# feat: guard against empty terms in createGameSession

## Problem

If `getGameTerms` returned an empty array — no vocabulary data matched the requested language, difficulty, and part of speech combination — `createGameSession` would create a session with zero questions and return it. The frontend would receive an empty `questions` array, attempt to render the first question, find nothing, and crash with no useful error message shown to the user.

## Options considered

### Option A — `NotFoundError` (404) ✅

Throw when `terms.length === 0` before any session is created. The combination of filters yielded no data — that's a "not found" situation.

Chosen because: the request is technically valid (all filter values are recognised), but the combination has no matching data. 404 is the correct semantic response.

### Option B — `ValidationError` (400)

Treat empty results as a bad request.

Rejected because: the client sent valid input. The problem is missing data, not invalid input. 400 would be misleading.

## Solution

Added a guard in `createGameSession` immediately after `getGameTerms`:

```ts
if (terms.length === 0) {
  throw new NotFoundError("No terms found for the given filters");
}
```

The error propagates through the controller's `try/catch` to the error handler, which returns a clean 404 response. No session is created.

## Files changed

- `apps/api/src/services/gameService.ts` — empty terms guard added
- `apps/api/src/services/gameService.test.ts` — pinning test added
- `apps/api/src/controllers/gameController.test.ts` — pinning test added at HTTP layer

## Commit

`feat: guard against empty terms in createGameSession`