diff --git a/patchoulene/README.md b/patchoulene/README.md
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+++ b/patchoulene/README.md
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+# patchoulene
+
+Here lies some openRuyi patch tracking scripts.
+
+## `ruyi-change-id.py`
+
+A Git `prepare-commit-msg` hook for inserting Change-Id trailers on otherwise unidentified commit messages.
+This hook checks the commit message on `git commit --amend [--signoff]` or `git rebase` "reword".
+If it has no recognized identifier, a Change-Id trailer is added.
+
+Usage:
+
+```console
+$ ln -s /path/to/patchoulene/ruyi-change-id.py .git/hooks/prepare-commit-msg
+```
diff --git a/patchoulene/ruyi-change-id.py b/patchoulene/ruyi-change-id.py
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+++ b/patchoulene/ruyi-change-id.py
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+#!/usr/bin/env python
+
+import datetime
+import os
+import re
+import shlex
+import subprocess
+import sys
+
+RE_CHERRY = re.compile(
+    r"^\(cherry picked from commit (?P<commit>[0-9a-f]+)\)$", re.MULTILINE
+)
+RE_CHERRY_TREE = re.compile(
+    r"^\(cherry picked from commit (?P<commit>[0-9a-f]+)[ \t]*\n[ \t]+(?P<tree>\S+)[ \t]+(?P<branch>\S+)\)$",
+    re.MULTILINE,
+)
+RE_LINK = re.compile(
+    r"^Link:[ \t]+(?:https://lore\.kernel\.org/r/|https://patch\.msgid\.link/)(?P<msgid>[^\s/]+)/?(?:[ \t]*$|[ \t]+#)",
+    re.MULTILINE,
+)
+RE_CHANGE_ID = re.compile(r"^(\w+-)*Change-Id:[ \t]+(\w+:\S+)$", re.MULTILINE)
+
+
+def message_subject(message: str) -> str:
+    """
+    The first line of a message, or the empty string if the message is empty.
+    """
+    return (message.split("\n", 1) + [""])[0]
+
+
+def clean_subject(subject: str) -> str:
+    """
+    Remove prefixes from commit message subject.
+    Vendor prefixes (except RUYI) are *not* removed.
+    """
+    return re.sub(r"^(?:(?:FROMLIST|FROMGIT|UPSTREAM|BACKPORT|RUYI): )*", "", subject)
+
+
+def sanitize_subject(subject: str) -> str:
+    """
+    Convert subject to a filename safe format.
+    This is a reimplementation of Git's log format %f.
+    """
+    # https://github.com/git/git/blob/v2.55.0/pretty.c#L947
+    # Safe chars: ASCII alphanumeric, '.', '_'
+
+    # Replace runs of other chars with one dash
+    subject = re.sub(r"[^a-zA-Z0-9._]+", "-", subject)
+    # Replace runs of multiple dots with one
+    subject = re.sub(r"\.+", ".", subject)
+    # Remove dashes at start or end
+    return re.sub("^-+|-+$", "", subject)
+
+
+def gen_change_id(message):
+    base = sanitize_subject(clean_subject(message_subject(message)))
+    datepart = datetime.datetime.now().strftime("%Y%m%d")
+    return f"{base}-{datepart}"
+
+
+def cur_branch() -> str | None:
+    """
+    Get the current branch or current rebasing in the Git repository in the current directory.
+    Returns the branch name, or None if none could be determined.
+    """
+    cur_branch = subprocess.check_output(
+        shlex.split("git rev-parse --abbrev-ref HEAD"), encoding="utf-8"
+    ).rstrip("\n")
+    if cur_branch != "HEAD" and cur_branch != "":
+        return cur_branch
+
+    rebase_head_file = subprocess.check_output(
+        shlex.split("git rev-parse --git-path rebase-merge/head-name"), encoding="utf-8"
+    ).rstrip("\n")
+    try:
+        with open(rebase_head_file, "r") as f:
+            rebase_head = f.read().rstrip("\n")
+    except FileNotFoundError:
+        return None
+
+    if not rebase_head.startswith("refs/"):
+        return None
+
+    rebase_head = subprocess.check_output(
+        shlex.split("git rev-parse --abbrev-ref") + [rebase_head], encoding="utf-8"
+    ).rstrip("\n")
+
+    return rebase_head
+
+
+def main():
+    msg_file, msg_source = sys.argv[1], sys.argv[2]
+
+    if msg_source != "commit":
+        return
+
+    if os.getenv("RUYI_FORCE_CHANGE_ID", "") == "":
+        b = cur_branch()
+        if b is None or not b.startswith("ruyi/"):
+            return
+
+    with open(msg_file, "r") as f:
+        commit_msg = f.read()
+
+    if message_subject(commit_msg).strip() == "":
+        return
+
+    # Skip if we have a recognized identifier
+    if any(
+        r.search(commit_msg) for r in [RE_CHERRY, RE_CHERRY_TREE, RE_LINK, RE_CHANGE_ID]
+    ):
+        return
+
+    # Okay, we need to insert an identifier.
+
+    # Find insertion point before last Signed-off-by and bracketed note.
+    #
+    # (insert here)
+    # [ Author: Something, something
+    #   something more. ]
+    # Signed-off-by: Author <author@example.com>
+    matches = list(
+        re.finditer(r"^(?:\[.*\n(?:\s.*\n)*)?Signed-off-by:", commit_msg, re.MULTILINE)
+    )
+    if not matches:
+        return
+    match = matches[-1]
+
+    change_id = gen_change_id(commit_msg)
+
+    # This is so the user still knows what happens after exiting the editor
+    print(f"(ruyi) Generated Change-Id: {change_id}", file=sys.stderr)
+
+    insert = f"""# !!! Automatically generated Change-Id
+Change-Id: ruyi:{change_id}
+# !!! ^^^^^^
+"""
+    new_msg = commit_msg[: match.span()[0]] + insert + commit_msg[match.span()[0] :]
+
+    with open(msg_file, "w") as f:
+        f.write(new_msg)
+
+
+if __name__ == "__main__":
+    main()