fix(135): from-lineup advanced overrides can express "clear to none"

CreateChannelFromLineupHandler resolved every advanced override with
advanced.X ?? template.X, so null always meant INHERIT and a channel could
not drop a template-set watermark / filler / preferred language. Add an
optional typed `clear` enum list to CreateChannelFromLineupAdvancedOptions:
omitted/null still inherits (byte-stable for existing clients), a field named
in `clear` is forced to none. Set+clear of the same field is a 422.

The enum (CreateChannelFromLineupClearField) lives in ErsatzTV.Core so the
OpenAPI string-enum scan renders it as a string enum, matching every sibling
advanced-options enum. Handler resolves clearable fields once via
ResolveClearable and validates set/clear conflicts via ValidateClear;
reference validation skips existence checks for cleared (null) refs.

SPA: the shared advancedOptions model re-adds a real "None" option to the five
id selects (watermark + fillers) in both the Channel Builder and the Auto-Tune
DetailPanel, routed through a CLEAR overrides sentinel that applyOverridesToRequest
folds into advanced.clear (never leaking onto the wire as a field value). The
backend enum also covers the preferred audio/subtitle language strings for
machine clients; the SPA text inputs keep "empty = inherit" (tri-state deferred).

Docs: api-conventions.md §2, spa-conventions.md §11, decisions.md record
api.from-lineup-clear-to-none; v1.json + generated TS regenerated.

fixes #135

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV.Application/Channels/Commands/CreateChannelFromLineup.cs

@@ -1,4 +1,4 @@
-using ErsatzTV.Application.Artworks;
+using ErsatzTV.Application.Artworks;
 using ErsatzTV.Core;
 using ErsatzTV.Core.Api.Channels;
 using ErsatzTV.Core.Api.LibraryBrowse;
@@ -43,7 +43,8 @@ public record CreateChannelFromLineupAdvancedOptions(
     ChannelIdleBehavior? IdleBehavior = null,
     bool? ShuffleScheduleItems = null,
     bool? RandomStartPoint = null,
-    FixedStartTimeBehavior? FixedStartTimeBehavior = null);
+    FixedStartTimeBehavior? FixedStartTimeBehavior = null,
+    IReadOnlyList<CreateChannelFromLineupClearField> Clear = null);
 
 public record CreateChannelFromLineupItem(
     LibraryBrowseMediaType MediaType,

ErsatzTV.Application/Channels/Commands/CreateChannelFromLineupHandler.cs

@@ -190,11 +190,21 @@ public class CreateChannelFromLineupHandler(
             return new NotFoundError($"Channel template {request.TemplateId} does not exist.");
         }
 
+        // "clear to none" (#135): a field named in advanced.Clear is forced to none even when the
+        // template sets one; both setting and clearing the same field is contradictory.
+        Either<BaseError, Unit> clearValidation = ValidateClear(advanced);
+        foreach (BaseError error in clearValidation.LeftToSeq())
+        {
+            return error;
+        }
+
+        ResolvedClearableOptions resolved = ResolveClearable(advanced, template);
+
         int ffmpegProfileId = advanced.FFmpegProfileId ?? template.FFmpegProfileId;
-        int? fallbackFillerId = advanced.FallbackFillerId ?? template.FallbackFillerId;
-        int? preRollFillerId = advanced.PreRollFillerId ?? template.PreRollFillerId;
-        int? midRollFillerId = advanced.MidRollFillerId ?? template.MidRollFillerId;
-        int? postRollFillerId = advanced.PostRollFillerId ?? template.PostRollFillerId;
+        int? fallbackFillerId = resolved.FallbackFillerId;
+        int? preRollFillerId = resolved.PreRollFillerId;
+        int? midRollFillerId = resolved.MidRollFillerId;
+        int? postRollFillerId = resolved.PostRollFillerId;
         PlaybackOrder playbackOrder = advanced.PlaybackOrder ?? PlaybackOrder.Chronological;
         ChannelPlayoutSource playoutSource = advanced.PlayoutSource ?? template.PlayoutSource;
 
@@ -207,8 +217,8 @@ public class CreateChannelFromLineupHandler(
 
         Either<BaseError, Unit> referenceValidation = await ValidateReferences(
             dbContext,
-            advanced,
-            template,
+            ffmpegProfileId,
+            resolved,
             cancellationToken);
         foreach (BaseError error in referenceValidation.LeftToSeq())
         {
@@ -272,6 +282,7 @@ public class CreateChannelFromLineupHandler(
             request,
             template,
             advanced,
+            resolved,
             name,
             number,
             group,
@@ -291,6 +302,7 @@ public class CreateChannelFromLineupHandler(
             playbackOrder,
             advanced,
             template,
+            resolved,
             fallbackFillerId,
             preRollFillerId,
             midRollFillerId,
@@ -383,6 +395,7 @@ public class CreateChannelFromLineupHandler(
         CreateChannelFromLineup request,
         ChannelTemplate template,
         CreateChannelFromLineupAdvancedOptions advanced,
+        ResolvedClearableOptions resolved,
         string name,
         string number,
         string group,
@@ -421,16 +434,14 @@ public class CreateChannelFromLineupHandler(
             PlayoutSource = advanced.PlayoutSource ?? template.PlayoutSource,
             PlayoutMode = advanced.PlayoutMode ?? template.PlayoutMode,
             StreamingMode = advanced.StreamingMode ?? template.StreamingMode,
-            WatermarkId = advanced.WatermarkId ?? template.WatermarkId,
+            WatermarkId = resolved.WatermarkId,
             FallbackFillerId = fallbackFillerId,
             Artwork = artwork,
             StreamSelectorMode = advanced.StreamSelectorMode ?? template.StreamSelectorMode,
             StreamSelector = advanced.StreamSelector ?? template.StreamSelector ?? string.Empty,
-            PreferredAudioLanguageCode =
-                advanced.PreferredAudioLanguageCode ?? template.PreferredAudioLanguageCode ?? string.Empty,
-            PreferredAudioTitle = advanced.PreferredAudioTitle ?? template.PreferredAudioTitle ?? string.Empty,
-            PreferredSubtitleLanguageCode =
-                advanced.PreferredSubtitleLanguageCode ?? template.PreferredSubtitleLanguageCode ?? string.Empty,
+            PreferredAudioLanguageCode = resolved.PreferredAudioLanguageCode,
+            PreferredAudioTitle = resolved.PreferredAudioTitle,
+            PreferredSubtitleLanguageCode = resolved.PreferredSubtitleLanguageCode,
             SubtitleMode = advanced.SubtitleMode ?? template.SubtitleMode,
             MusicVideoCreditsMode = advanced.MusicVideoCreditsMode ?? template.MusicVideoCreditsMode,
             MusicVideoCreditsTemplate =
@@ -462,6 +473,7 @@ public class CreateChannelFromLineupHandler(
         PlaybackOrder playbackOrder,
         CreateChannelFromLineupAdvancedOptions advanced,
         ChannelTemplate template,
+        ResolvedClearableOptions resolved,
         int? fallbackFillerId,
         int? preRollFillerId,
         int? midRollFillerId,
@@ -478,11 +490,9 @@ public class CreateChannelFromLineupHandler(
             MidRollFillerId = midRollFillerId,
             PostRollFillerId = postRollFillerId,
             FallbackFillerId = fallbackFillerId,
-            PreferredAudioLanguageCode =
-                advanced.PreferredAudioLanguageCode ?? template.PreferredAudioLanguageCode ?? string.Empty,
-            PreferredAudioTitle = advanced.PreferredAudioTitle ?? template.PreferredAudioTitle ?? string.Empty,
-            PreferredSubtitleLanguageCode =
-                advanced.PreferredSubtitleLanguageCode ?? template.PreferredSubtitleLanguageCode ?? string.Empty,
+            PreferredAudioLanguageCode = resolved.PreferredAudioLanguageCode,
+            PreferredAudioTitle = resolved.PreferredAudioTitle,
+            PreferredSubtitleLanguageCode = resolved.PreferredSubtitleLanguageCode,
             SubtitleMode = advanced.SubtitleMode ?? template.SubtitleMode
         };
 
