diff --git a/backend/internal/auth/middleware_test.go b/backend/internal/auth/middleware_test.go
index bac4856..c6733ed 100644
--- a/backend/internal/auth/middleware_test.go
+++ b/backend/internal/auth/middleware_test.go
@@ -63,7 +63,7 @@ func TestRequireSession_TamperedCookie(t *testing.T) {
 	if err != nil {
 		t.Fatalf("mint: %v", err)
 	}
-	cookie.Value = cookie.Value[:len(cookie.Value)-1] + "x"
+	cookie.Value = flipLastChar(cookie.Value)
 	req := httptest.NewRequest(http.MethodGet, "/", nil)
 	req.AddCookie(cookie)
 	rec := httptest.NewRecorder()
diff --git a/backend/internal/auth/session_test.go b/backend/internal/auth/session_test.go
index 7b0f3f1..8a645c3 100644
--- a/backend/internal/auth/session_test.go
+++ b/backend/internal/auth/session_test.go
@@ -41,12 +41,27 @@ func TestParseSessionCookie_Tampered(t *testing.T) {
 	if err != nil {
 		t.Fatalf("mint: %v", err)
 	}
-	cookie.Value = cookie.Value[:len(cookie.Value)-1] + "x"
+	cookie.Value = flipLastChar(cookie.Value)
 	if _, err := ParseSessionCookie(cookie, []byte(testSecret)); err == nil {
 		t.Fatal("expected error for tampered cookie")
 	}
 }
 
+// flipLastChar corrupts a signed token for tamper tests, guaranteeing the
+// last character actually changes -- blindly overwriting it with a fixed
+// character (e.g. "x") would occasionally be a no-op if that character
+// already happened to be there (it's derived from the token's embedded
+// timestamp, so this isn't as rare as it sounds), silently passing the
+// test without having tampered with anything.
+func flipLastChar(s string) string {
+	last := s[len(s)-1]
+	replacement := byte('x')
+	if last == replacement {
+		replacement = 'y'
+	}
+	return s[:len(s)-1] + string(replacement)
+}
+
 func TestParseSessionCookie_WrongSecret(t *testing.T) {
 	cookie, err := MintSessionCookie(Claims{Sub: "u1"}, []byte(testSecret), time.Hour, false)
 	if err != nil {
diff --git a/backend/internal/store/db.go b/backend/internal/store/db.go
index 9a1ed24..320aa11 100644
--- a/backend/internal/store/db.go
+++ b/backend/internal/store/db.go
@@ -24,7 +24,12 @@ type DB struct {
 // this is a pre-production app with no compatibility obligation to older
 // database files. Edit schema.sql directly to change the schema.
 func Open(path string) (*DB, error) {
-	sqlDB, err := sql.Open("sqlite", path+"?_pragma=foreign_keys(1)")
+	// WAL mode lets an external reader (sqlite3 CLI, DB Browser, DataGrip)
+	// inspect the file concurrently without "database is locked" errors
+	// while geniusrund is running -- the app's own queries are already
+	// fully serialized via SetMaxOpenConns(1) below, so this doesn't change
+	// in-process concurrency, only cross-process access to the same file.
+	sqlDB, err := sql.Open("sqlite", path+"?_pragma=foreign_keys(1)&_pragma=journal_mode(WAL)")
 	if err != nil {
 		return nil, fmt.Errorf("open sqlite database: %w", err)
 	}