diff --git a/mutations_add_comment_test.go b/mutations_add_comment_test.go
new file mode 100644
index 0000000..ba5e608
--- /dev/null
+++ b/mutations_add_comment_test.go
@@ -0,0 +1,77 @@
+package main
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+)
+
+// TestConcurrentComments_AllRetained — 100 goroutines add comments to the
+// same post. All should land, no path collisions, no lost writes.
+func TestConcurrentComments_AllRetained(t *testing.T) {
+	s, _, _ := newTestStore(t)
+	cp := &CreatePost{AuthorID: "scott", Body: "race test"}
+	must(t, Submit(context.Background(), s, cp))
+
+	const N = 100
+	var wg sync.WaitGroup
+	errCh := make(chan error, N)
+	for i := 0; i < N; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			m := &AddComment{PostID: cp.ResolvedPostID(), AuthorID: "hermes", Body: "concurrent"}
+			errCh <- Submit(context.Background(), s, m)
+		}()
+	}
+	wg.Wait()
+	close(errCh)
+	for err := range errCh {
+		if err != nil {
+			t.Fatalf("concurrent submit: %v", err)
+		}
+	}
+	comments := s.ListComments(cp.ResolvedPostID())
+	if len(comments) != N {
+		t.Fatalf("expected %d comments, got %d", N, len(comments))
+	}
+	// All paths must be unique and parseable.
+	seen := make(map[string]bool, N)
+	for _, c := range comments {
+		if seen[c.Path] {
+			t.Fatalf("duplicate path: %s", c.Path)
+		}
+		seen[c.Path] = true
+	}
+}
+
+// TestBumpOrder_AfterConcurrentComments — concurrent comments land in some
+// order, but the final post bump_at should be the time of the LAST comment.
+// With the per-author coarse lock, mutations serialize, so the order is
+// deterministic given the goroutine schedule. We don't assert exact order
+// (that would be flaky); we assert that the final bump_at is the latest.
+func TestBumpOrder_AfterConcurrentComments(t *testing.T) {
+	s, clock, _ := newTestStore(t)
+	cp := &CreatePost{AuthorID: "scott", Body: "bump order"}
+	must(t, Submit(context.Background(), s, cp))
+	initialBump := s.posts[cp.ResolvedPostID()].BumpedAt
+
+	const N = 10
+	var wg sync.WaitGroup
+	for i := 0; i < N; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			clock.Advance(time.Second)
+			m := &AddComment{PostID: cp.ResolvedPostID(), AuthorID: "hermes", Body: "x"}
+			_ = Submit(context.Background(), s, m)
+		}()
+	}
+	wg.Wait()
+
+	got, _ := s.GetPost(cp.ResolvedPostID())
+	if !got.BumpedAt.After(initialBump) {
+		t.Fatalf("post should have been bumped past %v, got %v", initialBump, got.BumpedAt)
+	}
+}