diff --git a/docs/release-notes.md b/docs/release-notes.md
index aa9b488c..8c2b6a9e 100644
--- a/docs/release-notes.md
+++ b/docs/release-notes.md
@@ -4,7 +4,7 @@
 ## Upcoming version
 * For players:
   * You no longer need .NET 5 installed to run SMAPI or the installer.
-  * The installer now detects when the game folder contains an incompatible older game version.
+  * The installer now detects when the game folder contains an incompatible legacy game version.
   * Updated for the latest Stardew Valley 1.5.5 hotfix.
 
 * For SMAPI maintainers:
diff --git a/src/SMAPI.Installer/Framework/InstallerContext.cs b/src/SMAPI.Installer/Framework/InstallerContext.cs
index 68df2001..bb973230 100644
--- a/src/SMAPI.Installer/Framework/InstallerContext.cs
+++ b/src/SMAPI.Installer/Framework/InstallerContext.cs
@@ -55,11 +55,11 @@ namespace StardewModdingAPI.Installer.Framework
             return this.GameScanner.LooksLikeGameFolder(dir);
         }
 
-        /// <summary>Get whether a folder seems to contain Stardew Valley 1.5.4 or earlier.</summary>
+        /// <summary>Get whether a folder seems to contain the game, and which version it contains if so.</summary>
         /// <param name="dir">The folder to check.</param>
-        public bool LooksLikeStardewValley154(DirectoryInfo dir)
+        public GameFolderType GetGameFolderType(DirectoryInfo dir)
         {
-            return this.GameScanner.LooksLikeStardewValley154(dir);
+            return this.GameScanner.GetGameFolderType(dir);
         }
     }
 }
diff --git a/src/SMAPI.Installer/InteractiveInstaller.cs b/src/SMAPI.Installer/InteractiveInstaller.cs
index 424fe42b..6694c257 100644
--- a/src/SMAPI.Installer/InteractiveInstaller.cs
+++ b/src/SMAPI.Installer/InteractiveInstaller.cs
@@ -10,6 +10,7 @@ using StardewModdingAPI.Installer.Framework;
 using StardewModdingAPI.Internal.ConsoleWriting;
 using StardewModdingAPI.Toolkit;
 using StardewModdingAPI.Toolkit.Framework;
+using StardewModdingAPI.Toolkit.Framework.GameScanning;
 using StardewModdingAPI.Toolkit.Framework.ModScanning;
 using StardewModdingAPI.Toolkit.Utilities;
 
@@ -633,18 +634,39 @@ namespace StardewModdingApi.Installer
             // use specified path
             if (specifiedPath != null)
             {
+                string errorPrefix = $"You specified --game-path \"{specifiedPath}\", but";
+
                 var dir = new DirectoryInfo(specifiedPath);
                 if (!dir.Exists)
                 {
-                    this.PrintError($"You specified --game-path \"{specifiedPath}\", but that folder doesn't exist.");
+                    this.PrintError($"{errorPrefix} that folder doesn't exist.");
                     return null;
                 }
-                if (!context.LooksLikeGameFolder(dir))
+
+                switch (context.GetGameFolderType(dir))
                 {
-                    this.PrintError($"You specified --game-path \"{specifiedPath}\", but that folder doesn't contain the Stardew Valley executable.");
-                    return null;
+                    case GameFolderType.Valid:
+                        return dir;
+
+                    case GameFolderType.Legacy154OrEarlier:
+                        this.PrintWarning($"{errorPrefix} that directory seems to have Stardew Valley 1.5.4 or earlier.");
+                        this.PrintWarning("Please update your game to the latest version to use SMAPI.");
+                        return null;
+
+                    case GameFolderType.LegacyCompatibilityBranch:
+                        this.PrintWarning($"{errorPrefix} that directory seems to have the Stardew Valley legacy 'compatibility' branch.");
+                        this.PrintWarning("Unfortunately SMAPI is only compatible with the full main version of the game.");
+                        this.PrintWarning("Please update your game to the main branch to use SMAPI.");
+                        return null;
+
+                    case GameFolderType.NoGameFound:
+                        this.PrintWarning($"{errorPrefix} that directory doesn't contain a Stardew Valley executable.");
+                        return null;
+
+                    default:
+                        this.PrintWarning($"{errorPrefix} that directory doesn't seem to contain a valid game install.");
+                        return null;
                 }
-                return dir;
             }
 
             // let user choose detected path
@@ -702,23 +724,32 @@ namespace StardewModdingApi.Installer
                     this.PrintWarning("That directory doesn't seem to exist.");
                     continue;
                 }
-                if (!context.LooksLikeGameFolder(directory))
+
+                switch (context.GetGameFolderType(directory))
                 {
-                    if (context.LooksLikeStardewValley154(directory))
-                    {
+                    case GameFolderType.Valid:
+                        this.PrintInfo("   OK!");
+                        return directory;
+
+                    case GameFolderType.Legacy154OrEarlier:
                         this.PrintWarning("That directory seems to have Stardew Valley 1.5.4 or earlier.");
                         this.PrintWarning("Please update your game to the latest version to use SMAPI.");
-                    }
-                    else
-                    {
-                        this.PrintWarning("That directory doesn't contain a Stardew Valley executable.");
-                    }
-                    continue;
-                }
+                        continue;
 
