId, ctx.fs)
+  local ok, bytes = pcall(ctx.storage.readBytes, ctx.storage, game, sk)
+  if ok and type(bytes) == "string" then return bytes end
+  return nil
+end
+
+local function readPath(ctx, path)
+  local at = classify(ctx, path)
+  if not at then return nil, "invalid path" end
+  local body = at.key and overlayRead(ctx, at.key)
+  if body then return body end
+  if at.rel then
+    local full = ownFull(ctx, at.rel)
+    local fs = ctx.fs
+    if full and fs and fs.read then
+      local packaged = fs.read(full)
+      if type(packaged) == "string" then return packaged end
+    end
+  end
+  body = at.key and storageRead(ctx, at.key)
+  if body then return body end
+  return nil, "could not open " .. tostring(path)
+end
+
+local function writePath(ctx, path, data)
+  local at = classify(ctx, path)
+  if not (at and at.key) then return false, "invalid path" end
+  return overlayWrite(ctx, at.key, data)
+end
+
+local function infoPath(ctx, path)
+  local at = classify(ctx, path)
+  if not at then return nil end
+  if at.key then
+    local info = overlayInfo(ctx, at.key)
+    if info then
+      return { type = info.type, size = info.size, modtime = info.modtime }
+    end
+  end
+  if at.rel then
+    local info = ownExists(ctx, at.rel)
+    if info then
+      return { type = info.type, size = info.size, modtime = info.modtime }
+    end
+  end
+  local stored = at.key and storageRead(ctx, at.key)
+  if stored then return { type = "file", size = #stored } end
+  return nil
+end
+
+local function listPath(ctx, path)
+  local at = classify(ctx, path)
+  local seen, out = {}, {}
+  local function add(name)
+    if name and name ~= "" and not seen[name] then
+      seen[name] = true
+      out[#out + 1] = name
+    end
+  end
+  if at and at.rel then
+    local full = ownFull(ctx, at.rel)
+    local fs = ctx.fs
+    if full and fs and fs.getDirectoryItems then
+      for _, name in ipairs(fs.getDirectoryItems(full) or {}) do add(name) end
+    end
+  end
+  if at and at.key then
+    local fs = persistFs(ctx)
+    if fs and fs.getDirectoryItems then
+      for _, name in ipairs(fs.getDirectoryItems(overlayPath(ctx, at.key)) or {}) do
+        add(name)
+      end
+    end
+  end
+  table.sort(out)
+  return out
+end
+
+-- ------- buffered file handles
+
+local function openBuffer(ctx, path, mode)
+  mode = tostring(mode or "r"):gsub("b", "")
+  local writable = mode:find("[wa+]") ~= nil
+  local body
+  if mode:sub(1, 1) == "w" then
+    body = ""
+  else
+    body = readPath(ctx, path)
+    if not body then
+      if not writable then return nil, "could not open " .. tostring(path) end
+      body = ""
+    end
+  end
+  local state = {
+    ctx = ctx, path = path, buf = body, pos = 1,
+    writable = writable, closed = false, dirty = false,
+  }
+  if mode:sub(1, 1) == "a" then state.pos = #body + 1 end
+  return state
+end
+
+local function bufferFlush(state)
+  if not (state.writable and state.dirty) then return true end
+  local ok, err = writePath(state.ctx, state.path, state.buf)
+  if ok then state.dirty = false end
+  return ok, err
+end
+
+local function bufferWrite(state, text)
+  if not state.writable then return false, "file is not open for writing" end
+  local head = state.buf:sub(1, state.pos - 1)
+  if #head < state.pos - 1 then head = head .. string.rep("\0", state.pos - 1 - #head) end
+  local tail = state.buf:sub(state.pos + #text)
+  state.buf = head .. text .. tail
+  state.pos = state.pos + #text
+  state.dirty = true
+  return true
+end
+
+local function bufferRead(state, fmt)
+  if fmt == nil then fmt = "*l" end
+  if type(fmt) == "number" then
+    if fmt == 0 then return state.pos <= #state.buf and "" or nil end
+    if state.pos > #state.buf then return nil end
+    local chunk = state.buf:sub(state.pos, state.pos + fmt - 1)
+    state.pos = state.pos + #chunk
+    return chunk
+  end
+  fmt = tostring(fmt):gsub("^%*", "")
+  if fmt == "a" then
+    local rest = state.buf:sub(state.pos)
+    state.pos = #state.buf + 1
+    return rest
+  end
+  if fmt == "l" or fmt == "L" then
+    if state.pos > #state.buf then return nil end
+    local nl = state.buf:find("\n", state.pos, true)
+    local line
+    if nl then
+      line = state.buf:sub(state.pos, fmt == "L" and nl or nl - 1)
+      state.pos = nl + 1
+    else
+      line = state.buf:sub(state.pos)
+      state.pos = #state.buf + 1
+    end
+    return line
+  end
+  if fmt == "n" then
+    local rest = state.buf:sub(state.pos)
+    local text, after = rest:match("^%s*(%-?%d+%.?%d*[eE]?[-+]?%d*)()")
+    if not text then return nil end
+    state.pos = state.pos + after - 1
+    return tonumber(text)
+  end
+  return nil
+end
+
+local function bufferSeek(state, whence, offset)
+  whence = whence or "cur"
+  offset = offset or 0
+  if whence == "set" then state.pos = offset + 1
+  elseif whence == "cur" then state.pos = state.pos + offset
+  elseif whence == "end" then state.pos = #state.buf + offset + 1
+  else return nil, "bad seek base" end
+  if state.pos < 1 then state.pos = 1 end
+  return state.pos - 1
+end
+
+local function bufferClose(state)
+  if state.closed then return true end
+  local ok, err = bufferFlush(state)
+  state.closed = true
+  return ok, err
+end
+
+local function ioFile(ctx, path, mode)
+  local state, err = openBuffer(ctx, path, mode)
+  if not state then return nil, err end
+  local file = {}
+  function file:read(...)
+    local count = select("#", ...)
+    if count == 0 then return bufferRead(state, "*l") end
+    local out = {}
+    for i = 1, count do out[i] = bufferRead(state, (select(i, ...))) end
+    return unpack(out, 1, count)
+  end
+  function file:write(...)
+    for i = 1, select("#", ...) do
+      local value = select(i, ...)
+      local ok, writeErr = bufferWrite(state, tostring(value))
+      if not ok then return nil, writeErr end
+    end
+    return file
+  end
+  function file:lines(fmt)
+    return function() return bufferRead(state, fmt or "*l") end
+  end
+  function file:seek(whence, offset) return bufferSeek(state, whence, offset) end
+  function file:flush() bufferFlush(state) return file end
+  function file:close() return bufferClose(state) end
+  function file:setvbuf() return true end
+  return file
+end
+
+local function loveFile(ctx, path, mode)
+  local state = nil
+  local file = {}
+  function file:open(openMode)
+    local opened, err = openBuffer(ctx, path, openMode or mode or "r")
+    if not opened then return false, err end
+    state = opened
+    return true
+  end
+  function file:isOpen() return state ~= nil and not state.closed end
+  function file:read(bytes)
+    if not state and not file:open("r") then return nil, 0 end
+    local body = bytes and bufferRead(state, bytes) or bufferRead(state, "*a")
+    if not body then return nil, 0 end
+    return body, #body
+  end
+  function file:write(data, size)
+    if not state and not file:open(mode or "w") then return false end
+    data = tostring(data)
+    if size then data = data:sub(1, size) end
+    local ok, err = bufferWrite(state, data)
+    if not ok then return false, err end
+    bufferFlush(state)
+    return true
+  end
+  function file:lines()
+    if not state then file:open("r") end
+    return function() return state and bufferRead(state, "*l") or nil end
+  end
+  function file:seek(offset)
+    if not state then return false end
+    bufferSeek(state, "set", offset or 0)
+    return true
+  end
+  function file:tell() return state and (state.pos - 1) or 0 end
+  function file:getSize()
+    if state then return #state.buf end
+    local body = readPath(ctx, path)
+    return body and #body or 0
+  end
+  function file:flush() if state then bufferFlush(state) end return true end
+  function file:close()
+    if state then bufferClose(state) end
+    state = nil
+    return true
+  end
+  function file:getFilename() return path end
+  function file:getMode() return mode or "c" end
+  function file:setBuffer() return true end
+  if mode and mode ~= "c" then file:open(mode) end
+  return file
+end
+
+-- ------- the love.filesystem stand-in
+
+local function realFilesystem()
+  return _G.love and _G.love.filesystem or nil
+end
+
+local function filesystemShim(ctx)
+  local fsShim = {}
+
+  local function readAdvice() return "reads now come from mod:read and mod.storage" end
