diff --git a/src/Orbit.Application/Accountability/Queries/GetAccountabilityPairsQuery.cs b/src/Orbit.Application/Accountability/Queries/GetAccountabilityPairsQuery.cs
index 10da3de3..57239cbb 100644
--- a/src/Orbit.Application/Accountability/Queries/GetAccountabilityPairsQuery.cs
+++ b/src/Orbit.Application/Accountability/Queries/GetAccountabilityPairsQuery.cs
@@ -99,8 +99,8 @@ private static AccountabilityPairDto BuildDto(
         Guid userId,
         Guid buddyId,
         User buddy,
-        IReadOnlyDictionary<Guid, List<AccountabilityPairHabit>> habitsByPair,
-        IReadOnlyDictionary<(Guid PairId, Guid UserId), DateOnly> lastCheckInByPairUser)
+        Dictionary<Guid, List<AccountabilityPairHabit>> habitsByPair,
+        Dictionary<(Guid PairId, Guid UserId), DateOnly> lastCheckInByPairUser)
     {
         var linkedHabits = habitsByPair.TryGetValue(pair.Id, out var habits)
             ? habits
diff --git a/src/Orbit.Application/Challenges/Commands/CreateChallengeCommand.cs b/src/Orbit.Application/Challenges/Commands/CreateChallengeCommand.cs
index bbba327f..a6130691 100644
--- a/src/Orbit.Application/Challenges/Commands/CreateChallengeCommand.cs
+++ b/src/Orbit.Application/Challenges/Commands/CreateChallengeCommand.cs
@@ -86,7 +86,7 @@ public async Task<Result<Guid>> Handle(CreateChallengeCommand request, Cancellat
         return Result.Success(challenge.Id);
     }
 
-    private async Task NotifyInvitedFriendsAsync(User requester, IReadOnlyList<Guid> invitedFriendIds, CancellationToken cancellationToken)
+    private async Task NotifyInvitedFriendsAsync(User requester, List<Guid> invitedFriendIds, CancellationToken cancellationToken)
     {
         if (invitedFriendIds.Count == 0)
             return;
diff --git a/src/Orbit.Application/Chat/Commands/ProcessUserChatCommand.Accumulator.cs b/src/Orbit.Application/Chat/Commands/ProcessUserChatCommand.Accumulator.cs
index 69a0fd8b..6e3ac80c 100644
--- a/src/Orbit.Application/Chat/Commands/ProcessUserChatCommand.Accumulator.cs
+++ b/src/Orbit.Application/Chat/Commands/ProcessUserChatCommand.Accumulator.cs
@@ -73,7 +73,7 @@ private void CollectRelatedSurfaces(AgentOperationResult operationResult)
     /// (e.g. describe_feature) by round-tripping it through JSON. Returns an empty sequence
     /// when the payload is null, not an object, or carries no usable surface IDs.
     /// </summary>
-    private static IEnumerable<string> ExtractRelatedSurfaces(object? payload)
+    private static List<string> ExtractRelatedSurfaces(object? payload)
     {
         if (payload is null)
             return [];
diff --git a/src/Orbit.Application/Chat/Commands/ProcessUserChatCommand.Tools.cs b/src/Orbit.Application/Chat/Commands/ProcessUserChatCommand.Tools.cs
index 0d70dff6..f9a0798c 100644
--- a/src/Orbit.Application/Chat/Commands/ProcessUserChatCommand.Tools.cs
+++ b/src/Orbit.Application/Chat/Commands/ProcessUserChatCommand.Tools.cs
@@ -88,7 +88,7 @@ public partial class ProcessUserChatCommandHandler
     /// reassemble results deterministically, independent of task-completion timing.
     /// </summary>
     private async Task<IReadOnlyDictionary<string, ToolCallOutcome>> ExecuteToolCallsAsync(
-        IReadOnlyList<AiToolCall> orderedCalls,
+        List<AiToolCall> orderedCalls,
         ProcessUserChatCommand request,
         CancellationToken cancellationToken)
     {
diff --git a/src/Orbit.Application/Chat/FeatureExplanations/FeatureExplanationService.cs b/src/Orbit.Application/Chat/FeatureExplanations/FeatureExplanationService.cs
index 5378bef3..afd55e9c 100644
--- a/src/Orbit.Application/Chat/FeatureExplanations/FeatureExplanationService.cs
+++ b/src/Orbit.Application/Chat/FeatureExplanations/FeatureExplanationService.cs
@@ -111,7 +111,7 @@ private static FeatureExplanation Parse(string content)
         return new FeatureExplanation(key, displayName, relatedCapabilities, relatedSurfaces, version, body);
     }
 
