Group character beats into arc-stage sections; reciprocal relationship types

Arc stages now group the beats that establish or pay off that stage of a
character's arc (many-to-many via ArcStageBeats), and carry a Result field
(renamed from Description) describing what the stage results in for the
character. Assigning a beat to a stage moves it out of any other stage of
the same character. New endpoint POST /api/arc-stages/{id}/beats, MCP tool
set_arc_stage_beats, and frontend grouping UI in CharacterArc/CharacterBeats.

Also records a relationship's reciprocal type so both characters' dossiers
show the correct direction (e.g. "sister" / "brother") instead of mirroring
the same label.

.editorconfig

@@ -104,6 +104,9 @@ dotnet_style_qualification_for_property = false:suggestion
 dotnet_style_qualification_for_method = false:warning
 dotnet_style_qualification_for_event = false:warning
 
+
+dotnet_diagnostic.CA1873.severity = silent
+
 [*.cs]
 csharp_using_directive_placement = outside_namespace:silent
 csharp_prefer_simple_using_statement = true:suggestion

src/Novelly.Api/Agent/NovelAgentToolset.cs

@@ -456,7 +456,7 @@ public class NovelAgentToolset(
                 var characterId = JsonInput.RequiredGuid(input, "character_id");
                 return await OrNotFound(
                     beats.ListForCharacterAsync(characterId, ct),
-                    list => list.Select(b => b.ToCharacterBeatResponse()),
+                    list => list.Select(b => b.ToCharacterBeatResponse(characterId)),
                     "Character",
                     characterId);
             });

src/Novelly.Api/Beats/Beat.cs

@@ -19,6 +19,8 @@ public class Beat
 
     public List<Character> Characters { get; set; } = [];
 
+    public List<CharacterArcStage> ArcStages { get; set; } = [];
+
     public string? WhatHappened { get; set; }
 
     public string? WhatsNext { get; set; }

src/Novelly.Api/Beats/BeatContracts.cs

@@ -84,7 +84,8 @@ public record CharacterBeatResponse(
     int SortOrder,
     string Title,
     string? WhatHappened,
-    string? WhatsNext);
+    string? WhatsNext,
+    Guid? ArcStageId);
 
 public record ReorderBeatsRequest(IReadOnlyList<Guid> BeatIds);
 
@@ -150,7 +151,7 @@ public static class BeatMapping
         [.. b.Tags.OrderBy(t => t.Name).Select(t => t.ToResponse())],
         b.UpdatedAt);
 
-    public static CharacterBeatResponse ToCharacterBeatResponse(this Beat b) => new(
+    public static CharacterBeatResponse ToCharacterBeatResponse(this Beat b, Guid characterId) => new(
         b.Id,
         b.ChapterId,
         b.Chapter?.Number ?? 0,
@@ -158,5 +159,6 @@ public static class BeatMapping
         b.SortOrder,
         b.Title,
         b.WhatHappened,
-        b.WhatsNext);
+        b.WhatsNext,
+        b.ArcStages.FirstOrDefault(s => s.CharacterId == characterId)?.Id);
 }

src/Novelly.Api/Beats/BeatEndpoints.cs

@@ -46,7 +46,7 @@ public static class BeatEndpoints
 
         app.MapGet("/api/characters/{characterId:guid}/beats", async (
                 Guid characterId, BeatService service, CancellationToken ct) =>
-                (await service.ListForCharacterAsync(characterId, ct))?.Select(b => b.ToCharacterBeatResponse()).ToList().ToApiResult())
+                (await service.ListForCharacterAsync(characterId, ct))?.Select(b => b.ToCharacterBeatResponse(characterId)).ToList().ToApiResult())
             .WithTags("Beats")
             .WithSummary("Every beat this character appears in, in manuscript order.");
 

src/Novelly.Api/Beats/BeatService.cs

@@ -68,6 +68,7 @@ public class BeatService(
 
         var beats = await db.Beats
             .Include(b => b.Chapter)
+            .Include(b => b.ArcStages)
             .Where(b => b.Characters.Any(c => c.Id == characterId))
             .ToListAsync(ct);
 

src/Novelly.Api/Characters/CharacterArcService.cs

@@ -12,7 +12,8 @@ public class CharacterArcService(
     ILogger<CharacterArcService> logger,
     IModelValidator<CreateArcStageRequest> createValidator,
     IModelValidator<UpdateArcStageRequest> updateValidator,
-    IModelValidator<ReorderArcStagesRequest> reorderValidator)
+    IModelValidator<ReorderArcStagesRequest> reorderValidator,
+    IModelValidator<SetArcStageBeatsRequest> setBeatsValidator)
 {
     public async Task<IReadOnlyList<CharacterArcStage>> ListAsync(Guid characterId, CancellationToken ct = default)
     {
@@ -70,7 +71,7 @@ public class CharacterArcService(
             CharacterId = characterId,
             Title = request.Title,
             SortOrder = request.SortOrder ?? await NextSortOrderAsync(characterId, ct),
-            Description = request.Description,
+            Result = request.Result,
             ChapterId = request.ChapterId
         };
 
@@ -107,7 +108,7 @@ public class CharacterArcService(
 
         stage.Title = Patch.Apply(stage.Title, request.Title) ?? stage.Title;
         stage.SortOrder = request.SortOrder ?? stage.SortOrder;
-        stage.Description = Patch.Apply(stage.Description, request.Description);
+        stage.Result = Patch.Apply(stage.Result, request.Result);
         stage.ChapterId = request.ChapterId ?? stage.ChapterId;
         stage.UpdatedAt = DateTimeOffset.UtcNow;
 
@@ -171,6 +172,67 @@ public class CharacterArcService(
         return await ListAsync(characterId, ct);
     }
 
