diff --git a/backend/tests/test_migrations.py b/backend/tests/test_migrations.py
new file mode 100644
index 0000000..fe225f9
--- /dev/null
+++ b/backend/tests/test_migrations.py
@@ -0,0 +1,134 @@
+"""Backward-compatibility tests for the legacy → multi-design migration.
+
+Simulates a database created by a pre-"designs" version of the app and asserts
+that running init_db() adopts all existing nodes/edges/canvas into a single
+default "Network Topology" design with no data loss. The rest of the test suite
+builds the *current* schema via create_all and never exercises this upgrade
+path, so this file guards real users upgrading in place.
+"""
+import os
+
+os.environ.setdefault("SECRET_KEY", "test-only-secret-key-not-for-production")
+
+import pytest
+from sqlalchemy.ext.asyncio import create_async_engine
+
+import app.db.database as database
+
+
+@pytest.fixture
+def legacy_engine(tmp_path, monkeypatch):
+    """Point the module-global engine + sqlite_path at a throwaway legacy DB."""
+    db_path = tmp_path / "legacy.db"
+    monkeypatch.setattr(database.settings, "sqlite_path", str(db_path))
+    engine = create_async_engine(f"sqlite+aiosqlite:///{db_path}")
+    monkeypatch.setattr(database, "engine", engine)
+    return db_path, engine
+
+
+async def _build_legacy_schema(engine) -> None:
+    """Create the pre-designs schema (no design_id, integer canvas_state PK)."""
+    async with engine.begin() as conn:
+        await conn.exec_driver_sql(
+            "CREATE TABLE nodes (id VARCHAR PRIMARY KEY, type VARCHAR, label VARCHAR, "
+            "status VARCHAR, services JSON, pos_x FLOAT, pos_y FLOAT)"
+        )
+        await conn.exec_driver_sql(
+            "CREATE TABLE edges (id VARCHAR PRIMARY KEY, source VARCHAR, target VARCHAR, type VARCHAR)"
+        )
+        await conn.exec_driver_sql(
+            "CREATE TABLE canvas_state (id INTEGER PRIMARY KEY, viewport JSON, "
+            "custom_style JSON, saved_at DATETIME)"
+        )
+        await conn.exec_driver_sql(
+            "INSERT INTO nodes (id, type, label, status, services, pos_x, pos_y) "
+            "VALUES ('n1','server','Old Server','online','[]',10,20)"
+        )
+        await conn.exec_driver_sql(
+            "INSERT INTO nodes (id, type, label, status, services, pos_x, pos_y) "
+            "VALUES ('n2','router','Old Router','offline','[]',30,40)"
+        )
+        await conn.exec_driver_sql(
+            "INSERT INTO edges (id, source, target, type) VALUES ('e1','n1','n2','ethernet')"
+        )
+        await conn.exec_driver_sql(
+            "INSERT INTO canvas_state (id, viewport, custom_style, saved_at) "
+            "VALUES (1, '{\"x\":5,\"y\":6,\"zoom\":2}', NULL, '2024-01-01 00:00:00')"
+        )
+
+
+async def test_legacy_canvas_migrates_into_default_design(legacy_engine):
+    db_path, engine = legacy_engine
+    await _build_legacy_schema(engine)
+
+    await database.init_db()
+
+    check = create_async_engine(f"sqlite+aiosqlite:///{db_path}")
+    try:
+        async with check.begin() as conn:
+            # Exactly one seeded default design.
+            designs = (await conn.exec_driver_sql(
+                "SELECT id, name, design_type, icon FROM designs"
+            )).fetchall()
+            assert len(designs) == 1
+            did, name, dtype, icon = designs[0]
+            assert name == "Network Topology"
+            assert dtype == "network"
+            assert icon == "dashboard"
+
+            # Every legacy node adopted into the default design, data preserved.
+            nodes = (await conn.exec_driver_sql(
+                "SELECT id, label, status, design_id FROM nodes ORDER BY id"
+            )).fetchall()
+            assert [(n[0], n[1], n[2]) for n in nodes] == [
+                ("n1", "Old Server", "online"),
+                ("n2", "Old Router", "offline"),
+            ]
+            assert all(n[3] == did for n in nodes)
+
+            # Legacy edge adopted too.
+            edge = (await conn.exec_driver_sql(
+                "SELECT design_id FROM edges WHERE id='e1'"
+            )).fetchone()
+            assert edge[0] == did
+
+            # canvas_state rebuilt with design_id PK; the old id=1 row maps to the
+            # default design and the viewport survives.
+            cs = (await conn.exec_driver_sql(
+                "SELECT design_id, viewport FROM canvas_state"
+            )).fetchall()
+            assert len(cs) == 1
+            assert cs[0][0] == did
+            assert "zoom" in (cs[0][1] or "")
+    finally:
+        await check.dispose()
+        await engine.dispose()
+
+
+async def test_migration_is_idempotent(legacy_engine):
+    """Running init_db twice must not duplicate the design or drop any data."""
+    db_path, engine = legacy_engine
+    await _build_legacy_schema(engine)
+
+    await database.init_db()
+    await database.init_db()  # second boot — should be a no-op
+
+    check = create_async_engine(f"sqlite+aiosqlite:///{db_path}")
+    try:
+        async with check.begin() as conn:
+            designs = (await conn.exec_driver_sql("SELECT id FROM designs")).fetchall()
+            assert len(designs) == 1
+            did = designs[0][0]
+
+            nodes = (await conn.exec_driver_sql(
+                "SELECT design_id FROM nodes"
+            )).fetchall()
+            assert len(nodes) == 2
+            assert all(n[0] == did for n in nodes)
+
+            cs = (await conn.exec_driver_sql("SELECT design_id FROM canvas_state")).fetchall()
+            assert len(cs) == 1
+            assert cs[0][0] == did
+    finally:
+        await check.dispose()
+        await engine.dispose()