-                // looks OK
-                this.PrintInfo("   OK!");
-                return directory;
+                    case GameFolderType.LegacyCompatibilityBranch:
+                        this.PrintWarning("That directory seems to have the Stardew Valley legacy 'compatibility' branch.");
+                        this.PrintWarning("Unfortunately SMAPI is only compatible with the full main version of the game.");
+                        this.PrintWarning("Please update your game to the main branch to use SMAPI.");
+                        continue;
+
+                    case GameFolderType.NoGameFound:
+                        this.PrintWarning("That directory doesn't contain a Stardew Valley executable.");
+                        continue;
+
+                    default:
+                        this.PrintWarning("That directory doesn't seem to contain a valid game install.");
+                        continue;
+                }
             }
         }
 
diff --git a/src/SMAPI.Toolkit/Framework/GameScanning/GameFolderType.cs b/src/SMAPI.Toolkit/Framework/GameScanning/GameFolderType.cs
new file mode 100644
index 00000000..d18af59b
--- /dev/null
+++ b/src/SMAPI.Toolkit/Framework/GameScanning/GameFolderType.cs
@@ -0,0 +1,21 @@
+namespace StardewModdingAPI.Toolkit.Framework.GameScanning
+{
+    /// <summary>The detected validity for a Stardew Valley game folder based on file structure heuristics.</summary>
+    public enum GameFolderType
+    {
+        /// <summary>The folder seems to contain a valid Stardew Valley 1.5.5+ install.</summary>
+        Valid,
+
+        /// <summary>The folder doesn't contain Stardew Valley.</summary>
+        NoGameFound,
+
+        /// <summary>The folder contains Stardew Valley 1.5.4 or earlier. This version uses XNA Framework and 32-bit .NET Framework 4.5.2 on Windows and Mono on Linux/macOS, and isn't compatible with current versions of SMAPI.</summary>
+        Legacy154OrEarlier,
+
+        /// <summary>The folder contains Stardew Valley from the game's legacy compatibility branch, which backports newer changes to the <see cref="Legacy154OrEarlier"/> format.</summary>
+        LegacyCompatibilityBranch,
+
+        /// <summary>The folder seems to contain Stardew Valley files, but they failed to load for unknown reasons (e.g. corrupted executable).</summary>
+        InvalidUnknown
+    }
+}
diff --git a/src/SMAPI.Toolkit/Framework/GameScanning/GameScanner.cs b/src/SMAPI.Toolkit/Framework/GameScanning/GameScanner.cs
index c7ebe6e0..37e4f263 100644
--- a/src/SMAPI.Toolkit/Framework/GameScanning/GameScanner.cs
+++ b/src/SMAPI.Toolkit/Framework/GameScanning/GameScanner.cs
@@ -55,36 +55,58 @@ namespace StardewModdingAPI.Toolkit.Framework.GameScanning
         /// <param name="dir">The folder to check.</param>
         public bool LooksLikeGameFolder(DirectoryInfo dir)
         {
-            return
-                dir.Exists
-                && dir.EnumerateFiles("Stardew Valley.dll").Any();
+            return this.GetGameFolderType(dir) == GameFolderType.Valid;
         }
 
-        /// <summary>Get whether a folder seems to contain Stardew Valley 1.5.4 or earlier.</summary>
+        /// <summary>Detect the validity of a game folder based on file structure heuristics.</summary>
         /// <param name="dir">The folder to check.</param>
-        public bool LooksLikeStardewValley154(DirectoryInfo dir)
+        public GameFolderType GetGameFolderType(DirectoryInfo dir)
         {
-            if (!dir.Exists || this.LooksLikeGameFolder(dir))
-                return false;
+            // no such folder
+            if (!dir.Exists)
+                return GameFolderType.NoGameFound;
 
-            // get legacy executable
-            FileInfo executable = new FileInfo(Path.Combine(dir.FullName, "Stardew Valley.exe"));
-            if (!executable.Exists)
-                executable = new FileInfo(Path.Combine(dir.FullName, "StardewValley.exe"));
-            if (!executable.Exists)
-                return false;
+            // apparently valid
+            if (dir.EnumerateFiles("Stardew Valley.dll").Any())
+                return GameFolderType.Valid;
 
-            // check if it's a standard .NET assembly
-            // This will fail in Stardew Valley 1.5.5+, where it's a binary wrapper around Stardew Valley.dll.
+            // doesn't contain any version of Stardew Valley
+            FileInfo executable = new(Path.Combine(dir.FullName, "Stardew Valley.exe"));
+            if (!executable.Exists)
+                executable = new(Path.Combine(dir.FullName, "StardewValley.exe")); // pre-1.5.5 Linux/macOS executable
+            if (!executable.Exists)
+                return GameFolderType.NoGameFound;
+
+            // get assembly version
+            Version version;
             try
             {
-                Version version = AssemblyName.GetAssemblyName(executable.FullName).Version;
-                return true;
+                version = AssemblyName.GetAssemblyName(executable.FullName).Version;
             }
             catch
             {
-                return false;
+                // The executable exists but it doesn't seem to be a valid assembly. This would
+                // happen with Stardew Valley 1.5.5+, but that should have been flagged as a valid
+                // folder before this point.
+                return GameFolderType.InvalidUnknown;
             }
+
+            // ignore Stardew Valley 1.5.5+ at this point
+            if (version.Major == 1 && version.Minor == 3 && version.Build == 37)
+                return GameFolderType.InvalidUnknown;
+
+            // incompatible version
+            if (version.Major == 1 && version.Minor < 4)
+            {
+                // Stardew Valley 1.5.4 and earlier have assembly versions <= 1.3.7853.31734
+                if (version.Minor < 3 || version.Build <= 7853)
+                    return GameFolderType.Legacy154OrEarlier;
+
+                // Stardew Valley 1.5.5+ legacy compatibility branch
+                return GameFolderType.LegacyCompatibilityBranch;
+            }
+
+            return GameFolderType.InvalidUnknown;
         }
 
         /*********