+    public async Task<CharacterArcStage?> SetBeatsAsync(
+        Guid stageId, SetArcStageBeatsRequest request, CancellationToken ct = default)
+    {
+        Guard.Default(stageId, nameof(stageId));
+        Guard.Null(request, nameof(request));
+        setBeatsValidator.Validate(request).ThrowIfInvalid(logger);
+
+        logger.LogInformation("Setting {Count} beats for arc stage {ArcStageId}", request.BeatIds.Count, stageId);
+
+        var stage = await FindAsync(stageId, ct);
+        if (stage is null)
+        {
+            return null;
+        }
+
+        var character = await db.Characters.FirstOrDefaultAsync(c => c.Id == stage.CharacterId, ct);
+        if (character is null)
+        {
+            logger.LogError("Arc stage {ArcStageId} references character {CharacterId} which does not exist", stageId, stage.CharacterId);
+            return null;
+        }
+
+        await access.RequireAsync(character.ProjectId, ProjectPermission.Write, ct);
+
+        var beats = await db.Beats
+            .Include(b => b.Characters)
+            .Include(b => b.ArcStages)
+            .Where(b => request.BeatIds.Contains(b.Id))
+            .ToListAsync(ct);
+
+        var missing = request.BeatIds.Where(id => beats.All(b => b.Id != id)).ToList();
+        if (missing.Count > 0)
+        {
+            logger.LogWarning("Rejected arc stage beat assignment: arc stage {ArcStageId} referenced missing beat {BeatId}", stageId, missing[0]);
+            return null;
+        }
+
+        var unrelated = beats.Where(b => b.Characters.All(c => c.Id != stage.CharacterId)).ToList();
+        if (unrelated.Count > 0)
+        {
+            logger.LogWarning(
+                "Rejected arc stage beat assignment: beat {BeatId} does not include character {CharacterId}",
+                unrelated[0].Id, stage.CharacterId);
+            throw new InvalidOperationException("A beat can only be grouped into an arc stage for a character who appears in it.");
+        }
+
+        foreach (var beat in beats)
+        {
+            foreach (var sibling in beat.ArcStages.Where(s => s.CharacterId == stage.CharacterId && s.Id != stageId).ToList())
+            {
+                beat.ArcStages.Remove(sibling);
+            }
+        }
+
+        stage.Beats = beats;
+        stage.UpdatedAt = DateTimeOffset.UtcNow;
+
+        await db.SaveChangesAsync(ct);
+        return (await FindAsync(stageId, ct))!;
+    }
+
     private async Task EnsureChapterIsInSameProjectAsync(
         Character character, Guid? chapterId, CancellationToken ct)
     {
@@ -212,7 +274,10 @@ public class CharacterArcService(
         await access.RequireAsync(projectId, permission, ct);
     }
 
-    private IQueryable<CharacterArcStage> Query() => db.CharacterArcStages.Include(s => s.Chapter);
+    private IQueryable<CharacterArcStage> Query() =>
+        db.CharacterArcStages
+            .Include(s => s.Chapter)
+            .Include(s => s.Beats).ThenInclude(b => b.Chapter);
 
     private async Task<CharacterArcStage?> FindAsync(Guid id, CancellationToken ct)
     {

src/Novelly.Api/Characters/CharacterArcStage.cs

@@ -1,5 +1,6 @@
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using Novelly.Api.Beats;
 using Novelly.Api.Chapters;
 
 namespace Novelly.Api.Characters;
@@ -15,11 +16,13 @@ public class CharacterArcStage
 
     public string Title { get; set; } = string.Empty;
 
-    public string? Description { get; set; }
+    public string? Result { get; set; }
 
     public Guid? ChapterId { get; set; }
     public Chapter? Chapter { get; init; }
 
+    public List<Beat> Beats { get; set; } = [];
+
     public DateTimeOffset CreatedAt { get; init; } = DateTimeOffset.UtcNow;
     public DateTimeOffset UpdatedAt { get; set; } = DateTimeOffset.UtcNow;
 }
@@ -33,5 +36,8 @@ public class CharacterArcStageEntityTypeConfiguration : IEntityTypeConfiguration
 
         entity.HasOne(s => s.Chapter).WithMany()
             .HasForeignKey(s => s.ChapterId).OnDelete(DeleteBehavior.SetNull);
+
+        entity.HasMany(s => s.Beats).WithMany(b => b.ArcStages)
+            .UsingEntity(join => join.ToTable("ArcStageBeats"));
     }
 }

src/Novelly.Api/Characters/CharacterContracts.cs

@@ -1,3 +1,4 @@
+using Novelly.Api.Beats;
 using Novelly.Api.Common.Validation;
 using Novelly.Api.Tags;
 
@@ -162,7 +163,8 @@ file static class CharacterValidation
 public record CreateRelationshipRequest(
     Guid RelatedCharacterId,
     string RelationshipType,
-    string? Description = null);
+    string? Description = null,
+    string? ReciprocalRelationshipType = null);
 
 public class CreateRelationshipRequestValidator : IModelValidator<CreateRelationshipRequest>
 {
@@ -175,6 +177,7 @@ public class CreateRelationshipRequestValidator : IModelValidator<CreateRelation
 
         result.AddRequiredTextErrors("RelationshipType", "Relationship Type", model.RelationshipType, 100);
         result.AddOptionalTextErrors("Description", "Description", model.Description, 2000);
+        result.AddOptionalTextErrors("ReciprocalRelationshipType", "Reciprocal Relationship Type", model.ReciprocalRelationshipType, 100);
 
         return result;
     }
@@ -205,16 +208,17 @@ public record ArcStageResponse(
     Guid CharacterId,
     int SortOrder,
     string Title,
-    string? Description,
+    string? Result,
     Guid? ChapterId,
     int? ChapterNumber,
     string? ChapterTitle,
+    IReadOnlyList<CharacterBeatResponse> Beats,
     DateTimeOffset UpdatedAt);
 
 public record CreateArcStageRequest(
     string Title,
     int? SortOrder = null,
-    string? Description = null,
+    string? Result = null,
     Guid? ChapterId = null);
 
 public class CreateArcStageRequestValidator : IModelValidator<CreateArcStageRequest>
@@ -224,7 +228,7 @@ public class CreateArcStageRequestValidator : IModelValidator<CreateArcStageRequ
         var result = new ValidationResult();
 
         result.AddRequiredTextErrors("Title", "Title", model.Title, 200);
-        ArcStageValidation.OptionalFields(model.SortOrder, model.Description, result);
+        ArcStageValidation.OptionalFields(model.SortOrder, model.Result, result);
 
