diff --git a/GerbilManager.Tests/SaleAdTests.cs b/GerbilManager.Tests/SaleAdTests.cs
new file mode 100644
index 0000000..0032df1
--- /dev/null
+++ b/GerbilManager.Tests/SaleAdTests.cs
@@ -0,0 +1,225 @@
+using System.Net;
+using System.Text;
+using System.Text.Json;
+using GerbilManagerWebAPI.SaleAd;
+using Microsoft.Extensions.Options;
+
+namespace GerbilManager.Tests
+{
+    /// <summary>
+    /// FEAT-12a: sale-ad backend — prompt assembly, the 503-unconfigured path,
+    /// the OpenAI-compatible wire handling against a stubbed provider, and the
+    /// provider config matrix (Gemini / Groq / Mistral / Ollama base URLs).
+    /// </summary>
+    public class SaleAdTests
+    {
+        private static SaleAdRequest SampleRequest() => new(
+            Animals:
+            [
+                new SaleAdAnimal("Balu", "CP-Agouti", "2024-03-12", "ruhig, nimmt Leckerlis aus der Hand"),
+                new SaleAdAnimal("Benny", "Schwarz Schecke", "2024-03-12", "neugieriger Entdecker"),
+            ],
+            StatusLine: "FREI",
+            Hints: "bitte die Bruder-Dynamik betonen");
+
+        // ── Prompt assembly ────────────────────────────────────────────────
+
+        [Fact]
+        public void UserPrompt_enthält_Tiere_Status_und_Hinweise()
+        {
+            var prompt = SaleAdPromptBuilder.BuildUserPrompt(SampleRequest());
+
+            Assert.Contains("Status-Zeile: FREI", prompt);
+            Assert.Contains("Name: Balu", prompt);
+            Assert.Contains("Farbschlag: CP-Agouti", prompt);
+            Assert.Contains("Name: Benny", prompt);
+            Assert.Contains("ruhig, nimmt Leckerlis aus der Hand", prompt);
+            Assert.Contains("Wünsche/Hinweise: bitte die Bruder-Dynamik betonen", prompt);
+        }
+
+        [Fact]
+        public void UserPrompt_formatiert_Geburtsdatum_deutsch()
+        {
+            var prompt = SaleAdPromptBuilder.BuildUserPrompt(SampleRequest());
+            Assert.Contains("geboren am 12.03.2024", prompt);
+            Assert.DoesNotContain("2024-03-12", prompt);
+        }
+
+        [Fact]
+        public void UserPrompt_lässt_fehlende_Angaben_weg()
+        {
+            var request = new SaleAdRequest(
+                [new SaleAdAnimal("Mysti", null, null, null)], "RESERVIERT", "");
+            var prompt = SaleAdPromptBuilder.BuildUserPrompt(request);
+
+            Assert.Contains("Name: Mysti", prompt);
+            Assert.DoesNotContain("Farbschlag:", prompt);
+            Assert.DoesNotContain("geboren am", prompt);
+            Assert.DoesNotContain("Notizen:", prompt);
+            Assert.DoesNotContain("Wünsche/Hinweise:", prompt);
+        }
+
+        [Fact]
+        public void SystemPrompt_enthält_Stilregeln_und_beide_Beispiele()
+        {
+            var prompt = SaleAdPromptBuilder.BuildSystemPrompt();
+
+            Assert.Contains("Zucht der kleinen Chaoten", prompt);
+            Assert.Contains("NIEMALS Preise", prompt);
+            Assert.Contains("KEINE Fakten erfinden", prompt);
+            // few-shot: die beiden Beispiel-Inserate (Status-Zeilen der Vorlage)
+            Assert.Contains("Status: FREI", prompt);
+            Assert.Contains("Status: LOCKER RESERVIERT Anna", prompt);
+            Assert.Contains("Großer und kleiner Bruder Dynamik", prompt);
+        }
+
+        [Theory]
+        [InlineData("2024-03-12", "12.03.2024")]
+        [InlineData("kaputt", "kaputt")] // unparsebar -> unverändert durchreichen
+        public void FormatGermanDate_konvertiert_ISO_nach_TT_MM_JJJJ(string input, string expected)
+            => Assert.Equal(expected, SaleAdPromptBuilder.FormatGermanDate(input));
+
+        // ── 503: unkonfiguriert ────────────────────────────────────────────
+
+        [Theory]
+        [InlineData(null, "key", "model")]
+        [InlineData("https://api.example.com/v1", null, "model")]
+        [InlineData("https://api.example.com/v1", "key", null)]
+        [InlineData(null, null, null)]
+        public async Task GenerateAsync_meldet_NotConfigured_wenn_eine_Einstellung_fehlt(
+            string? baseUrl, string? apiKey, string? model)
+        {
+            var service = CreateService(baseUrl, apiKey, model,
+                new StubHandler(_ => throw new InvalidOperationException("darf nicht aufgerufen werden")));
+
+            var result = await service.GenerateAsync(SampleRequest());
+
+            Assert.Equal(SaleAdStatus.NotConfigured, result.Status);
+            Assert.Null(result.Text);
+        }
+
+        // ── Fake-Server: OpenAI-kompatible Antwort wird geparst ────────────
+
+        [Fact]
+        public async Task GenerateAsync_parst_die_Chat_Completion_des_Stubs()
+        {
+            var handler = new StubHandler(_ => Canned("Status: FREI\n\nWunderbares Duo …"));
+            var service = CreateService("https://api.groq.com/openai/v1", "k", "llama-3.3-70b", handler);
+
+            var result = await service.GenerateAsync(SampleRequest());
+
+            Assert.Equal(SaleAdStatus.Ok, result.Status);
+            Assert.Equal("Status: FREI\n\nWunderbares Duo …", result.Text);
+        }
+
+        [Fact]
+        public async Task GenerateAsync_sendet_Modell_Messages_und_Bearer_Key()
+        {
+            var handler = new StubHandler(_ => Canned("ok"));
+            var service = CreateService("https://api.example.com/v1", "geheim", "test-model", handler);
+