@@ -526,20 +536,21 @@ public class CreateChannelFromLineupHandler(
 
     private static async Task<Either<BaseError, Unit>> ValidateReferences(
         TvContext dbContext,
-        CreateChannelFromLineupAdvancedOptions advanced,
-        ChannelTemplate template,
+        int ffmpegProfileId,
+        ResolvedClearableOptions resolved,
         CancellationToken cancellationToken)
     {
-        int ffmpegProfileId = advanced.FFmpegProfileId ?? template.FFmpegProfileId;
         if (!await dbContext.FFmpegProfiles.AnyAsync(p => p.Id == ffmpegProfileId, cancellationToken))
         {
             return new NotFoundError($"FFmpegProfile {ffmpegProfileId} does not exist.");
         }
 
+        // Validate the post-clear effective ids: a cleared reference resolves to null and skips the
+        // existence check (there is nothing to point at).
         Either<BaseError, Unit> channelReferences = await ValidateChannelReferences(
             dbContext,
-            advanced.WatermarkId ?? template.WatermarkId,
-            advanced.FallbackFillerId ?? template.FallbackFillerId,
+            resolved.WatermarkId,
+            resolved.FallbackFillerId,
             cancellationToken);
         foreach (BaseError error in channelReferences.LeftToSeq())
         {
@@ -548,9 +559,9 @@ public class CreateChannelFromLineupHandler(
 
         Either<BaseError, Unit> itemFillers = await ValidateItemFillers(
             dbContext,
-            advanced.PreRollFillerId ?? template.PreRollFillerId,
-            advanced.MidRollFillerId ?? template.MidRollFillerId,
-            advanced.PostRollFillerId ?? template.PostRollFillerId,
+            resolved.PreRollFillerId,
+            resolved.MidRollFillerId,
+            resolved.PostRollFillerId,
             cancellationToken);
         foreach (BaseError error in itemFillers.LeftToSeq())
         {
@@ -560,6 +571,80 @@ public class CreateChannelFromLineupHandler(
         return Unit.Default;
     }
 
+    // A field named in advanced.Clear must not also carry a set value: that request is contradictory.
+    // A null/empty set value alongside a clear is fine (redundant, not conflicting). (#135)
+    private static Either<BaseError, Unit> ValidateClear(CreateChannelFromLineupAdvancedOptions advanced)
+    {
+        if (advanced.Clear is null || advanced.Clear.Count == 0)
+        {
+            return Unit.Default;
+        }
+
+        var cleared = advanced.Clear.ToHashSet();
+
+        (CreateChannelFromLineupClearField Field, bool HasSetValue)[] checks =
+        [
+            (CreateChannelFromLineupClearField.Watermark, advanced.WatermarkId.HasValue),
+            (CreateChannelFromLineupClearField.FallbackFiller, advanced.FallbackFillerId.HasValue),
+            (CreateChannelFromLineupClearField.PreRollFiller, advanced.PreRollFillerId.HasValue),
+            (CreateChannelFromLineupClearField.MidRollFiller, advanced.MidRollFillerId.HasValue),
+            (CreateChannelFromLineupClearField.PostRollFiller, advanced.PostRollFillerId.HasValue),
+            (CreateChannelFromLineupClearField.PreferredAudioLanguage,
+                !string.IsNullOrEmpty(advanced.PreferredAudioLanguageCode)),
+            (CreateChannelFromLineupClearField.PreferredAudioTitle,
+                !string.IsNullOrEmpty(advanced.PreferredAudioTitle)),
+            (CreateChannelFromLineupClearField.PreferredSubtitleLanguage,
+                !string.IsNullOrEmpty(advanced.PreferredSubtitleLanguageCode))
+        ];
+
+        foreach ((CreateChannelFromLineupClearField field, bool hasSetValue) in checks)
+        {
+            if (cleared.Contains(field) && hasSetValue)
+            {
+                return BaseError.New(
+                    $"Advanced option '{field}' cannot be both set and cleared in the same request");
+            }
+        }
+
+        return Unit.Default;
+    }
+
+    // Compute the effective value of every clearable field once: cleared -> none, else the advanced
+    // override coalesced with the template value (the historical omitted=inherit contract). (#135)
+    private static ResolvedClearableOptions ResolveClearable(
+        CreateChannelFromLineupAdvancedOptions advanced,
+        ChannelTemplate template)
+    {
+        System.Collections.Generic.HashSet<CreateChannelFromLineupClearField> cleared = advanced.Clear is null
+            ? []
+            : advanced.Clear.ToHashSet();
+
+        int? Id(CreateChannelFromLineupClearField field, int? adv, int? tmpl) =>
+            cleared.Contains(field) ? null : adv ?? tmpl;
+
+        string Str(CreateChannelFromLineupClearField field, string adv, string tmpl) =>
+            cleared.Contains(field) ? string.Empty : adv ?? tmpl ?? string.Empty;
+
+        return new ResolvedClearableOptions(
+            Id(CreateChannelFromLineupClearField.Watermark, advanced.WatermarkId, template.WatermarkId),
+            Id(CreateChannelFromLineupClearField.FallbackFiller, advanced.FallbackFillerId, template.FallbackFillerId),
+            Id(CreateChannelFromLineupClearField.PreRollFiller, advanced.PreRollFillerId, template.PreRollFillerId),
+            Id(CreateChannelFromLineupClearField.MidRollFiller, advanced.MidRollFillerId, template.MidRollFillerId),
+            Id(CreateChannelFromLineupClearField.PostRollFiller, advanced.PostRollFillerId, template.PostRollFillerId),
+            Str(
+                CreateChannelFromLineupClearField.PreferredAudioLanguage,
+                advanced.PreferredAudioLanguageCode,
+                template.PreferredAudioLanguageCode),
+            Str(
+                CreateChannelFromLineupClearField.PreferredAudioTitle,
+                advanced.PreferredAudioTitle,
+                template.PreferredAudioTitle),
+            Str(
+                CreateChannelFromLineupClearField.PreferredSubtitleLanguage,
+                advanced.PreferredSubtitleLanguageCode,
+                template.PreferredSubtitleLanguageCode));
+    }
+
     private static async Task<Either<BaseError, Unit>> ValidateChannelReferences(
         TvContext dbContext,
         int? watermarkId,
@@ -803,4 +888,16 @@ public class CreateChannelFromLineupHandler(
         Playlist Playlist,
         ProgramSchedule ProgramSchedule,
         Playout Playout);
+
+    // Effective values for the clearable advanced fields after applying advanced.Clear + template
+    // coalescing (#135). Strings coalesce to string.Empty (never null); ids stay nullable.
+    private sealed record ResolvedClearableOptions(
+        int? WatermarkId,
+        int? FallbackFillerId,
+        int? PreRollFillerId,
+        int? MidRollFillerId,
+        int? PostRollFillerId,
+        string PreferredAudioLanguageCode,
+        string PreferredAudioTitle,
+        string PreferredSubtitleLanguageCode);
 }