-    private static IReadOnlyList<string> ParseInlineList(string value)
+    private static List<string> ParseInlineList(string value)
     {
         var trimmed = value.Trim();
         if (trimmed.Length < 2 || trimmed[0] != '[' || trimmed[^1] != ']')
diff --git a/src/Orbit.Application/Goals/Commands/RestoreGoalCommand.cs b/src/Orbit.Application/Goals/Commands/RestoreGoalCommand.cs
index 9db5e337..0dbdb0b4 100644
--- a/src/Orbit.Application/Goals/Commands/RestoreGoalCommand.cs
+++ b/src/Orbit.Application/Goals/Commands/RestoreGoalCommand.cs
@@ -29,7 +29,7 @@ public async Task<Result> Handle(RestoreGoalCommand request, CancellationToken c
             g => g.Id == request.GoalId && g.UserId == request.UserId,
             cancellationToken);
 
-        var goal = goals.FirstOrDefault();
+        var goal = goals.Count > 0 ? goals[0] : null;
         if (goal is null || !goal.IsDeleted)
             return Result.Failure(ErrorMessages.GoalNotFound);
 
diff --git a/src/Orbit.Application/Goals/Queries/GetGoalByIdQuery.cs b/src/Orbit.Application/Goals/Queries/GetGoalByIdQuery.cs
index 6660062f..60a9d31c 100644
--- a/src/Orbit.Application/Goals/Queries/GetGoalByIdQuery.cs
+++ b/src/Orbit.Application/Goals/Queries/GetGoalByIdQuery.cs
@@ -55,7 +55,7 @@ public async Task<Result<GoalDetailDto>> Handle(GetGoalByIdQuery request, Cancel
             q => q.Include(g => g.ProgressLogs)
                   .Include(g => g.Habits).ThenInclude(h => h.Logs.Where(l => l.Date >= streakWindowStart)),
             cancellationToken);
-        var goal = goals.FirstOrDefault();
+        var goal = goals.Count > 0 ? goals[0] : null;
 
         if (goal is null)
             return Result.Failure<GoalDetailDto>(ErrorMessages.GoalNotFound);
diff --git a/src/Orbit.Application/Goals/Queries/GetGoalDetailQuery.cs b/src/Orbit.Application/Goals/Queries/GetGoalDetailQuery.cs
index b8f71b0b..8d9913f1 100644
--- a/src/Orbit.Application/Goals/Queries/GetGoalDetailQuery.cs
+++ b/src/Orbit.Application/Goals/Queries/GetGoalDetailQuery.cs
@@ -36,7 +36,7 @@ public async Task<Result<GoalDetailWithMetricsResponse>> Handle(GetGoalDetailQue
             q => q.Include(g => g.ProgressLogs)
                   .Include(g => g.Habits).ThenInclude(h => h.Logs.Where(l => l.Date >= streakWindowStart)),
             cancellationToken);
-        var goal = goals.FirstOrDefault();
+        var goal = goals.Count > 0 ? goals[0] : null;
 
         if (goal is null)
             return Result.Failure<GoalDetailWithMetricsResponse>(ErrorMessages.GoalNotFound);
diff --git a/src/Orbit.Application/Goals/Queries/GetGoalMetricsQuery.cs b/src/Orbit.Application/Goals/Queries/GetGoalMetricsQuery.cs
index ed3463b7..c7ef7c75 100644
--- a/src/Orbit.Application/Goals/Queries/GetGoalMetricsQuery.cs
+++ b/src/Orbit.Application/Goals/Queries/GetGoalMetricsQuery.cs
@@ -30,7 +30,7 @@ public async Task<Result<GoalMetrics>> Handle(GetGoalMetricsQuery request, Cance
             q => q.Include(g => g.ProgressLogs)
                   .Include(g => g.Habits).ThenInclude(h => h.Logs.Where(l => l.Date >= streakWindowStart)),
             cancellationToken);
-        var goal = goals.FirstOrDefault();
+        var goal = goals.Count > 0 ? goals[0] : null;
 
