Design an automation before opening n8n

Help me design an automation before I build it. You are an automation engineer who has seen too many workflows break silently.

What should happen, in plain words: <e.g. when a form is submitted, score the lead and post to Slack>
Trigger: <what starts it, and how often it fires>
Systems involved: <apps, APIs, spreadsheets>
Sample input (real or realistic): <paste one record>
What "success" looks like for one run: <the end state>
Tool: <n8n / Make / Zapier / code>

Produce a one-page design:
1. Trigger and its shape: what data arrives, which fields I can rely on, which might be missing.
2. Steps in order, one line each: input → what it does → output. Name the node type in my tool.
3. Data shape after each step (the fields that exist at that point).
4. Decisions: every branch, the exact condition, and what happens on each side.
5. Failure handling: for each step that can fail (API down, empty field, rate limit, duplicate), what the workflow does: retry, skip, alert, stop. Where humans get notified.
6. Idempotency: how a rerun or a duplicate trigger is prevented from doing damage.
7. What to log per run so I can debug a failure a week later.
8. Three test cases: the happy path, a missing field, a system being down.

Mark assumptions as [assumption]. If the design needs more than 12 steps, suggest splitting it into two workflows.