+
+  function fsShim.read(a, b, c)
+    local path, size = a, b
+    if (a == "string" or a == "data") and type(b) == "string" then
+      path, size = b, c
+    end
+    note(ctx, "love.filesystem.read", readAdvice())
+    local body, err = readPath(ctx, path)
+    if not body then return nil, err end
+    if size and size >= 0 then body = body:sub(1, size) end
+    return body, #body
+  end
+
+  function fsShim.write(path, data, size)
+    note(ctx, "love.filesystem.write",
+      "writes are rerouted to this mod's private compat storage; migrate to mod.storage")
+    data = tostring(data)
+    if size then data = data:sub(1, size) end
+    local ok, err = writePath(ctx, path, data)
+    if not ok then return false, err end
+    return true
+  end
+
+  function fsShim.append(path, data, size)
+    note(ctx, "love.filesystem.append",
+      "writes are rerouted to this mod's private compat storage; migrate to mod.storage")
+    data = tostring(data)
+    if size then data = data:sub(1, size) end
+    local existing = readPath(ctx, path) or ""
+    local ok, err = writePath(ctx, path, existing .. data)
+    if not ok then return false, err end
+    return true
+  end
+
+  function fsShim.lines(path)
+    note(ctx, "love.filesystem.lines", readAdvice())
+    local body = readPath(ctx, path)
+    if not body then error("could not open " .. tostring(path), 2) end
+    local pos = 1
+    return function()
+      if pos > #body then return nil end
+      local nl = body:find("\n", pos, true)
+      local line
+      if nl then
+        line = body:sub(pos, nl - 1)
+        pos = nl + 1
+      else
+        line = body:sub(pos)
+        pos = #body + 1
+      end
+      return line
+    end
+  end
+
+  function fsShim.load(path)
+    note(ctx, "love.filesystem.load",
+      "the chunk is compiled into this mod's sandbox; prefer require or mod:read plus load")
+    local body = readPath(ctx, path)
+    if not body then return nil, "could not open " .. tostring(path) end
+    if not ctx.compile then return nil, "no sandbox is bound yet" end
+    return ctx.compile(body, "@" .. tostring(path))
+  end
+
+  function fsShim.getInfo(path, a, b)
+    local info = infoPath(ctx, path)
+    local filter = type(a) == "string" and a or nil
+    local into = type(a) == "table" and a or (type(b) == "table" and b or nil)
+    if not info then return nil end
+    if filter and info.type ~= filter then return nil end
+    if into then
+      into.type, into.size, into.modtime = info.type, info.size, info.modtime
+      return into
+    end
+    return info
+  end
+
+  function fsShim.getDirectoryItems(path)
+    note(ctx, "love.filesystem.getDirectoryItems", "use mod.assets:list")
+    return listPath(ctx, path)
+  end
+
+  function fsShim.createDirectory(path)
+    local at = classify(ctx, path)
+    if not (at and at.key) then return false end
+    local fs = persistFs(ctx)
+    if not (fs and fs.createDirectory) then return false end
+    ensureParent(fs, overlayPath(ctx, at.key) .. "/.")
+    fs.createDirectory(overlayPath(ctx, at.key))
+    return true
+  end
+
+  function fsShim.remove(path)
+    local at = classify(ctx, path)
+    if not (at and at.key) then return false end
+    return overlayRemove(ctx, at.key)
+  end
+
+  function fsShim.exists(path) return infoPath(ctx, path) ~= nil end
+
+  function fsShim.isFile(path)
+    local info = infoPath(ctx, path)
+    return info ~= nil and info.type == "file"
+  end
+
+  function fsShim.isDirectory(path)
+    local info = infoPath(ctx, path)
+    return info ~= nil and info.type == "directory"
+  end
+
+  function fsShim.getSize(path)
+    local info = infoPath(ctx, path)
+    if not info then return nil, "could not open " .. tostring(path) end
+    return info.size or 0
+  end
+
+  function fsShim.getLastModified(path)
+    local info = infoPath(ctx, path)
+    if not info then return nil, "could not open " .. tostring(path) end
+    return info.modtime or 0
+  end
+
+  local function virtual(call)
+    note(ctx, call, "paths are virtual now; everything under the returned root "
+      .. "lands in this mod's private compat storage")
+    return ctx.virtualRoot
+  end
+
+  function fsShim.getSaveDirectory() return virtual("love.filesystem.getSaveDirectory") end
+  function fsShim.getWorkingDirectory() return virtual("love.filesystem.getWorkingDirectory") end
+  function fsShim.getUserDirectory() return virtual("love.filesystem.getUserDirectory") end
+  function fsShim.getAppdataDirectory() return virtual("love.filesystem.getAppdataDirectory") end
+  function fsShim.getSourceBaseDirectory() return virtual("love.filesystem.getSourceBaseDirectory") end
+  function fsShim.getRealDirectory() return ctx.virtualRoot end
+
+  function fsShim.getIdentity() return ctx.modId end
+
+  function fsShim.setIdentity()
+    note(ctx, "love.filesystem.setIdentity",
+      "a mod cannot repoint the save directory; the call does nothing")
+    return false
+  end
+
+  function fsShim.getRequirePath() return "" end
+  function fsShim.setRequirePath() return false end
+  function fsShim.getCRequirePath() return "" end
+  function fsShim.setCRequirePath() return false end
+
+  function fsShim.mount()
+    note(ctx, "love.filesystem.mount",
+      "mounting is refused; ship the files inside your mod and use mod:read")
+    return false
+  end
+  fsShim.unmount = fsShim.mount
+
+  function fsShim.newFile(path, mode) return loveFile(ctx, path, mode) end
+
+  function fsShim.newFileData(a, b)
+    local real = realFilesystem()
+    if type(a) == "string" and b == nil then
+      note(ctx, "love.filesystem.newFileData", readAdvice())
+      local body = readPath(ctx, a)
+      if not body then return nil, "could not open " .. tostring(a) end
+      if real and real.newFileData then return real.newFileData(body, a) end
+      return nil, "no filesystem"
+    end
+    if real and real.newFileData then return real.newFileData(a, b) end
+    return nil, "no filesystem"
+  end
+
+  function fsShim.isFused()
+    local real = realFilesystem()
+    return real and real.isFused and real.isFused() or false
+  end
+
+  function fsShim.areSymlinksEnabled() return false end
+  function fsShim.setSymlinksEnabled() return false end
+  function fsShim.init() return false end
+  function fsShim.setSource() return false end
+
+  return setmetatable(fsShim, { __index = function(_, key)
+    note(ctx, "love.filesystem." .. tostring(key),
+      "there is no compat stand-in for it; use mod.storage or mod:read")
+    return nil
+  end })
+end
+
+-- ------- the rest of the removed surface
+
+local function systemShim(ctx)
+  local function real() return _G.love and _G.love.system or nil end
+  return {
+    getOS = function()
+      local sys = real()
+      return sys and sys.getOS and sys.getOS() or "Unknown"
+    end,
+    getPowerInfo = function()
+      local sys = real()
+      if not (sys and sys.getPowerInfo) then return "unknown", nil, nil end
+      return sys.getPowerInfo()
+    end,
+    getProcessorCount = function()
+      local sys = real()
+      return sys and sys.getProcessorCount and sys.getProcessorCount() or 1
+    end,
+    getClipboardText = function()
+      note(ctx, "love.system.getClipboardText",
+        "clipboard access stays sandboxed; the call returns an empty string")
+      return ""
+    end,
+    setClipboardText = function()
+      note(ctx, "love.system.setClipboardText",
+        "clipboard access stays sandboxed; the call does nothing")
+      return false
+    end,
+    openURL = function()
+      note(ctx, "love.system.openURL",
+        "launching a URL stays sandboxed; the call does nothing")
+      return false
+    end,
+    vibrate = function(...)
+      local sys = real()
+      if sys and sys.vibrate then return sys.vibrate(...) end
+      return false
+    end,
+  }
+end
+
+local function eventShim(ctx)
+  local function real() return _G.love and _G.love.event or nil end
+  local shim = {
+    quit = function()
+      note(ctx, "love.event.quit",
+        "a mod cannot close the game out from under the player; the call does nothing")
+      return false
+    end,
+  }
+  shim.push = function(name, ...)
+    if name == "quit" then return shim.quit() end
+    local ev = real()
+    if ev and ev.push then return ev.push(name, ...) end
+    return false
+  end