         return result;
     }
@@ -233,7 +237,7 @@ public class CreateArcStageRequestValidator : IModelValidator<CreateArcStageRequ
 public record UpdateArcStageRequest(
     string? Title = null,
     int? SortOrder = null,
-    string? Description = null,
+    string? Result = null,
     Guid? ChapterId = null);
 
 public class UpdateArcStageRequestValidator : IModelValidator<UpdateArcStageRequest>
@@ -243,7 +247,7 @@ public class UpdateArcStageRequestValidator : IModelValidator<UpdateArcStageRequ
         var result = new ValidationResult();
 
         result.AddUnclearableTextErrors("Title", "Title", model.Title, "an arc stage", 200);
-        ArcStageValidation.OptionalFields(model.SortOrder, model.Description, result);
+        ArcStageValidation.OptionalFields(model.SortOrder, model.Result, result);
 
         return result;
     }
@@ -251,13 +255,13 @@ public class UpdateArcStageRequestValidator : IModelValidator<UpdateArcStageRequ
 
 file static class ArcStageValidation
 {
-    public static void OptionalFields(int? sortOrder, string? description, ValidationResult result)
+    public static void OptionalFields(int? sortOrder, string? result_, ValidationResult result)
     {
         if (sortOrder is < 0)
             result.AddError("SortOrder", "'Sort Order' must be zero or greater.");
 
-        if (description is { Length: > 20000 })
+        if (result_ is { Length: > 20000 })
-            result.AddError("Description", "'Description' must be 20,000 characters or fewer.");
+            result.AddError("Result", "'Result' must be 20,000 characters or fewer.");
     }
 }
 
@@ -276,6 +280,21 @@ public class ReorderArcStagesRequestValidator : IModelValidator<ReorderArcStages
     }
 }
 
+public record SetArcStageBeatsRequest(IReadOnlyList<Guid> BeatIds);
+
+public class SetArcStageBeatsRequestValidator : IModelValidator<SetArcStageBeatsRequest>
+{
+    public ValidationResult Validate(SetArcStageBeatsRequest model)
+    {
+        var result = new ValidationResult();
+
+        if (model.BeatIds is null)
+            result.AddError("BeatIds", "'Beat Ids' must not be null.");
+
+        return result;
+    }
+}
+
 
 public static class CharacterMapping
 {
@@ -305,9 +324,13 @@ public static class CharacterMapping
         s.CharacterId,
         s.SortOrder,
         s.Title,
-        s.Description,
+        s.Result,
         s.ChapterId,
         s.Chapter?.Number,
         s.Chapter?.Title,
+        [.. s.Beats
+            .OrderBy(b => b.Chapter?.Number ?? 0)
+            .ThenBy(b => b.SortOrder)
+            .Select(b => b.ToCharacterBeatResponse(s.CharacterId))],
         s.UpdatedAt);
 }

src/Novelly.Api/Characters/CharacterEndpoints.cs

@@ -107,6 +107,12 @@ public static class CharacterEndpoints
                 await service.DeleteAsync(id, ct) ? Results.NoContent() : Results.NotFound())
             .WithSummary("Delete an arc stage.");
 
+        arcStages.MapPost("/{id:guid}/beats", async (
+                Guid id, SetArcStageBeatsRequest request, CharacterArcService service, CancellationToken ct) =>
+                (await service.SetBeatsAsync(id, request, ct))?.ToResponse().ToApiResult())
+            .WithSummary("Set which beats belong to this arc stage, replacing its current set. "
+                         + "A beat moved into this stage leaves any other stage of the same character it was in.");
+
         return app;
     }
 }

src/Novelly.Api/Characters/CharacterService.cs

@@ -214,6 +214,14 @@ public class CharacterService(
             Description = request.Description
         });
 
+        db.CharacterRelationships.Add(new CharacterRelationship
+        {
+            CharacterId = request.RelatedCharacterId,
+            RelatedCharacterId = characterId,
+            RelationshipType = request.ReciprocalRelationshipType ?? request.RelationshipType,
+            Description = request.Description
+        });
+
         await db.SaveChangesAsync(ct);
         return (await FindAsync(characterId, ct))!;
     }
@@ -235,7 +243,12 @@ public class CharacterService(
 
         await access.RequireAsync(relationship.Character!.ProjectId, ProjectPermission.Write, ct);
 
+        var reciprocals = await db.CharacterRelationships
+            .Where(r => r.CharacterId == relationship.RelatedCharacterId && r.RelatedCharacterId == relationship.CharacterId)
+            .ToListAsync(ct);
+
         db.CharacterRelationships.Remove(relationship);
+        db.CharacterRelationships.RemoveRange(reciprocals);
         await db.SaveChangesAsync(ct);
         return true;
     }
@@ -338,6 +351,9 @@ public class CharacterService(
             .Include(c => c.Tags)
             .Include(c => c.ArcStages)
             .ThenInclude(s => s.Chapter)
+            .Include(c => c.ArcStages)
+            .ThenInclude(s => s.Beats)
+            .ThenInclude(b => b.Chapter)
             .Include(c => c.SameCharacterAs)
             .Include(c => c.OtherIdentities)
             .Include(c => c.RevealedInChapter);

src/Novelly.Api/Data/Migrations/20260818022141_AddArcStageResultAndBeats.cs

@@ -0,0 +1,61 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Novelly.Api.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddArcStageResultAndBeats : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.RenameColumn(
+                name: "Description",
+                table: "CharacterArcStages",
+                newName: "Result");
+
+            migrationBuilder.CreateTable(
+                name: "ArcStageBeats",
+                columns: table => new
+                {
+                    ArcStagesId = table.Column<Guid>(type: "TEXT", nullable: false),
+                    BeatsId = table.Column<Guid>(type: "TEXT", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_ArcStageBeats", x => new { x.ArcStagesId, x.BeatsId });
+                    table.ForeignKey(
+                        name: "FK_ArcStageBeats_Beats_BeatsId",
+                        column: x => x.BeatsId,
+                        principalTable: "Beats",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                    table.ForeignKey(
+                        name: "FK_ArcStageBeats_CharacterArcStages_ArcStagesId",
+                        column: x => x.ArcStagesId,
+                        principalTable: "CharacterArcStages",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ArcStageBeats_BeatsId",
+                table: "ArcStageBeats",
+                column: "BeatsId");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "ArcStageBeats");
+
+            migrationBuilder.RenameColumn(
+                name: "Result",
+                table: "CharacterArcStages",
+                newName: "Description");
+        }
+    }
+}

src/Novelly.Api/Data/Migrations/NovelDbContextModelSnapshot.cs

@@ -32,6 +32,21 @@ namespace Novelly.Api.Data.Migrations
                     b.ToTable("BeatCharacters", (string)null);
                 });
 
