{"version":"2.1.321","anchor":"fast-mode-now-needs-an-explicit-opt-in-on-fable-52-mythos","canonical_anchor":"fable-52-and-mythos-52-model-recognition-added","heading":"Fable 5.2 and Mythos 5.2 arrive, and fast mode changes around them","tier":"use","area":"Models","scope":"individual","heads_up":true,"url":"https:\/\/changelogs.core-directive.com\/v\/2.1.321\/e\/fast-mode-now-needs-an-explicit-opt-in-on-fable-52-mythos","release_url":"https:\/\/changelogs.core-directive.com\/v\/2.1.321","markdown":"### Fable 5.2 and Mythos 5.2 arrive, and fast mode changes around them\n\nClaude Code recognises the Fable 5.2 and Mythos 5.2 models, needs an explicit opt-in for fast mode on them, and keeps your model when fast mode supports it\n\n**Unclear.** It is not known where Claude Code asks you to choose fast mode for these models.\n\n**What**\n\nClaude Code now recognises two new models, Fable 5.2 and Mythos 5.2. Neither was known before.\n\n- `claude-fable-5-2` and `claude-mythos-5-2` are new model IDs, with the short names `fable-5-2` and `mythos-5-2`.\n\n- The model label shows \"5.2\" for Fable 5.2. The system prompt, the standing instructions Claude Code sends to the model, now lists Fable 5.2 as the latest Fable model.\n\n- On Google Vertex, a new environment variable, `VERTEX_REGION_CLAUDE_FABLE_5_2`, overrides the region used for this model. An environment variable is a named value set in your shell before you start Claude Code.\n\n- A set of guidance names appears with the new models: `silent_turn_reminder`, `original_games_guidance`, `finishing_work_guidance`, `memory_guidance` and `timed_silent_turn_reminder`.\n\nFast mode gives faster responses at a higher price. It changes in three ways:\n\n- On Fable 5.2 and Mythos 5.2, fast mode is allowed only if you chose it in this session or the config key `fastModeChosenOnFableOrMythos` is true. Both models get their own fast mode pricing entry. Before, fast mode had no such opt-in and fast pricing covered only Opus models.\n\n- When fast mode is turned on remotely through `apply_flag_settings`, the session keeps its current model if that model supports fast mode. Otherwise it falls back to the default fast model, as it always did before. If the workspace refuses, the error begins \"fast mode: workspace did not accept apply_flag_settings\".\n\n- Turning fast mode off in the settings screen now runs an extra step. Before, it only cleared `fastMode` and set the state to off.\n\n**Why**\n\nIf you use Fable 5.2 or Mythos 5.2, you will not end up in fast mode, and paying its higher price, unless you chose it. If your model supports fast mode, turning fast mode on no longer moves you to Opus.\n\n- Area: Models\n- Tier: Use it now\n- Useful: 5\/5\n- Signal: 4\/5\n- Scope: individual\n- Heads-up: yes"}