ErsatzTV.Core/Api/Channels/CreateChannelFromLineupClearField.cs

@@ -0,0 +1,20 @@
+namespace ErsatzTV.Core.Api.Channels;
+
+// The "clear to none" signal for POST /api/v1/channels/from-lineup (#135). For these
+// template-inheritable advanced fields a null/omitted override means INHERIT the template value;
+// naming the field here forces it to NONE on the new channel even when the template sets one.
+// Omitting the field entirely keeps the historical omitted=inherit behavior stable for existing
+// clients. Sending both a set value and a clear for the same field is a validation error (see
+// CreateChannelFromLineupHandler). Lives in Core so the OpenAPI string-enum scan (Startup
+// UseStringEnumSchemas) renders it as a string enum, matching every sibling advanced-options enum.
+public enum CreateChannelFromLineupClearField
+{
+    Watermark,
+    FallbackFiller,
+    PreRollFiller,
+    MidRollFiller,
+    PostRollFiller,
+    PreferredAudioLanguage,
+    PreferredAudioTitle,
+    PreferredSubtitleLanguage
+}

ErsatzTV.Tests/Application/Channels/CreateChannelFromLineupHandlerTests.cs

@@ -304,6 +304,110 @@ public class CreateChannelFromLineupHandlerTests
         item.MidRollFillerId.ShouldBe(3);
     }
 
+    [Test]
+    public async Task Clear_Should_Force_Template_Inherited_Values_To_None()
+    {
+        await SeedTemplateDependencies();
+        await SeedTemplate();
+        await SeedMovie(42);
+
+        // Give the template a watermark + preferred audio language so "clear" has something to drop
+        // (the base template has fillers 2-5 but no watermark / audio language).
+        await using (TvContext context = _db.CreateContext())
+        {
+            context.ChannelWatermarks.Add(new ChannelWatermark { Id = 21, Name = "wm", Image = "wm.png" });
+            ChannelTemplate template = await context.ChannelTemplates.SingleAsync();
+            template.WatermarkId = 21;
+            template.PreferredAudioLanguageCode = "eng";
+            await context.SaveChangesAsync();
+        }
+
+        var advanced = new CreateChannelFromLineupAdvancedOptions(
+            PlaybackOrder: PlaybackOrder.Shuffle,
+            Clear:
+            [
+                CreateChannelFromLineupClearField.Watermark,
+                CreateChannelFromLineupClearField.FallbackFiller,
+                CreateChannelFromLineupClearField.PreRollFiller,
+                CreateChannelFromLineupClearField.MidRollFiller,
+                CreateChannelFromLineupClearField.PostRollFiller,
+                CreateChannelFromLineupClearField.PreferredAudioLanguage
+            ]);
+
+        Either<BaseError, CreateChannelFromLineupResponseModel> result =
+            await MakeHandler().Handle(MakeRequest(advanced: advanced), CancellationToken.None);
+        RightOf(result);
+
+        await using TvContext assert = _db.CreateContext();
+        DomainChannel channel = await assert.Channels.SingleAsync();
+
+        // Cleared -> none, even though the template supplies a value.
+        channel.WatermarkId.ShouldBeNull();
+        channel.FallbackFillerId.ShouldBeNull();
+        channel.PreferredAudioLanguageCode.ShouldBe(string.Empty);
+
+        ProgramScheduleItem item = await assert.ProgramScheduleItems.SingleAsync();
+        item.PreRollFillerId.ShouldBeNull();
+        item.MidRollFillerId.ShouldBeNull();
+        item.PostRollFillerId.ShouldBeNull();
+        item.FallbackFillerId.ShouldBeNull();
+        item.PreferredAudioLanguageCode.ShouldBe(string.Empty);
+    }
+
+    [Test]
+    public async Task Clear_And_Set_Same_Field_Should_Be_Validation_Error()
+    {
+        await SeedTemplateDependencies();
+        await SeedTemplate();
+        await SeedMovie(42);
+
+        await using (TvContext context = _db.CreateContext())
+        {
+            context.FillerPresets.Add(MakeFiller(6, FillerKind.Fallback));
+            await context.SaveChangesAsync();
+        }
+
+        var advanced = new CreateChannelFromLineupAdvancedOptions(
+            PlaybackOrder: PlaybackOrder.Shuffle,
+            FallbackFillerId: 6,
+            Clear: [CreateChannelFromLineupClearField.FallbackFiller]);
+
+        Either<BaseError, CreateChannelFromLineupResponseModel> result =
+            await MakeHandler().Handle(MakeRequest(advanced: advanced), CancellationToken.None);
+
+        BaseError error = LeftOf(result);
+        error.ShouldNotBeOfType<NotFoundError>();
+        error.Value.ShouldContain("cannot be both set and cleared");
+    }
+
+    [Test]
+    public async Task Clear_And_Set_String_Field_Conflicts_But_Empty_String_Is_Redundant()
+    {
+        await SeedTemplateDependencies();
+        await SeedTemplate();
+        await SeedMovie(42);
+
+        // A real value + clear on the same string field is contradictory.
+        var conflicting = new CreateChannelFromLineupAdvancedOptions(
+            PlaybackOrder: PlaybackOrder.Shuffle,
+            PreferredAudioLanguageCode: "eng",
+            Clear: [CreateChannelFromLineupClearField.PreferredAudioLanguage]);
+
+        BaseError error = LeftOf(
+            await MakeHandler().Handle(MakeRequest(advanced: conflicting), CancellationToken.None));
+        error.Value.ShouldContain("cannot be both set and cleared");
+
+        // An empty string + clear is redundant, not conflicting -> the create still succeeds. (This
+        // runs second: the conflicting request above returned Left without persisting, so number "12"
+        // is still free.)
+        var redundant = new CreateChannelFromLineupAdvancedOptions(
+            PlaybackOrder: PlaybackOrder.Shuffle,
+            PreferredAudioLanguageCode: "",
+            Clear: [CreateChannelFromLineupClearField.PreferredAudioLanguage]);
+
+        RightOf(await MakeHandler().Handle(MakeRequest(advanced: redundant), CancellationToken.None));
+    }
+
     [Test]
     public async Task Should_Return_Validation_Error_When_Not_Exactly_One_Id_Provided()
     {

