diff --git a/lagoon/attach/file_test.go b/lagoon/attach/file_test.go
index 79943c8..c24cc2b 100644
--- a/lagoon/attach/file_test.go
+++ b/lagoon/attach/file_test.go
@@ -1,6 +1,10 @@
 package attach
 
-import "testing"
+import (
+	"testing"
+
+	"gocloud.dev/blob/memblob"
+)
 
 func TestFileTableName(t *testing.T) {
 	if got := (File{}).TableName(); got != "system_files" {
@@ -17,3 +21,44 @@ func TestFileSelfRegisters(t *testing.T) {
 	}
 	t.Fatal("File must self-register via attach.Register")
 }
+
+// WR-05 pin: the force-delete prefix is thumb_<id>_ with its trailing
+// underscore, so ID 4 cannot match thumbs of ID 40/41/42 that share a
+// partition. "thumb_4_" is not a string prefix of "thumb_42_".
+func TestDeleteKeysThumbPrefixIsIDDelimited(t *testing.T) {
+	ctx := t.Context()
+	bucket := memblob.OpenBucket(nil)
+	defer bucket.Close()
+	const disk = "abc123xyz000.jpg"
+	part := PartitionDirectory(disk)
+	doomed := []string{
+		part + disk,
+		part + ThumbFilename(4, 200, 200, 0, 0, "auto", "jpg"),
+		part + ThumbFilename(4, 50, 50, 0, 0, "crop", "jpg"),
+	}
+	survivors := []string{
+		part + "abc123xyz111.jpg",
+		part + ThumbFilename(40, 200, 200, 0, 0, "auto", "jpg"),
+		part + ThumbFilename(41, 200, 200, 0, 0, "auto", "jpg"),
+		part + ThumbFilename(42, 4, 4, 0, 0, "auto", "jpg"),
+		part + ThumbFilename(14, 200, 200, 0, 0, "auto", "jpg"),
+	}
+	for _, key := range append(append([]string{}, doomed...), survivors...) {
+		if err := bucket.WriteAll(ctx, key, []byte("x"), nil); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := DeleteKeys(ctx, bucket, blobKeysFor(File{ID: 4, DiskName: disk})); err != nil {
+		t.Fatal(err)
+	}
+	for _, key := range doomed {
+		if ok, err := bucket.Exists(ctx, key); err != nil || ok {
+			t.Fatalf("%s must be deleted (exists=%v err=%v)", key, ok, err)
+		}
+	}
+	for _, key := range survivors {
+		if ok, err := bucket.Exists(ctx, key); err != nil || !ok {
+			t.Fatalf("%s must survive deleting file 4 (exists=%v err=%v)", key, ok, err)
+		}
+	}
+}