+            modelBuilder.Entity("BeatCharacterArcStage", b =>
+                {
+                    b.Property<Guid>("ArcStagesId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("BeatsId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("ArcStagesId", "BeatsId");
+
+                    b.HasIndex("BeatsId");
+
+                    b.ToTable("ArcStageBeats", (string)null);
+                });
+
             modelBuilder.Entity("BeatTag", b =>
                 {
                     b.Property<Guid>("BeatsId")
@@ -398,7 +413,7 @@ namespace Novelly.Api.Data.Migrations
                     b.Property<long>("CreatedAt")
                         .HasColumnType("INTEGER");
 
-                    b.Property<string>("Description")
+                    b.Property<string>("Result")
                         .HasColumnType("TEXT");
 
                     b.Property<int>("SortOrder")
@@ -856,6 +871,21 @@ namespace Novelly.Api.Data.Migrations
                         .IsRequired();
                 });
 
+            modelBuilder.Entity("BeatCharacterArcStage", b =>
+                {
+                    b.HasOne("Novelly.Api.Characters.CharacterArcStage", null)
+                        .WithMany()
+                        .HasForeignKey("ArcStagesId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.HasOne("Novelly.Api.Beats.Beat", null)
+                        .WithMany()
+                        .HasForeignKey("BeatsId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
             modelBuilder.Entity("BeatTag", b =>
                 {
                     b.HasOne("Novelly.Api.Beats.Beat", null)

src/Novelly.Api/Imports/ImportAgentToolset.cs

@@ -359,7 +359,7 @@ public class ImportAgentToolset(
                     characterId,
                     new CreateArcStageRequest(
                         JsonInput.RequiredString(input, "title"),
-                        Description: JsonInput.String(input, "description"),
+                        Result: JsonInput.String(input, "description"),
                         ChapterId: JsonInput.Guid(input, "chapter_id")), ct);
 
                 return created is null

src/Novelly.Api/Projects/ProjectContracts.cs

@@ -77,11 +77,9 @@ public class UpdateProjectRequestValidator : IModelValidator<UpdateProjectReques
 
 file static class ProjectValidation
 {
-    public static void Title(string title, ValidationResult result) =>
+    public static void Title(string title, ValidationResult result) => result.AddRequiredTextErrors("Title", "Title", title, 200);
-        result.AddRequiredTextErrors("Title", "Title", title, 200);
 
-    public static void OptionalFields(
+    public static void OptionalFields(string? author, string? genre, string? logline, string? synopsis, string? notes, int? targetWordCount, ValidationResult result)
-        string? author, string? genre, string? logline, string? synopsis, string? notes, int? targetWordCount, ValidationResult result)
     {
         if (author is { Length: > 200 })
             result.AddError("Author", "'Author' must be 200 characters or fewer.");

src/Novelly.Api/Projects/ProjectService.cs

@@ -42,10 +42,7 @@ public class ProjectService(
         logger.LogInformation("Getting project {ProjectId}", id);
 
         var project = await FindAsync(id, ct);
-        if (project is null)
+        if (project is null) return null;
-        {
-            return null;
-        }
 
         await access.RequireAsync(id, ProjectPermission.Read, ct);
         return project;
@@ -85,10 +82,7 @@ public class ProjectService(
         logger.LogInformation("Updating project {ProjectId}", id);
 
         var project = await FindAsync(id, ct);
-        if (project is null)
+        if (project is null) return null;
-        {
-            return null;
-        }
 
         await access.RequireAsync(id, ProjectPermission.Write, ct);
 
@@ -113,10 +107,7 @@ public class ProjectService(
         logger.LogInformation("Deleting project {ProjectId}", id);
 
         var project = await FindAsync(id, ct);
-        if (project is null)
+        if (project is null) return false;
-        {
-            return false;
-        }
 
         await access.RequireAsync(id, ProjectPermission.DeleteContent, ct);
 

src/Novelly.Api/Users/NovellyUser.cs

@@ -6,9 +6,9 @@ namespace Novelly.Api.Users;
 
 public class NovellyUser : IdentityUser<Guid>
 {
-    public string DisplayName { get; set; } = string.Empty;
+    public string DisplayName { get; init; } = string.Empty;
     public GlobalRole GlobalRole { get; set; } = GlobalRole.Reviewer;
-    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset CreatedAt { get; init; } = DateTimeOffset.UtcNow;
 }
 
 public class NovellyUserEntityTypeConfiguration : IEntityTypeConfiguration<NovellyUser>

src/Novelly.Mcp/Tools/CharacterTools.cs

@@ -147,11 +147,11 @@ public static class CharacterTools
         [Description("Id of the character whose arc to add to.")] Guid characterId,
         [Description("A short handle for the change, three to five words.")] string title,
         CancellationToken ct,
-        [Description("What shifts in the character here, and what it costs them.")] string? description = null,
+        [Description("What this stage of the arc results in for the character — what shifts, and what it costs them.")] string? result = null,
         [Description("Id of the chapter where this stage lands, if it is pinned to one.")] Guid? chapterId = null,
         [Description("Position in the arc. Appended to the end when omitted.")] int? sortOrder = null) =>
         api.PostAsync($"/api/characters/{characterId}/arc",
-            new { title, sortOrder, description, chapterId }, ct);
+            new { title, sortOrder, result, chapterId }, ct);
 