+  return setmetatable(shim, { __index = function(_, key)
+    local ev = real()
+    return ev and ev[key] or nil
+  end })
+end
+
+local function ioShim(ctx)
+  local stream = {
+    write = function(self, ...)
+      for i = 1, select("#", ...) do io.write(tostring((select(i, ...)))) end
+      return self
+    end,
+    flush = function(self) return self end,
+    close = function() return true end,
+    read = function() return nil end,
+    lines = function() return function() return nil end end,
+    seek = function() return 0 end,
+    setvbuf = function() return true end,
+  }
+  local out = setmetatable({}, { __index = stream })
+  local shim = {
+    open = function(path, mode)
+      note(ctx, "io.open",
+        "the handle is backed by mod:read and this mod's private compat storage")
+      local file, err = ioFile(ctx, path, mode)
+      if not file then return nil, err, 2 end
+      return file
+    end,
+    lines = function(path, fmt)
+      if path == nil then return function() return nil end end
+      note(ctx, "io.lines",
+        "the handle is backed by mod:read and this mod's private compat storage")
+      local file, err = ioFile(ctx, path, "r")
+      if not file then error(err, 2) end
+      return file:lines(fmt)
+    end,
+    close = function(file) if file and file.close then return file:close() end return true end,
+    type = function(file)
+      if type(file) == "table" and file.read and file.close then return "file" end
+      return nil
+    end,
+    read = function() return nil end,
+    write = function(...) return out:write(...) end,
+    input = function() return out end,
+    output = function() return out end,
+    stdout = out,
+    stderr = out,
+    stdin = setmetatable({}, { __index = stream }),
+    popen = function()
+      return refuse(ctx, "io.popen", "spawning a process is refused")
+    end,
+    tmpfile = function()
+      return ioFile(ctx, ctx.virtualRoot .. "/io_tmpfile", "w+")
+    end,
+  }
+  return shim
+end
+
+local function osShim(ctx)
+  local HOME = { HOME = true, APPDATA = true, LOCALAPPDATA = true,
+                 USERPROFILE = true, XDG_DATA_HOME = true,
+                 XDG_CONFIG_HOME = true, TMPDIR = true, TEMP = true, TMP = true }
+  return {
+    getenv = function(name)
+      note(ctx, "os.getenv",
+        "the environment is hidden; home-like names answer with this mod's virtual root")
+      if type(name) == "string" and HOME[name:upper()] then
+        return ctx.virtualRoot
+      end
+      return nil
+    end,
+    remove = function(path)
+      note(ctx, "os.remove", "the delete lands in this mod's private compat storage")
+      local at = classify(ctx, path)
+      if not (at and at.key and overlayRemove(ctx, at.key)) then
+        return nil, tostring(path) .. ": no such file"
+      end
+      return true
+    end,
+    rename = function(from, to)
+      note(ctx, "os.rename", "the move lands in this mod's private compat storage")
+      local body = readPath(ctx, from)
+      if not body then return nil, tostring(from) .. ": no such file" end
+      local ok, err = writePath(ctx, to, body)
+      if not ok then return nil, err end
+      local at = classify(ctx, from)
+      if at and at.key then overlayRemove(ctx, at.key) end
+      return true
+    end,
+    tmpname = function()
+      return ctx.virtualRoot .. "/tmp/os_tmpname"
+    end,
+    execute = function()
+      note(ctx, "os.execute", "running a command is refused")
+      return false
+    end,
+    exit = function()
+      note(ctx, "os.exit",
+        "a mod cannot end the process; the call does nothing")
+      return false
+    end,
+    setlocale = function() return "C" end,
+  }
+end
+
+local function packageShim(ctx)
+  local shim = { path = "", cpath = "", preload = {}, loaded = {}, loaders = {} }
+  return setmetatable(shim, {
+    __index = function(_, key)
+      note(ctx, "package." .. tostring(key),
+        "the module loader is sandboxed; use require for the supported engine modules")
+      return nil
+    end,
+  })
+end
+
+-- ------- love callbacks
+
+local CALLBACKS = {
+  load = true, update = true, draw = true, quit = true, lowmemory = true,
+  threaderror = true, keypressed = true, keyreleased = true,
+  textinput = true, textedited = true, mousemoved = true,
+  mousepressed = true, mousereleased = true, wheelmoved = true,
+  mousefocus = true, touchpressed = true, touchreleased = true,
+  touchmoved = true, joystickadded = true, joystickremoved = true,
+  joystickpressed = true, joystickreleased = true, joystickaxis = true,
+  joystickhat = true, gamepadpressed = true, gamepadreleased = true,
+  gamepadaxis = true, focus = true, visible = true, resize = true,
+  filedropped = true, directorydropped = true, displayrotated = true,
+  audiodevicechanged = true, localechanged = true,
+}
+
+local REFUSED = {
+  run = "love.run is the engine's fixed-step loop; use mod.hooks and mod.events",
+  errorhandler = "love.errorhandler is how a crash reaches the player; "
+    .. "use mod.events",
+}
+
+-- ------- assembly
+
+function LegacyCompat.new(opts)
+  opts = opts or {}
+  local ctx = {
+    modId = opts.modId or "mod",
+    modPath = opts.modPath,
+    fs = opts.fs,
+    game = opts.game,
+    compile = nil,
+  }
+  ctx.virtualRoot = "/pokeport/" .. ctx.modId
+
+  local filesystem = filesystemShim(ctx)
+  local io_ = ioShim(ctx)
+  local system = systemShim(ctx)
+  local event = eventShim(ctx)
+
+  local compat = {
+    ctx = ctx,
+    love = { filesystem = filesystem, system = system, event = event },
+    os = osShim(ctx),
+    globals = {
+      io = io_,
+      package = packageShim(ctx),
+      loadfile = function(path)
+        note(ctx, "loadfile",
+          "the chunk is compiled into this mod's sandbox; prefer require or mod:read")
+        local body = readPath(ctx, path)
+        if not body then return nil, "could not open " .. tostring(path) end
+        if not ctx.compile then return nil, "no sandbox is bound yet" end
+        return ctx.compile(body, "@" .. tostring(path))
+      end,
+    },
+    modules = {
+      io = io_,
+      ["love.filesystem"] = filesystem,
+      ["love.system"] = system,
+      ["love.event"] = event,
+    },
+  }
+
+  compat.globals.dofile = function(path)
+    local chunk, err = compat.globals.loadfile(path)
+    if not chunk then error(err or ("could not open " .. tostring(path)), 2) end
+    return chunk()
+  end
+
+  function compat.module(name)
+    if type(name) ~= "string" then return nil end
+    local substitute = compat.modules[name]
+    if substitute then
+      note(ctx, ('require("%s")'):format(name),
+        "it now answers with the compat stand-in, not the real module")
+      return substitute
+    end
+    if name == "os" then
+      note(ctx, 'require("os")',
+        "it now answers with the compat stand-in, not the real module")
+      return compat.osTable
+    end
+    return nil
+  end
+
+  -- true when the assignment is a callback chain, which is what a mod
+  -- wrapping love.mousemoved did before the sandbox; false plus a reason for
+  -- the two names the engine owns, false alone for a module table.
+  function compat.assign(key, value)
+    if REFUSED[key] then
+      note(ctx, ("love.%s assignment"):format(key), REFUSED[key])
+      return false, ("[%s] %s"):format(ctx.modId, REFUSED[key])
+    end
+    if not CALLBACKS[key] then return false end
+    if value ~= nil and type(value) ~= "function" then return false end
+    note(ctx, ("love.%s assignment"):format(key),
+      "the callback lands on the real love table the way it did before the "
+      .. "sandbox; prefer mod.hooks and mod.events")
+    return true
+  end
+
+  function compat.bind(env, compile)
+    ctx.compile = compile
+    compat.osTable = env and env.os or nil
+  end
+
+  return compat
+end
+
+return LegacyCompat
diff --git a/src/mods/Loader.lua b/src/mods/Loader.lua
index 1a46dbf9..09bb7e2c 100644
--- a/src/mods/Loader.lua
+++ b/src/mods/Loader.lua
@@ -20,6 +20,7 @@ local Semver = require("src.mods.Semver")
 local Events = require("src.mods.Events")
 local Gen2Compat = require("src.mods.Gen2Compat")
 local Hooks = require("src.mods.Hooks")
+local LegacyCompat = require("src.mods.LegacyCompat")
 local Runtime = require("src.mods.Runtime")
 local Steps = require("src.mods.Steps")
 
