{"version":"2.1.312","anchor":"lean-prompt-model-check-made-deterministic-claude-mythos-5","canonical_anchor":"lean-prompt-model-check-made-deterministic-claude-mythos-5","heading":"claude-mythos-5 is no longer automatically excluded from the lean prompt","tier":"internal","area":"System Prompt","scope":"individual","heads_up":false,"url":"https:\/\/changelogs.core-directive.com\/v\/2.1.312\/e\/lean-prompt-model-check-made-deterministic-claude-mythos-5","release_url":"https:\/\/changelogs.core-directive.com\/v\/2.1.312","markdown":"### `claude-mythos-5` is no longer automatically excluded from the lean prompt\n\nClaude Code no longer rules out the lean prompt for `claude-mythos-5` up front and now checks it by model name like other models\n\n**Unclear.** Whether `claude-mythos-5` now actually receives the lean prompt depends on the rest of the check, which is not known.\n\n**What**\n\nClaude Code can send the model a leaner variant of its instructions, and it decides which models get it. Two things changed in that decision:\n\n- `claude-mythos-5`, and models matched by a related family check, are no longer ruled out at the very start. They now go through the same name-based checks as other models, which look for names such as claude-3-, haiku and sonnet.\n\n- The model name is now worked out in a fixed, repeatable way before those checks run.\n\nA remote setting called `lean_prompt` still takes priority over all of this when it is sent.\n\n**Why**\n\nThis can change which version of the instructions `claude-mythos-5` and similar models receive.\n\n- Area: System Prompt\n- Tier: Under the hood\n- Useful: 3\/5\n- Signal: 4\/5\n- Scope: individual\n- Heads-up: no"}