Scheduler createTask rejects recurrence: null for one-off schedules

[dcramer]

When a model includes recurrence: null in a scheduler_slackScheduleCreateTask call with schedule_kind: "one_off", the TypeBox schema rejects it because null is not in the create tool's recurrence union — even though downstream logic (buildRecurrence) already treats null as absent, and the updateTask schema correctly accepts null to remove recurrence.

Root cause

createTask recurrence schema:

recurrence: Type.Optional(
  Type.Union([
    Type.Literal("daily"),
    Type.Literal("weekly"),
    Type.Literal("monthly"),
    Type.Literal("yearly"),
  ])
)

updateTask recurrence schema includes Type.Null() in the union. createTask does not.

validateCreateScheduleKind also rejects any recurrence !== undefined, meaning a model that passes recurrence: null will fail schema validation before reaching the custom error, and a model that passes a valid frequency string (e.g. "daily") with schedule_kind: "one_off" hits the semantic rejection.

buildRecurrence already short-circuits on null:

if (args.input.recurrence === null) {
  return undefined;
}

So the schema and the downstream are inconsistent.

Fix

• Add Type.Null() to the recurrence union in createSlackScheduleCreateTaskTool.
• Update validateCreateScheduleKind to treat null the same as absent: input.recurrence !== undefined && input.recurrence !== null.
• Keep rejection for schedule_kind: "one_off" + valid frequency string (semantically contradictory).

Test cases

• schedule_kind: "one_off" + omitted recurrence → succeeds (unchanged).
• schedule_kind: "one_off" + recurrence: null → succeeds after fix.
• schedule_kind: "one_off" + recurrence: "daily" → continues to fail.
• schedule_kind: "recurring" + null/omitted recurrence → continues to fail.

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