Your Prompts Have a Version Number
Anthropic ships a manual with its models now. The prompting guide has a section per model, a list of behaviors that changed, and migration notes for prompts written against the previous generation. Most teams running AI in production have never opened it.
A post that made the rounds when Fable 5 launched put it plainly: before you run the same prompts you used on the last model, read the guide. We'd push it one step further: migration notes exist because a prompt is code, and the compiler changes underneath it a few times a year.
Old prompts carry old scar tissue
Every production prompt accumulates workarounds for the model it was written against. The ALL-CAPS instruction the old model kept ignoring. The prefilled response forcing a format. The three examples added to stop a specific misbehavior in March.
New models follow instructions more literally and support structured outputs directly, so the scar tissue stops being harmless. A compensation for a bug that no longer exists is now an instruction the model will dutifully obey, and you won't know what it costs you until something reads strangely in front of a customer.
Anthropic's own golden rule for prompts is worth stealing for every instruction your automation carries: "Show your prompt to a colleague with minimal context on the task and ask them to follow it. If they'd be confused, Claude will be too."
How we handle it
In the systems we run, prompts sit in version control next to the code, and each one records the model it was written for. A model upgrade is a change to both, and it goes through the same eval harness as any release. The migration checklist is short:
- Read the vendor's model-specific notes. They exist because behavior changed.
- Delete workarounds before adding anything. Most migrations should make prompts shorter.
- Run the eval suite and compare, then ship the number, with the new model and the pruned prompt together.
A prompt without a version number is a prompt nobody maintains. Read the vendor's notes before you bump the model.