openprose__prose/packages/std/patterns/guard.prose.md

name, kind

name	kind
guard	pattern

Guard

Check before delegating. Fail-fast pattern.

Metadata

• version: 0.2.0
• role: coordinator

Slots

• guard
• target

Config

• None. The guard and target are slot bindings; the task brief is a system input.

Invariants

• The target runs only when the guard explicitly approves
• The target receives the original brief unchanged
• Unparseable guard output blocks delegation
• The guard's reason is returned when the pattern blocks

Shape

• self: delegate to guard, if cleared delegate to target, otherwise return guard's reason
• delegates:
  • guard: evaluate whether the task should proceed
  • target: execute the task if guard approves
• prohibited: none

Requires

• Pattern instance receives: guard: string -- service or system name for the guard target: string -- service or system name for the target task_brief: string -- the task (goes to both guard and target)

Returns

• Guard receives the brief and returns structured JSON: { proceed: boolean, reason: string }
• If proceed: target receives the ORIGINAL brief unchanged
• If blocked: return immediately with the guard's reason — no delegation to target
• The guard does NOT modify the brief based on output — binary pass/block
• result: target output if proceeded, or guard reason if blocked
• proceeded: boolean

Delegation

let decision = call guard
  task_brief: task_brief
  prompt: "Return { proceed: boolean, reason: string }. Block if the decision is ambiguous."

if decision does not explicitly proceed:
  return {
    result: decision.reason,
    proceeded: false
  }

let result = call target
  task_brief: task_brief

return {
  result: result,
  proceeded: true
}

Notes

The guard does not know it gates access to another service. The target does not know a guard was consulted. The guard is a binary decision point — not a filter that modifies the brief. Useful when delegation is expensive and preconditions should be checked cheaply first.

The guard's contract promises structured JSON output ({ proceed, reason }). The delegation code parses that structure directly. If the guard returns unparseable output, the pattern fails safe by blocking — a guard that cannot clearly approve is a guard that has not approved.

Different from refine: a guard makes a one-shot binary decision before work begins. Refinement improves work that has already been done.