diff --git a/changelog.d/18135.bugfix b/changelog.d/18135.bugfix
new file mode 100644
index 00000000000..998914a8116
--- /dev/null
+++ b/changelog.d/18135.bugfix
@@ -0,0 +1 @@
+Fix user directory search when using a legacy module with a `check_username_for_spam` callback. Broke in v1.122.0.
diff --git a/synapse/module_api/callbacks/spamchecker_callbacks.py b/synapse/module_api/callbacks/spamchecker_callbacks.py
index a2f328cafed..9bc572422b8 100644
--- a/synapse/module_api/callbacks/spamchecker_callbacks.py
+++ b/synapse/module_api/callbacks/spamchecker_callbacks.py
@@ -19,6 +19,7 @@
 #
 #
 
+import functools
 import inspect
 import logging
 from typing import (
@@ -297,6 +298,7 @@ def wrapper(
                         "Bad signature for callback check_registration_for_spam",
                     )
 
+            @functools.wraps(wrapped_func)
             def run(*args: Any, **kwargs: Any) -> Awaitable:
                 # Assertion required because mypy can't prove we won't change `f`
                 # back to `None`. See