diff --git a/internal/db/connect.go b/internal/db/connect.go
index e31d220c7454a3ccc4241b336bedbc7bf30d5fba..ef800c716efc44b137c163a188f599366f1c66e3 100644
--- a/internal/db/connect.go
+++ b/internal/db/connect.go
@@ -50,6 +50,13 @@ func Connect(ctx context.Context, dataDir string) (*sql.DB, error) {
 		return nil, err
 	}
 
+	// Serialize all access through a single connection. SQLite serializes
+	// writes at the file level anyway, and allowing multiple pool
+	// connections to interleave writes/checkpoints (especially under
+	// concurrent sub-agents) has caused WAL/header desync resulting in
+	// SQLITE_NOTADB (26) on the next open.
+	db.SetMaxOpenConns(1)
+
 	if err = db.PingContext(ctx); err != nil {
 		db.Close()
 		return nil, fmt.Errorf("failed to connect to database: %w", err)
diff --git a/internal/db/connect_modernc.go b/internal/db/connect_modernc.go
index 39c7faa42516297d4df497821baa0be56835be15..2e9676d6420c6fbda69cb924595a6e8a55740a5c 100644
--- a/internal/db/connect_modernc.go
+++ b/internal/db/connect_modernc.go
@@ -17,6 +17,9 @@ func openDB(dbPath string) (*sql.DB, error) {
 	for name, value := range pragmas {
 		params.Add("_pragma", fmt.Sprintf("%s(%s)", name, value))
 	}
+	// Use BEGIN IMMEDIATE so writers acquire the reserved lock up front,
+	// preventing deferred-to-writer upgrade deadlocks.
+	params.Set("_txlock", "immediate")
 
 	dsn := fmt.Sprintf("file:%s?%s", dbPath, params.Encode())
 	db, err := sql.Open("sqlite", dsn)
diff --git a/internal/db/connect_ncruces.go b/internal/db/connect_ncruces.go
index 12946e29439862653e0253adc9eb6e2869bea559..d90833350030f08be364f940d483e0afb0b63eb5 100644
--- a/internal/db/connect_ncruces.go
+++ b/internal/db/connect_ncruces.go
@@ -11,7 +11,11 @@ import (
 )
 
 func openDB(dbPath string) (*sql.DB, error) {
-	db, err := driver.Open(dbPath, func(c *sqlite3.Conn) error {
+	// Use BEGIN IMMEDIATE so writers acquire the reserved lock up front,
+	// preventing deferred-to-writer upgrade deadlocks. The "file:" prefix
+	// is required for the ncruces driver to parse query parameters.
+	dsn := fmt.Sprintf("file:%s?_txlock=immediate", dbPath)
+	db, err := driver.Open(dsn, func(c *sqlite3.Conn) error {
 		// Set pragmas for better performance via _pragma query params.
 		// Format: PRAGMA name = value;
 		for name, value := range pragmas {