@@ -1288,12 +1289,24 @@ function Loader:_modEnv(mod)
   local id = mod.manifest.id
   local env = self.modEnv[id]
   if not env then
-    env = Sandbox.envFor({ modId = id, permissions = mod.manifest.permissionSet })
+    local loader = self
+    local compat = LegacyCompat.new({
+      modId = id, modPath = mod.path, fs = self.fs,
+      game = function() return loader:_game() end,
+    })
+    env = Sandbox.envFor({ modId = id, permissions = mod.manifest.permissionSet,
+      compat = compat })
     self.modEnv[id] = env
   end
   return env
 end
 
+-- Which pre-sandbox calls each loaded mod actually took, for the manager's
+-- "needs updating" badge; nil id answers for every mod.
+function Loader:legacyReport(modId)
+  return LegacyCompat.report(modId)
+end
+
 function Loader:_loadMod(mod)
   local path = SafePath.join(mod.path, mod.manifest.entry, "manifest entry")
   local chunk, err = Sandbox.loadFile(self.fs, path, self:_modEnv(mod))
diff --git a/src/mods/Sandbox.lua b/src/mods/Sandbox.lua
index b6cb61dd..328b0547 100644
--- a/src/mods/Sandbox.lua
+++ b/src/mods/Sandbox.lua
@@ -73,11 +73,15 @@ local BLOCKED_LOVE = {
     .. "the step bridge", event = true,
 }
 
