Core_Be/AI/GeminiClient.cs

using System.Text.Json;

namespace Fitness_core_BE.AI
{
    public class GeminiClient : IAiClient
    {
        private readonly HttpClient _http;
        private readonly IConfiguration _config;

        public GeminiClient(HttpClient http, IConfiguration configuration)
        {
            _http = http;
            _config = configuration;
        }

        public async Task<string> GenerateJsonAsync( string systemPrompt, object payload)
        {
            string apiKey = _config["Gemini:ApiKey"];

            if (string.IsNullOrEmpty(apiKey))
                throw new Exception("Gemini API key is not configured.");

            var requestBody = new
            {
                contents = new[]
                {
            new
            {
                parts = new[]
                {
                    new { text = systemPrompt },
                    new { text = JsonSerializer.Serialize(payload) }
                }
            }
        },
                generationConfig = new
                {
                    temperature = 0.4,
                    maxOutputTokens = 10000
                }
            };

            var response = await _http.PostAsJsonAsync(
                $"https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash:generateContent?key={apiKey}",
                requestBody
            );

            if (!response.IsSuccessStatusCode)
            {
                var error = await response.Content.ReadAsStringAsync();
                throw new Exception($"Gemini API Error: {error}");
            }

            var jsonString = await response.Content.ReadAsStringAsync();

            using var doc = JsonDocument.Parse(jsonString);

            var text = doc.RootElement
                .GetProperty("candidates")[0]
                .GetProperty("content")
                .GetProperty("parts")[0]
                .GetProperty("text")
                .GetString();

            return text ?? throw new Exception("No response text from Gemini.");
        }
    }
}