services/domain/Big.Api/Program.cs

@@ -23,6 +23,7 @@ builder.Services.AddTransient<IExternalWorkerClient>(sp => sp.GetRequiredService
 builder.Services.AddHttpClient<IAclClient, AclHttpClient>();
 
 builder.Services.AddScoped<SubmitRegistration>();
+builder.Services.AddScoped<ApproveRegistration>();
 builder.Services.AddScoped<OpenZaakWorker>();
 builder.Services.AddScoped<OpenZaakJobProcessor>();
 
@@ -42,6 +43,18 @@ app.MapPost("/registrations", async (SubmitRegistrationRequest body, SubmitRegis
     return Results.Accepted($"/registrations/{id}", new RegistrationResponse(id.ToString(), RegistrationStatus.Ingediend.ToString(), null));
 });
 
+// Temporary admin endpoint (S-09b): approve a registration — the behandelaar's decision, until the
+// behandel-portal exists (S-12). The zaak's final status is set via the ACL, which flows back over
+// NRC to the projection, making the entry publicly visible as INGESCHREVEN. Idempotent.
+app.MapPost("/registrations/{id}/approve", async (string id, ApproveRegistration approve, CancellationToken ct) =>
+{
+    if (!Guid.TryParse(id, out var guid))
+        return Results.NotFound();
+
+    await approve.HandleAsync(new ApproveRegistrationCommand(new RegistrationId(guid)), ct);
+    return Results.NoContent();
+});
+
 // Read a registration. Its zaak URL appears once the worker has opened the zaak (eventually).
 app.MapGet("/registrations/{id}", async (string id, IRegistrationStore store, CancellationToken ct) =>
 {

services/domain/Big.Application/ApproveRegistration.cs

@@ -0,0 +1,36 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>A behandelaar's decision to approve a registration, in domain language.</summary>
+public sealed record ApproveRegistrationCommand(RegistrationId RegistrationId);
+
+/// <summary>
+/// The approve use case (S-09b): set the registration's zaak to its final status via the ACL (§8.1),
+/// then advance the aggregate to INGESCHREVEN. Idempotent — a redelivered or repeated approval of an
+/// already-approved registration is a no-op, so the ACL is not asked to set the status twice. The
+/// zaak status is the projection's source of truth (it flows back over NRC); the aggregate transition
+/// keeps the domain's own view consistent.
+/// </summary>
+public sealed class ApproveRegistration(IRegistrationStore store, IAclClient acl)
+{
+    public async Task HandleAsync(ApproveRegistrationCommand command, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(command);
+
+        var registration = await store.GetAsync(command.RegistrationId, ct)
+            ?? throw new InvalidOperationException($"No registration {command.RegistrationId} to approve.");
+
+        // A repeated approval is a no-op: don't set the zaak status a second time.
+        if (registration.Status == RegistrationStatus.Ingeschreven)
+            return;
+
+        if (registration.ZaakUrl is null)
+            throw new InvalidOperationException(
+                $"Registration {command.RegistrationId} has no zaak yet; it cannot be approved.");
+
+        await acl.ApproveZaakAsync(registration.ZaakUrl, ct);
+        registration.Approve();
+        await store.SaveAsync(registration, ct);
+    }
+}

services/domain/Big.Application/Ports.cs

@@ -25,6 +25,12 @@ public interface IWorkflowClient
 public interface IAclClient
 {
     Task<Uri> OpenZaakAsync(string bsn, CancellationToken ct = default);
+
+    /// <summary>
+    /// Record the approval on the given zaak. The ACL translates this to the ZGW concept — setting
+    /// the zaak's final status — which OpenZaak notifies over NRC; the domain never names statustypen.
+    /// </summary>
+    Task ApproveZaakAsync(Uri zaakUrl, CancellationToken ct = default);
 }
 
 /// <summary>

services/domain/Big.Infrastructure/AclHttpClient.cs

@@ -22,7 +22,18 @@ public sealed class AclHttpClient(HttpClient http, AclOptions options) : IAclCli
         return new Uri(opened.ZaakUrl);
     }
 
+    public async Task ApproveZaakAsync(Uri zaakUrl, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(zaakUrl);
+
+        using var response = await http.PostAsJsonAsync(
+            new Uri(options.BaseUrl, "statussen"), new SetStatusRequest(zaakUrl.ToString()), ct);
+        response.EnsureSuccessStatusCode();
+    }
+
     private sealed record OpenZaakRequest([property: JsonPropertyName("bsn")] string Bsn);
 
     private sealed record OpenZaakResponse([property: JsonPropertyName("zaakUrl")] string ZaakUrl);
+
+    private sealed record SetStatusRequest([property: JsonPropertyName("zaakUrl")] string ZaakUrl);
 }

services/domain/Big.Tests/AclHttpClientTests.cs

@@ -41,4 +41,27 @@ public class AclHttpClientTests
         var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => client.OpenZaakAsync("123456782"));
         Assert.Contains("empty", ex.Message, StringComparison.OrdinalIgnoreCase);
     }
+
+    [Fact]
+    public async Task Approves_a_zaak_by_posting_its_url_to_statussen()
+    {
+        var capture = new RequestCapture();
+        var client = Client(capture.Responds(HttpStatusCode.NoContent));
+
+        await client.ApproveZaakAsync(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+
+        Assert.Equal(HttpMethod.Post, capture.Seen!.Method);
+        Assert.Equal("http://acl/statussen", capture.Seen.RequestUri!.ToString());
+        Assert.Contains("\"zaakUrl\":\"http://openzaak/zaken/api/v1/zaken/abc\"", capture.Body);
+    }
+
+    [Fact]
+    public async Task Approve_throws_when_the_acl_rejects_the_request()
+    {
+        var capture = new RequestCapture();
+        var client = Client(capture.Responds(HttpStatusCode.BadGateway));
+
+        await Assert.ThrowsAsync<HttpRequestException>(
+            () => client.ApproveZaakAsync(new Uri("http://openzaak/zaken/api/v1/zaken/abc")));
+    }
 }