diff --git a/codex-lens/tests/test_storage_concurrency.py b/codex-lens/tests/test_storage_concurrency.py
index 7338c6de..6f2608a5 100644
--- a/codex-lens/tests/test_storage_concurrency.py
+++ b/codex-lens/tests/test_storage_concurrency.py
@@ -67,6 +67,16 @@ def dir_index_store(tmp_path):
     store.close()
 
 
+@pytest.fixture()
+def writable_store(tmp_path):
+    """Create a fresh SQLiteStore for concurrent write tests."""
+    db_path = tmp_path / "writes.db"
+    store = SQLiteStore(db_path)
+    store.initialize()
+    yield store
+    store.close()
+
+
 class TestConcurrentReads:
     """Concurrent read tests for storage managers."""
 
@@ -345,3 +355,248 @@ class TestConcurrentReads:
 
         assert not errors
         assert results == [10] * 10
+
+
+class TestConcurrentWrites:
+    """Concurrent write tests for SQLiteStore."""
+
+    def test_concurrent_inserts_commit_all_rows(self, writable_store):
+        """Concurrent inserts from 10 threads should commit all rows."""
+        thread_count = 10
+        files_per_thread = 10
+        errors = []
+        lock = threading.Lock()
+
+        def worker(thread_index: int):
+            try:
+                for i in range(files_per_thread):
+                    path = f"/write/thread_{thread_index}/file_{i}.py"
+                    indexed_file = IndexedFile(
+                        path=path,
+                        language="python",
+                        symbols=[Symbol(name=f"sym_{thread_index}_{i}", kind="function", range=(1, 1))],
+                    )
+                    content = f"# write_token_{thread_index}_{i}\nprint({i})\n"
+                    writable_store.add_file(indexed_file, content)
+            except Exception as exc:
+                with lock:
+                    errors.append(exc)
+
+        threads = [threading.Thread(target=worker, args=(i,)) for i in range(thread_count)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        assert not errors
+        stats = writable_store.stats()
+        assert stats["files"] == thread_count * files_per_thread
+        assert stats["symbols"] == thread_count * files_per_thread
+
+    def test_concurrent_updates_same_file_serializes(self, writable_store):
+        """Concurrent updates to the same file should serialize and not lose writes."""
+        target_path = "/write/shared.py"
+        base = IndexedFile(
+            path=target_path,
+            language="python",
+            symbols=[Symbol(name="base", kind="function", range=(1, 1))],
+        )
+        writable_store.add_file(base, "print('base')\n")
+
+        update_contents = []
+        errors = []
+        lock = threading.Lock()
+
+        def worker(version: int):
+            try:
+                content = f"print('v{version}')\n"
+                indexed_file = IndexedFile(
+                    path=target_path,
+                    language="python",
+                    symbols=[Symbol(name=f"v{version}", kind="function", range=(1, 1))],
+                )
+                writable_store.add_file(indexed_file, content)
+                with lock:
+                    update_contents.append(content)
+            except Exception as exc:
+                with lock:
+                    errors.append(exc)
+
+        threads = [threading.Thread(target=worker, args=(i,)) for i in range(5)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        assert not errors
+
+        resolved = str(Path(target_path).resolve())
+        rows = writable_store.execute_query("SELECT content FROM files WHERE path=?", (resolved,))
+        assert len(rows) == 1
+        assert rows[0]["content"] in set(update_contents)
+
+    def test_wal_mode_is_active_for_thread_connections(self, writable_store):
+        """PRAGMA journal_mode should be WAL for all thread-local connections."""
+        modes = []
+        errors = []
+        lock = threading.Lock()
+
+        def worker():
+            try:
+                conn = writable_store._get_connection()
+                mode = conn.execute("PRAGMA journal_mode").fetchone()[0]
+                with lock:
+                    modes.append(str(mode).lower())
+            except Exception as exc:
+                with lock:
+                    errors.append(exc)
+
+        threads = [threading.Thread(target=worker) for _ in range(10)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        assert not errors
+        assert modes
+        assert all(mode == "wal" for mode in modes)
+
+    def test_transaction_isolation_reader_sees_committed_state(self, writable_store):
+        """Readers should not see uncommitted writer updates and should not block."""
+        target_path = "/write/isolation.py"
+        indexed_file = IndexedFile(path=target_path, language="python", symbols=[])
+        writable_store.add_file(indexed_file, "print('original')\n")
+        resolved = str(Path(target_path).resolve())
