diff --git a/hgext/logtoprocess.py b/hgext/logtoprocess.py
--- a/hgext/logtoprocess.py
+++ b/hgext/logtoprocess.py
@@ -113,10 +113,8 @@
                     # try to format the log message given the remaining
                     # arguments
                     try:
-                        # Python string formatting with % either uses a
-                        # dictionary *or* tuple, but not both. If we have
-                        # keyword options, assume we need a mapping.
-                        formatted = msg[0] % (opts or msg[1:])
+                        # Format the message as blackbox does
+                        formatted = msg[0] % msg[1:]
                     except (TypeError, KeyError):
                         # Failed to apply the arguments, ignore
                         formatted = msg[0]
diff --git a/tests/test-logtoprocess.t b/tests/test-logtoprocess.t
--- a/tests/test-logtoprocess.t
+++ b/tests/test-logtoprocess.t
@@ -19,7 +19,7 @@
   > )
   > @command(b'foo', [])
   > def foo(ui, repo):
-  >     ui.log('foo', 'a message: %(bar)s\n', bar='spam')
+  >     ui.log('foo', 'a message: %s\n', 'spam')
   > EOF
   $ cp $HGRCPATH $HGRCPATH.bak
   $ cat >> $HGRCPATH << EOF
@@ -39,7 +39,7 @@
   > foo=(echo 'logtoprocess foo output:';
   >     echo "\$EVENT";
   >     echo "\$MSG1";
-  >     echo "\$OPT_BAR") > $TESTTMP/foo.log
+  >     echo "\$MSG2") > $TESTTMP/foo.log
   > EOF
 
 Running a command triggers both a ui.log('command') and a