task dependency types

Q Which dependency_type values does a TaskDependency take, and what do the type, offset_days and shift_ratio do to the dependent Task's dates?

Endpoint POST /entity/task_dependencies ; PUT /entity/task_dependencies/<id> ; GET /entity/tasks/<id>

Docs claim Silent. The schema types dependency_type as plain text with no valid_values.

Actual

POST /entity/task_dependencies {"task": D, "dependent_task": U, "dependency_type": "zz_bogus"} -> 400 code 104
  "Create failed for [TaskDependency]: Validation failed: Dependency type Dependency type 'zz_bogus'
   is invalid, accepted values are finish-to-start-next-day, start-to-start, finish-to-finish,
   start-to-finish-next-day."
dependency_type omitted -> 201, stored "finish-to-start-next-day"
null -> 400, the same message with ''     "START-TO-START" -> 400, the same message

{"task": D, "dependent_task": U} -> cached_display_name "Task <D> dependent on Task <U>";
  D.upstream_tasks = [U]

U runs Mon 2026-03-02 .. Fri 2026-03-06; each D starts 2026-02-16, two working days long
  type                      none                 offset_days 2        offset_days -1       shift_ratio 0.5
  finish-to-start-next-day  03-09..03-10         03-11..03-12         03-06..03-09         03-09..03-10
  start-to-start            03-02..03-03         03-04..03-05         02-27..03-02         03-02..03-03
  finish-to-finish          03-05..03-06         03-09..03-10         03-04..03-05         03-05..03-06
  start-to-finish-next-day  02-26..02-27         03-02..03-03         02-25..02-26         02-26..02-27

PUT on a live finish-to-start row: offset_days 3 -> D 03-12..03-13; type start-to-start -> 03-05..03-06;
  offset_days null -> 03-02..03-03
a second row for the same pair -> 400 "... Validation failed: There is already a connection between the entities."
task == dependent_task, or the reverse edge -> 400 "... Can't create this dependency as it causes a loop."

Teaches

• The field names read backwards. task is the downstream Task and dependent_task is the one it depends on: the row reads Task <task> dependent on Task <dependent_task>, and task gets dependent_task in its upstream_tasks.
• offset_days is working days on top of the type and may be negative: finish-to-start-next-day with 2 moved Monday to Wednesday, with -1 it put a start on the upstream's Friday and its end on the next Monday.
• The dependent snaps to the constraint in both directions. Every D started two weeks early and was moved later; setting offset_days back to null moved a D earlier. It is placement, not a minimum.
• PUT on a live row reschedules at once, so a type or offset can be corrected in place.
• shift_ratio 0.5 was accepted with 201 and changed no date in any of the four types. Its effect is unmeasured beyond that.
• The server rejects a duplicate pair, a self-loop and a two-Task cycle with a 400; a three-Task cycle is the same 400, inside _batch too (probe 107). pinned stayed false throughout (probe 087 for pinned Tasks).
• offset_days null and 0 place the Task alike but are two stored values: a filter on one misses the other, and a template apply treats them as a difference (probe 105).