diff --git a/SVSim.EmulatedEntrypoint/Controllers/MissionController.cs b/SVSim.EmulatedEntrypoint/Controllers/MissionController.cs
index 0d729f90..ef5cb212 100644
--- a/SVSim.EmulatedEntrypoint/Controllers/MissionController.cs
+++ b/SVSim.EmulatedEntrypoint/Controllers/MissionController.cs
@@ -116,6 +116,23 @@ public class MissionController : SVSimController
         return Ok(dto);
     }
 
+    [HttpPost("buy_additional_right")]
+    public async Task<IActionResult> BuyAdditionalRight(BaseRequest _, CancellationToken ct)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+
+        // The client-side task class is absent from the decompilation; the URL is
+        // registered but no Parse() is documented. Spec marks the shape as INFERRED.
+        // Safe stub: refresh MissionInfoDetail and let the client re-render — same
+        // shape as /mission/info. No currency debit until we see a real wire call.
+        var viewer = await LoadViewer(viewerId, ct);
+        await _state.EnsureCurrentAsync(viewer.Id, ct);
+        await _db.SaveChangesAsync(ct);
+
+        var dto = await _assembler.BuildAsync(viewer, ct);
+        return Ok(dto);
+    }
+
     private Task<Viewer> LoadViewer(long viewerId, CancellationToken ct) =>
         _db.Viewers
             .Include(v => v.MissionData)
diff --git a/SVSim.UnitTests/Controllers/MissionControllerBuyAdditionalRightTests.cs b/SVSim.UnitTests/Controllers/MissionControllerBuyAdditionalRightTests.cs
new file mode 100644
index 00000000..7d2d2b90
--- /dev/null
+++ b/SVSim.UnitTests/Controllers/MissionControllerBuyAdditionalRightTests.cs
@@ -0,0 +1,25 @@
+using System.Net;
+using System.Text;
+using NUnit.Framework;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Controllers;
+
+public class MissionControllerBuyAdditionalRightTests
+{
+    [Test]
+    public async Task BuyAdditionalRight_returns_ok()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync(tutorialState: 0);
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        // Body shape is INFERRED — we send the bare base envelope and expect the server
+        // to refresh MissionInfoDetail.
+        var requestJson = """{"viewer_id":"0","steam_id":0,"steam_session_ticket":""}""";
+        var response = await client.PostAsync("/mission/buy_additional_right",
+            new StringContent(requestJson, Encoding.UTF8, "application/json"));
+
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+    }
+}