A Gemini Gem behaves the way you wrote it on day one. After a while, it doesn't quite anymore — model updates, your work shifting, and accumulated example clutter all play a part.
This article isn't about writing your first Gem. It's about rewriting one that has stopped pulling its weight, written by someone who has shipped several Gems into daily workflows.
Five Symptoms That Mean It's Time to Rewrite
These are the signals I trust:
First, the same prompt returns three differently-structured answers across three runs. If the format used to be steady and now wobbles, something inside the instruction has stopped landing.
Second, a single keyword derails the response. If typing "Stripe" suddenly drops the Gem into generic Stripe-API explanations regardless of context, the priority order in the instructions has broken down.
Third, your tone setting (formal/casual/professional) stops being honored. Model updates can quietly weaken tone directives, which is a textbook reason to rewrite.
Fourth, exception rules that used to fire reliably ("answer in English when the user writes in English") get ignored. That's a sign your instruction's ordering or emphasis needs review.
Fifth, your work has shifted but the Gem still operates on stale assumptions. The Gem isn't broken; you are overdue.
The Real Cause Is Almost Always Bloat
In every Gem I've maintained for more than a few weeks, the leading cause of decay is exception accumulation. The Gem starts at 200 characters of clean intent. Then "in this case do X, in this case do Y" gets stitched in week after week, until the instruction passes 2,000 characters and contradicts itself in places.
In that state, the model can't tell which guideline to prioritize, and quality slides. Rewriting requires the courage to discard most of the accumulated exceptions and keep only the few that genuinely still matter.
A Four-Step Safe Migration
Don't overwrite production directly. Walk through this:
Step 1: back up the current instruction. Paste it into an editor and save as gems-backup-YYYY-MM-DD.txt. Without a known-good rollback target, you'll psychologically resist rewriting.
Step 2: write the Gem's purpose as a single sentence. The exercise of distilling the most important goal forces the rewrite to have an axis.
Step 3: rebuild the instruction in "core directive → examples → exceptions" order. Cap examples at two or three, and keep exceptions to ones that have genuinely happened in production.
Step 4: save the rewrite under a separate Gem name and run it parallel to the old one for a week. Fold any rough edges back in before you swap to the rewrite as primary.
Example: Post-Rewrite Instruction
For reference, a simplified version of my current writing-support Gem looks like this:
# Role
Editing partner for Japanese-language technical articles.
# Core principles
- Always use the polite "desu/masu" form. No casual form.
- Define technical terms briefly on first appearance.
- Always specify a language in code fences.
# Output format
- Prefer flowing prose to bullet lists.
- Surface no more than three suggestions at a time.
# Exceptions
- Switch to casual form only when the user explicitly asks.
- Reply in English if the user writes in English.
Trimming the instruction to roughly this size cuts the risk of the model dropping directives substantially.
Make a Habit of Checking After Model Updates
Within a day or two of a Gemini model update, I poke the Gems I rely on most and confirm they still behave. After 3.2 shipped, I noticed one of mine had a weakened tone directive and rewrote it that day. A small post-update check reliably saves the future detective work of "when did this start drifting?"
When You Share a Gem with a Team
If multiple people use the same Gem, advance notice on rewrites is mandatory. Otherwise you end up with one teammate on the new instruction and others on the old, and the Gem's behavior appears to vary by person.
When sharing the diff, paste the before-and-after texts side by side in Slack or Notion. That single move handles both review and consensus at once.
A Concrete Next Step
Tomorrow, just count the characters in the Gem you use most often. If you're past 1,500, a rewrite is probably overdue. Shorter Gems behave more reliably than longer ones — almost always.