diff --git a/script/ci-custom.py b/script/ci-custom.py
index 25a0cf2127..7d0680a491 100755
--- a/script/ci-custom.py
+++ b/script/ci-custom.py
@@ -525,6 +525,29 @@ def lint_constants_usage():
     return errs
 
 
+# Maximum allowed CONF_ constants in esphome/const.py.
+# This file is frozen — new constants go in esphome/components/const/__init__.py.
+# Decrease this number when constants are moved out of const.py.
+CONST_PY_MAX_CONF = 1011
+
+
+@lint_content_check(include=["esphome/const.py"])
+def lint_const_py_frozen(fname, content):
+    """Block new CONF_ constants from being added to esphome/const.py.
+
+    New constants should go in esphome/components/const/__init__.py instead.
+    """
+    count = sum(1 for line in content.splitlines() if line.startswith("CONF_"))
+    if count > CONST_PY_MAX_CONF:
+        return (
+            "esphome/const.py is frozen. "
+            "Add new constants to esphome/components/const/__init__.py instead."
+        )
+    if count < CONST_PY_MAX_CONF:
+        return f"CONST_PY_MAX_CONF in ci-custom.py should be updated to {count}."
+    return None
+
+
 def relative_cpp_search_text(fname: Path, content) -> str:
     parts = fname.parts
     integration = parts[2]