diff --git a/Assets/Tests/Editor/ThirdPartyToolMigrationFileServiceTests.cs b/Assets/Tests/Editor/ThirdPartyToolMigrationFileServiceTests.cs
index 49fd5afb34..b04cecea34 100644
--- a/Assets/Tests/Editor/ThirdPartyToolMigrationFileServiceTests.cs
+++ b/Assets/Tests/Editor/ThirdPartyToolMigrationFileServiceTests.cs
@@ -37,6 +37,13 @@ public void ThirdPartyToolMigrationPreview_WhenInputFilePathsMutate_KeepsSnapsho
             Assert.That(preview.FilePaths, Is.EqualTo(new[] { "Assets/VendorTools/HelloTool.cs" }));
         }
 
+        [Test]
+        public void ThirdPartyToolMigrationPreview_WhenFilePathsIsNull_Throws()
+        {
+            // Verifies that preview snapshots fail fast when file paths are missing.
+            Assert.Throws<ArgumentNullException>(() => new ThirdPartyToolMigrationPreview(0, 0, null));
+        }
+
         [Test]
         public void ThirdPartyToolMigrationResult_WhenInputFilePathsMutate_KeepsSnapshot()
         {
@@ -53,6 +60,13 @@ public void ThirdPartyToolMigrationResult_WhenInputFilePathsMutate_KeepsSnapshot
             Assert.That(result.FilePaths, Is.EqualTo(new[] { "Assets/VendorTools/HelloTool.cs" }));
         }
 
+        [Test]
+        public void ThirdPartyToolMigrationResult_WhenFilePathsIsNull_Throws()
+        {
+            // Verifies that result snapshots fail fast when file paths are missing.
+            Assert.Throws<ArgumentNullException>(() => new ThirdPartyToolMigrationResult(0, 0, null));
+        }
+
         [Test]
         public void ThirdPartyToolMigrationSourceFileCache_WhenFileIsReadTwice_UsesCachedSource()
         {
diff --git a/Assets/Tests/Editor/ThirdPartyToolMigrationRulesTests.cs b/Assets/Tests/Editor/ThirdPartyToolMigrationRulesTests.cs
index dece29f92d..b36d2226e6 100644
--- a/Assets/Tests/Editor/ThirdPartyToolMigrationRulesTests.cs
+++ b/Assets/Tests/Editor/ThirdPartyToolMigrationRulesTests.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Linq;
 
 using NUnit.Framework;
@@ -12,6 +13,48 @@ namespace io.github.hatayama.UnityCliLoop.Tests.Editor
     /// </summary>
     public sealed class ThirdPartyToolMigrationRulesTests
     {
+        [Test]
+        public void RemovedLegacyPlayerLoopTimingSignature_WhenOriginalParametersIsNull_Throws()
+        {
+            // Verifies that timing signatures fail fast when original parameter snapshots are missing.
+            Assert.Throws<ArgumentNullException>(() => new RemovedLegacyPlayerLoopTimingSignature(
+                "Run",
+                "SampleTool",
+                null,
+                Array.Empty<RemovedLegacyPlayerLoopTimingParameter>()));
+        }
+
+        [Test]
+        public void RemovedLegacyPlayerLoopTimingSignature_WhenRemovedParametersIsNull_Throws()
+        {
+            // Verifies that timing signatures fail fast when removed parameter snapshots are missing.
+            Assert.Throws<ArgumentNullException>(() => new RemovedLegacyPlayerLoopTimingSignature(
+                "Run",
+                "SampleTool",
+                Array.Empty<LegacyPlayerLoopTimingParameterDeclaration>(),
+                null));
+        }
+
+        [Test]
+        public void ThirdPartyToolMigrationContentResult_WhenContentIsNull_Throws()
+        {
+            // Verifies that migration content results fail fast when rewritten content is missing.
+            Assert.Throws<ArgumentNullException>(() => new ThirdPartyToolMigrationContentResult(
+                null,
+                0,
+                Array.Empty<RemovedLegacyPlayerLoopTimingSignature>()));
+        }
+
+        [Test]
+        public void ThirdPartyToolMigrationContentResult_WhenRemovedSignaturesIsNull_Throws()
+        {
+            // Verifies that migration content results fail fast when removed timing signatures are missing.
+            Assert.Throws<ArgumentNullException>(() => new ThirdPartyToolMigrationContentResult(
+                string.Empty,
+                0,
+                null));
+        }
+
         [Test]
         public void MigrateCSharpSource_WhenLegacyToolApiIsUsed_RewritesToV3Contracts()
         {
diff --git a/Packages/src/Editor/Domain/ThirdPartyToolMigrationData.cs b/Packages/src/Editor/Domain/ThirdPartyToolMigrationData.cs
index 6b0ec7f355..032ad6a2c4 100644
--- a/Packages/src/Editor/Domain/ThirdPartyToolMigrationData.cs
+++ b/Packages/src/Editor/Domain/ThirdPartyToolMigrationData.cs
@@ -96,7 +96,12 @@ internal static class ThirdPartyToolMigrationFilePathSnapshot
     {
         internal static string[] Copy(string[] filePaths)
         {
-            if (filePaths == null || filePaths.Length == 0)
+            if (filePaths == null)
+            {
+                throw new ArgumentNullException(nameof(filePaths));
+            }
+
+            if (filePaths.Length == 0)
             {
                 return Array.Empty<string>();
             }
diff --git a/Packages/src/Editor/Domain/ThirdPartyToolMigrationRuleData.cs b/Packages/src/Editor/Domain/ThirdPartyToolMigrationRuleData.cs
index b0352877e6..7a409a6910 100644
--- a/Packages/src/Editor/Domain/ThirdPartyToolMigrationRuleData.cs
+++ b/Packages/src/Editor/Domain/ThirdPartyToolMigrationRuleData.cs
@@ -22,8 +22,9 @@ public RemovedLegacyPlayerLoopTimingSignature(
             MethodName = methodName;
             DeclaringTypeName = declaringTypeName;
             OriginalParameters =
-                originalParameters ?? Array.Empty<LegacyPlayerLoopTimingParameterDeclaration>();
-            RemovedParameters = removedParameters ?? Array.Empty<RemovedLegacyPlayerLoopTimingParameter>();
+                originalParameters ?? throw new ArgumentNullException(nameof(originalParameters));
+            RemovedParameters =
+                removedParameters ?? throw new ArgumentNullException(nameof(removedParameters));
         }
 
         public string MethodName { get; }
@@ -93,11 +94,11 @@ public ThirdPartyToolMigrationContentResult(
                 removedPlayerLoopTimingSignatures != null,
                 "removedPlayerLoopTimingSignatures must not be null");
 
-            Content = content ?? string.Empty;
+            Content = content ?? throw new ArgumentNullException(nameof(content));
             ReplacementCount = replacementCount;
             RemovedPlayerLoopTimingSignatures =
                 removedPlayerLoopTimingSignatures ??
-                Array.Empty<RemovedLegacyPlayerLoopTimingSignature>();
+                throw new ArgumentNullException(nameof(removedPlayerLoopTimingSignatures));
         }
 
         public string Content { get; }