ostring(writeErr or "write failed")
+    end
+    total = total + #chunk
+    if #chunk < chunkBytes then break end
+  end
+  input:close()
+  output:close()
+
+  if input.size and total ~= input.size then
+    cleanupDestination()
+    return nil, ("source read ended early (expected %d bytes, copied %d)"):format(input.size, total)
+  end
+  local storedSizeErr = RequiredImports.sizeError(spec, total, true)
+  if storedSizeErr then cleanupDestination(); return nil, storedSizeErr end
+  local digest = md5:final()
+  CacheFs.prefix = savedPrefix
+  local accepted, detail = RequiredImports.acceptStoredDigest(
+    manifest, importId, digest, love.filesystem)
+  if not accepted then
+    CacheFs.prefix = ""
+    CacheFs.remove(destination)
+    CacheFs.prefix = savedPrefix
+    return nil, detail
+  end
+  return true, detail
+end
+
 local function readDroppedFile(file)
   local ok, openError = file:open("r")
   if not ok then return nil, openError end
@@ -1228,11 +1324,13 @@ local function chooseRequiredFile()
       "$d=New-Object System.Windows.Forms.OpenFileDialog;",
       "$d.Title='" .. prompt .. "';",
       "$d.Filter='All files (*.*)|*.*';",
+      -- Required imports can be multi-gigabyte optical-disc images. Do NOT
+      -- stage them through %TEMP%: that doubles free-space requirements and a
+      -- failed Copy-Item can leave a plausible-looking truncated temp file.
+      -- Stream the selected source directly into the mod-owned destination.
       "if($d.ShowDialog() -eq 'OK'){",
-      "$t=Join-Path $env:TEMP 'pokeport_required_import.bin';",
-      "Copy-Item -LiteralPath $d.FileName -Destination $t -Force;",
       "[Console]::OutputEncoding=[Text.Encoding]::UTF8;",
-      "[Console]::Write($t)}",
+      "[Console]::Write($d.FileName)}",
     })
     return commandOutput(
       'powershell -NoProfile -STA -Command "' .. script .. '"')
@@ -2069,8 +2170,13 @@ function RomImporter:_importRequiredSource(modId, importId, source, confirmed)
     return nil
   end
   local RequiredImports = require("src.mods.RequiredImports")
-  local info = love.filesystem.getInfo(source, "file")
-  local size = info and info.size or externalFileSize(source)
+  -- A desktop picker returns a host path. Ask the host file handle first;
+  -- love.filesystem.getInfo is only authoritative for virtual/save paths.
+  local size = externalFileSize(source)
+  if not size then
+    local info = love.filesystem.getInfo(source, "file")
+    size = info and info.size or nil
+  end
   local sizeErr = RequiredImports.sizeError(spec, size, false)
   if sizeErr then
     requiredImportNotice(self, modId, importId, sizeErr)
@@ -2093,6 +2199,20 @@ function RomImporter:_importRequiredSource(modId, importId, source, confirmed)
     }
     return nil
   end
+  if type(size) == "number" and size > RequiredImports.LARGE_WARN_BYTES
+      and spec.format ~= "n64" then
+    local ok, result = streamRequiredImport(manifest, importId, source)
+    if ok then
+      self.requiredImportNotice = nil
+      self.modNotice = { ok = true, text = "Imported " .. tostring(importId)
+        .. " for " .. tostring(manifest.name or manifest.id) .. "." }
+      self:_refreshMods()
+      return true
+    end
+    requiredImportNotice(self, modId, importId, result)
+    self.modNotice = nil
+    return nil
+  end
   local data = love.filesystem.read(source)
   if not data then data = readExternalPath(source) end
   if not data then
diff --git a/src/mods/Loader.lua b/src/mods/Loader.lua
index 8ec4767..ddfc3f1 100644
--- a/src/mods/Loader.lua
+++ b/src/mods/Loader.lua
@@ -962,6 +962,8 @@ function Loader:_api(mod)
   local Storage = engineRequire("src.mods.Storage")
   local storage = Storage and Storage.new(modId, loader.fs)
   local Checkpoint = engineRequire("src.core.Checkpoint")
+  local ImportAccess = engineRequire("src.mods.ImportAccess")
+  local importApi, installCache = ImportAccess.new(mod.manifest, loader.fs)
   local api = {
     id = modId,
     version = mod.manifest.version,
@@ -1161,6 +1163,12 @@ function Loader:_api(mod)
     -- checkpoint. The
     -- engine binds version/playthrough/mod scope and portable persistence;
     -- callers never receive paths or a raw filesystem handle.
+    -- Read-only bounded access to this mod's manifest-declared, launcher-validated
+    -- imports. No host path is exposed; large sources are read in bounded ranges.