-local loveProxy
-local function loveFacade()
-  if loveProxy or not _G.love then return loveProxy end
-  loveProxy = setmetatable({}, {
+-- Per-mod, because the compat overrides (src/mods/LegacyCompat.lua) are backed
+-- by that mod's own overlay and must not be shared.
+local function loveFacade(compat)
+  if not _G.love then return nil end
+  local overrides = compat and compat.love
+  return setmetatable({}, {
     __index = function(_, key)
+      local override = overrides and overrides[key]
+      if override ~= nil then return override end
       local hint = BLOCKED_LOVE[key]
       if hint then
         error(("love.%s is not available to mods%s"):format(key,
@@ -85,11 +89,20 @@ local function loveFacade()
       end
       return _G.love[key]
     end,
-    __newindex = function(_, key)
+    -- a callback chain lands on the real table (compat.assign decides which
+    -- names qualify); a module table never does
+    __newindex = function(_, key, value)
+      if compat then
+        local allowed, reason = compat.assign(key, value)
+        if allowed then
+          _G.love[key] = value
+          return
+        end
+        if reason then error(reason, 2) end
+      end
       error(("mods cannot assign love.%s"):format(tostring(key)), 2)
     end,
   })
-  return loveProxy
 end
 
 -- ------- the environment
@@ -177,8 +190,12 @@ end
 -- Runtime.modRequire is how the loader's gate identifies the caller for the
 -- Gen 2 facade once Runtime.currentMod has gone back to nil (a mod requiring
 -- lazily from an event handler).
-local function sandboxedRequire(modId, permissionSet)
+local function sandboxedRequire(modId, permissionSet, compat)
   return function(name, ...)
+    -- the compat stand-in answers first, so a legacy require("io") gets the
+    -- rerouted table instead of the denial below (src/mods/LegacyCompat.lua)
+    local substitute = compat and compat.module(name)
+    if substitute ~= nil then return substitute end
     local denial = Sandbox.moduleDenial(name, permissionSet)
     if denial then error(("[%s] %s"):format(modId or "mod", denial), 2) end
     local previous = Runtime.modRequire
@@ -192,15 +209,23 @@ end
 
 function Sandbox.envFor(opts)
   opts = opts or {}
+  local compat = opts.compat
   local env = baseGlobals()
-  env.love = loveFacade()
-  env.require = sandboxedRequire(opts.modId, opts.permissions)
+  env.love = loveFacade(compat)
+  env.require = sandboxedRequire(opts.modId, opts.permissions, compat)
   local loader = sandboxedLoad(env)
   env.load = loader
   env.loadstring = loader
   -- a mod's globals are its own: two mods no longer share a namespace, and
   -- neither can reach the engine's
   env._G = env
+  if compat then
+    for key, value in pairs(compat.globals) do env[key] = value end
+    for key, value in pairs(compat.os) do env.os[key] = value end
+    compat.bind(env, function(source, chunkname)
+      return Sandbox.compile(source, chunkname, env)
+    end)
+  end
   return env
 end
 
diff --git a/tests/fs_io.lua b/tests/fs_io.lua
index 688bb790..f05a34af 100644
--- a/tests/fs_io.lua
+++ b/tests/fs_io.lua
@@ -138,6 +138,15 @@ function FsIo.new(rootDir)
     return loadfile(abs(path))
   end
 