         if (goal is null)
             return Result.Failure<GoalMetrics>(ErrorMessages.GoalNotFound);
diff --git a/src/Orbit.Application/Habits/Commands/MoveHabitParentCommand.cs b/src/Orbit.Application/Habits/Commands/MoveHabitParentCommand.cs
index 6950fb10..6be782ea 100644
--- a/src/Orbit.Application/Habits/Commands/MoveHabitParentCommand.cs
+++ b/src/Orbit.Application/Habits/Commands/MoveHabitParentCommand.cs
@@ -61,7 +61,7 @@ public async Task<Result> Handle(MoveHabitParentCommand request, CancellationTok
         return Result.Success();
     }
 
-    private static bool WouldCreateCycle(Guid habitId, Guid targetParentId, IReadOnlyDictionary<Guid, Habit> habitsById)
+    private static bool WouldCreateCycle(Guid habitId, Guid targetParentId, Dictionary<Guid, Habit> habitsById)
     {
         var currentId = targetParentId;
         while (habitsById.TryGetValue(currentId, out var current))
@@ -73,7 +73,7 @@ private static bool WouldCreateCycle(Guid habitId, Guid targetParentId, IReadOnl
         return false;
     }
 
-    private static int GetDepth(Guid habitId, IReadOnlyDictionary<Guid, Habit> habitsById)
+    private static int GetDepth(Guid habitId, Dictionary<Guid, Habit> habitsById)
     {
         var depth = 0;
         var currentId = habitsById.TryGetValue(habitId, out var habit) ? habit.ParentHabitId : null;
diff --git a/src/Orbit.Application/Habits/Queries/GetHabitWidgetQuery.cs b/src/Orbit.Application/Habits/Queries/GetHabitWidgetQuery.cs
index 025465ce..ee3e0913 100644
--- a/src/Orbit.Application/Habits/Queries/GetHabitWidgetQuery.cs
+++ b/src/Orbit.Application/Habits/Queries/GetHabitWidgetQuery.cs
@@ -90,7 +90,7 @@ private async Task<IReadOnlyList<Habit>> LoadWidgetHabits(
             cancellationToken);
     }
 
-    private static bool ShouldShowTomorrow(IReadOnlyList<HabitWidgetItem> todayItems)
+    private static bool ShouldShowTomorrow(List<HabitWidgetItem> todayItems)
     {
         return todayItems.Count == 0 || todayItems.All(item => item.IsCompleted);
     }
diff --git a/src/Orbit.Application/Habits/Queries/GetRescheduleSuggestionQuery.cs b/src/Orbit.Application/Habits/Queries/GetRescheduleSuggestionQuery.cs
index 3fd82349..cd260d3b 100644
--- a/src/Orbit.Application/Habits/Queries/GetRescheduleSuggestionQuery.cs
+++ b/src/Orbit.Application/Habits/Queries/GetRescheduleSuggestionQuery.cs
@@ -49,7 +49,7 @@ public async Task<Result<RescheduleSuggestionResponse>> Handle(
             q => q.Include(h => h.Logs.Where(l => l.Date >= logWindowStart && l.Date <= userToday)),
             cancellationToken);
 
-        var habit = habits.FirstOrDefault();
+        var habit = habits.Count > 0 ? habits[0] : null;
         if (habit is null)
             return Result.Failure<RescheduleSuggestionResponse>(ErrorMessages.HabitNotFound);
 
diff --git a/src/Orbit.Application/Habits/Services/RetrospectiveMetricsCalculator.cs b/src/Orbit.Application/Habits/Services/RetrospectiveMetricsCalculator.cs
index 41d56ac6..f9a7526d 100644
--- a/src/Orbit.Application/Habits/Services/RetrospectiveMetricsCalculator.cs
+++ b/src/Orbit.Application/Habits/Services/RetrospectiveMetricsCalculator.cs
@@ -121,7 +121,7 @@ private static RetrospectiveHabitStat BuildHabitStat(Habit habit, int scheduledC
             scheduledCount,
             habit.FrequencyUnit is null);
 
