Fable 5.2 and Mythos 5.2 arrive, and fast mode changes around them

What

Claude Code now recognises two new models, Fable 5.2 and Mythos 5.2. Neither was known before.

• claude-fable-5-2 and claude-mythos-5-2 are new model IDs, with the short names fable-5-2 and mythos-5-2.
• 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.
• 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.
• 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.

Fast mode gives faster responses at a higher price. It changes in three ways:

• 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.
• 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".
• Turning fast mode off in the settings screen now runs an extra step. Before, it only cleared fastMode and set the state to off.

Why

If 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.