diff --git a/CLAUDE.md b/CLAUDE.md
index 97ac8db..28a47c9 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -327,6 +327,9 @@ comparisons and nothing else decides how it reports or exits.
   reference protobuf implementation.
 - **float-vectors** — `test/float_vectors_test.lua`, differential against the C
   float cast and `struct.pack`.
+- **lpack** — `test/lpack_test.lua`, `pb.selftest()` with `string.pack`/`unpack`
+  replaced by Control4's lpack shape before load, so the vendored bitn has to
+  decline them and pack bytes itself.
 
 Each runs once per math mode it asks for: native `math.frexp`/`math.ldexp`, and
 again with them cleared so the module's own fallbacks are bound.
diff --git a/test/lpack_stub.lua b/test/lpack_stub.lua
new file mode 100644
index 0000000..5adb9f8
--- /dev/null
+++ b/test/lpack_stub.lua
@@ -0,0 +1,83 @@
+--- @diagnostic disable: duplicate-set-field
+-- Installs string.pack/string.unpack in the shape Control4's LuaJIT ships (lpack),
+-- as measured on a controller: single-letter codes with no size suffix, '<' and
+-- '=' little-endian, '>' big-endian, a NUL ends the format, a digit or unknown
+-- code raises, and unpack(data, fmt, pos) returns the next position before the
+-- values, with nil for a value the data cannot supply. Loaded ahead of bitn so a
+-- byte helper that assumes the 5.3 dialect fails here rather than on a controller.
+local SIZE = { b = 1, B = 1, c = 1, h = 2, H = 2, i = 4, I = 4, l = 4, L = 4 }
+
+local function pack_int(value, size, big_endian)
+  value = value % 2 ^ (8 * size)
+  local out = {}
+  for _ = 1, size do
+    out[#out + 1] = string.char(value % 256)
+    value = math.floor(value / 256)
+  end
+  if big_endian then
+    local reversed = {}
+    for i = size, 1, -1 do
+      reversed[#reversed + 1] = out[i]
+    end
+    out = reversed
+  end
+  return table.concat(out)
+end
+
+local function unpack_int(data, pos, size, big_endian)
+  if #data < pos + size - 1 then
+    return pos, nil
+  end
+  local value = 0
+  for i = 0, size - 1 do
+    local weight = big_endian and (size - 1 - i) or i
+    value = value + string.byte(data, pos + i) * 2 ^ (8 * weight)
+  end
+  return pos + size, value
+end
+
+local function codes(fmt)
+  local out = {}
+  for c in fmt:gmatch(".") do
+    if c == "\0" then
+      break
+    end
+    out[#out + 1] = c
+  end
+  return out
+end
+
+function string.pack(fmt, ...)
+  local args, i, out, big_endian = { ... }, 0, {}, false
+  for _, c in ipairs(codes(fmt)) do
+    if c == "<" or c == "=" then
+      big_endian = false
+    elseif c == ">" then
+      big_endian = true
+    elseif SIZE[c] then
+      i = i + 1
+      out[#out + 1] = pack_int(args[i], SIZE[c], big_endian)
+    else
+      error("lpack stub: unsupported code '" .. c .. "'")
+    end
+  end
+  return table.concat(out)
+end
+
+function string.unpack(data, fmt, pos)
+  pos = pos or 1
+  local out, n, big_endian = {}, 0, false
+  for _, c in ipairs(codes(fmt)) do
+    if c == "<" or c == "=" then
+      big_endian = false
+    elseif c == ">" then
+      big_endian = true
+    elseif SIZE[c] then
+      n = n + 1
+      pos, out[n] = unpack_int(data, pos, SIZE[c], big_endian)
+    else
+      error("lpack stub: unsupported code '" .. c .. "'")
+    end
+  end
+  return pos, (table.unpack or unpack)(out, 1, n)
+end
diff --git a/test/lpack_test.lua b/test/lpack_test.lua
new file mode 100644
index 0000000..f28a28d
--- /dev/null
+++ b/test/lpack_test.lua
@@ -0,0 +1,11 @@
+-- @test-name Selftest under lpack
+-- @test-modes native
+--
+-- Control4's LuaJIT has string.pack/unpack as lpack, a different dialect from 5.3.
+-- The vendored bitn must decline it and take the byte-math path.
+
+local here = debug.getinfo(1, "S").source:match("^@(.*[/\\])") or "./"
+dofile(here .. "lpack_stub.lua")
+
+local pb = require("protobuf")
+os.exit(pb.selftest() and 0 or 1)