-    private static IReadOnlyList<int> BuildWeeklyConsistency(int[] weekdayScheduled, int[] weekdayCompleted)
+    private static int[] BuildWeeklyConsistency(int[] weekdayScheduled, int[] weekdayCompleted)
     {
         var consistency = new int[7];
         for (var i = 0; i < 7; i++)
diff --git a/src/Orbit.Application/Profile/Queries/ExportUserDataQuery.cs b/src/Orbit.Application/Profile/Queries/ExportUserDataQuery.cs
index 13c2cc0e..49143ba4 100644
--- a/src/Orbit.Application/Profile/Queries/ExportUserDataQuery.cs
+++ b/src/Orbit.Application/Profile/Queries/ExportUserDataQuery.cs
@@ -162,7 +162,7 @@ private static ExportedApiKey MapApiKey(ApiKey apiKey)
             apiKey.IsRevoked);
     }
 
-    private static ExportedHabit MapHabit(Habit habit, IReadOnlyDictionary<Guid, List<HabitLog>> logsByHabit)
+    private static ExportedHabit MapHabit(Habit habit, Dictionary<Guid, List<HabitLog>> logsByHabit)
     {
         var logs = logsByHabit.TryGetValue(habit.Id, out var habitLogs)
             ? habitLogs.Select(l => new ExportedHabitLog(l.Date, l.Value, l.Note, l.CreatedAtUtc)).ToList()
@@ -187,7 +187,7 @@ private static ExportedHabit MapHabit(Habit habit, IReadOnlyDictionary<Guid, Lis
 
     private static ExportedGoal MapGoal(
         Goal goal,
-        IReadOnlyDictionary<Guid, List<GoalProgressLog>> progressByGoal,
+        Dictionary<Guid, List<GoalProgressLog>> progressByGoal,
         IReadOnlyDictionary<Guid, int> freshStreakValues)
     {
         var progress = progressByGoal.TryGetValue(goal.Id, out var goalLogs)
diff --git a/src/Orbit.Application/Profile/Queries/GetPublicProfileQuery.cs b/src/Orbit.Application/Profile/Queries/GetPublicProfileQuery.cs
index eeb44f3b..ea8bab1b 100644
--- a/src/Orbit.Application/Profile/Queries/GetPublicProfileQuery.cs
+++ b/src/Orbit.Application/Profile/Queries/GetPublicProfileQuery.cs
@@ -39,7 +39,7 @@ public async Task<Result<PublicProfileView>> Handle(GetPublicProfileQuery reques
             return Result.Failure<PublicProfileView>(ErrorMessages.UserNotFound);
 
         var matches = await userRepository.FindAsync(u => u.PublicProfileSlug == request.Slug, cancellationToken);
-        var user = matches.FirstOrDefault();
+        var user = matches.Count > 0 ? matches[0] : null;
         if (user is null)
             return Result.Failure<PublicProfileView>(ErrorMessages.UserNotFound);
 
diff --git a/src/Orbit.Application/Social/Queries/GetFriendProfileQuery.cs b/src/Orbit.Application/Social/Queries/GetFriendProfileQuery.cs
index 9247c61c..a3795892 100644
--- a/src/Orbit.Application/Social/Queries/GetFriendProfileQuery.cs
+++ b/src/Orbit.Application/Social/Queries/GetFriendProfileQuery.cs
@@ -60,7 +60,7 @@ public async Task<Result<FriendProfileView>> Handle(GetFriendProfileQuery reques
             return Result.Failure<FriendProfileView>(ErrorMessages.UserNotFound);
 
         var matches = await userRepository.FindAsync(u => u.Id == request.FriendUserId, cancellationToken);
-        var friend = matches.FirstOrDefault();
+        var friend = matches.Count > 0 ? matches[0] : null;
         if (friend is null)
             return Result.Failure<FriendProfileView>(ErrorMessages.UserNotFound);
 
@@ -101,7 +101,7 @@ public async Task<Result<FriendProfileView>> Handle(GetFriendProfileQuery reques
             sharedChallenges));
     }
 
