diff --git a/src/core/ModCompat.lua b/src/core/ModCompat.lua
index ee5e198fd..2d4109584 100644
--- a/src/core/ModCompat.lua
+++ b/src/core/ModCompat.lua
@@ -100,6 +100,13 @@ ModCompat.ALIASES = {
   ["src.render.GbcPalette"] = "src.render.GBCFX",
 }
 
+-- Extend the data vocabulary without changing loader permissions or public
+-- module objects. Existing aliases retain their native identities.
+local Compatibility = require("src.mods.Gen1Compat")
+for name, target in pairs(Compatibility.ALIASES) do
+  if ModCompat.ALIASES[name] == nil then ModCompat.ALIASES[name] = target end
+end
+
 local seen = {}
 local installed = false
 
@@ -107,6 +114,8 @@ local installed = false
 -- matters because "the target is missing too" is a build problem, not a mod
 -- problem, and should not be reported as an unknown module.
 function ModCompat.resolve(name)
+  local missing = Compatibility.unavailable(name)
+  if missing then return nil, missing end
   local target = ModCompat.ALIASES[name]
   if not target then return nil, nil end
   local ok, value = pcall(require, target)
@@ -128,7 +137,23 @@ function ModCompat.install()
   if installed then return false end
   local loaders = package.loaders or package.searchers
   if type(loaders) ~= "table" then return false end
-  loaders[#loaders + 1] = function(name)
+  local searcher
+  searcher = function(name)
+    local missing = Compatibility.unavailable(name)
+    if missing then
+      -- A later host/plugin provider still outranks an unsupported descriptor.
+      local following = false
+      for _, candidate in ipairs(loaders) do
+        if candidate == searcher then following = true
+        elseif following then
+          local provider, origin = candidate(name)
+          if type(provider) == "function" then return provider, origin end
+        end
+      end
+      -- Raise before require installs its in-progress sentinel. Retrying the
+      -- import must retain the diagnostic instead of reporting a fake loop.
+      error(missing, 0)
+    end
     if not ModCompat.ALIASES[name] then
       -- the standard "not found" contract: a string the require error joins
       return "\n\tno mod-compat alias for '" .. tostring(name) .. "'"
@@ -142,6 +167,7 @@ function ModCompat.install()
       return value
     end
   end
+  loaders[#loaders + 1] = searcher
   installed = true
   return true
 end
diff --git a/src/mods/Gen1Compat.lua b/src/mods/Gen1Compat.lua
new file mode 100644
index 000000000..fd427e7db
--- /dev/null
+++ b/src/mods/Gen1Compat.lua
@@ -0,0 +1,33 @@
+-- Compatibility spellings, kept separate from the loader's execution policy.
+-- Only interchangeable native objects belong in ALIASES. Unsupported modules
+-- fail at import with an actionable diagnostic instead of a misleading facade.
+local C = {}
+C.ALIASES = {
+  ['src.world.gen2.WorldAPI'] = 'src.world.WorldAPI',
+  ['src.world.gen2.Follower'] = 'src.world.OptionalFollower',
+}
+C.UNAVAILABLE = {
+  ['src.import.pmd.Sprite'] = { 'mystery_dungeon_follower' },
+  ['src.world.game3.Follower'] = { 'mystery_dungeon_follower' },
+  ['src.core.gen2.Unown'] = { 'mystery_dungeon_follower' },
+  ['src.core.game3.pokemon'] = { 'mystery_dungeon_follower' },
+  ['src.core.game3.player'] = { 'mystery_dungeon_follower' },
+}
+-- Concrete host modules and registered providers always take precedence.
+function C.hasProvider(name)
+  if type(name) ~= 'string' then return false end
+  if type(package.loaded[name]) == 'table' or type(package.preload[name]) == 'function' then return true end
+  if package.searchpath then
+    if package.searchpath(name, package.path or '') or package.searchpath(name, package.cpath or '') then return true end
+  end
+  local path = name:gsub('%.', '/') .. '.lua'
+  return love and love.filesystem and love.filesystem.getInfo
+    and love.filesystem.getInfo(path, 'file') ~= nil or false
+end
+function C.unavailable(name)
+  local consumers = C.UNAVAILABLE[name]
+  if not consumers or C.hasProvider(name) then return nil end
+  return ('Compatibility module %s is unavailable in this engine; affected mods: %s')
+    :format(name, table.concat(consumers, ', '))
+end
+return C
diff --git a/tests/engine/compat_module_aliases.lua b/tests/engine/compat_module_aliases.lua
new file mode 100644
index 000000000..583ba2234
--- /dev/null
+++ b/tests/engine/compat_module_aliases.lua
@@ -0,0 +1,39 @@
+local T=require('tests.harness')
+love=require('tests.love_stub')
+local C=require('src.core.ModCompat')
+C.install()
+T.eq(require('src.world.gen2.WorldAPI'),require('src.world.WorldAPI'),'world API alias preserves object identity')
+if loadfile('src/world/OptionalFollower.lua') then
+ T.eq(require('src.world.gen2.Follower'),require('src.world.OptionalFollower'),'Gen2 follower alias preserves object identity')
+else
+ T.raises(function()require('src.world.gen2.Follower')end,'src.world.OptionalFollower','alias identifies an unavailable native follower provider')
+end
+local names=require('src.mods.Gen1Compat')
+for name in pairs(names.UNAVAILABLE)do
+ if names.hasProvider(name)then
+  local ok,module=pcall(require,name)
+  T.check(ok,'concrete host provider loads '..name)
+  T.eq(type(module),'table','concrete host provider remains a module '..name)
+  T.eq(names.unavailable(name),nil,'existing provider is not marked unavailable '..name)
+  T.eq(require(name),module,'existing provider retains require identity '..name)
+  T.check(names.hasProvider(name),'provider remains available '..name)
+ else
+  local ok,reason=pcall(require,name)
+  T.check(not ok,'unsupported import fails '..name)
+  T.check(tostring(reason):find(name,1,true)~=nil,'failure identifies module '..name)
+  T.check(tostring(reason):find('mystery_dungeon_follower',1,true)~=nil,'failure identifies affected consumer '..name)
+  T.check(not tostring(reason):find('no file ',1,true),'failure does not masquerade as a filesystem search '..name)
+  local again,detail=pcall(require,name)
+  T.check(not again and reason==detail,'repeat import retains the exact diagnostic '..name)
+ end
+end
+
+local providers=package.loaders or package.searchers
+local provided={marker='native-late-provider'}
+local late=function(name)if name=='src.import.pmd.Sprite'then return function()return provided end end end
+providers[#providers+1]=late
+T.eq(require('src.import.pmd.Sprite'),provided,'late concrete provider outranks the missing-module descriptor')
+T.check(names.hasProvider('src.import.pmd.Sprite'),'loaded provider is recognized')
+T.eq(names.unavailable('src.import.pmd.Sprite'),nil,'loaded provider has no missing-module diagnostic')
+table.remove(providers);package.loaded['src.import.pmd.Sprite']=nil
+T.finish()