+  function fs.createDirectory(path)
+    os.execute(("mkdir -p %q"):format(abs(path)))
+    return true
+  end
+
+  function fs.remove(path)
+    return os.remove(abs(path)) ~= nil
+  end
+
   function fs.getDirectoryItems(path)
     return FsIo.listDir(abs(path))
   end
diff --git a/tests/modkit/cases/sandbox.lua b/tests/modkit/cases/sandbox.lua
index e021e1bb..4c7aba9b 100644
--- a/tests/modkit/cases/sandbox.lua
+++ b/tests/modkit/cases/sandbox.lua
@@ -1,7 +1,8 @@
--- T4: the mod sandbox (src/mods/Sandbox.lua).  A mod's own chunks run against
--- an environment with no io, no os beyond the clock, and no way to name a path
--- outside its own directory, so a mod cannot reach the player's filesystem.
--- Every case here is an escape a mod would actually try.
+-- T4: the mod sandbox (src/mods/Sandbox.lua) and the compat reroute over it
+-- (src/mods/LegacyCompat.lua).  A mod's own chunks still cannot name a path
+-- outside their own directory: the pre-sandbox globals are back as stand-ins
+-- whose reads come from the mod's own files and whose writes land in a private
+-- per-mod overlay.  Every case here is an escape a mod would actually try.
 
 package.path = "./?.lua;./?/init.lua;" .. package.path
 
@@ -9,6 +10,7 @@ local T = require("tests.modkit")
 local Manifest = require("src.mods.Manifest")
 local Sandbox = require("src.mods.Sandbox")
 local SafePath = require("src.mods.SafePath")
+local LegacyCompat = require("src.mods.LegacyCompat")
 
 local function manifest(id, extra)
   return ('{"id":"%s","name":"%s","version":"1.0.0","entry":"main.lua",'
@@ -20,16 +22,14 @@ end
 local PROBE = [[
   local mod = ...
   local out = mod.exports
-  out.io = io
-  out.package = package
-  out.dofile = dofile
-  out.loadfile = loadfile
+  out.io = type(io)
+  out.package = type(package)
+  out.dofile = type(dofile)
+  out.loadfile = type(loadfile)
   out.setfenv = setfenv
   out.getfenv = getfenv
   out.debug = debug
-  out.osGetenv = os.getenv
-  out.osExecute = os.execute
-  out.osRemove = os.remove
+  out.osGetenv = type(os.getenv)
   out.osTime = type(os.time)
   out.stringOk = ("a"):rep(3)
 
@@ -38,30 +38,86 @@ local PROBE = [[
     if ok then return false end
     return tostring(err)
   end
-  out.requireIo = attempt(require, "io")
-  out.requireOs = attempt(require, "os")
+  out.requireIoIsShim = select(2, pcall(require, "io")) == io
+  out.requireLoveFsIsShim =
+    select(2, pcall(require, "love.filesystem")) == love.filesystem
   out.requireDebug = attempt(require, "debug")
   out.requirePackage = attempt(require, "package")
   out.requireFfi = attempt(require, "ffi")
-  out.requireLoveFs = attempt(require, "love.filesystem")
   out.requireSocket = attempt(require, "socket")
-  -- called from a nested Lua frame rather than straight off pcall, which is
-  -- the shape a stack-walking gate reads differently
-  out.requireIoNested = attempt(function() return require("io") end)
   out.requireSemver = select(2, pcall(require, "src.mods.Semver"))
 
-  out.loveFilesystem = attempt(function() return love.filesystem end)
   out.loveThread = attempt(function() return love.thread end)
-  out.loveSystem = attempt(function() return love.system end)
   out.loveGraphics = type(love.graphics)
   out.loveAssign = attempt(function() love.filesystem = {} end)
+  -- the callback chain a mod wrapping the mouse writes: it has to reach the
+  -- real love table or the wrap silently never fires
+  out.chainedInner = false
+  local inner = love.mousemoved
+  love.mousemoved = function(...)
+    out.chainedInner = true
+    if inner then return inner(...) end
+  end
+  out.assignRun = attempt(function() love.run = function() end end)
+  out.assignGarbage = attempt(function() love.mousemoved = 7 end)
+  out.powerInfo = type(love.system.getPowerInfo)
+  out.openUrl = love.system.openURL("https://example.com")
+  out.eventQuit = love.event.quit()
+  out.popen = select(1, io.popen("ls"))
+
+  -- the reroute: a write anywhere a mod used to name must land in the mod's
+  -- own overlay, and reading it back must see the write and nothing else
+  local escape = io.open("/etc/hosts", "w")
+  out.escapeOpened = escape ~= nil
+  if escape then
+    escape:write("pwned")
+    escape:close()
+  end
+  local reread = io.open("/etc/hosts", "r")
+  out.escapeReadBack = reread and reread:read("*a") or nil
+  if reread then reread:close() end
+
+  out.homeEnv = os.getenv("HOME")
+  out.saveDir = love.filesystem.getSaveDirectory()
+
+  love.filesystem.write("cfg/settings.txt", "x=1\ny=2\n")
+  out.roundTrip = love.filesystem.read("cfg/settings.txt")
+  out.roundTripInfo = love.filesystem.getInfo("cfg/settings.txt")
+  local lines = {}
+  for line in love.filesystem.lines("cfg/settings.txt") do
+    lines[#lines + 1] = line
+  end
+  out.roundTripLines = lines
+  love.filesystem.append("cfg/settings.txt", "z=3\n")
+  out.appended = love.filesystem.read("cfg/settings.txt")
+
+  -- an absolute path built off the reported save directory comes back to the
+  -- same overlay, which is what a legacy mod's own path joining does
+  love.filesystem.write(out.saveDir .. "/cfg/settings.txt", "rooted")
+  out.rootedRead = love.filesystem.read("cfg/settings.txt")
+
+  -- the mod's own packaged files still read through the old call
+  out.ownThroughLove = love.filesystem.read("mods/fix_sandbox/data/note.txt")
+  out.ownThroughIo = (function()
+    local f = io.open("data/note.txt", "r")
+    if not f then return nil end
+    local body = f:read("*a")
+    f:close()
+    return body
+  end)()
+
+  -- copy-on-write: writing over a packaged path shadows it, it does not
+  -- rewrite the shipped file
+  love.filesystem.write("mods/fix_sandbox/data/note.txt", "shadowed")
+  out.shadowed = love.filesystem.read("mods/fix_sandbox/data/note.txt")
+  out.shadowedOwn = mod:read("data/note.txt")
 