-    private static IReadOnlyList<int> BuildWeeklyActivity(IEnumerable<Habit> habits, DateOnly today)
+    private static int[] BuildWeeklyActivity(IEnumerable<Habit> habits, DateOnly today)
     {
         var windowStart = today.AddDays(-(ActivityWindowDays - 1));
         var counts = new int[ActivityWindowDays];
diff --git a/src/Orbit.Application/Social/Services/FriendGraphService.cs b/src/Orbit.Application/Social/Services/FriendGraphService.cs
index c45aa5e9..6bd3340a 100644
--- a/src/Orbit.Application/Social/Services/FriendGraphService.cs
+++ b/src/Orbit.Application/Social/Services/FriendGraphService.cs
@@ -23,7 +23,7 @@ public class FriendGraphService(
             var matches = await userRepository.FindAsync(
                 u => u.Handle != null && u.Handle.ToLower() == normalized,
                 cancellationToken);
-            return matches.FirstOrDefault();
+            return matches.Count > 0 ? matches[0] : null;
         }
 
         if (!string.IsNullOrWhiteSpace(referralCode))
@@ -32,7 +32,7 @@ public class FriendGraphService(
             var matches = await userRepository.FindAsync(
                 u => u.ReferralCode == normalized,
                 cancellationToken);
-            return matches.FirstOrDefault();
+            return matches.Count > 0 ? matches[0] : null;
         }
 
         return null;
diff --git a/src/Orbit.Application/Tags/Commands/RestoreTagCommand.cs b/src/Orbit.Application/Tags/Commands/RestoreTagCommand.cs
index eac8f325..890e441f 100644
--- a/src/Orbit.Application/Tags/Commands/RestoreTagCommand.cs
+++ b/src/Orbit.Application/Tags/Commands/RestoreTagCommand.cs
@@ -23,7 +23,7 @@ public async Task<Result> Handle(RestoreTagCommand request, CancellationToken ca
             t => t.Id == request.TagId && t.UserId == request.UserId,
             cancellationToken);
 
-        var tag = tags.FirstOrDefault();
+        var tag = tags.Count > 0 ? tags[0] : null;
         if (tag is null || !tag.IsDeleted)
             return Result.Failure(ErrorMessages.TagNotFound);
 
diff --git a/src/Orbit.Application/Tags/Queries/SuggestTagsQuery.cs b/src/Orbit.Application/Tags/Queries/SuggestTagsQuery.cs
index 8a9e2b97..12f7842d 100644
--- a/src/Orbit.Application/Tags/Queries/SuggestTagsQuery.cs
+++ b/src/Orbit.Application/Tags/Queries/SuggestTagsQuery.cs
@@ -53,7 +53,7 @@ public async Task<Result<SuggestTagsResponse>> Handle(
         return Result.Success(new SuggestTagsResponse(suggestions));
     }
 
-    private static IReadOnlyList<SuggestedTag> MapSuggestions(
+    private static List<SuggestedTag> MapSuggestions(
         IReadOnlyList<string> suggestedNames,
         IReadOnlyList<Tag> existingTags)
     {
diff --git a/src/Orbit.Infrastructure/AI/ContentModerationService.cs b/src/Orbit.Infrastructure/AI/ContentModerationService.cs
index f242b09c..85c431b5 100644
--- a/src/Orbit.Infrastructure/AI/ContentModerationService.cs
+++ b/src/Orbit.Infrastructure/AI/ContentModerationService.cs
@@ -44,7 +44,7 @@ public async Task<ModerationResult> CheckTextAsync(string text, CancellationToke
             }
 
             var payload = await response.Content.ReadFromJsonAsync<ModerationResponse>(SerializerOptions, cancellationToken);
-            var result = payload?.Results?.FirstOrDefault();
+            var result = payload?.Results is { Count: > 0 } results ? results[0] : null;
             if (result is null)
                 return Unavailable;
 
