Within agents.md, an idea. Feel free to steal if you work on this:

    Model.Claude*:
      Do this one thing

    Model.Claude.Opus.4.8:
      Extra specific instructions

    Model.*:
      This is relevant to everyone

    Harness.ClaudeCode*:
      Why not, let’s allow harness specific instructions so you can do things differently depending how you’re working

I did this at one point with Jinja templates.

I wrote an agent launcher sort of bash script. Pass in the command to start the agent, the script checks if there’s a Jinja file in a special directory matching that name, and builds it to AGENTS.md. Then it launches the agent.

I was trying to use it as a sort of janky RAG. I had a bunch of snippets (one for DB architecture, one for how load balancing works, etc), and my Jinja files were mostly a list of snippets to pull in. Voila, a bunch of agents that share little pieces of info but have a single source of truth.

I never got a ton of value tbh, it was very good at just grepping the snippets.

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Seriously, though, this is a great idea. Good communicators know to tailor their discourse to their audience. LLMs are just as sensitive to small variations in prompts, and just as inconsistent between them, as are humans.

It's not a bad idea, although at that point it probably shouldn't be a .md file anymore. You'd want to use YAML or something.