diff --git a/backend/pyproject.toml b/backend/pyproject.toml
index 5bfdc8df..d42edc28 100644
--- a/backend/pyproject.toml
+++ b/backend/pyproject.toml
@@ -41,6 +41,7 @@ dev = [
   "pytest-cov==6.0.0",
   "coverage[toml]==7.6.10",
   "ruff==0.6.9",
+  "aiosqlite==0.21.0",
 ]
 
 [tool.mypy]
diff --git a/backend/tests/test_task_dependencies_integration.py b/backend/tests/test_task_dependencies_integration.py
new file mode 100644
index 00000000..2dc72bdc
--- /dev/null
+++ b/backend/tests/test_task_dependencies_integration.py
@@ -0,0 +1,163 @@
+from __future__ import annotations
+
+from uuid import UUID, uuid4
+
+import pytest
+from fastapi import HTTPException
+from sqlalchemy.ext.asyncio import AsyncEngine, create_async_engine
+from sqlmodel import SQLModel
+from sqlmodel.ext.asyncio.session import AsyncSession
+
+from app.models.boards import Board
+from app.models.task_dependencies import TaskDependency
+from app.models.tasks import Task
+from app.services import task_dependencies as td
+
+
+async def _make_engine() -> AsyncEngine:
+    # Single shared in-memory db per engine.
+    engine = create_async_engine("sqlite+aiosqlite:///:memory:")
+    async with engine.begin() as conn:
+        await conn.run_sync(SQLModel.metadata.create_all)
+    return engine
+
+
+async def _make_session(engine: AsyncEngine) -> AsyncSession:
+    return AsyncSession(engine)
+
+
+async def _seed_board_and_tasks(
+    session: AsyncSession, *, board_id: UUID, task_ids: list[UUID]
+) -> None:
+    session.add(Board(id=board_id, name="b", slug="b"))
+    for tid in task_ids:
+        session.add(Task(id=tid, board_id=board_id, title=f"t-{tid}", description=None))
+    await session.commit()
+
+
+@pytest.mark.asyncio
+async def test_validate_dependency_update_rejects_self_dependency() -> None:
+    engine = await _make_engine()
+    async with await _make_session(engine) as session:
+        board_id = uuid4()
+        task_id = uuid4()
+        await _seed_board_and_tasks(session, board_id=board_id, task_ids=[task_id])
+
+        with pytest.raises(HTTPException) as exc:
+            await td.validate_dependency_update(
+                session,
+                board_id=board_id,
+                task_id=task_id,
+                depends_on_task_ids=[task_id],
+            )
+        assert exc.value.status_code == 422
+
+
+@pytest.mark.asyncio
+async def test_validate_dependency_update_404s_when_dependency_missing() -> None:
+    engine = await _make_engine()
+    async with await _make_session(engine) as session:
+        board_id = uuid4()
+        task_id = uuid4()
+        dep_id = uuid4()
+        await _seed_board_and_tasks(session, board_id=board_id, task_ids=[task_id])
+
+        with pytest.raises(HTTPException) as exc:
+            await td.validate_dependency_update(
+                session,
+                board_id=board_id,
+                task_id=task_id,
+                depends_on_task_ids=[dep_id],
+            )
+        assert exc.value.status_code == 404
+        detail = exc.value.detail
+        assert isinstance(detail, dict)
+        assert detail["missing_task_ids"] == [str(dep_id)]
+
+
+@pytest.mark.asyncio
+async def test_validate_dependency_update_detects_cycle() -> None:
+    engine = await _make_engine()
+    async with await _make_session(engine) as session:
+        board_id = uuid4()
+        a, b = uuid4(), uuid4()
+        await _seed_board_and_tasks(session, board_id=board_id, task_ids=[a, b])
+
+        # existing edge a -> b
+        session.add(TaskDependency(board_id=board_id, task_id=a, depends_on_task_id=b))
+        await session.commit()
+
+        # update b -> a introduces cycle
+        with pytest.raises(HTTPException) as exc:
+            await td.validate_dependency_update(
+                session,
+                board_id=board_id,
+                task_id=b,
+                depends_on_task_ids=[a],
+            )
+        assert exc.value.status_code == 409
+
+
+@pytest.mark.asyncio
+async def test_dependency_queries_and_replace_and_dependents() -> None:
+    engine = await _make_engine()
+    async with await _make_session(engine) as session:
+        board_id = uuid4()
+        t1, t2, t3 = uuid4(), uuid4(), uuid4()
+        await _seed_board_and_tasks(session, board_id=board_id, task_ids=[t1, t2, t3])
+
+        # seed deps: t1 depends on t2 then t3
+        session.add(TaskDependency(board_id=board_id, task_id=t1, depends_on_task_id=t2))
+        session.add(TaskDependency(board_id=board_id, task_id=t1, depends_on_task_id=t3))
+        await session.commit()
+
+        # cover empty input short-circuit
+        assert await td.dependency_ids_by_task_id(session, board_id=board_id, task_ids=[]) == {}
+
+        deps_map = await td.dependency_ids_by_task_id(session, board_id=board_id, task_ids=[t1, t2])
+        assert deps_map[t1] == [t2, t3]
+        assert deps_map.get(t2, []) == []
+
+        # mark t2 done, t3 not
+        task2 = await session.get(Task, t2)
+        assert task2 is not None
+        task2.status = td.DONE_STATUS
+        await session.commit()
+
+        # cover empty input short-circuit
+        assert await td.dependency_status_by_id(session, board_id=board_id, dependency_ids=[]) == {}
+
+        status_map = await td.dependency_status_by_id(session, board_id=board_id, dependency_ids=[t2, t3])
+        assert status_map[t2] == td.DONE_STATUS
+        assert status_map[t3] != td.DONE_STATUS
+
+        blocked = await td.blocked_by_for_task(session, board_id=board_id, task_id=t1)
+        assert blocked == [t3]
+
+        # cover early return when no deps provided
+        assert (
+            await td.blocked_by_for_task(session, board_id=board_id, task_id=t1, dependency_ids=[])
+            == []
+        )
+
+        # replace deps with duplicates (deduped) -> [t3]
+        out = await td.replace_task_dependencies(
+            session,
+            board_id=board_id,
+            task_id=t1,
+            depends_on_task_ids=[t3, t3],
+        )
+        await session.commit()
+        assert out == [t3]
+
+        deps_map2 = await td.dependency_ids_by_task_id(session, board_id=board_id, task_ids=[t1])
+        assert deps_map2[t1] == [t3]
+
+        dependents = await td.dependent_task_ids(session, board_id=board_id, dependency_task_id=t3)
+        assert dependents == [t1]
+
+        # also exercise explicit dependency_ids passed
+        blocked2 = await td.blocked_by_for_task(
+            session, board_id=board_id, task_id=t1, dependency_ids=[t3]
+        )
+        assert blocked2 == [t3]
diff --git a/backend/uv.lock b/backend/uv.lock
index e7ee66fe..aab4bb9c 100644
--- a/backend/uv.lock
+++ b/backend/uv.lock
@@ -2,6 +2,18 @@ version = 1
 revision = 3
 requires-python = ">=3.12"
 
