diff --git a/tox.ini b/tox.ini
index 41432eea..9f3be10f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -89,6 +89,9 @@ deps =
     ansible10: ansible~=10.0
     ansible11: ansible~=11.0
     ansible12: ansible>=12.0a
+    # Avoid yaml callback bug, https://github.com/mitogen-hq/mitogen/issues/1284
+    ansible12: ansible!=12.0a6
+    ansible12: ansible-core!=2.19b6
 install_command =
     python -m pip --no-python-version-warning --disable-pip-version-check install {opts} {packages}
 commands_pre =