diff --git a/app/api/routers/tasks.py b/app/api/routers/tasks.py
index e5c5df7..a250e6a 100644
--- a/app/api/routers/tasks.py
+++ b/app/api/routers/tasks.py
@@ -16,6 +16,7 @@ from app.models.notification import Notification as NotificationModel
 from app.api.deps import get_current_user_or_apikey
 from app.api.rbac import check_project_role, check_permission, ensure_can_edit_task
 from app.services.activity import log_activity
+from app.services.dependency_check import check_task_deps
 
 router = APIRouter(tags=["Tasks"])
 
@@ -293,7 +294,7 @@ def transition_task(
     # P5.1: enforce state-machine
     _check_transition(old_status, new_status)
 
-    # P5.2: pending -> open requires milestone to be undergoing (dependencies checked later)
+    # P5.2: pending -> open requires milestone to be undergoing + task deps satisfied
     if old_status == "pending" and new_status == "open":
         milestone = db.query(Milestone).filter(Milestone.id == task.milestone_id).first()
         if milestone:
@@ -303,6 +304,13 @@ def transition_task(
                     status_code=400,
                     detail=f"Cannot open task: milestone is '{ms_status}', must be 'undergoing'",
                 )
+        # P4.3: check task-level depend_on
+        dep_result = check_task_deps(db, task.depend_on)
+        if not dep_result.ok:
+            raise HTTPException(
+                status_code=400,
+                detail=f"Cannot open task: {dep_result.reason}",
+            )
 
     # P5.3: open -> undergoing requires assignee AND operator must be the assignee
     if old_status == "open" and new_status == "undergoing":
diff --git a/app/services/dependency_check.py b/app/services/dependency_check.py
index 690065c..f7f81cb 100644
--- a/app/services/dependency_check.py
+++ b/app/services/dependency_check.py
@@ -111,3 +111,38 @@ def check_milestone_deps(
             result.blockers.append(f"Dependent tasks not {required_status}: {incomplete_tasks}")
 
     return result
+
+
+def check_task_deps(
+    db: Session,
+    depend_on: str | None,
+    *,
+    required_status: str = "completed",
+) -> DepCheckResult:
+    """Check whether a task's depend_on tasks are all satisfied.
+
+    Parameters
+    ----------
+    db:
+        Active DB session.
+    depend_on:
+        JSON-encoded list of task IDs (from the task's ``depend_on`` field).
+    required_status:
+        The status dependees must have reached (default ``"completed"``).
+
+    Returns
+    -------
+    DepCheckResult with ``ok=True`` if all deps satisfied, else ``ok=False``
+    with human-readable ``blockers``.
+    """
+    result = DepCheckResult()
+    task_ids = _parse_json_ids(depend_on)
+    if task_ids:
+        dep_tasks: Sequence[Task] = (
+            db.query(Task).filter(Task.id.in_(task_ids)).all()
+        )
+        incomplete = [t.id for t in dep_tasks if _task_status(t) != required_status]
+        if incomplete:
+            result.ok = False
+            result.blockers.append(f"Dependent tasks not {required_status}: {incomplete}")
+    return result