diff --git a/CHANGELOG.md b/CHANGELOG.md
index ab7b1435..5d52d712 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,12 @@
 # Changelog
 
+## v0.5.1 - 2026-06-05
+
+Hotfix for Windows IL2CPP builds in the Unity package.
+
+### Magic.Unity
+- IL2CPP workaround generation selects signature assemblies from the player compilation reference set (`CompilationPipeline`) instead of a Unity name-prefix check, so editor-only desktop assemblies (`Mono.WebBrowser`, reached through a `csc.rsp` `System.Web` reference on Windows) no longer leak into emitted signatures and break the build. Keep and skip decisions are logged, and a degenerate reference set fails the build instead of silently emitting no workarounds - [#23](https://github.com/flybot-sg/magic/issues/23).
+
 ## v0.5.0 - 2026-06-04
 
 Consumer quality-of-life fixes from the 0.4.0 rollout.
diff --git a/magic-unity/Editor/GenerateGenericWorkaroundMethods.cs b/magic-unity/Editor/GenerateGenericWorkaroundMethods.cs
index 3bc258d1..6fae0abf 100644
--- a/magic-unity/Editor/GenerateGenericWorkaroundMethods.cs
+++ b/magic-unity/Editor/GenerateGenericWorkaroundMethods.cs
@@ -5,6 +5,7 @@
 using Mono.Cecil;
 using Mono.Cecil.Rocks;
 using Mono.Cecil.Cil;
+using UnityEditor.Compilation;
 
 namespace Magic.Unity
 {
@@ -21,10 +22,35 @@ struct DynamicCallSiteInfo
         static List<MethodDefinition> AllMethods = new List<MethodDefinition>();
         static TypeDefinition MagicRuntimeDelegateHelpers = null;
 
-        static bool ShouldCollectReferencedAssembly(AssemblyDefinition assy)
+        static HashSet<string> PlayerReferenceNames = null;
+
+        // The collected assemblies feed AllMethods, the pool of candidate dynamic
+        // dispatch targets whose GetMethodDelegateFast instantiations get emitted
+        // into the shipped .clj.dlls. A signature referencing an assembly absent
+        // from the player build breaks the IL2CPP build; the type-reference
+        // closure resolves against the editor's full desktop BCL, which on
+        // Windows reaches editor-only assemblies like Mono.WebBrowser. So collect
+        // an assembly only if player scripts compile against it. Desktop-only BCL
+        // assemblies are never player compilation references, while UnityEngine
+        // modules, the player BCL profile, plugins (including .clj.dlls), and
+        // user script assemblies all are.
+        static HashSet<string> CollectPlayerReferenceNames()
         {
-            return !assy.FullName.StartsWith("Unity") || assy.FullName.StartsWith("UnityEngine");
+            var names = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+            foreach (var assembly in CompilationPipeline.GetAssemblies(AssembliesType.PlayerWithoutTestAssemblies))
+            {
+                names.Add(assembly.name);
+                foreach (var reference in assembly.allReferences)
+                {
+                    names.Add(System.IO.Path.GetFileNameWithoutExtension(reference));
+                }
+            }
+            return names;
+        }
 
+        static bool ShouldCollectReferencedAssembly(AssemblyDefinition assy)
+        {
+            return PlayerReferenceNames.Contains(assy.Name.Name);
         }
 
         static HashSet<AssemblyDefinition> CollectAllReferencedAssemblies(AssemblyDefinition assydef, HashSet<AssemblyDefinition> seen = null)
@@ -51,7 +77,7 @@ static HashSet<AssemblyDefinition> CollectAllReferencedAssemblies(AssemblyDefini
                         }
                         else
                         {
-                            UnityEngine.Debug.Log($"[CollectAllReferencedAssemblies] Skip {resolved.Module.Assembly}");
+                            UnityEngine.Debug.Log($"[CollectAllReferencedAssemblies] Skip {resolved.Module.Assembly} (not a player compilation reference)");
                         }
                     }
                 }
@@ -66,6 +92,18 @@ static HashSet<AssemblyDefinition> CollectAllReferencedAssemblies(AssemblyDefini
 
         public static void Init()
         {
+            PlayerReferenceNames = CollectPlayerReferenceNames();
+            // A degenerate reference set would silently turn workaround
+            // generation into a no-op: the build stays green and devices throw
+            // ExecutionEngineException at runtime. Fail the build instead. Any
+            // sane player reference set contains a core library and at least
+            // one UnityEngine module.
+            if (!(PlayerReferenceNames.Contains("mscorlib") || PlayerReferenceNames.Contains("netstandard"))
+                || !PlayerReferenceNames.Any(n => n.StartsWith("UnityEngine")))
+            {
+                throw new InvalidOperationException($"[Magic.Unity] player compilation reference set looks degenerate ({PlayerReferenceNames.Count} entries), refusing to generate IL2CPP workarounds from it");
+            }
+            UnityEngine.Debug.Log($"[CollectAllReferencedAssemblies] player compilation references: {string.Join(",", PlayerReferenceNames.OrderBy(n => n))}");
             var assemblyCSharp = AssemblyDefinition.ReadAssembly("Library/ScriptAssemblies/Assembly-CSharp.dll");
             var referencedAssemblies = CollectAllReferencedAssemblies(assemblyCSharp);
             UnityEngine.Debug.Log($"[CollectAllReferencedAssemblies] {string.Join(",", referencedAssemblies)}");
diff --git a/magic-unity/package.json b/magic-unity/package.json
index 25423779..15682bb6 100644
--- a/magic-unity/package.json
+++ b/magic-unity/package.json
@@ -1,6 +1,6 @@
 {
     "name": "sg.flybot.magic.unity",
-    "version": "0.5.0",
+    "version": "0.5.1",
     "displayName": "MAGIC Unity Integration",
     "description": "The integration of the MAGIC Clojure compiler into Unity",
     "unity": "2021.2",
diff --git a/version.edn b/version.edn
index 879b88c2..03443473 100644
--- a/version.edn
+++ b/version.edn
@@ -1 +1 @@
-{:version "0.5.0"}
+{:version "0.5.1"}