diff --git a/src/Orbit.Infrastructure/Services/AgentTargetOwnershipService.cs b/src/Orbit.Infrastructure/Services/AgentTargetOwnershipService.cs
index 71b9d6b6..dc06abae 100644
--- a/src/Orbit.Infrastructure/Services/AgentTargetOwnershipService.cs
+++ b/src/Orbit.Infrastructure/Services/AgentTargetOwnershipService.cs
@@ -36,7 +36,7 @@ private static OwnershipCheck CreateCheck(string resourceName, OwnershipResult r
     private static async Task<OwnershipResult> AllOwnedAsync<TEntity>(
         IQueryable<TEntity> queryable,
         Guid userId,
-        IReadOnlyCollection<Guid> ids,
+        List<Guid> ids,
         CancellationToken cancellationToken)
         where TEntity : class
     {
diff --git a/src/Orbit.Infrastructure/Services/AiHabitSuggestionService.cs b/src/Orbit.Infrastructure/Services/AiHabitSuggestionService.cs
index f74213b3..0e98dcda 100644
--- a/src/Orbit.Infrastructure/Services/AiHabitSuggestionService.cs
+++ b/src/Orbit.Infrastructure/Services/AiHabitSuggestionService.cs
@@ -124,7 +124,7 @@ internal static Result<HabitSetupSuggestion> MapSuggestion(HabitSuggestionDto? d
         return quantity is { } value && value >= 1 ? value : 1;
     }
 
-    private static IReadOnlyList<DayOfWeek> SanitizeDays(
+    private static List<DayOfWeek> SanitizeDays(
         IReadOnlyList<string>? days, FrequencyUnit? frequencyUnit, int? frequencyQuantity, bool isFlexible)
     {
         if (isFlexible || days is null || frequencyUnit != FrequencyUnit.Day || frequencyQuantity != 1)
@@ -146,7 +146,7 @@ private static IReadOnlyList<DayOfWeek> SanitizeDays(
         return time.ToString("HH\\:mm", CultureInfo.InvariantCulture);
     }
 
-    private static IReadOnlyList<string> SanitizeTitles(IReadOnlyList<string>? values, int cap, int maxLength)
+    private static List<string> SanitizeTitles(IReadOnlyList<string>? values, int cap, int maxLength)
     {
         if (values is null)
             return [];
diff --git a/src/Orbit.Infrastructure/Services/GoalDeadlineNotificationService.cs b/src/Orbit.Infrastructure/Services/GoalDeadlineNotificationService.cs
index 1141a834..1c29b6b6 100644
--- a/src/Orbit.Infrastructure/Services/GoalDeadlineNotificationService.cs
+++ b/src/Orbit.Infrastructure/Services/GoalDeadlineNotificationService.cs
@@ -125,7 +125,7 @@ private async Task<Dictionary<Guid, int>> ComputeFreshStreakValuesAsync(OrbitDbC
         return freshValues;
     }
 
-    private static decimal EffectiveCurrentValue(Goal goal, IReadOnlyDictionary<Guid, int> freshStreakValues) =>
+    private static decimal EffectiveCurrentValue(Goal goal, Dictionary<Guid, int> freshStreakValues) =>
         freshStreakValues.TryGetValue(goal.Id, out var fresh) ? fresh : goal.CurrentValue;
 
     private async Task ProcessGoalDeadlineAsync(
diff --git a/src/Orbit.Infrastructure/Services/PushNotificationService.cs b/src/Orbit.Infrastructure/Services/PushNotificationService.cs
index 080ea836..8eab8ff8 100644
--- a/src/Orbit.Infrastructure/Services/PushNotificationService.cs
+++ b/src/Orbit.Infrastructure/Services/PushNotificationService.cs
@@ -69,7 +69,7 @@ private async Task SendFcm(
         }
 
         const int FcmBatchSize = 500;
-        var subsList = subs as IList<Domain.Entities.PushSubscription> ?? subs.ToList();
+        var subsList = subs as List<Domain.Entities.PushSubscription> ?? subs.ToList();
         for (int offset = 0; offset < subsList.Count; offset += FcmBatchSize)
         {
             var chunk = subsList.Skip(offset).Take(FcmBatchSize).ToList();
diff --git a/tests/Orbit.Application.Tests/Accountability/AccountabilityValidatorsTests.cs b/tests/Orbit.Application.Tests/Accountability/AccountabilityValidatorsTests.cs
index b970ba52..c76d4428 100644
--- a/tests/Orbit.Application.Tests/Accountability/AccountabilityValidatorsTests.cs
+++ b/tests/Orbit.Application.Tests/Accountability/AccountabilityValidatorsTests.cs
@@ -11,7 +11,7 @@ public class AccountabilityValidatorsTests
     private static readonly Guid UserId = Guid.NewGuid();
     private static readonly Guid PairId = Guid.NewGuid();
 
