5.4 -- Review the system prompt

Context

A system prompt is not sacred text. Your context changes, your projects evolve, your needs become clearer. A prompt written 3 months ago no longer reflects your reality today. If you don't review it, it accumulates obsolete instructions and the agent gradually degrades.

Frequency

• Complete review: every month.
• Spot adjustments: when a trigger appears (see below).
• Condensation: every 3 months (the prompt naturally grows, you need to compress it).

Triggers for a review

You should review the prompt when:

1. A project ends. Instructions tied to that project clutter the prompt.
2. A tool changes. You migrate from X to Y but the prompt still mentions X.
3. You correct the agent regularly on the same point. A signal that the prompt doesn't cover this case.
4. The prompt exceeds 300 words. Time to condense.
5. You change models. Each model interprets differently. Test and adjust.
6. An incident occurred. The agent did something it shouldn't have. Add a rule or clarify.

What to preserve

Certain parts of the prompt are stable. Don't touch them unless there's a strong reason:

• The mission (who the agent is, who it's for).
• Tone and format (once calibrated, it holds).
• Boundaries (prohibitions don't change).
• Language (stable once defined).

What to rewrite

What changes most often:

• Context: active projects, stack, tools.
• Operational rules: workflows, priorities, processes.
• Access: which tools the agent can use.

Review method

1. Reread the current prompt

Read it as if for the first time. Mark: - [OBSOLETE]: what is no longer true. - [VAGUE]: what could be more precise. - [MISSING]: what should be there but isn't. - [OK]: what is good.

2. Delete before adding

The rule: for every line added, find a line to remove. The prompt must stay short.

3. Test

After the review, test with 5 typical requests. Compare the responses with responses from before. If quality drops, roll back.

4. Version control

Keep previous versions. A simple file:

system-prompt-v1.md    (2026-01-15)
system-prompt-v2.md    (2026-02-20)
system-prompt-v3.md    (2026-04-01)  <- current

If a revision degrades performance, you can revert to the previous version.

Common mistakes

Never reviewing. The prompt from January 2026 is still active in December. It mentions finished projects, replaced tools, obsolete rules.

Reviewing too often. Every day you change something. The agent never has time to "stabilize" and you can't evaluate the impact of a change.

Adding without deleting. The prompt grows from 150 to 500 words in 3 months. Performance drops, cost increases.

No version control. You modify the prompt and no longer know what was there before. Impossible to roll back.

Steps

1. Schedule a monthly review (set a reminder).
2. Reread the prompt using the OBSOLETE/VAGUE/MISSING/OK system.
3. Delete what is obsolete.
4. Clarify what is vague.
5. Add what is missing.
6. Condense if > 300 words.
7. Test with 5 requests.
8. Save the old version.

Checklist

• [ ] The prompt's last review was less than a month ago.
• [ ] The prompt is under 300 words.
• [ ] No instruction references a finished project or replaced tool.
• [ ] Previous versions are saved.
• [ ] 5 test requests produce correct responses after the review.