     [McpServerTool(Name = "update_arc_stage")]
     [Description("Revise a stage of a character's arc. Only the fields you supply change.")]
@@ -160,11 +160,11 @@ public static class CharacterTools
         [Description("The arc stage's id.")] Guid arcStageId,
         CancellationToken ct,
         [Description("New title for the stage.")] string? title = null,
-        [Description("What shifts in the character here.")] string? description = null,
+        [Description("What this stage of the arc results in for the character.")] string? result = null,
         [Description("Id of the chapter where this stage lands.")] Guid? chapterId = null,
         [Description("Position in the arc.")] int? sortOrder = null) =>
         api.PatchAsync($"/api/arc-stages/{arcStageId}",
-            new { title, sortOrder, description, chapterId }, ct);
+            new { title, sortOrder, result, chapterId }, ct);
 
     [McpServerTool(Name = "delete_arc_stage")]
     [Description("Remove a stage from a character's arc.")]
@@ -184,17 +184,34 @@ public static class CharacterTools
         CancellationToken ct) =>
         api.PostAsync($"/api/characters/{characterId}/arc/reorder", new { stageIds }, ct);
 
+    [McpServerTool(Name = "set_arc_stage_beats")]
+    [Description("Set which beats belong to an arc stage, replacing its current set. This groups the "
+                 + "chapter-level beats that establish or pay off this stage of the character's arc. A "
+                 + "beat moved into this stage leaves any other stage of the same character it was in. "
+                 + "Each beat must already include this character.")]
+    public static Task<CallToolResult> SetArcStageBeats(
+        NovelApiClient api,
+        [Description("The arc stage's id.")] Guid arcStageId,
+        [Description("Beat ids that belong to this stage, replacing whatever was there before.")] string[] beatIds,
+        CancellationToken ct) =>
+        api.PostAsync($"/api/arc-stages/{arcStageId}/beats", new { beatIds }, ct);
+
     [McpServerTool(Name = "relate_characters")]
-    [Description("Record a relationship from one character to another in the same project.")]
+    [Description("Record a relationship between two characters in the same project. Creates both directions "
+                 + "at once — characterId's side and relatedCharacterId's side — so the pair always shows up "
+                 + "on both dossiers.")]
     public static Task<CallToolResult> RelateCharacters(
         NovelApiClient api,
         [Description("Id of the character the relationship belongs to.")] Guid characterId,
         [Description("Id of the character they are related to.")] Guid relatedCharacterId,
-        [Description("How they are related, e.g. 'sister', 'rival', 'former mentor'.")] string relationshipType,
+        [Description("How characterId is related to relatedCharacterId, e.g. 'sister', 'rival', 'former mentor'.")] string relationshipType,
         CancellationToken ct,
+        [Description("How relatedCharacterId is related back to characterId, if different — e.g. 'brother' for "
+                     + "'sister'. Defaults to relationshipType when the relation is symmetric, like 'rival'.")]
+        string? reciprocalRelationshipType = null,
         [Description("What the relationship is like, and where it is headed.")] string? description = null) =>
         api.PostAsync($"/api/characters/{characterId}/relationships",
-            new { relatedCharacterId, relationshipType, description }, ct);
+            new { relatedCharacterId, relationshipType, reciprocalRelationshipType, description }, ct);
 
     [McpServerTool(Name = "link_character_identity")]
     [Description("Record that this character is really another character — e.g. a character introduced "

src/Novelly.Web/src/api/hooks.ts

@@ -204,6 +204,40 @@ export function useUnlinkCharacterIdentity(projectId: string) {
   })
 }
 
+export function useAddRelationship(projectId: string) {
+  const qc = useQueryClient()
+  return useMutation({
+    mutationFn: ({
+      id,
+      relatedCharacterId,
+      relationshipType,
+      reciprocalRelationshipType,
+      description,
+    }: {
+      id: string
+      relatedCharacterId: string
+      relationshipType: string
+      reciprocalRelationshipType?: string | null
+      description?: string | null
+    }) =>
+      api.post<Character>(`/api/characters/${id}/relationships`, {
+        relatedCharacterId,
+        relationshipType,
+        reciprocalRelationshipType,
+        description,
+      }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: keys.characters(projectId) }),
+  })
+}
+
+export function useRemoveRelationship(projectId: string) {
+  const qc = useQueryClient()
+  return useMutation({
+    mutationFn: (relationshipId: string) => api.delete(`/api/characters/relationships/${relationshipId}`),
+    onSuccess: () => qc.invalidateQueries({ queryKey: keys.characters(projectId) }),
+  })
+}
+
 export const useCharacterBeats = (characterId: string | undefined) =>
   useQuery({
     queryKey: keys.characterBeats(characterId ?? ''),
@@ -214,7 +248,7 @@ export const useCharacterBeats = (characterId: string | undefined) =>
 export function useCreateArcStage(projectId: string) {
   const qc = useQueryClient()
   return useMutation({
-    mutationFn: ({ characterId, ...body }: { characterId: string; title: string; description?: string; chapterId?: string }) =>
+    mutationFn: ({ characterId, ...body }: { characterId: string; title: string; result?: string; chapterId?: string }) =>
       api.post<ArcStage>(`/api/characters/${characterId}/arc`, body),
     onSuccess: () => qc.invalidateQueries({ queryKey: keys.characters(projectId) }),
   })
@@ -223,12 +257,24 @@ export function useCreateArcStage(projectId: string) {
 export function useUpdateArcStage(projectId: string) {
   const qc = useQueryClient()
   return useMutation({
-    mutationFn: ({ id, ...body }: { id: string; title?: string; description?: string; chapterId?: string }) =>
+    mutationFn: ({ id, ...body }: { id: string; title?: string; result?: string; chapterId?: string }) =>
       api.patch<ArcStage>(`/api/arc-stages/${id}`, body),
     onSuccess: () => qc.invalidateQueries({ queryKey: keys.characters(projectId) }),
   })
 }
 
+export function useSetArcStageBeats(projectId: string, characterId: string | undefined) {
+  const qc = useQueryClient()
+  return useMutation({
+    mutationFn: ({ id, beatIds }: { id: string; beatIds: string[] }) =>
+      api.post<ArcStage>(`/api/arc-stages/${id}/beats`, { beatIds }),
+    onSuccess: () => {
+      qc.invalidateQueries({ queryKey: keys.characters(projectId) })
+      qc.invalidateQueries({ queryKey: keys.characterBeats(characterId ?? '') })
+    },
+  })
+}
+
 export function useDeleteArcStage(projectId: string) {
   const qc = useQueryClient()
   return useMutation({

src/Novelly.Web/src/api/types.ts

@@ -151,10 +151,11 @@ export interface ArcStage {
   characterId: string
   sortOrder: number
   title: string
-  description: string | null
+  result: string | null
   chapterId: string | null
   chapterNumber: number | null
   chapterTitle: string | null
+  beats: CharacterBeat[]
   updatedAt: string
 }
 