-    private static IReadOnlyList<Guid> Habits(int count) =>
+    private static List<Guid> Habits(int count) =>
         Enumerable.Range(0, count).Select(_ => Guid.NewGuid()).ToList();
 
     [Fact]
diff --git a/tests/Orbit.Application.Tests/Challenges/ChallengeValidatorsTests.cs b/tests/Orbit.Application.Tests/Challenges/ChallengeValidatorsTests.cs
index b635c512..55bd54ef 100644
--- a/tests/Orbit.Application.Tests/Challenges/ChallengeValidatorsTests.cs
+++ b/tests/Orbit.Application.Tests/Challenges/ChallengeValidatorsTests.cs
@@ -10,7 +10,7 @@ public class ChallengeValidatorsTests
     private static readonly Guid UserId = Guid.NewGuid();
     private static readonly Guid ChallengeId = Guid.NewGuid();
 
-    private static IReadOnlyList<Guid> Habits(int count) =>
+    private static List<Guid> Habits(int count) =>
         Enumerable.Range(0, count).Select(_ => Guid.NewGuid()).ToList();
 
     [Theory]
diff --git a/tests/Orbit.Application.Tests/Commands/Chat/ProcessUserChatCommandHandlerTests.cs b/tests/Orbit.Application.Tests/Commands/Chat/ProcessUserChatCommandHandlerTests.cs
index 88224341..c4c964d0 100644
--- a/tests/Orbit.Application.Tests/Commands/Chat/ProcessUserChatCommandHandlerTests.cs
+++ b/tests/Orbit.Application.Tests/Commands/Chat/ProcessUserChatCommandHandlerTests.cs
@@ -246,7 +246,7 @@ private static IAiTool FakeTool(string name)
         return tool;
     }
 
-    private static IReadOnlyList<string> ToolNames(IReadOnlyList<object> declarations) =>
+    private static List<string> ToolNames(IReadOnlyList<object> declarations) =>
         declarations.Select(declaration => (string)declaration.GetType().GetProperty("name")!.GetValue(declaration)!).ToList();
 
     [Fact]
diff --git a/tests/Orbit.Application.Tests/Commands/Profile/ApplyOnboardingCommandHandlerTests.cs b/tests/Orbit.Application.Tests/Commands/Profile/ApplyOnboardingCommandHandlerTests.cs
index e08e30e9..80dedd6d 100644
--- a/tests/Orbit.Application.Tests/Commands/Profile/ApplyOnboardingCommandHandlerTests.cs
+++ b/tests/Orbit.Application.Tests/Commands/Profile/ApplyOnboardingCommandHandlerTests.cs
@@ -20,7 +20,7 @@ public class ApplyOnboardingCommandHandlerTests
     private readonly IUserDateService _userDateService = Substitute.For<IUserDateService>();
     private readonly IAppConfigService _appConfig = Substitute.For<IAppConfigService>();
     private readonly IUnitOfWork _unitOfWork = Substitute.For<IUnitOfWork>();
-    private readonly IMemoryCache _cache = new MemoryCache(new MemoryCacheOptions());
+    private readonly MemoryCache _cache = new MemoryCache(new MemoryCacheOptions());
 
     private static readonly Guid UserId = Guid.NewGuid();
     private static readonly DateOnly Today = new(2026, 7, 5);
diff --git a/tests/Orbit.Domain.Tests/Generators/OrbitArbitraries.cs b/tests/Orbit.Domain.Tests/Generators/OrbitArbitraries.cs
index d1f9a189..99ec4b49 100644
--- a/tests/Orbit.Domain.Tests/Generators/OrbitArbitraries.cs
+++ b/tests/Orbit.Domain.Tests/Generators/OrbitArbitraries.cs
@@ -81,7 +81,7 @@ private static Habit Build(
             HabitOwnerId, "Property Habit", unit, quantity,
             Days: days, IsBadHabit: isBadHabit, DueDate: dueDate, IsFlexible: isFlexible)).Value;
 