ErsatzTV/Controllers/Api/Requests/CreateChannelFromLineupRequest.cs

@@ -2,6 +2,7 @@
 
 using ErsatzTV.Application.Artworks;
 using ErsatzTV.Application.Channels;
+using ErsatzTV.Core.Api.Channels;
 using ErsatzTV.Core.Api.LibraryBrowse;
 using ErsatzTV.Core.Domain;
 using ErsatzTV.Core.Scheduling;
@@ -64,7 +65,8 @@ public record CreateChannelFromLineupAdvancedOptionsRequest(
     ChannelIdleBehavior? IdleBehavior = null,
     bool? ShuffleScheduleItems = null,
     bool? RandomStartPoint = null,
-    FixedStartTimeBehavior? FixedStartTimeBehavior = null)
+    FixedStartTimeBehavior? FixedStartTimeBehavior = null,
+    List<CreateChannelFromLineupClearField>? Clear = null)
 {
     public CreateChannelFromLineupAdvancedOptions ToCommand() =>
         new(
@@ -91,7 +93,8 @@ public record CreateChannelFromLineupAdvancedOptionsRequest(
             IdleBehavior,
             ShuffleScheduleItems,
             RandomStartPoint,
-            FixedStartTimeBehavior);
+            FixedStartTimeBehavior,
+            Clear);
 }
 
 public record CreateChannelFromLineupItemRequest(

ErsatzTV/wwwroot/openapi/v1.json

@@ -24544,9 +24544,31 @@
                 "$ref": "#/components/schemas/FixedStartTimeBehavior"
               }
             ]
+          },
+          "clear": {
+            "type": [
+              "null",
+              "array"
+            ],
+            "items": {
+              "$ref": "#/components/schemas/CreateChannelFromLineupClearField"
+            }
           }
         }
       },
+      "CreateChannelFromLineupClearField": {
+        "enum": [
+          "Watermark",
+          "FallbackFiller",
+          "PreRollFiller",
+          "MidRollFiller",
+          "PostRollFiller",
+          "PreferredAudioLanguage",
+          "PreferredAudioTitle",
+          "PreferredSubtitleLanguage"
+        ],
+        "type": "string"
+      },
       "CreateChannelFromLineupItemRequest": {
         "required": [
           "mediaType",

docs/api-conventions.md

@@ -102,6 +102,15 @@ Exemplars:
   (`GetAllHealthCheckResultsForApi(bool Refresh = false)`) which forces a fresh run past the service's TTL
   result cache — the cached poll path is the default, the flag is the explicit opt-out (see `decisions.md`
   2026-07-19, #431).
+- **"Clear to none" vs "inherit" on a coalescing DTO: a typed `clear` enum list, not null.** When a
+  create/patch DTO resolves a field as `request.X ?? inherited.X` (e.g. `advanced.X ?? template.X`),
+  `null` already means *inherit*, so it cannot also mean *set to none*. Add an optional `clear` field
+  typed as a **list of a string enum** naming the fields to force to none — additive, so omitted =
+  inherit stays byte-stable for existing clients. Validate that a field is not both set and cleared
+  (reject as 422). **Define the enum in `ErsatzTV.Core`** (not the Application command) so
+  `Startup.UseStringEnumSchemas` renders it as a string enum in the spec — an Application-layer enum
+  shows as a bare `integer`. Exemplar: `CreateChannelFromLineupClearField` on
+  `POST /api/v1/channels/from-lineup` (`decisions.md` 2026-07-21, `api.from-lineup-clear-to-none`, #135).
 - **Evolving a frozen DTO: deprecate-in-place, add the richer field, never remove.** `/api/v1` is
   frozen-additive (#286), so when a response field's shape needs to grow, keep the old member
   populated (mark it deprecated in an XML/`//` comment) and add the replacement alongside. Exemplar:

docs/decisions.md

@@ -3427,3 +3427,17 @@ leaving the shared tree dirty is the one outcome that would make this script a n
 **`UnbalancedReleases` can under-count, and that is documented rather than fixed.** It only increments when a release finds the pool already empty. An over-release while the count is positive — e.g. one cancelling out a coexisting leak — decrements a real-looking slot and is never recorded, so the two bugs hide each other. There are no false positives (non-zero still means the contract broke), but zero does not prove correctness. Exact accounting would need per-owner tokens, which the #536 "ownership is a discipline, not a token" decision deliberately avoids; the docstring now states the limitation instead.
 