+[[package]]
+name = "aiosqlite"
+version = "0.21.0"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "typing-extensions" },
+]
+sdist = { url = "https://files.pythonhosted.org/packages/13/7d/8bca2bf9a247c2c5dfeec1d7a5f40db6518f88d314b8bca9da29670d2671/aiosqlite-0.21.0.tar.gz", hash = "sha256:131bb8056daa3bc875608c631c678cda73922a2d4ba8aec373b19f18c17e7aa3", size = 13454, upload-time = "2025-02-03T07:30:16.235Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/f5/10/6c25ed6de94c49f88a91fa5018cb4c0f3625f31d5be9f771ebe5cc7cd506/aiosqlite-0.21.0-py3-none-any.whl", hash = "sha256:2549cf4057f95f53dcba16f2b64e8e2791d7e1adedb13197dd8ed77bb226d7d0", size = 15792, upload-time = "2025-02-03T07:30:13.6Z" },
+]
+
 [[package]]
 name = "alembic"
 version = "1.13.2"
@@ -572,6 +584,7 @@ dependencies = [
 
 [package.optional-dependencies]
 dev = [
+    { name = "aiosqlite" },
     { name = "black" },
     { name = "coverage" },
     { name = "flake8" },
@@ -586,6 +599,7 @@ dev = [
 
 [package.metadata]
 requires-dist = [
+    { name = "aiosqlite", marker = "extra == 'dev'", specifier = "==0.21.0" },
     { name = "alembic", specifier = "==1.13.2" },
     { name = "black", marker = "extra == 'dev'", specifier = "==24.10.0" },
     { name = "coverage", extras = ["toml"], marker = "extra == 'dev'", specifier = "==7.6.10" },