086 batch tasks with dependencies: Flow PT REST finding

Q Can one _batch create Tasks and link them to each other? If not, what is the fewest calls, and does each route schedule the dependents?

Endpoint POST /entity/_batch ; POST /entity/tasks ; GET /entity/tasks/<id>

Docs claim Silent on dependencies, and on whether a batch can create a TaskDependency.

Actual

one batch: create x1, create x2 with upstream_tasks [{"type": "Task", "id": "$0"}]
  -> 400 code 104 "Invalid field value, update failed [5 - Update failed for [Task.upstream_tasks]:
     Value is not legal.]"          id -1: the same.  Tasks left on the Shot: 0

route 1  batch 1: 4 Task creates, each 2026-03-02..03-03     -> 200 in 0.9s
         batch 2: 3 TaskDependency creates                    -> 200 in 0.4s
           a2 on a1 start-to-start offset_days 1, a3 on a2 (type omitted), a4 on a3 finish-to-finish
         a1 03-02..03-03  a2 03-03..03-04  a3 03-05..03-06  a4 03-05..03-06
         rows read back: (start-to-start, 1), (finish-to-start-next-day, None), (finish-to-finish, None)
route 2  batch 2: 2 Task updates of upstream_tasks            -> 200 in 0.6s
         b1 03-02..03-03  b2 03-04..03-05  b3 03-06..03-09    both rows finish-to-start-next-day
route 3  batch: 2 Task creates with upstream_tasks [c1]       -> 200
         c1 03-02..03-03  c2 03-02..03-03  c3 03-02..03-03    rows: 2 x finish-to-start-next-day
         dependency_violation [False, False, False]  pinned [False, False, False]
         the same body as a plain POST /entity/tasks: c4 03-02..03-03
         a later PUT of an unrelated field on c2: dates unchanged
rollback batch: one good TaskDependency, one closing a loop
  -> 400 "Create failed for [TaskDependency]: Can't create this dependency as it causes a loop."
     rows among the Tasks: 3 before, 3 after

Teaches

route	calls	dependency type	dependents scheduled
one batch with a placeholder id	1	n/a	400, nothing created
Task creates, then `TaskDependency` creates	2	any, with `offset_days`	yes
Task creates, then `upstream_tasks` updates	2	`finish-to-start-next-day` only	yes
level by level, `upstream_tasks` in the create body	one per level	`finish-to-start-next-day` only	no

A link made at create time does not schedule. upstream_tasks in a create body, batched or not, writes the TaskDependency row and leaves the dates as sent, with dependency_violation false. A later write to another field does not fix them. Write the link as a separate update or row.
"entity": "TaskDependency" is a valid batch entity, so route 1 is the one that takes a type and an offset: it is how to copy a template's edges (probe 085 for what each type does).
A loop in a dependency batch rolls the whole batch back like any other failure (recipe 002).
Recipe 016_create_tasks_with_dependencies is route 1 as code.