 **Negative control (inherited from #231/#250).** A dedicated test hammers unbalanced releases on an empty pool while reader threads sample the count; none may ever observe a value below zero. Reinstating the pre-#539 decrement-first body makes it fail (`sawNegative > 0` — the readers catch the transient `−1`); verified. As with the #536 tests, break the primitive by reverting the real body, **not** `if (true)` (CS0219 under warnings-as-errors leaves `--no-build` running a stale, still-fixed dll).
+## 2026-07-21 — `from-lineup` advanced overrides express "clear to none" via a typed `clear` enum list (#135)
+
+`key: api.from-lineup-clear-to-none` · `status: active` · `since: 2026-07-21` · `supersedes: none` · `superseded-by: none`
+**Rule:** `POST /api/v1/channels/from-lineup` (and the Auto-Tune per-channel `advanced`, which reuses the same DTO) distinguishes *inherit* from *clear-to-none* with a typed `clear` enum list on `advanced`. A field left null/omitted still inherits the template value (unchanged for every existing client); naming a field in `clear` forces it to none on the new channel even when the template sets one. Sending both a set value and a clear for the same field is a validation error.
+**Signals:** clear to none, inherit vs none, advanced override, watermark/filler clear, template-minus-one-setting · paths: `ErsatzTV.Core/Api/Channels/CreateChannelFromLineupClearField.cs`, `ErsatzTV.Application/Channels/Commands/CreateChannelFromLineupHandler.cs`, `ErsatzTV/Controllers/Api/Requests/CreateChannelFromLineupRequest.cs`, `web/src/builder/advancedOptions.tsx` · issues: #135, #89, #385, #386
+**Mechanics:** `CreateChannelFromLineupHandler.ResolveClearable`/`ValidateClear`; SPA `applyOverridesToRequest`/`collectClears`/`CLEAR` sentinel; api-conventions.md §2, spa-conventions.md
+
+**Why a `clear` list, not a sentinel or per-field flags.** The gap (found in #89 review) was that the handler resolved every advanced override with `advanced.X ?? template.X`, so a client sending `null` always *inherited*. That is correct for the common path but leaves "this channel should have NO watermark / pre-roll filler even though the template has one" inexpressible. The fix had to keep `omitted = inherit` byte-stable for existing clients (`/api/v1` is frozen-additive, #286), so it is a new optional field, not a reshaping of the existing ones. A `{set, value}` wrapper per field would have rewritten every field's wire type; a reserved `0` sentinel is magic and asymmetric between int ids and strings; parallel `clearX` bools add one field per clearable. A single **typed enum list** is additive, self-documenting, type-checked (an invalid value is a 400 at model binding), covers ids and strings with one mechanism, and extends by adding an enum member. The clearable set is the eight template-inheritable fields where "none" is meaningful: watermark, the four fillers, and the preferred audio/subtitle language + audio title.
+
+**Set + clear of the same field is rejected, not silently resolved.** The SPA never produces that state (a select is inherit, a value, or None), so the check exists to keep hand-crafted / machine-client requests unambiguous rather than picking a winner. A null/empty set value alongside a clear is fine (redundant, not conflicting).
+
+**The enum lives in `ErsatzTV.Core`, not the Application command, on purpose.** The OpenAPI string-enum pass (`Startup.UseStringEnumSchemas`) scans the Core assembly wholesale; an enum defined in `ErsatzTV.Application` renders as a bare `integer` in the spec while every sibling advanced-options enum (`PlaybackOrder`, `ChannelSubtitleMode`, …) is a string enum. Placing `CreateChannelFromLineupClearField` in `ErsatzTV.Core/Api/Channels/` makes the wire contract a string enum by construction, matching its siblings.
+
+**SPA is id-fields-first; the API is complete ahead of the UI.** The Channel Builder + Auto-Tune DetailPanel re-add a real "None" option to the five id selects (watermark + fillers) — the pickers #89 had degraded to "Inherit"-only — routed through a `CLEAR` overrides sentinel folded into `advanced.clear` at request-build time (`applyOverridesToRequest`, so the sentinel never leaks as a field value). The three string clear-fields are covered by the backend enum for machine clients (MCP) but the SPA text inputs keep "empty = inherit"; adding a tri-state to those inputs is deferred, not blocked. This is the deliberate "REST API is a real audience" posture (`rest-api-purpose-mcp-and-new-ui`).

docs/decisions/README.md

@@ -12,6 +12,7 @@ the link for rationale. Superseded/retired history lives in `archive/`. Regenera
 | `api.async-op-contract` | Queue-triggering `/api/*` endpoints normalize onto one contract — 202 Accepted (queued), 404 (missing entity), 409 (lock held), 422 (domain precondition) — with Trakt as the reference implementation; playout list/detail GETs also carry an `isLocked` observability flag as the HTTP-observable substitute for a live push channel. | 2026-07-11 | [link](../decisions.md#2026-07-11--async-op-api-contract-normalization--playout-build-observability--f9-scan-endpoints-235) |
 | `api.channel-health-signal` | Channel health rides `ChannelResponseModel`/`ChannelListItem` DTOs as a raw `int PlayoutCount` fact (free — `GetAll` already `Include`s `Playouts`), not a new endpoint, not `/channels/state` (runtime-liveness cadence), and not a derived `ChannelHealth` enum (would freeze policy before the #383/#384 auto-tune status taxonomy lands). | 2026-07-17 | [link](../decisions.md#2026-07-17--channel-health-on-the-api--the-raw-playoutcount-fact-on-the-list-dto-not-a-derived-status-enum-72) |
 | `api.decode-by-id` | Endpoints that decode/expand opaque stored state accept a database row id and resolve it server-side rather than round-tripping client-supplied serialized state. | 2026-07-07 | [link](../decisions.md#2026-07-07--decode-style-endpoints-take-a-row-id-and-look-up-server-side) |
+| `api.from-lineup-clear-to-none` | `POST /api/v1/channels/from-lineup` (and the Auto-Tune per-channel `advanced`, which reuses the same DTO) distinguishes *inherit* from *clear-to-none* with a typed `clear` enum list on `advanced`. A field left null/omitted still inherits the template value (unchanged for every existing client); naming a field in `clear` forces it to none on the new channel even when the template sets one. Sending both a set value and a clear for the same field is a validation error. | 2026-07-21 | [link](../decisions.md#2026-07-21--from-lineup-advanced-overrides-express-clear-to-none-via-a-typed-clear-enum-list-135) |
 | `api.healthcheck-remediation-dto` | Health-check remediation is server-declared `{Kind, Target}` metadata on an additive DTO field; the SPA renders/acts on it, it doesn't derive labels itself. | 2026-07-17 | [link](../decisions.md#2026-07-17--health-check-remediation-is-server-declared-kind-target-on-an-additive-dto-the-spa-acts-on-it-164) |
 | `api.healthcheck-ttl-cache` | Health-check results are held in a 30s TTL cache inside `HealthCheckService`; a non-forced `GET /api/v1/health` returns the cached list, and `?refresh=true` (or a forced internal caller) bypasses it to run fresh. | 2026-07-19 | [link](../decisions.md#2026-07-19--health-check-results-are-ttl-cached-refreshtrue-forces-a-fresh-run-431) |
 | `api.logs-sort-params` | `GET /api/logs` takes allow-listed `sortField` (`timestamp`\|`level`) and `sortDirection` (`asc`\|`desc`) query params, normalized (not rejected) on an unrecognized value. | 2026-07-11 | [link](../decisions.md#2026-07-11--logs-column-sorting-allow-listed-sortfieldsortdirection-on-get-apilogs) |