+
+        writer_started = threading.Event()
+        reader_done = threading.Event()
+        errors = []
+        lock = threading.Lock()
+        observed = {"reader": None}
+        updated_content = "print('updated')\n"
+
+        def writer():
+            try:
+                conn = writable_store._get_connection()
+                conn.execute("BEGIN IMMEDIATE")
+                conn.execute(
+                    "UPDATE files SET content=? WHERE path=?",
+                    (updated_content, resolved),
+                )
+                writer_started.set()
+                reader_done.wait(timeout=5)
+                conn.commit()
+            except Exception as exc:
+                with lock:
+                    errors.append(exc)
+
+        def reader():
+            try:
+                writer_started.wait(timeout=5)
+                conn = writable_store._get_connection()
+                row = conn.execute("SELECT content FROM files WHERE path=?", (resolved,)).fetchone()
+                observed["reader"] = row[0] if row else None
+                reader_done.set()
+            except Exception as exc:
+                with lock:
+                    errors.append(exc)
+
+        threads = [threading.Thread(target=writer), threading.Thread(target=reader)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        assert not errors
+        assert observed["reader"] == "print('original')\n"
+
+        rows = writable_store.execute_query("SELECT content FROM files WHERE path=?", (resolved,))
+        assert rows[0]["content"] == updated_content
+
+    def test_batch_insert_performance_and_counts(self, writable_store):
+        """Batch inserts across threads should not lose rows."""
+        thread_count = 10
+        files_per_thread = 100
+        errors = []
+        lock = threading.Lock()
+
+        def worker(thread_index: int):
+            try:
+                files = []
+                for i in range(files_per_thread):
+                    path = f"/write/batch_{thread_index}/file_{i}.py"
+                    indexed_file = IndexedFile(
+                        path=path,
+                        language="python",
+                        symbols=[
+                            Symbol(name=f"sym_{thread_index}_{i}", kind="function", range=(1, 1))
+                        ],
+                    )
+                    content = f"# batch_token_{thread_index}_{i}\nprint({i})\n"
+                    files.append((indexed_file, content))
+
+                writable_store.add_files(files)
+            except Exception as exc:
+                with lock:
+                    errors.append(exc)
+
+        start = time.time()
+        threads = [threading.Thread(target=worker, args=(i,)) for i in range(thread_count)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+        duration = max(time.time() - start, 1e-6)
+
+        assert not errors
+        stats = writable_store.stats()
+        assert stats["files"] == thread_count * files_per_thread
+        assert stats["symbols"] == thread_count * files_per_thread
+        assert (thread_count * files_per_thread) / duration > 0
+
+    def test_mixed_read_write_operations_no_errors(self, writable_store):
+        """Mixed reader and writer threads should complete without exceptions."""
+        writer_threads = 5
+        reader_threads = 10
+        writes_per_writer = 20
+        reads_per_reader = 50
+
+        errors = []
+        lock = threading.Lock()
+        target_paths = [
+            f"/write/mixed_{w}/file_{i}.py"
+            for w in range(writer_threads)
+            for i in range(writes_per_writer)
+        ]
+
+        def writer(worker_index: int):
+            try:
+                for i in range(writes_per_writer):
+                    path = f"/write/mixed_{worker_index}/file_{i}.py"
+                    indexed_file = IndexedFile(path=path, language="python", symbols=[])
+                    writable_store.add_file(indexed_file, f"# mixed\nprint({i})\n")
+            except Exception as exc:
+                with lock:
+                    errors.append(exc)
+
+        def reader(worker_index: int):
+            try:
+                for i in range(reads_per_reader):
+                    path = target_paths[(worker_index + i) % len(target_paths)]
+                    writable_store.file_exists(path)
+            except Exception as exc:
+                with lock:
+                    errors.append(exc)
+
+        threads = [
+            *[threading.Thread(target=writer, args=(i,)) for i in range(writer_threads)],
+            *[threading.Thread(target=reader, args=(i,)) for i in range(reader_threads)],
+        ]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        assert not errors
+        stats = writable_store.stats()
+        assert stats["files"] == writer_threads * writes_per_writer