Premortem

Premortem

A premortem is the opposite of a postmortem: imagine the plan has already failed, then work backwards to find out why — before you've spent anything. It surfaces failure modes and hidden assumptions that optimism hides. This is destructive stress-testing; use [[brainstorming]] when you instead need to generate options.

Procedure

1. State the plan and a horizon. One or two sentences, plus a date ("in 6 months").
2. Assert the failure. "It is now 6 months later and this was a clear, costly failure." Write from that future as fact, not possibility — this licenses honesty that "what could go wrong?" suppresses.
3. Brainstorm causes across categories. Force coverage, don't just list the obvious:
   • Technical — it didn't scale, broke under load, the approach didn't fit the problem
   • Scope/effort — far bigger than estimated, never finished, gold-plated the wrong part
   • Dependencies — an external API/library/team/data source failed or changed
   • Ops/people — nobody maintained it, the one person who understood it left, no one adopted it
   • Data — wrong/missing/dirty data, migration corrupted state
   • Security/safety — a hole, a permission, a destructive action with no guard
4. Rank by likelihood × impact. Sort so the few that matter rise to the top.
5. Surface hidden assumptions. For each top risk, name the belief it depends on ("assumes the API stays stable", "assumes one repo per agent"). These are the load-bearing guesses.
6. Revise. Produce a changed plan that defuses the top risks, plus early-warning signals to watch so you catch the failure early if it starts.

Output

• Plan & horizon
• Top failure modes (ranked, with likelihood × impact)
• Hidden assumptions behind each
• Revised plan + early-warning signals to monitor

Common mistakes

• Listing vague risks ("it might be hard") instead of specific, mechanism-level failures.
• Only covering the technical category — most real failures are scope, people, or dependency.
• Stopping at the risk list. The value is the revised plan and the signals to watch.
• Treating it as theater — if every risk is "low likelihood", you're not being honest about the failure.