docs/spa-conventions.md

@@ -431,14 +431,23 @@ string-keyed dispatcher, or generic screen-action framework participates in norm
   `window.confirm` guard belongs on *screen navigation / unload* while any uncommitted edit exists,
   not on panel close. Show an "Edited" badge on customised rows so the pending edits are visible.
 - **Advanced channel-options overrides are shared, not duplicated** (`web/src/builder/advancedOptions.tsx`).
-  The 24-field `CreateChannelFromLineupAdvancedOptionsRequest` override model — the enum catalogs,
-  `ADVANCED_KEYS`, `effectiveValue`, and the **INHERIT/omit** field adapters (`useAdvancedOverrides`) —
-  is one module consumed by both the Channel Builder and the Auto-Tune DetailPanel. The contract
-  (a select on `INHERIT` or a cleared text input **omits** the field so the create handler coalesces
-  it with the template value; there is no "None" — #135) must not be re-implemented per screen. Each
-  screen writes its own field JSX over the shared hook; `playbackOrder`/`playoutMode` are surfaced as
-  dedicated controls (Builder state / Auto-Tune's Shuffle + Always-playing toggles) and merged into
-  `advanced` at create, not carried in the override map.
+  The `CreateChannelFromLineupAdvancedOptionsRequest` override model — the enum catalogs,
+  `ADVANCED_KEYS`, `effectiveValue`, and the field adapters (`useAdvancedOverrides`) — is one module
+  consumed by both the Channel Builder and the Auto-Tune DetailPanel, and must not be re-implemented
+  per screen. Each screen writes its own field JSX over the shared hook; `playbackOrder`/`playoutMode`
+  are surfaced as dedicated controls (Builder state / Auto-Tune's Shuffle + Always-playing toggles) and
+  merged into `advanced` at create, not carried in the override map.
+- **Three per-field states: INHERIT / set / CLEAR** (#135, `api.from-lineup-clear-to-none`). A select on
+  `INHERIT` (or a cleared text input) **omits** the field so the create handler coalesces it with the
+  template value. A concrete value overrides the template. The **`CLEAR` sentinel** ("None") is stored in
+  the overrides map like any override but is folded into the request's `advanced.clear` list at build
+  time by `applyOverridesToRequest`/`collectClears` — so the sentinel never reaches the wire as a field
+  value, and `effectiveValue` reads a `CLEAR` override as none. **Build the request only through
+  `applyOverridesToRequest`**, never a raw spread of the overrides map (a spread leaks the `CLEAR`
+  sentinel). Today only the five id selects (watermark + fillers) expose a "None" option; the backend
+  `clear` enum also covers the preferred audio/subtitle language strings for machine clients, but the SPA
+  text inputs keep "empty = inherit" (a tri-state text control is deferred) — API-ahead-of-UI, matching
+  the "REST API is a real audience" posture.
 
 ## 12. Reusable rule builder (`web/src/builder/rules/`)
 

web/src/api/channels.ts

@@ -11,6 +11,7 @@ export type BulkMoveChannelsToGroupRequest = components['schemas']['BulkMoveChan
 export type BulkDeleteChannelsRequest = components['schemas']['BulkDeleteChannelsRequest'];
 export type CreateChannelFromLineupRequest = components['schemas']['CreateChannelFromLineupRequest'];
 export type CreateChannelFromLineupResponseModel = components['schemas']['CreateChannelFromLineupResponseModel'];
+export type CreateChannelFromLineupClearField = components['schemas']['CreateChannelFromLineupClearField'];
 export type CreateChannelRequest = components['schemas']['CreateChannelRequest'];
 
 export interface ChannelsScreenData {

web/src/api/generated/v1.d.ts

@@ -347,7 +347,9 @@ export interface components {
     "shuffleScheduleItems"?: null | boolean;
     "randomStartPoint"?: null | boolean;
     "fixedStartTimeBehavior"?: null | components["schemas"]["FixedStartTimeBehavior"];
+    "clear"?: null | Array<components["schemas"]["CreateChannelFromLineupClearField"]>;
   };
+    "CreateChannelFromLineupClearField": "Watermark" | "FallbackFiller" | "PreRollFiller" | "MidRollFiller" | "PostRollFiller" | "PreferredAudioLanguage" | "PreferredAudioTitle" | "PreferredSubtitleLanguage";
     "CreateChannelFromLineupItemRequest": {
     "mediaType": components["schemas"]["LibraryBrowseMediaType"];
     "collectionType": components["schemas"]["CollectionType"];

web/src/builder/ChannelBuilder.tsx

@@ -76,6 +76,7 @@ import {
 } from '../components';
 import {
   ADVANCED_KEYS,
+  applyOverridesToRequest,
   FIXED_START_TIME_BEHAVIORS,
   IDLE_BEHAVIORS,
   MUSIC_VIDEO_CREDITS_MODES,
@@ -940,15 +941,13 @@ function ChannelBuilder({
         }
       }
 