@@ -167,6 +168,7 @@ export interface CharacterBeat {
   title: string
   whatHappened: string | null
   whatsNext: string | null
+  arcStageId: string | null
 }
 
 export interface Character {

src/Novelly.Web/src/components/CharacterArc.tsx

@@ -1,10 +1,12 @@
 import { useState } from 'react'
 import { Link } from 'react-router-dom'
 import {
+  useCharacterBeats,
   useChapters,
   useCreateArcStage,
   useDeleteArcStage,
   useReorderArcStages,
+  useSetArcStageBeats,
   useUpdateArcStage,
 } from '../api/hooks'
 import type { ArcStage, Character } from '../api/types'
@@ -24,12 +26,14 @@ export function CharacterArc({
   canDelete: boolean
 }) {
   const { data: chapters } = useChapters(projectId)
+  const { data: beats } = useCharacterBeats(character.id)
   const create = useCreateArcStage(projectId)
   const reorder = useReorderArcStages(projectId)
 
   const [title, setTitle] = useState('')
 
   const stages = character.arcStages
+  const unassignedBeats = (beats ?? []).filter((b) => b.arcStageId === null)
 
   const submit = (e: React.FormEvent) => {
     e.preventDefault()
@@ -56,8 +60,9 @@ export function CharacterArc({
         )}
       </div>
       <p className="mb-3 text-xs muted">
-        The changes {character.name} goes through, in order. Pin a stage to the chapter it
+        The sections {character.name}&rsquo;s arc breaks into, in order — each one a short span of
-        lands in and it links into that outline.
+        beats and what it results in for them. Pin a section to the chapter it lands in and it
+        links into that outline.
       </p>
 
       {stages.length > 0 && (
@@ -68,6 +73,7 @@ export function CharacterArc({
               projectId={projectId}
               stage={stage}
               chapters={chapters ?? []}
+              unassignedBeats={unassignedBeats}
               canMoveUp={index > 0}
               canMoveDown={index < stages.length - 1}
               onMove={(delta) => move(index, delta)}
@@ -78,11 +84,18 @@ export function CharacterArc({
         </ol>
       )}
 
+      {unassignedBeats.length > 0 && (
+        <p className="mt-3 text-xs muted">
+          {unassignedBeats.length} beat{unassignedBeats.length === 1 ? '' : 's'} not yet grouped
+          into a section — add {character.name} to a section above, or check the Beats list below.
+        </p>
+      )}
+
       {canCreate && (
         <form onSubmit={submit} className="mt-3 flex gap-2">
           <input
             className="input flex-1"
-            placeholder="Add a stage — three to five words, e.g. “she stops covering for him”"
+            placeholder="Add a section — a short title, e.g. “spoiled noble”"
             value={title}
             onChange={(e) => setTitle(e.target.value)}
           />
@@ -105,6 +118,7 @@ function ArcStageRow({
   projectId,
   stage,
   chapters,
+  unassignedBeats,
   canMoveUp,
   canMoveDown,
   onMove,
@@ -114,6 +128,7 @@ function ArcStageRow({
   projectId: string
   stage: ArcStage
   chapters: { id: string; number: number; title: string }[]
+  unassignedBeats: { id: string; chapterNumber: number; sortOrder: number; title: string }[]
   canMoveUp: boolean
   canMoveDown: boolean
   onMove: (delta: number) => void
@@ -122,6 +137,16 @@ function ArcStageRow({
 }) {
   const update = useUpdateArcStage(projectId)
   const remove = useDeleteArcStage(projectId)
+  const setBeats = useSetArcStageBeats(projectId, stage.characterId)
+
+  const addBeat = (beatId: string) => {
+    if (!beatId) return
+    setBeats.mutate({ id: stage.id, beatIds: [...stage.beats.map((b) => b.id), beatId] })
+  }
+
+  const removeBeat = (beatId: string) => {
+    setBeats.mutate({ id: stage.id, beatIds: stage.beats.filter((b) => b.id !== beatId).map((b) => b.id) })
+  }
 
   return (
     <li
@@ -138,14 +163,59 @@ function ArcStageRow({
             readOnly={!canWrite}
           />
           <AutoField
-            value={stage.description}
+            value={stage.result}
             multiline
             rows={2}
-            placeholder="What shifts here, and what it costs them."
+            placeholder="What this results in for them — what shifts, and what it costs."
-            onCommit={(description) => update.mutate({ id: stage.id, description })}
+            onCommit={(result) => update.mutate({ id: stage.id, result })}
             readOnly={!canWrite}
           />
 
+          {stage.beats.length > 0 && (
+            <ul className="grid gap-1">
+              {stage.beats.map((beat) => (
+                <li
+                  key={beat.id}
+                  className="flex items-center gap-2 rounded px-2 py-1 text-xs"
+                  style={{ background: 'var(--surface-1, rgba(0,0,0,0.015))' }}
+                >
+                  <Link
+                    className="shrink-0 tabular-nums underline"
+                    style={{ color: 'var(--accent)' }}
+                    to={`/projects/${projectId}/chapters/${beat.chapterId}#beat-${beat.id}`}
+                  >
+                    {beat.chapterNumber}.{beat.sortOrder}
+                  </Link>
+                  <span className="min-w-0 flex-1 truncate">{beat.title}</span>
+                  {canWrite && (
+                    <button
+                      className="btn shrink-0 px-1.5 py-0 text-xs"
+                      onClick={() => removeBeat(beat.id)}
+                      aria-label={`Remove beat ${beat.title} from this section`}
+                    >
+                      ✕
+                    </button>
+                  )}
+                </li>
+              ))}
+            </ul>
+          )}
+
+          {canWrite && unassignedBeats.length > 0 && (
+            <select
+              className="input py-1 text-xs"
+              value=""
+              onChange={(e) => addBeat(e.target.value)}
+            >
+              <option value="">Add a beat to this section…</option>
+              {unassignedBeats.map((beat) => (
+                <option key={beat.id} value={beat.id}>
+                  {beat.chapterNumber}.{beat.sortOrder} — {beat.title}
+                </option>
+              ))}
+            </select>
+          )}
+
           <div className="flex flex-wrap items-center gap-2">
             <select
               className="input max-w-[16rem] py-1 text-xs"

