diff --git a/setup.py b/setup.py
index 214efd8..bfa78ae 100644
--- a/setup.py
+++ b/setup.py
@@ -9,6 +9,11 @@ from setuptools import setup
 
 ROOT = Path(__file__).parent.resolve()
 
+if sys.maxsize <= 2**32:
+    raise RuntimeError(
+        "aeg does not support 32-bit platforms; build with a 64-bit Python"
+    )
+
 libaegis_static = ROOT / "libaegis/zig-out/lib" / (
     "aegis.lib" if sys.platform == "win32" else "libaegis.a"
 )
diff --git a/src/aeg/aegis_cdef.h b/src/aeg/aegis_cdef.h
index 6c4a394..e120a03 100644
--- a/src/aeg/aegis_cdef.h
+++ b/src/aeg/aegis_cdef.h
@@ -1,8 +1,5 @@
 /* This file is generated with tools/generate.py. Do not edit. */
 
-typedef unsigned char uint8_t;
-typedef unsigned long size_t;
-
 /* aegis.h */
 int aegis_init(void);
 int aegis_verify_16(const uint8_t *x, const uint8_t *y);
diff --git a/tools/generate.py b/tools/generate.py
index fff240a..3b27bc7 100755
--- a/tools/generate.py
+++ b/tools/generate.py
@@ -74,12 +74,12 @@ def format_declaration(decl: str, max_width: int = 100) -> str:
 
 
 def generate_cdef(include_dir: pathlib.Path) -> str:
+    # cffi pre-defines uint8_t, size_t, etc. with the correct platform sizes.
+    # Do not typedef them here: "unsigned long" matches size_t on LP64 but is
+    # 4 bytes vs 8 on Win64 (LLP64), breaking struct layout verification.
     lines = [
         "/* This file is generated with tools/generate.py. Do not edit. */",
         "",
-        "typedef unsigned char uint8_t;",
-        "typedef unsigned long size_t;",
-        "",
     ]
 
     headers = [