-      // advanced null/omitted = inherit template value (handler coalesces with ??);
-      // clear-to-none is not expressible — see #135. Overrides only ever holds
-      // real values (inherited fields are absent), so we copy present keys as-is.
-      const advanced: CreateChannelFromLineupRequest['advanced'] = { playbackOrder, playoutMode };
-      for (const key of ADVANCED_KEYS) {
-        if (key in overrides) {
-          (advanced as Record<string, unknown>)[key] = overrides[key];
-        }
-      }
+      // advanced null/omitted = inherit template value (handler coalesces with ??); a field set to
+      // the CLEAR sentinel is translated into advanced.clear (force to none) — see #135.
+      // applyOverridesToRequest copies present set-values and attaches `clear`.
+      const advanced: CreateChannelFromLineupRequest['advanced'] = applyOverridesToRequest(
+        { playbackOrder, playoutMode },
+        overrides
+      );
 
       const body: CreateChannelFromLineupRequest = {
         name: trimmedName,

web/src/builder/advancedOptions.test.ts

@@ -0,0 +1,64 @@
+import { describe, expect, it } from 'vitest';
+import {
+  applyOverridesToRequest,
+  CLEAR,
+  collectClears,
+  effectiveValue,
+  type Overrides
+} from './advancedOptions';
+import type { ChannelTemplate } from '../api';
+
+// Minimal template stub: only the fields the assertions read.
+const template = {
+  watermarkId: 21,
+  preRollFillerId: 2,
+  preferredAudioLanguageCode: 'eng'
+} as unknown as ChannelTemplate;
+
+describe('advancedOptions clear-to-none (#135)', () => {
+  it('effectiveValue reads a CLEAR override as none, an absent key as the template value', () => {
+    const overrides: Overrides = { watermarkId: CLEAR, preRollFillerId: 7 };
+    // Cleared -> null (renders "None").
+    expect(effectiveValue('watermarkId', template, overrides)).toBeNull();
+    // Set -> the override value.
+    expect(effectiveValue('preRollFillerId', template, overrides)).toBe(7);
+    // Absent -> inherits the template value.
+    expect(effectiveValue('preferredAudioLanguageCode', template, overrides)).toBe('eng');
+  });
+
+  it('collectClears maps CLEAR-valued clearable keys to their backend enum values', () => {
+    const overrides: Overrides = {
+      watermarkId: CLEAR,
+      fallbackFillerId: CLEAR,
+      preferredAudioLanguageCode: CLEAR,
+      preRollFillerId: 7
+    };
+    expect(collectClears(overrides).sort()).toEqual(
+      ['FallbackFiller', 'PreferredAudioLanguage', 'Watermark'].sort()
+    );
+  });
+
+  it('applyOverridesToRequest copies set values and folds CLEAR into advanced.clear (never as a value)', () => {
+    const overrides: Overrides = { watermarkId: CLEAR, preRollFillerId: 7, midRollFillerId: 3 };
+    const advanced = applyOverridesToRequest({ playbackOrder: 'Shuffle' as const }, overrides);
+
+    // Set values copied through.
+    expect(advanced).toMatchObject({ playbackOrder: 'Shuffle', preRollFillerId: 7, midRollFillerId: 3 });
+    // The CLEAR sentinel never leaks onto the wire as a field value.
+    expect('watermarkId' in advanced).toBe(false);
+    // It becomes a clear-list entry instead.
+    expect((advanced as { clear?: string[] }).clear).toEqual(['Watermark']);
+  });
+
+  it('applyOverridesToRequest omits the clear key entirely when nothing is cleared', () => {
+    const advanced = applyOverridesToRequest({ playbackOrder: 'Chronological' as const }, { preRollFillerId: 7 });
+    expect('clear' in advanced).toBe(false);
+  });
+
+  it('a non-clearable key holding the CLEAR sentinel is copied through, never silently dropped', () => {
+    // Only reachable if a user literally typed the sentinel into a non-clearable text input.
+    const advanced = applyOverridesToRequest({}, { streamSelector: CLEAR });
+    expect((advanced as { streamSelector?: unknown }).streamSelector).toBe(CLEAR);
+    expect('clear' in advanced).toBe(false);
+  });
+});

web/src/builder/advancedOptions.tsx

@@ -2,13 +2,19 @@
 // a channel template's baseline, used by BOTH the manual Channel Builder
 // (ChannelBuilder.tsx) and the Auto-Tune per-channel DetailPanel (AutoTuneScreen.tsx).
 //
-// The tricky, bug-prone part is the INHERIT/omit contract (see #135): a field left
-// on "Inherit from template" is OMITTED from the request entirely (not sent as null),
-// because the create handler coalesces a missing value with the template's value and
-// the API cannot express "clear to none". Both consumers MUST honour that, so it lives
-// here once. Consumers render their own field JSX; only the model + adapters are shared.
+// The tricky, bug-prone part is the INHERIT/omit vs CLEAR contract (see #135). Three states
+// per advanced field:
+//   - INHERIT: the field is OMITTED from the request; the create handler coalesces the missing
+//     value with the template's value (the historical default for every existing client).
+//   - a set value: sent as-is, overriding the template.
+//   - CLEAR ("None"): the field is added to `advanced.clear`, forcing it to none on the new
+//     channel even when the template sets one. Only the id fields (watermark + the four fillers)
+//     expose a "None" option in the SPA today; the backend `clear` enum also covers the preferred
+//     audio/subtitle language strings for machine clients (API-ahead-of-UI — see spa-conventions).
+// Both consumers MUST honour that, so it lives here once. Consumers render their own field JSX;
+// only the model + adapters are shared.
 
-import type { ChannelTemplate } from '../api';
+import type { ChannelTemplate, CreateChannelFromLineupClearField } from '../api';
 
 // ---- Enum unions (hand-listed from generated v1.d.ts; keep in sync) --------
 export const PLAYBACK_ORDERS = [
@@ -93,19 +99,74 @@ export type AdvancedKey = (typeof ADVANCED_KEYS)[number];
 
 export type Overrides = Partial<Record<AdvancedKey, unknown>>;
 
