diff --git a/lib/Plerd.pm b/lib/Plerd.pm
index eba0ec5..1f0739e 100644
--- a/lib/Plerd.pm
+++ b/lib/Plerd.pm
@@ -829,11 +829,19 @@ sub _build_recent_posts {
 sub _build_posts {
     my $self = shift;
 
-    my @posts = sort { $b->date <=> $a->date }
-                map { Plerd::Post->new( plerd => $self, source_file => $_ ) }
-                sort { $a->basename cmp $b->basename }
-                grep { /\.markdown$|\.md$/ }
-                $self->source_directory->children
+    # Newest publication date first, ties broken by source basename, using the
+    # same comparator as _ordered_basenames so a full publish and an incremental
+    # publish agree on order. The inner sort guarantees order regardless of
+    # readdir order, and the grep keeps both .md and .markdown sources.
+    my @posts =
+        sort {
+            $b->date <=> $a->date
+            || $a->source_file->basename cmp $b->source_file->basename
+        }
+        map { Plerd::Post->new( plerd => $self, source_file => $_ ) }
+        sort { $a->basename cmp $b->basename }
+        grep { /\.markdown$|\.md$/ }
+        $self->source_directory->children
     ;
 
     return \@posts;
diff --git a/t/post_sort.t b/t/post_sort.t
new file mode 100644
index 0000000..d2add96
--- /dev/null
+++ b/t/post_sort.t
@@ -0,0 +1,62 @@
+use warnings;
+use strict;
+use Test::More;
+use Path::Class::Dir;
+use Path::Class::File;
+use URI;
+
+use FindBin;
+use lib "$FindBin::Bin/../lib";
+
+use_ok( 'Plerd' );
+use Plerd::Init;
+
+# Regression test for GitHub issue #50: the post list must come out in a
+# deterministic order -- newest publication date first, ties broken by source
+# basename -- even when the source directory mixes .md and .markdown files.
+# This matches the ordering the incremental publisher derives from the post
+# index (see Plerd::_ordered_basenames), so both code paths agree.
+
+my $blog_dir = Path::Class::Dir->new( "$FindBin::Bin/post_sort_blog" );
+$blog_dir->rmtree;
+Plerd::Init::initialize( $blog_dir->stringify, 0 );
+
+my $source_dir = Path::Class::Dir->new( $blog_dir, 'source' );
+
+# Two posts share a date (forcing the basename tiebreak) and use different
+# extensions; a third is older. Explicit timestamps keep dates exact.
+my %sources = (
+    '2020-06-20-bravo.md'        => '2020-06-20T10:00:00',
+    '2020-06-20-alpha.markdown'  => '2020-06-20T10:00:00',
+    '2019-01-01-charlie.md'      => '2019-01-01T10:00:00',
+);
+for my $basename ( keys %sources ) {
+    Path::Class::File->new( $source_dir, $basename )->spew(
+        iomode => '>:encoding(utf8)',
+        "title: $basename\ntime: $sources{ $basename }\n\nBody.\n",
+    );
+}
+
+my $plerd = Plerd->new(
+    path         => $blog_dir->stringify,
+    title        => 'Test Blog',
+    author_name  => 'Nobody',
+    author_email => 'nobody@example.com',
+    base_uri     => URI->new( 'http://blog.example.com/' ),
+);
+
+my @order = map { $_->source_file->basename } @{ $plerd->posts };
+
+is_deeply(
+    \@order,
+    [
+        '2020-06-20-alpha.markdown',  # same date as bravo, basename sorts first
+        '2020-06-20-bravo.md',
+        '2019-01-01-charlie.md',      # oldest, last
+    ],
+    'Posts sort newest-first with a stable basename tiebreak across extensions.'
+);
+
+$blog_dir->rmtree;
+
+done_testing();