Model defaults belong to config.toml. Use /model and choose…

Error message

Model defaults belong to config.toml. Use /model and choose Remember as my default, or /config model <id> --save.

What it means

Settings::set deliberately refuses the `default_model`/`model` key: model defaults are owned by config.toml, not session settings. The bail message redirects the user to the /model picker ('Remember as my default') or `/config model <id> --save`. This is an unsupported-operation guard, not a value-format error.

Solutions

Use the /model command in the TUI and pick 'Remember as my default'.
Write the model id into config.toml via `/config model <id> --save`.
Remove default_model from any settings write path; it is not a settable session setting.

Example fix

// before
settings.set("default_model", "glm-5.3-flash")?;
// after
// in the TUI:
//   /config model glm-5.3-flash --save
// or edit ~/.config/codewhale/config.toml: model = "glm-5.3-flash"

Defensive patterns

Strategy: fallback

Validate before calling

fn is_model_setting_key(key: &str) -> bool {
    matches!(key, "default_model" | "model") // always rejected by Settings::set
}

Try / catch

match settings.set(key, value) {
    Err(e) if e.to_string().contains("Model defaults belong to config.toml") => {
        // route to the model config path instead of settings
        write_model_to_config_toml(value)?;
    }
    r => r?,
}

Prevention

Never route model defaults through Settings::set; use /config model <id> --save or config.toml.
Filter model-related keys out of generic settings-write helpers.
Update migration scripts that predate this separation.

When it happens

Trigger: Any call to Settings::set("default_model", ...) or Settings::set("model", ...) with any value — the key itself is rejected unconditionally.

Common situations: Users attempting `/config model <id>` without --save, scripts migrating settings files that used to carry default_model, or confusing the settings layer with the config layer.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of Hmbown/CodeWhale@73e0f67d83 (2026-09-22). Data as JSON: /api/errors/849f944a04d5580e. Report an issue: GitHub.

Appendix: source

Thrown at crates/tui/src/settings.rs:1748

                let max: usize = value.parse().map_err(|_| {
                    anyhow::anyhow!(
                        "Failed to update setting: invalid max history '{value}'. Expected a positive number."
                    )
                })?;
                self.max_input_history = max;
            }
            "default_model" | "model" => {
                anyhow::bail!(
                    "Model defaults belong to config.toml. Use /model and choose Remember as my default, or /config model <id> --save."
                );
            }
            "reasoning_effort" | "effort" => {
                self.reasoning_effort = normalize_reasoning_effort_setting(value)?;
            }
            "permission_posture" | "permissions" => {
                self.permission_posture = normalize_permission_posture(value);
                if self.permission_posture.is_none() {
                    anyhow::bail!(
                        "Failed to update setting: invalid permission posture '{value}'. Expected: ask, auto-review, or full-access."
                    );
                }
            }
            "sandbox_mode" | "sandbox" | "filesystem_sandbox" => {
                self.sandbox_mode = normalize_sandbox_mode(value);
                if self.sandbox_mode.is_none() {
                    anyhow::bail!(
                        "Failed to update setting: invalid sandbox_mode '{value}'. Expected: read-only, workspace-write, danger-full-access, or external-sandbox."
                    );
                }
            }
            _ => {
                anyhow::bail!("Failed to update setting: unknown setting '{key}'.");
            }
        }
        if let Some(canonical) = canonical {
            self.provenance

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