-// The current effective value for an advanced field = user override (if any)
-// else the selected template's value.
+// Sentinel select value meaning "inherit from the template" — the field is
+// omitted from `advanced` entirely (not sent as null). See #135.
+export const INHERIT = '__inherit__';
+
+// Sentinel override value meaning "clear to none" — the field is added to the request's
+// `advanced.clear` list, forcing it to none even when the template sets one (#135). Stored in the
+// overrides map (a clear IS a divergence from the template) and translated at request-build time.
+export const CLEAR = '__clear__';
+
+// AdvancedKeys whose "None" maps to a backend clear-field. Only these can be cleared; every other
+// key set to CLEAR is ignored by collectClears. The id keys drive the SPA "None" option today; the
+// string keys are covered so a machine client (MCP) can clear them via the same enum.
+export const CLEARABLE_FIELDS: Partial<Record<AdvancedKey, CreateChannelFromLineupClearField>> = {
+  watermarkId: 'Watermark',
+  fallbackFillerId: 'FallbackFiller',
+  preRollFillerId: 'PreRollFiller',
+  midRollFillerId: 'MidRollFiller',
+  postRollFillerId: 'PostRollFiller',
+  preferredAudioLanguageCode: 'PreferredAudioLanguage',
+  preferredAudioTitle: 'PreferredAudioTitle',
+  preferredSubtitleLanguageCode: 'PreferredSubtitleLanguage'
+};
+
+// The current effective value for an advanced field = user override (if any) else the selected
+// template's value. A CLEAR override reads as none (null), so pickers/previews render "None".
 export function effectiveValue(key: AdvancedKey, template: ChannelTemplate, overrides: Overrides): unknown {
   if (key in overrides) {
-    return overrides[key];
+    return overrides[key] === CLEAR ? null : overrides[key];
   }
   return (template as unknown as Record<string, unknown>)[key];
 }
 
-// Sentinel select value meaning "inherit from the template" — the field is
-// omitted from `advanced` entirely (not sent as null). The API cannot express
-// clear-to-none, so there is no "None" choice; see #135.
-export const INHERIT = '__inherit__';
+// The backend clear-field enum values for every key set to CLEAR in the overrides map.
+export function collectClears(overrides: Overrides): CreateChannelFromLineupClearField[] {
+  const clears: CreateChannelFromLineupClearField[] = [];
+  for (const key of ADVANCED_KEYS) {
+    if (overrides[key] === CLEAR) {
+      const field = CLEARABLE_FIELDS[key];
+      if (field) {
+        clears.push(field);
+      }
+    }
+  }
+  return clears;
+}
+
+// Fold an overrides map into an `advanced` request object: copy every set (non-CLEAR) override, then
+// attach `clear` for any CLEAR-valued key. Shared by both consumers so the CLEAR sentinel never leaks
+// onto the wire as a field value. `base` carries consumer-specific extras (playbackOrder/playoutMode).
+export function applyOverridesToRequest<T extends Record<string, unknown>>(base: T, overrides: Overrides): T {
+  for (const key of ADVANCED_KEYS) {
+    if (!(key in overrides)) {
+      continue;
+    }
+    // A CLEAR sentinel is folded into `clear` below, but only for clearable fields. Any other key
+    // holding the sentinel is only reachable if a user literally typed it into a text input — copy
+    // it through as its value rather than silently dropping the field from the request.
+    if (overrides[key] === CLEAR && key in CLEARABLE_FIELDS) {
+      continue;
+    }
+    (base as Record<string, unknown>)[key] = overrides[key];
+  }
+  const clear = collectClears(overrides);
+  if (clear.length > 0) {
+    (base as Record<string, unknown>).clear = clear;
+  }
+  return base;
+}
 
 // Templates carry no playbackOrder; the Shuffle toggle *is* the template's
 // declared shuffle intent (template.shuffleScheduleItems -> Shuffle/Chronological).
@@ -146,25 +207,30 @@ export function useAdvancedOverrides(
 
   const templateValueOf = (key: AdvancedKey): unknown => (template as unknown as Record<string, unknown>)[key];
 
-  // Override selects: the first option is always "Inherit from template" (the
-  // INHERIT sentinel -> field omitted from the request). There is deliberately
-  // NO "None" option for filler/watermark ids: the create handler coalesces
-  // null with the template value, so clear-to-none is not expressible (#135).
+  // Override selects: the first option is always "Inherit from template" (the INHERIT sentinel ->
+  // field omitted from the request), followed by an explicit "None" (the CLEAR sentinel -> field
+  // added to advanced.clear, forcing it to none even when the template sets one) for clearable
+  // fields, then the concrete rows. "None" is distinct from "Inherit": inherit keeps the template
+  // value, None drops it (#135).
   const idSelectOptions = (key: AdvancedKey, rows: Array<{ id: number; name: string | null }>) => {
     const inherited = templateValueOf(key) as number | null;
     return [
       inheritOption(inherited == null ? null : fillerName(rows, inherited)),
+      ...(key in CLEARABLE_FIELDS ? [{ value: CLEAR, label: 'None' }] : []),
       ...rows.map((row) => ({ value: String(row.id), label: row.name ?? `#${row.id}` }))
     ];
   };
 
-  // Selected value for an override select: the override when present, else INHERIT.
+  // Selected value for an override select: the override when present (CLEAR renders as the "None"
+  // option since String(CLEAR) === CLEAR), else INHERIT.
   const selValue = (key: AdvancedKey): string => (key in overrides ? String(overrides[key]) : INHERIT);
   const onSelect =
     (key: AdvancedKey, map: (raw: string) => unknown) => (event: React.ChangeEvent<HTMLSelectElement>) => {
       const raw = event.target.value;
       if (raw === INHERIT) {
         removeOverride(key);
+      } else if (raw === CLEAR) {
+        setOverride(key, CLEAR);
       } else {
         setOverride(key, map(raw));
       }

web/src/screens/AutoTuneScreen.tsx

@@ -48,6 +48,7 @@ import {
 } from '../api';
 import { registerNavigationGuard } from '../navigationGuard';
 import {
+  applyOverridesToRequest,
   FIXED_START_TIME_BEHAVIORS,
   IDLE_BEHAVIORS,
   MUSIC_VIDEO_CREDITS_MODES,
@@ -325,9 +326,11 @@ export function AutoTuneScreen() {
           }
           channel.logo = logo;
         }
-        const advanced: Overrides & { playbackOrder?: PlaybackOrder; playoutMode?: PlayoutMode } = {
-          ...(override?.advanced ?? {})
-        };
+        // applyOverridesToRequest copies set overrides and folds any CLEAR sentinels into
+        // advanced.clear, so the sentinel never leaks onto the wire as a field value (#135).
+        const advanced = applyOverridesToRequest<
+          Overrides & { playbackOrder?: PlaybackOrder; playoutMode?: PlayoutMode }
+        >({}, override?.advanced ?? {});
         if (override?.playbackOrder != null) {
           advanced.playbackOrder = override.playbackOrder;
         }