diff --git a/.claude/settings.json b/.claude/settings.json
new file mode 100644
index 0000000000..d47520dc0e
--- /dev/null
+++ b/.claude/settings.json
@@ -0,0 +1,16 @@
+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Edit|Write|MultiEdit",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "python3 \"$CLAUDE_PROJECT_DIR/tools/rules/hook.py\"",
+            "statusMessage": "Checking house rules"
+          }
+        ]
+      }
+    ]
+  }
+}
diff --git a/tools/clangtidy/README.md b/tools/clangtidy/README.md
index 5312d55742..9d52e7158f 100644
--- a/tools/clangtidy/README.md
+++ b/tools/clangtidy/README.md
@@ -6,8 +6,16 @@ git diff -U0 HEAD | clang-tidy-diff.py -p1 -path out/clang-tidy -j 10 -quiet
 run-clang-tidy -p out/clang-tidy -j 10 -quiet <files>
 ```
 
-The second line is the gate: it reports only the lines a commit touches. Both
-runners ship with LLVM, under `share/clang` next to the binary.
+Both runners ship with LLVM, under `share/clang`. Needs clang-tidy 20 or
+newer: an older one ignores `CustomFunctions` without a word.
 
-Needs clang-tidy 20 or newer -- an older one ignores `CustomFunctions`, which
-the bans in `.clang-tidy` are built on, without a word.
+The commit hook, once per clone, submodules included:
+
+```bash
+for repo in . $(git config -f .gitmodules --get-regexp path \
+        | awk '{print $2}' | grep -v ThirdParty); do
+    hooks=$(git -C "$repo" rev-parse --git-path hooks)/pre-commit
+    printf '#!/bin/sh\nexec python3 %s/tools/rules/hook.py --staged\n' \
+        "$PWD" > "$hooks" && chmod +x "$hooks"
+done
+```
diff --git a/tools/rules/hook.py b/tools/rules/hook.py
new file mode 100644
index 0000000000..eb6ae5ca09
--- /dev/null
+++ b/tools/rules/hook.py
@@ -0,0 +1,87 @@
+#!/usr/bin/env python3
+"""Refuse a function .clang-tidy bans: as an edit hook, or --staged."""
+import json
+import os
+import re
+import subprocess
+import sys
+
+HERE = os.path.dirname(os.path.abspath(__file__))
+ROOT = os.path.dirname(os.path.dirname(HERE))
+SOURCES = (".cpp", ".h", ".mm", ".cxx")
+
+
+def banned():
+    """Name and replacement of every entry in CustomFunctions."""
+    with open(os.path.join(ROOT, ".clang-tidy"), encoding="utf-8") as stream:
+        return re.findall(r"\^::(\w+)\$,([^,;]+),", stream.read())
+
+
+def written(payload):
+    """The text this call put into the file, across all the edit shapes."""
+    tool = payload.get("tool_input") or {}
+    parts = [tool.get("new_string") or "", tool.get("content") or ""]
+    parts += [edit.get("new_string") or ""
+        for edit in tool.get("edits") or []]
+    return "\n".join(parts)
+
+
+def code_only(text):
+    """The same text with comments and string literals taken out."""
+    text = re.sub(r"//[^\n]*", "", text)
+    text = re.sub(r"/\*.*?\*/", "", text, flags=re.S)
+    return re.sub(r'"(?:[^"\\]|\\.)*"', '""', text)
+
+
+def complain(path, found, where):
+    for name, into in found:
+        print(f"{where}{name} is banned in this repository -- write {into}"
+            " instead.", file=sys.stderr)
+    print(f"The rule lives in .clang-tidy. Fix {os.path.basename(path)} now,"
+        " before anything else.", file=sys.stderr)
+
+
+def staged():
+    """Every banned name added by the staged change, with its line."""
+    diff = subprocess.run(["git", "diff", "--cached", "-U0"], cwd=ROOT,
+        capture_output=True, text=True).stdout
+    path, number, bad = "", 0, []
+    for line in diff.split("\n"):
+        if line.startswith("+++ b/"):
+            path, number = line[6:], 0
+        elif line.startswith("@@"):
+            number = int(re.search(r"\+(\d+)", line).group(1)) - 1
+        elif line.startswith("+") and not line.startswith("+++"):
+            number += 1
+            if not path.endswith(SOURCES):
+                continue
+            found = [(name, into) for name, into in banned()
+                if re.search(r"\b" + name + r"\s*\(", code_only(line[1:]))]
+            if found:
+                complain(path, found, f"{path}:{number}: ")
+                bad += found
+    return 1 if bad else 0
+
+
+def main():
+    if "--staged" in sys.argv:
+        return staged()
+    try:
+        payload = json.load(sys.stdin)
+    except (ValueError, OSError):
+        return 0
+    path = ((payload.get("tool_response") or {}).get("filePath")
+        or (payload.get("tool_input") or {}).get("file_path") or "")
+    if not path.endswith(SOURCES):
+        return 0
+    text = code_only(written(payload))
+    found = [(name, into) for name, into in banned()
+        if re.search(r"\b" + name + r"\s*\(", text)]
+    if not found:
+        return 0
+    complain(path, found, "")
+    return 2
+
+
+if __name__ == "__main__":
+    sys.exit(main())