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16 min · agents

The plan → act → check loop

Goal: run agents in a simple loop so mistakes stay small.

Best for: first agent runs where you want a safety rhythm, not a one-shot “do everything.”

Think of an agent like a junior teammate: first they propose a plan, then they do a small action, then you both check the result. That loop—plan → act → check—is the beginner-safe default.

If you skip plan, you get surprise actions. If you skip check, errors pile up. Keep each “act” tiny.

Why beginners skip this

Jumping straight to action feels fast until silent wrong steps stack. Plan → one act → check keeps blast radius small. Mid-week cost shows up as rework, not as a missing bookmark.

Step 1 — Demand a plan with no side effects

First message: ask only for a numbered plan. Ban tool use until you say “go.”

Goal: Propose a plan only—no tools, no edits.
Task: [task]
Done checklist: [paste]
Format: 5–8 numbered steps · which need my approval · estimated risk (low/med)
Boundaries: Do not call tools yet.
Example

Filled prompt

Task: rotate API keys in staging.
Plan 5 bullets → Act (list key names only) → Check.
Refuse Act2 until Check1 green. No prod.

What good output looks like

  • Done boxes for “run agents in a simple loop so mistakes stay small” are explicit and checkable
  • Allow-list tools only; never-do lines honored
  • Stops before send/delete without human gate
  • Pass/fail: Plan→Act one→Check visible for agents-plan-act-check-loop
  • Pass/fail dated (Asia/Shanghai) sits next to the artifact

Step 2 — Approve one small act

Reply: “Do step 1 only. Then stop and show me evidence.” Evidence = quote, file list, or description—not just “done.”

Execute ONLY step [N] from the plan.
Then stop.
Show: what you did · evidence · what you will do next if I say go.
Do not continue automatically.
Example

Filled prompt

Here is a skimpy plan: “gather links, draft brief, email stakeholders.”
Rewrite into Plan → Act → Check with explicit CHECKPOINT before email.
Stakeholder list is internal #ops only; no external send.

What good output looks like

  • Allow-list tools only; never-do lines honored
  • Stops before send/delete without human gate
  • Pass/fail: Plan→Act one→Check visible for agents-plan-act-check-loop
  • Done boxes for “run agents in a simple loop so mistakes stay small” are explicit and checkable

Step 3 — Check, then loop

You spot-check. If good: “Go step 2 only.” If bad: revise the plan. Never say “finish everything” on day one.

I checked step [N]. Feedback: [ok / fix this…]
Revise the remaining plan in ≤6 steps.
Wait for my go before any action.
Example

Filled prompt

Task: rename feature flag in repo.
Force Plan (5 bullets) → Act (one step) → Check (pass/fail).
I will refuse Act on step 2 until Check on step 1 is green.
Return the loop card filled for this task.

What good output looks like

  • Stops before send/delete without human gate
  • Done boxes for “run agents in a simple loop so mistakes stay small” are explicit and checkable
  • Pass/fail: Plan→Act one→Check visible for agents-plan-act-check-loop
  • Allow-list tools only; never-do lines honored

Keep it real

Run this guide’s Practice now on work you will actually ship this week—not a fictional scenario. Dated notes beat perfect theory.

Do not bookmark-only

Finish Practice now, then tick this guide on your path. Titles unread are not skills.

Close the loop

After Practice now, write one line: what you will refuse next time. That line is the habit.

One more pass

Re-read your Save-this-card line. If it is vague, rewrite it until a stranger could apply it.

Red flag

Acting without a checked plan; skipping the check to “finish faster”; irreversible step with no human gate—stop the run.

Save this card

Card: plan → one act → check. Block the next act until check is green. Keep this beside the tool; refuse to start if a box is blank.

1. What I shipped: ___ 2. What I refused: ___ 3. What I will never-do again: ___ (date: Asia/Shanghai)

  • [ ] Tools/writes paused
  • [ ] Refuse-line dated
  • [ ] Artifact path saved
  • [ ] Next link chosen (or stop)

Before you start

Write Done (checkboxes) and the human gate on paper before any run for “run agents in a simple loop so mistakes.” Allow-list tools; leave send/delete off. Guide: agents-plan-act-check-loop. If you only have a blank practice box, stop and fetch the real job first.

  • Goal in one line: ___
  • Allowed tools: ___
  • Forbidden: ___
  • Human approve needed for: ___

How to use this week

This week: run one live task as plan → one action → check. Block step 2 until step 1’s check is green. Save the loop card with today’s Asia/Shanghai date on a live artifact—not a blank demo. Write one refuse-line and date the note (Asia/Shanghai). If you only skimmed agents-plan-act-check-loop, put 25 minutes on the calendar and redo once.

Pitfalls

PitfallFix
“Do the whole plan” in one goOne step per approval at first
Trusting “done” with no evidenceRequire artifacts
Changing the goal mid-loop silentlyUpdate Done checklist explicitly
Example

Filled prompt

Job: run agents in a simple loop so mistakes stay small
Prior fail: [what missed the check]
Repair: keep the same Done; add one constraint + one concrete example from my notes.
Boundaries: draft only; label Unknowns [?]; no silent send.
Return: fixed output + the one-minute pass/fail I will mark.

What good output looks like

  • Names the prior fail without inventing a new job
  • Adds one constraint tied to “run agents in a simple loop so mistakes stay small”
  • Still draft-only / Unknowns labeled
  • Pass/fail: you mark green on the same checklist (agents-plan-act-check-loop #4)

Practice now

Live card: write Plan / Act / Check for a 20-min chore; refuse Act until Plan fits on one screen (Asia/Shanghai).

Today: run one real task as plan → one approved act → check. Do not start step two until check passes. Write Done + allow-list first. Date pass/fail (Asia/Shanghai).

Today card (copy) — agents-plan-act-check-loop: 1. Artifact I will produce: ___ 2. Pass/fail check: ___ 3. Refuse-line if tempted to skip verify/send: ___ (date: Asia/Shanghai) 4. One never-do from this guide: ___

  • [ ] Artifact path written
  • [ ] Refuse example would say "yes"
  • [ ] Dated Asia/Shanghai

One sentence takeaway

Plan, act one step, check—refuse the next act until check passes—date a refuse-line this week, or nothing changed

  • You can point to a dated refuse-line
  • You have one artifact path from tonight
  • You can teach the red flag in one sentence

Next

Human-in-the-loop checkpoints · Subagents and handoffs · Failure modes and retries · Logging what matters · Define done and permissions

Pair with today: open the first Next link (agents-human-in-the-loop-checkpoints) only after you dated a refuse-line for this guide (Asia/Shanghai).

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