src/Novelly.Web/src/components/CharacterBeats.tsx

@@ -1,17 +1,21 @@
 import { Link } from 'react-router-dom'
 import { useCharacterBeats } from '../api/hooks'
+import type { ArcStage } from '../api/types'
 import { ErrorNote, Spinner } from './ui'
 
 export function CharacterBeats({
   projectId,
   characterId,
   characterName,
+  arcStages,
 }: {
   projectId: string
   characterId: string
   characterName: string
+  arcStages: ArcStage[]
 }) {
   const { data: beats, isPending, error } = useCharacterBeats(characterId)
+  const stageTitleById = new Map(arcStages.map((s) => [s.id, s.title]))
 
   return (
     <section className="card mt-6 p-5">
@@ -33,6 +37,7 @@ export function CharacterBeats({
               <tr className="text-left text-xs uppercase muted">
                 <th className="py-1 pr-3 font-semibold">Chapter</th>
                 <th className="py-1 pr-3 font-semibold">Beat</th>
+                <th className="py-1 pr-3 font-semibold">Arc section</th>
                 <th className="py-1 pr-3 font-semibold">What happened</th>
                 <th className="py-1 font-semibold">What&rsquo;s next</th>
               </tr>
@@ -51,6 +56,9 @@ export function CharacterBeats({
                     <div className="text-xs muted">{beat.chapterTitle}</div>
                   </td>
                   <td className="py-2 pr-3 font-medium">{beat.title}</td>
+                  <td className="py-2 pr-3 muted">
+                    {beat.arcStageId ? (stageTitleById.get(beat.arcStageId) ?? '—') : '—'}
+                  </td>
                   <td className="py-2 pr-3 muted">{beat.whatHappened}</td>
                   <td className="py-2 muted">{beat.whatsNext}</td>
                 </tr>

src/Novelly.Web/src/pages/CharacterDetailPage.tsx

@@ -1,11 +1,13 @@
 import { useState } from 'react'
 import { Link, useNavigate, useParams } from 'react-router-dom'
 import {
+  useAddRelationship,
   useChapters,
   useCharacters,
   useDeleteCharacter,
   useLinkCharacterIdentity,
   useProject,
+  useRemoveRelationship,
   useTags,
   useUnlinkCharacterIdentity,
   useUpdateCharacter,
@@ -84,6 +86,8 @@ function CharacterSheet({
   const remove = useDeleteCharacter(projectId)
   const linkIdentity = useLinkCharacterIdentity(projectId)
   const unlinkIdentity = useUnlinkCharacterIdentity(projectId)
+  const addRelationship = useAddRelationship(projectId)
+  const removeRelationship = useRemoveRelationship(projectId)
   const [confirmingDelete, setConfirmingDelete] = useState(false)
   const patch = (body: Partial<Omit<Character, 'tags' | 'aliases'>> & { tags?: string[]; aliases?: string[] }) =>
     update.mutate({ id: character.id, ...body })
@@ -233,21 +237,6 @@ function CharacterSheet({
         />
       </div>
 
-      {character.relationships.length > 0 && (
-        <div className="mt-6">
-          <h3 className="label">Relationships</h3>
-          <ul className="grid gap-1 text-sm">
-            {character.relationships.map((relationship) => (
-              <li key={relationship.id}>
-                <span className="font-medium">{relationship.relatedCharacterName}</span>
-                <span className="muted"> — {relationship.relationshipType}</span>
-                {relationship.description && <span className="muted">: {relationship.description}</span>}
-              </li>
-            ))}
-          </ul>
-        </div>
-      )}
-
       <div className="mt-6">
         <IdentitySection
           character={character}
@@ -273,6 +262,23 @@ function CharacterSheet({
       )}
     </div>
 
+    <RelationshipsSection
+      character={character}
+      allCharacters={allCharacters ?? []}
+      canWrite={canWrite}
+      onAdd={(relatedCharacterId, relationshipType, reciprocalRelationshipType, description) =>
+        addRelationship.mutate({
+          id: character.id,
+          relatedCharacterId,
+          relationshipType,
+          reciprocalRelationshipType,
+          description,
+        })
+      }
+      onRemove={(relationshipId) => removeRelationship.mutate(relationshipId)}
+      error={addRelationship.error}
+    />
+
     {(character.importance === 'Main' || character.arcStages.length > 0) && (
       <CharacterArc
         projectId={projectId}
@@ -287,6 +293,7 @@ function CharacterSheet({
       projectId={projectId}
       characterId={character.id}
       characterName={character.name}
+      arcStages={character.arcStages}
     />
 