   -- the multi-file pattern mods/timekeepers_hut uses: a chunk loaded from the
   -- mod's own source must inherit the sandbox, not the real globals
   local child = load("return io, os.getenv, _G")
   local childIo, childGetenv, childG = child()
-  out.childIo = childIo
-  out.childGetenv = childGetenv
+  out.childIoIsShim = childIo == io
+  out.childGetenvIsShim = childGetenv == os.getenv
   out.childSharesEnv = childG == _G
 
   out.readEscape = attempt(function() return mod:read("../../secret.txt") end)
@@ -95,58 +151,121 @@ local FILES = {
   ["mods/fix_sandbox/assets/sprites/walk.png"] = "png",
 }
 
+LegacyCompat.reset()
+local savedMouseMoved = love.mousemoved
 local run = T.sdk.loadMods({ "mods/fix_sandbox" }, { fs = T.sdk.memfs(FILES) })
+local installedMouseMoved = love.mousemoved
+love.mousemoved = savedMouseMoved
 T.eq(#run.errors, 0,
   "the probe mod loads clean (" .. tostring(run.errors[1]) .. ")")
 local out = run.loader.exports.fix_sandbox or {}
 
--- ------- the standard library a mod does not get
+-- ------- the pre-sandbox globals are stand-ins, not the real thing
 
-T.eq(out.io, nil, "io is absent from the mod environment")
-T.eq(out.package, nil, "package is absent, so package.loaded is unreachable")
-T.eq(out.dofile, nil, "dofile is absent")
-T.eq(out.loadfile, nil, "loadfile is absent")
-T.eq(out.setfenv, nil, "setfenv is absent, so a mod cannot swap its own env")
-T.eq(out.getfenv, nil, "getfenv is absent, so a mod cannot read the real _G out")
-T.eq(out.debug, nil, "the debug library is absent")
-T.eq(out.osGetenv, nil, "os.getenv is absent -- it is how the report's exploit "
-  .. "found the user's home directory")
-T.eq(out.osExecute, nil, "os.execute is absent")
-T.eq(out.osRemove, nil, "os.remove is absent")
-T.eq(out.osTime, "function", "os.time still works: the clock is not the hole")
+T.eq(out.io, "table", "io is present again, as the compat stand-in")
+T.eq(out.package, "table", "so is package, as an inert stub")
+T.eq(out.dofile, "function", "dofile routes through the stand-in")
+T.eq(out.loadfile, "function", "so does loadfile")
+T.eq(out.osGetenv, "function", "os.getenv answers rather than crashing the mod")
+T.eq(out.osTime, "function", "os.time still works: the clock was never the hole")
 T.eq(out.stringOk, "aaa", "the safe standard library is intact")
 
--- ------- require, the one call that would undo all of the above
+-- what stays gone: there is no rerouted stand-in for these, so faking one
+-- would be the hole rather than a compat shim
+T.eq(out.setfenv, nil, "setfenv is still absent, so a mod cannot swap its own env")
+T.eq(out.getfenv, nil, "getfenv is still absent, so a mod cannot read the real _G out")
+T.eq(out.debug, nil, "the debug library is still absent")
+T.check(out.loveThread ~= false, "love.thread is still refused: it opens a full Lua state")
+T.check(out.requireFfi ~= false, "require(\"ffi\") is still refused: it is arbitrary C")
+T.check(out.requireDebug ~= false, "require(\"debug\") is still refused")
+T.check(out.requirePackage ~= false, "require(\"package\") is still refused")
+T.eq(out.popen, nil, "io.popen refuses rather than spawning a process")
+T.eq(out.openUrl, false, "love.system.openURL does nothing")
+T.eq(out.eventQuit, false, "love.event.quit cannot close the game on the player")
 
-T.check(out.requireIo and out.requireIo:find("not available to mods", 1, true),
-  "require(\"io\") is refused: " .. tostring(out.requireIo))
-T.check(out.requireOs ~= false, "require(\"os\") is refused")
-T.check(out.requireDebug ~= false, "require(\"debug\") is refused")
-T.check(out.requirePackage ~= false, "require(\"package\") is refused")
-T.check(out.requireFfi ~= false, "require(\"ffi\") is refused: it is arbitrary C")
-T.check(out.requireLoveFs ~= false, "require(\"love.filesystem\") is refused")
-T.check(out.requireIoNested ~= false,
-  "require(\"io\") from a nested frame is refused the same way")
+-- ------- require answers with the same stand-ins
+
+T.check(out.requireIoIsShim, "require(\"io\") hands back the same compat table")
+T.check(out.requireLoveFsIsShim,
+  "require(\"love.filesystem\") hands back the same compat table")
 T.check(out.requireSocket and out.requireSocket:find("network", 1, true),
-  "a network module names the permission it needs: " .. tostring(out.requireSocket))
+  "a network module still names the permission it needs: " .. tostring(out.requireSocket))
 T.eq(type(out.requireSemver), "table",
   "the supported engine requires still resolve")
 
