diff --git a/SVSim.EmulatedEntrypoint/Controllers/ImmutableDataController.cs b/SVSim.EmulatedEntrypoint/Controllers/ImmutableDataController.cs
index 7fd34477..711a587f 100644
--- a/SVSim.EmulatedEntrypoint/Controllers/ImmutableDataController.cs
+++ b/SVSim.EmulatedEntrypoint/Controllers/ImmutableDataController.cs
@@ -29,7 +29,8 @@ public class ImmutableDataController : SVSimController
     {
         if (!_provider.IsAvailable)
         {
-            return StatusCode(503, "card-master payload not available");
+            // 500 not 503: blob missing is operator error (config / build), not transient — no retry will help.
+            return StatusCode(500, "card-master payload not available");
         }
         return Ok(new CardMasterResponse { CardMaster = _provider.Base64Blob });
     }
diff --git a/SVSim.UnitTests/SVSim.UnitTests.csproj b/SVSim.UnitTests/SVSim.UnitTests.csproj
index 40a39464..4ac8a48a 100644
--- a/SVSim.UnitTests/SVSim.UnitTests.csproj
+++ b/SVSim.UnitTests/SVSim.UnitTests.csproj
@@ -65,6 +65,11 @@
         <Link>Data\build-deck\build_deck_series_master.csv</Link>
         <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       </Content>
+      <!-- CardMasterPayloadProvider reads from AppContext.BaseDirectory + Data/. Content does
+           NOT flow across a ProjectReference, so link the blob explicitly here as a defensive
+           mirror of the host csproj's own link (today SDK behavior happens to copy it, but a
+           future cleanup could break that implicit flow and tests would 500 silently). -->
+      <None Include="..\SVSim.EmulatedEntrypoint\Data\card_master_2026-06-03.txt" Link="Data\card_master_2026-06-03.txt" CopyToOutputDirectory="PreserveNewest" />
     </ItemGroup>
 
 </Project>