The host's settings for rho, config :rho, ..., each with a default that works on a computer. :data_dir, :default_model and :system_prompt may be given as {module, function, args} (or a 0-arity function), called each time the value is read, so a host can keep them in its own settings and change them at run time.

• :data_dir: where rho keeps sessions, artifacts, todo lists, the cost ledger, usage.json, settings.json, the trimmed model catalog and (with the default credential store) the sign-ins. Default: MOB_DATA_DIR when set (a mob app's private files dir), else the OS's per-user data dir for "rho" (:filename.basedir/2). Created on first read.
• :credential_store: a Rho.CredentialStore module. Default Rho.CredentialStore.File (0600 files under the data dir); a phone app plugs in the platform's secure store (Keystore / Keychain).
• :settings: a Rho.Settings module (fallback model, daily cost cap). Default Rho.Settings.File (settings.json in the data dir).
• :default_model: a req_llm spec, default "anthropic:claude-haiku-4-5".
• :system_prompt: the loop's system prompt when a loop isn't given one. Default Rho.default_system_prompt/0.
• :namespace: the prefix of the customType of the loop's notices in a session ("<namespace>.notice", .error, .aside). Default "rho"; Operator's sessions use "operator".
• :open_url: how Rho.Auth.Login opens the provider's sign-in page, fn url -> :ok end or an MFA taking the URL last. Default Rho.Auth.Login.open_in_browser/1 (open / xdg-open / start); a mob app sets {Mob.Device, :open_url, []}.
• :app_name: what the sign-in pages call the app ("Switch back to … to finish signing in"). Default "the app".
• :hints: where the user fixes a failed call, as Rho.LLM.describe/1 and Rho.Auth.Login say it: sign_in: (default "from the app's settings", as in "sign in again from …") and switch_model: (default "in the app's settings"); and, for Dyn (Rho.Dyn.agent_guide/0, dyn_propose), approve:, how the human approves a proposed generation (default "on the phone with the screen lock (fingerprint, face, PIN, pattern, password or passcode)", as Rho.Dyn.Approval.Biometric behind an approve control).
• :tools, :tool_sources: Rho.ToolRegistry's core tools and registry sources, when Rho.Supervisor isn't given them.
• :dyn: the Dyn layer (agent-written code, Rho.Dyn), off by default; its keys are Rho.Dyn.Config's.