 -- ------- the love facade
 
-T.check(out.loveFilesystem and out.loveFilesystem:find("mod.storage", 1, true),
-  "love.filesystem is refused and names the replacement")
-T.check(out.loveThread ~= false, "love.thread is refused: it opens a full Lua state")
-T.check(out.loveSystem and out.loveSystem:find("mod.device:powerInfo()", 1, true),
-  "love.system is refused and names the scoped power replacement")
 T.eq(out.loveGraphics, "table", "the rest of love passes through")
-T.check(out.loveAssign ~= false, "a mod cannot assign into the love facade")
+T.check(out.loveAssign ~= false,
+  "a mod cannot replace a love module table: " .. tostring(out.loveAssign))
+T.eq(out.powerInfo, "function",
+  "love.system reads through to the same information mod.device exposes")
+
+-- a wrapped callback has to land on the real table or the wrap never fires
+T.eq(type(installedMouseMoved), "function",
+  "love.mousemoved assigned by a mod reaches the real love table")
+T.check(installedMouseMoved ~= savedMouseMoved,
+  "and it is the mod's wrapper, not the one that was there")
+T.check(out.assignRun and out.assignRun:find("fixed-step loop", 1, true),
+  "love.run stays refused: it is the engine's own loop ("
+  .. tostring(out.assignRun) .. ")")
+T.check(out.assignGarbage ~= false,
+  "and a callback slot only takes a function")
+
+-- ------- containment: every rerouted write lands in the mod's own overlay
+
+T.check(out.escapeOpened, "io.open on an absolute path outside the tree opens")
+T.eq(out.escapeReadBack, "pwned", "and reads its own write back")
+T.eq(FILES["/etc/hosts"], nil,
+  "but nothing was written outside the game tree")
+T.eq(FILES["mod_compat/fix_sandbox/etc/hosts"], "pwned",
+  "the bytes went to this mod's private overlay instead")
+T.eq(out.homeEnv, "/pokeport/fix_sandbox",
+  "os.getenv(\"HOME\") answers with the mod's virtual root, not the real one")
+T.eq(out.saveDir, "/pokeport/fix_sandbox",
+  "and so does the reported save directory")
+
+T.eq(out.roundTrip, "x=1\ny=2\n", "a love.filesystem write reads back")
+T.eq(out.roundTripInfo and out.roundTripInfo.type, "file",
+  "and getInfo sees it")
+T.same(out.roundTripLines, { "x=1", "y=2" }, "lines() walks it")
+T.eq(out.appended, "x=1\ny=2\nz=3\n", "append extends it")
+T.eq(out.rootedRead, "rooted",
+  "a path joined to the reported save directory routes to the same key")
+T.eq(FILES["mod_compat/fix_sandbox/cfg/settings.txt"], "rooted",
+  "one key in the overlay, under the mod's own id, however it was named")
+
+-- ------- reads still see the mod's packaged files
+
+T.eq(out.ownThroughLove, "own file",
+  "love.filesystem.read of a path inside the mod reads the shipped file")
+T.eq(out.ownThroughIo, "own file", "and so does io.open on a relative path")
+T.eq(out.shadowed, "shadowed", "a write over a packaged path shadows it")
+T.eq(FILES["mods/fix_sandbox/data/note.txt"], "own file",
+  "without rewriting what the mod shipped")
+T.eq(out.shadowedOwn, "own file",
+  "and mod:read still reports the packaged bytes")
+
+-- ------- the reroute is reported, not silent
+
+do
+  local report = run.loader:legacyReport("fix_sandbox")
+  local calls = {}
+  for _, row in ipairs(report) do calls[row.call] = row end
+  T.check(calls["io.open"], "io.open is recorded against the mod")
+  T.check(calls["love.filesystem.write"], "so is love.filesystem.write")
+  T.check(calls["os.getenv"], "and os.getenv")
+  T.check(calls["love.filesystem.write"].count >= 2,
+    "with a count, so a manager can rank the worst offenders")
+  T.check(calls["io.open"].advice and #calls["io.open"].advice > 0,
+    "each row carries the advice the warning printed")
+end
 
 -- ------- env propagation and isolation
 
-T.eq(out.childIo, nil,
+T.check(out.childIoIsShim,
   "a chunk a mod load()s inherits the sandbox (5.1 would hand it the real _G)")
-T.eq(out.childGetenv, nil, "the child chunk gets the same reduced os")
+T.check(out.childGetenvIsShim, "the child chunk gets the same rerouted os")
 T.check(out.childSharesEnv, "the child chunk shares the mod's own globals table")
 T.check(out.globalsAreOwn, "a mod's globals write to its own table")
 T.eq(_G.SANDBOX_LEAK, nil, "and never reach the engine's _G")
@@ -187,6 +306,36 @@ T.check(out.infoEscape and out.infoEscape:find("must stay inside", 1, true),
   "mod:info cannot climb either")
 run.release()
 
+-- ------- two mods never share an overlay
+
+do
+  local files = {
+    ["mods/one/manifest.json"] = manifest("one"),
+    ["mods/one/main.lua"] = [[
+      local mod = ...
+      love.filesystem.write("shared.txt", "from one")
+      mod.exports.mine = love.filesystem.read("shared.txt")
+    ]],
+    ["mods/two/manifest.json"] = manifest("two"),
+    ["mods/two/main.lua"] = [[
+      local mod = ...
+      mod.exports.peek = love.filesystem.read("shared.txt")
+      mod.exports.climb = love.filesystem.read("../one/shared.txt")
+    ]],
+  }
+  local pair = T.sdk.loadMods({ "mods/one", "mods/two" },
+    { fs = T.sdk.memfs(files) })
+  T.eq(#pair.errors, 0, "both mods load (" .. tostring(pair.errors[1]) .. ")")
+  T.eq(pair.loader.exports.one.mine, "from one", "the first mod sees its write")
+  T.eq(pair.loader.exports.two.peek, nil,
+    "the second mod, naming the same path, sees nothing")
+  T.eq(files["mod_compat/one/shared.txt"], "from one",
+    "because the overlay is keyed by mod id")
+  T.eq(pair.loader.exports.two.climb, nil,
+    "and a climb out of the overlay resolves inside it, not into the neighbour")
+  pair.release()
+end
+
 -- ------- the grammar itself
 
 for _, bad in ipairs({ "../x", "a/../../x", "/etc/hosts", "C:/Windows/x",
@@ -226,11 +375,11 @@ do
   bytecodeRun.release()
 end
 
--- ------- the sandbox is not opt-in
+-- ------- the sandbox with no compat layer is still closed
 
 do
   local env = Sandbox.envFor({ modId = "probe" })
-  T.eq(env.io, nil, "a bare Sandbox.envFor is already closed")
+  T.eq(env.io, nil, "a bare Sandbox.envFor has no io")
   T.eq(env._G, env, "_G points at the sandbox, not the real globals")
   T.check(not pcall(env.require, "io"), "and its require refuses io")
 end