+            await service.GenerateAsync(SampleRequest());
+
+            Assert.NotNull(handler.LastRequest);
+            Assert.Equal("Bearer", handler.LastRequest!.Headers.Authorization?.Scheme);
+            Assert.Equal("geheim", handler.LastRequest.Headers.Authorization?.Parameter);
+
+            using var body = JsonDocument.Parse(handler.LastRequestBody!);
+            Assert.Equal("test-model", body.RootElement.GetProperty("model").GetString());
+            var messages = body.RootElement.GetProperty("messages");
+            Assert.Equal(2, messages.GetArrayLength());
+            Assert.Equal("system", messages[0].GetProperty("role").GetString());
+            Assert.Equal("user", messages[1].GetProperty("role").GetString());
+        }
+
+        [Fact]
+        public async Task GenerateAsync_meldet_UpstreamError_bei_Provider_HTTP_Fehler()
+        {
+            var handler = new StubHandler(_ => new HttpResponseMessage(HttpStatusCode.TooManyRequests)
+            {
+                Content = new StringContent("rate limited"),
+            });
+            var service = CreateService("https://api.example.com/v1", "k", "m", handler);
+
+            var result = await service.GenerateAsync(SampleRequest());
+
+            Assert.Equal(SaleAdStatus.UpstreamError, result.Status);
+        }
+
+        [Fact]
+        public async Task GenerateAsync_meldet_UpstreamError_bei_leerer_Antwort()
+        {
+            var handler = new StubHandler(_ => Canned(""));
+            var service = CreateService("https://api.example.com/v1", "k", "m", handler);
+
+            var result = await service.GenerateAsync(SampleRequest());
+
+            Assert.Equal(SaleAdStatus.UpstreamError, result.Status);
+        }
+
+        // ── Provider-Konfigurationsmatrix: BaseUrl -> /chat/completions ────
+
+        [Theory]
+        [InlineData("https://generativelanguage.googleapis.com/v1beta/openai",
+                    "https://generativelanguage.googleapis.com/v1beta/openai/chat/completions")] // Gemini
+        [InlineData("https://api.groq.com/openai/v1",
+                    "https://api.groq.com/openai/v1/chat/completions")] // Groq
+        [InlineData("https://api.mistral.ai/v1",
+                    "https://api.mistral.ai/v1/chat/completions")] // Mistral
+        [InlineData("http://truenas:11434/v1",
+                    "http://truenas:11434/v1/chat/completions")] // Ollama (lokal)
+        [InlineData("https://api.groq.com/openai/v1/",
+                    "https://api.groq.com/openai/v1/chat/completions")] // trailing slash
+        public async Task Konfigurationsmatrix_baut_die_richtige_Completions_URL(
+            string baseUrl, string expectedUri)
+        {
+            Assert.Equal(new Uri(expectedUri), SaleAdService.BuildCompletionsUri(baseUrl));
+
+            // und der Stub sieht den Aufruf tatsächlich auf dieser URL
+            var handler = new StubHandler(_ => Canned("ok"));
+            var service = CreateService(baseUrl, "k", "m", handler);
+            await service.GenerateAsync(SampleRequest());
+            Assert.Equal(new Uri(expectedUri), handler.LastRequest!.RequestUri);
+        }
+
+        // ── Helfer ─────────────────────────────────────────────────────────
+
+        private static SaleAdService CreateService(
+            string? baseUrl, string? apiKey, string? model, StubHandler handler)
+        {
+            var options = Options.Create(new AiOptions { BaseUrl = baseUrl, ApiKey = apiKey, Model = model });
+            return new SaleAdService(new HttpClient(handler), options);
+        }
+
+        private static HttpResponseMessage Canned(string content)
+        {
+            var completion = new
+            {
+                choices = new[] { new { message = new { role = "assistant", content } } },
+            };
+            return new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = new StringContent(JsonSerializer.Serialize(completion),
+                    Encoding.UTF8, "application/json"),
+            };
+        }
+
+        /// <summary>HttpMessageHandler-Stub, der Request + Body zur Inspektion festhält.</summary>
+        private sealed class StubHandler(Func<HttpRequestMessage, HttpResponseMessage> respond)
+            : HttpMessageHandler
+        {
+            public HttpRequestMessage? LastRequest { get; private set; }
+            public string? LastRequestBody { get; private set; }
+
+            protected override async Task<HttpResponseMessage> SendAsync(
+                HttpRequestMessage request, CancellationToken cancellationToken)
+            {
+                LastRequest = request;
+                LastRequestBody = request.Content is null
+                    ? null
+                    : await request.Content.ReadAsStringAsync(cancellationToken);
+                return respond(request);
+            }
+        }
+    }
+}
diff --git a/GerbilManagerWebAPI/GerbilManagerWebAPI.csproj b/GerbilManagerWebAPI/GerbilManagerWebAPI.csproj
index af6567e..cd572d9 100644
--- a/GerbilManagerWebAPI/GerbilManagerWebAPI.csproj
+++ b/GerbilManagerWebAPI/GerbilManagerWebAPI.csproj
@@ -30,4 +30,9 @@
     <EmbeddedResource Include="Contracts\Templates\Abgabevertrag.docx" />
   </ItemGroup>
 
+  <ItemGroup>
+    <!-- FEAT-12a: Tests prüfen interne Bausteine (z. B. die Completions-URL). -->
+    <InternalsVisibleTo Include="GerbilManager.Tests" />
+  </ItemGroup>
+
 </Project>