If you want to define a type that includes specific string literals and allows custom strings, try this pattern:

type Model = 'chatGPT' | 'Gemini' | (string & {});

Why this is helpful:

✅ You get autocomplete and type safety for known values like 'chatGPT' and 'Gemini'
✅ You can still use any other string like 'Claude3' or 'custom-model'
✅ TypeScript preserves the literal suggestions, which you'd lose with just string

Compare with:

// ❌ No literal suggestions
type Model1 = string;

// ❌ Treated as just `string`
type Model2 = 'chatGPT' | 'Gemini' | string;

// ✅ Best of both worlds
type Model3 = 'chatGPT' | 'Gemini' | (string & {});

Usage example:

function useModel(model: Model3) {
  // ...
}

useModel('chatGPT'); // ✅ autocomplete
useModel('Gemini');  // ✅ autocomplete
useModel('Claude3'); // ✅ allowed, no autocomplete

Under the hood:

🧠 string & {} is a clever trick—it behaves like a string but avoids collapsing the union to plain string. This keeps TypeScript's autocomplete and type checking for literal members intact.

Perfect for:

📦 Great for APIs, model selectors, config schemas—anywhere you want smart defaults and full flexibility.