-    private static IReadOnlyList<DayOfWeek> DaysFromMask(int mask) =>
+    private static List<DayOfWeek> DaysFromMask(int mask) =>
         Enumerable.Range(0, 7).Where(bit => (mask & (1 << bit)) != 0).Select(bit => (DayOfWeek)bit).ToList();
 
     private static Gen<Habit> OneTimeGen =>
diff --git a/tests/Orbit.Infrastructure.Tests/Services/SystemPromptBuilderTests.cs b/tests/Orbit.Infrastructure.Tests/Services/SystemPromptBuilderTests.cs
index aaa5fb94..995abbb7 100644
--- a/tests/Orbit.Infrastructure.Tests/Services/SystemPromptBuilderTests.cs
+++ b/tests/Orbit.Infrastructure.Tests/Services/SystemPromptBuilderTests.cs
@@ -16,7 +16,7 @@ private static string BuildPrompt(
         bool hasImage = false, IReadOnlyList<Tag>? userTags = null,
         DateOnly? userToday = null, IReadOnlyDictionary<Guid, HabitMetrics>? habitMetrics = null)
     {
-        ISystemPromptBuilder builder = new SystemPromptBuilder();
+        var builder = new SystemPromptBuilder();
         var request = new PromptBuildRequest(habits, facts, hasImage, UserTags: userTags, UserToday: userToday ?? new DateOnly(2026, 3, 20), HabitMetrics: habitMetrics);
         return builder.BuildStatic(request) + builder.BuildDynamic(request);
     }
@@ -162,7 +162,7 @@ public void Build_IncludesEncouragingTone()
     [Fact]
     public void BuildStatic_OrdersEncouragingToneAfterIdentityAndBeforeRules()
     {
-        ISystemPromptBuilder builder = new SystemPromptBuilder();
+        var builder = new SystemPromptBuilder();
         var staticPrompt = builder.BuildStatic(new PromptBuildRequest(Array.Empty<Habit>(), Array.Empty<UserFact>()));
 
         staticPrompt.Should().Contain("Tone and Encouragement");
@@ -198,7 +198,7 @@ public void Build_WithFactContainingControlCharacters_SanitizesPromptData()
     [Fact]
     public void BuildStatic_IsRequestInvariant_AndExcludesDynamicHabitIndex()
     {
-        ISystemPromptBuilder builder = new SystemPromptBuilder();
+        var builder = new SystemPromptBuilder();
         var habit = Habit.Create(new HabitCreateParams(TestUserId, "Morning Run", FrequencyUnit.Day, 1, DueDate: DateOnly.FromDateTime(DateTime.UtcNow))).Value;
         var withHabit = new PromptBuildRequest([habit], Array.Empty<UserFact>());
         var empty = new PromptBuildRequest(Array.Empty<Habit>(), Array.Empty<UserFact>());
@@ -214,7 +214,7 @@ public void BuildStatic_IsRequestInvariant_AndExcludesDynamicHabitIndex()
     [Fact]
     public void BuildDynamic_ContainsUserData_AndExcludesStaticRules()
     {
-        ISystemPromptBuilder builder = new SystemPromptBuilder();
+        var builder = new SystemPromptBuilder();
         var habit = Habit.Create(new HabitCreateParams(TestUserId, "Morning Run", FrequencyUnit.Day, 1, DueDate: DateOnly.FromDateTime(DateTime.UtcNow))).Value;
         var request = new PromptBuildRequest([habit], Array.Empty<UserFact>(), UserToday: new DateOnly(2026, 3, 20));
 
diff --git a/tests/Orbit.Infrastructure.Tests/Services/UserDateServiceTests.cs b/tests/Orbit.Infrastructure.Tests/Services/UserDateServiceTests.cs
index 9dd4c8d3..99d95e13 100644
--- a/tests/Orbit.Infrastructure.Tests/Services/UserDateServiceTests.cs
+++ b/tests/Orbit.Infrastructure.Tests/Services/UserDateServiceTests.cs
@@ -117,6 +117,6 @@ public async Task GetUserWeekStartDayAsync_SecondInstanceSharingCache_ReadsCache
         await secondInstanceRepo.DidNotReceive().GetByIdAsync(UserId, Arg.Any<CancellationToken>());
     }
 
-    private static IDistributedCache NewDistributedCache() =>
+    private static MemoryDistributedCache NewDistributedCache() =>
         new MemoryDistributedCache(Options.Create(new MemoryDistributedCacheOptions()));
 }