+    imports = importApi,
+    -- Installation-scoped generated data, independent from Pokémon save slots.
+    -- This is where ROM-derived caches belong; mod.storage remains playthrough-scoped.
+    cache = installCache,
     storage = {
       context = function(_, game) return storage:context(game) end,
       selected = function(_, game) return storage:selected(game) end,
diff --git a/src/mods/RequiredImports.lua b/src/mods/RequiredImports.lua
index 806e916..25e8aa8 100644
--- a/src/mods/RequiredImports.lua
+++ b/src/mods/RequiredImports.lua
@@ -120,6 +120,36 @@ local function accepts(spec, digest)
   return false
 end
 
+local function specById(manifest, importId)
+  for _, candidate in ipairs(allSpecs(manifest)) do
+    if candidate.id == importId then return candidate end
+  end
+  return nil
+end
+
+RequiredImports.spec = specById
+
+local function streamDigest(fs, path, chunkBytes)
+  if not (fs and fs.newFile) then return nil, "streaming file access is unavailable" end
+  local file, makeErr = fs.newFile(path)
+  if not file then return nil, makeErr or "could not open stored import" end
+  local ok, openErr = file:open("r")
+  if not ok then return nil, openErr or "could not open stored import" end
+  local MD5 = require("src.mods.StreamMD5")
+  local ctx = MD5.new()
+  chunkBytes = chunkBytes or (4 * 1024 * 1024)
+  while true do
+    local data, readErr = file:read(chunkBytes)
+    if data and #data > 0 then ctx:update(data) end
+    if not data or #data < chunkBytes then
+      if readErr then file:close(); return nil, readErr end
+      break
+    end
+  end
+  file:close()
+  return ctx:final()
+end
+
 function RequiredImports.path(manifest, spec)
   return manifest.path .. "/baseroms/" .. spec.file
 end
@@ -180,6 +210,35 @@ local function removeReceipt(manifest, spec, fs)
   end
 end
 
+-- Finalize a caller-streamed import after the destination bytes have already
+-- been copied into the engine-owned baseroms path. This keeps large imports
+-- out of a single Lua string while preserving the same size/MD5 receipt rules.
+function RequiredImports.acceptStoredDigest(manifest, importId, digest, fs)
+  fs = fs or (love and love.filesystem)
+  local spec = specById(manifest, importId)
+  if not spec then return nil, "unknown required import: " .. tostring(importId) end
+  digest = tostring(digest or ""):lower()
+  if not accepts(spec, digest) then
+    return nil, ("MD5 mismatch (got %s)"):format(digest ~= "" and digest or "unavailable")
+  end
+  local path = RequiredImports.path(manifest, spec)
+  local info = fs and fs.getInfo and fs.getInfo(path, "file") or nil
+  if not info then return nil, "copied import is missing" end
+  local sizeErr = RequiredImports.sizeError(spec, info.size, true)
+  if sizeErr then return nil, sizeErr end
+  if love and fs == love.filesystem then
+    local savedPrefix = CacheFs.prefix
+    CacheFs.prefix = ""
+    CacheFs.remove(removedMarker(manifest, spec))
+    CacheFs.prefix = savedPrefix
+  elseif fs and fs.remove then
+    fs.remove(removedMarker(manifest, spec))
+  end
+  writeReceipt(manifest, spec, digest, info, fs)
+  return true, digest
+end
+
+
 -- Validate bytes against a declaration.  The returned data is canonicalized
 -- (notably for N64 byte order/header variants) and is what must be stored.
 function RequiredImports.validateData(spec, data, hashFn)
@@ -217,6 +276,22 @@ function RequiredImports.validateStored(manifest, spec, fs, hashFn)
   local cached = cachedDigest(manifest, spec, fs, info)
   if cached then return true, cached, true end
   removeReceipt(manifest, spec, fs)
+  -- Large raw imports (GameCube discs, future optical images, etc.) must never
+  -- be materialized into one Lua string merely because their validation
+  -- receipt was lost. Stream the MD5 directly from the installed file. N64
+  -- sources stay on the canonicalization path because byte-order/header
+  -- normalization is part of their validation contract.
+  if info.size and info.size > RequiredImports.LARGE_WARN_BYTES
+      and spec.format ~= "n64" and fs.newFile then
+    local digest, hashErr = streamDigest(fs, path)
+    if not digest then return nil, hashErr end
+    if not accepts(spec, digest) then
+      return nil, ("MD5 mismatch (got %s)"):format(digest)
+    end
+    info = fs.getInfo(path, "file") or info
+    writeReceipt(manifest, spec, digest, info, fs)
+    return true, digest, false
+  end
   if not fs.read then return nil, "file could not be read" end
   local data = fs.read(path)
   local normalized, detail = RequiredImports.validateStoredData(spec, data, hashFn)