Designing a Privacy-Focused AI Browser Platform with Flutter, Laravel, Gemini, VPN, and Remote Configuration

<?php

namespace App\Http\Controllers\Api\V1;

use App\Http\Controllers\Controller;
use App\Models\AiPrompt;
use App\Services\AiQuotaService;
use Illuminate\Http\JsonResponse;
use Illuminate\Http\Client\ConnectionException;
use Illuminate\Http\Request;
use Illuminate\Support\Facades\Http;
use Illuminate\Support\Facades\Log;
use Illuminate\Validation\Rule;

class AiProxyController extends Controller
{
    public function __construct(private readonly AiQuotaService $quota) {}

    // Route security (routes/api.php):
    // Route::post('/v1/ai/ask', [AiProxyController::class, 'ask'])
    //    ->middleware(['auth:sanctum', 'throttle:ai']); // authentication + rate limiting

    public function ask(Request $request): JsonResponse
    {
        // Validation: cap sizes so one request cannot send a huge, costly prompt.
        $data = $request->validate([
            'mode'            => ['required', Rule::in(['search', 'chat', 'summarize', 'translate', 'page_question'])],
            'question'        => ['nullable', 'string', 'max:1000'],
            'page_text'       => ['nullable', 'string', 'max:20000'],
            'target_language' => ['nullable', 'string', 'size:2'],
        ]);

        $user = $request->user();
        $prompt = AiPrompt::where('key', $data['mode'])->where('is_active', true)->firstOrFail();

        // Quota: throws QuotaExceededException (rendered as HTTP 429) when the allowance is used up.
        $remaining = $this->quota->consume($user);

        try {
            // Secrets: the key comes from .env via config/services.php, never from the client.
            $response = Http::withHeaders(['x-goog-api-key' => config('services.gemini.key')])
                ->timeout(20)
                ->post(
                    sprintf(
                        'https://generativelanguage.googleapis.com/v1beta/models/%s:generateContent',
                        config('services.gemini.model')
                    ),
                    [
                        'systemInstruction' => ['parts' => [['text' => $prompt->template]]],
                        'contents' => [[
                            'role' => 'user',
                            'parts' => [['text' => $this->buildUserText($data)]],
                        ]],
                    ]
                );
        } catch (ConnectionException $e) {
            $this->quota->refund($user);
            Log::warning('Gemini connection failed', ['user' => $user->id]);
            return response()->json(['error' => ['code' => 'ai_unavailable', 'message' => 'AI is temporarily unavailable.']], 503);
        }

        if ($response->failed()) {
            // Error handling: refund the credit and log provider details server-side only.
            $this->quota->refund($user);
            Log::error('Gemini error', ['status' => $response->status()]);
            return response()->json(['error' => ['code' => 'ai_unavailable', 'message' => 'AI is temporarily unavailable.']], 502);
        }

        $answer = $response->json('candidates.0.content.parts.0.text');

        return response()->json(['data' => [
            'answer' => $answer,
            'sources' => [], // Filled by the retrieval step you choose for search mode (not shown here).
            'quota' => ['remaining' => $remaining],
            'notice' => 'AI can make mistakes. Check important facts against the sources.',
        ]]);
    }

    private function buildUserText(array $data): string
    {
        $question = $data['question'] ?? '';
        $page = $data['page_text'] ?? '';

        // Prompt-injection defense: label page text as data, and say so in the stored system prompt.
        return match ($data['mode']) {
            'summarize'     => "Summarize the content inside <page> tags.\n<page>{$page}</page>",
            'translate'     => "Translate the content inside <page> tags into {$data['target_language']}.\n<page>{$page}</page>",
            'page_question' => "Question: {$question}\n<page>{$page}</page>",
            default         => $question,
        };
    }
}