     <OpenQuestions
@@ -311,6 +318,160 @@ function CharacterSheet({
   )
 }
 
+function RelationshipsSection({
+  character,
+  allCharacters,
+  canWrite,
+  onAdd,
+  onRemove,
+  error,
+}: {
+  character: Character
+  allCharacters: Character[]
+  canWrite: boolean
+  onAdd: (
+    relatedCharacterId: string,
+    relationshipType: string,
+    reciprocalRelationshipType: string | null,
+    description: string | null,
+  ) => void
+  onRemove: (relationshipId: string) => void
+  error: unknown
+}) {
+  const [adding, setAdding] = useState(false)
+  const [targetId, setTargetId] = useState('')
+  const [relationshipType, setRelationshipType] = useState('')
+  const [reciprocalRelationshipType, setReciprocalRelationshipType] = useState('')
+  const [description, setDescription] = useState('')
+
+  const candidates = allCharacters.filter(
+    (c) => c.id !== character.id && !character.relationships.some((r) => r.relatedCharacterId === c.id),
+  )
+  const targetName = candidates.find((c) => c.id === targetId)?.name ?? 'them'
+
+  const submit = (e: React.FormEvent) => {
+    e.preventDefault()
+    if (!targetId || !relationshipType.trim()) return
+    onAdd(targetId, relationshipType.trim(), reciprocalRelationshipType.trim() || null, description.trim() || null)
+    setAdding(false)
+    setTargetId('')
+    setRelationshipType('')
+    setReciprocalRelationshipType('')
+    setDescription('')
+  }
+
+  return (
+    <section id="character-relationships" className="card mt-6 p-5">
+      <h3 className="mb-3 text-sm font-semibold">Relationships</h3>
+
+      {character.relationships.length > 0 && (
+        <ul className="mb-3 grid gap-1 text-sm">
+          {character.relationships.map((relationship) => (
+            <li key={relationship.id} className="flex items-start justify-between gap-2">
+              <div>
+                <span className="font-medium">{character.name}</span>
+                <span className="muted"> is {relationship.relatedCharacterName}&rsquo;s </span>
+                <span className="font-medium">{relationship.relationshipType}</span>
+                {relationship.description && <span className="muted">: {relationship.description}</span>}
+              </div>
+              {canWrite && (
+                <button
+                  type="button"
+                  id={`remove-relationship-${relationship.id}`}
+                  className="shrink-0 opacity-60 transition hover:opacity-100"
+                  aria-label={`Remove relationship with ${relationship.relatedCharacterName}`}
+                  onClick={() => onRemove(relationship.id)}
+                >
+                  ✕
+                </button>
+              )}
+            </li>
+          ))}
+        </ul>
+      )}
+
+      {character.relationships.length === 0 && !adding && (
+        <p className="mb-3 text-sm muted">No relationships recorded yet.</p>
+      )}
+
+      {canWrite && candidates.length > 0 && (
+        <>
+          {!adding ? (
+            <button className="btn" id="add-relationship-button" onClick={() => setAdding(true)}>
+              Add relationship…
+            </button>
+          ) : (
+            <form onSubmit={submit} className="card grid gap-2 p-3">
+              <label className="block">
+                <span className="label">Related to…</span>
+                <select
+                  id="add-relationship-character-select"
+                  className="input"
+                  value={targetId}
+                  onChange={(e) => setTargetId(e.target.value)}
+                  autoFocus
+                >
+                  <option value="">Select a character…</option>
+                  {candidates.map((c) => (
+                    <option key={c.id} value={c.id}>
+                      {c.name}
+                    </option>
+                  ))}
+                </select>
+              </label>
+              <label className="block">
+                <span className="label">{character.name} is {targetName}&rsquo;s…</span>
+                <input
+                  id="add-relationship-type-input"
+                  className="input"
+                  placeholder="e.g. sister, rival, servant"
+                  value={relationshipType}
+                  onChange={(e) => setRelationshipType(e.target.value)}
+                />
+              </label>
+              <label className="block">
+                <span className="label">
+                  {targetName} is {character.name}&rsquo;s… (optional, defaults to the same)
+                </span>
+                <input
+                  id="add-relationship-reciprocal-type-input"
+                  className="input"
+                  placeholder="e.g. brother, rival, employer"
+                  value={reciprocalRelationshipType}
+                  onChange={(e) => setReciprocalRelationshipType(e.target.value)}
+                />
+              </label>
+              <label className="block">
+                <span className="label">Note (optional)</span>
+                <input
+                  id="add-relationship-description-input"
+                  className="input"
+                  value={description}
+                  onChange={(e) => setDescription(e.target.value)}
+                />
+              </label>
+              <div className="flex justify-end gap-2">
+                <button type="button" className="btn" onClick={() => setAdding(false)}>
+                  Cancel
+                </button>
+                <button className="btn btn-primary" disabled={!targetId || !relationshipType.trim()}>
+                  Add
+                </button>
+              </div>
+            </form>
+          )}
+        </>
+      )}
+
+      {error !== null && error !== undefined && (
+        <div className="mt-3">
+          <ErrorNote error={error} />
+        </div>
+      )}
+    </section>
+  )
+}
+
 function IdentitySection({
   character,
   allCharacters,

tests/Novelly.Api.Tests/CharacterArcTests.cs

@@ -98,14 +98,14 @@ public class CharacterArcTests : ServiceTestFixture
     public async Task A_character_dossier_carries_its_arc()
     {
         await Arcs.CreateAsync(_characterId, new CreateArcStageRequest(
-            "She trusts the map", Description: "Because her mother drew it."));
+            "She trusts the map", Result: "Because her mother drew it."));
 
         var character = (await Characters.GetAsync(_characterId))!;
 
         Assert.Multiple(() =>
         {
             Assert.That(character.ArcStages, Has.Count.EqualTo(1));
-            Assert.That(character.ArcStages[0].Description, Does.Contain("her mother drew it"));
+            Assert.That(character.ArcStages[0].Result, Does.Contain("her mother drew it"));
         });
     }
 
@@ -211,4 +211,71 @@ public class CharacterArcTests : ServiceTestFixture
     [Test]
     public async Task Asking_for_the_beats_of_a_character_who_does_not_exist_returns_null_rather_than_throwing() =>
         Assert.That(await Beats.ListForCharacterAsync(Guid.NewGuid()), Is.Null);
+
+    [Test]
+    public async Task An_arc_stage_groups_the_beats_assigned_to_it()
+    {
+        var chapter = await Chapters.CreateAsync(_projectId, new CreateChapterRequest("Landfall"));
+        var spoiled = await Beats.CreateAsync(chapter.Id, new CreateBeatRequest("Snaps at the crew", CharacterIds: [_characterId]));
+        var humbled = await Beats.CreateAsync(chapter.Id, new CreateBeatRequest("Learns to swab a deck", CharacterIds: [_characterId]));
+        var stage = await Arcs.CreateAsync(_characterId, new CreateArcStageRequest("Spoiled noble"));
+
+        var updated = await Arcs.SetBeatsAsync(stage.Id, new SetArcStageBeatsRequest([spoiled!.Id]));
+
+        Assert.That(updated!.Beats.Select(b => b.Id), Is.EqualTo(new[] { spoiled.Id }));
+        Assert.That(humbled, Is.Not.Null);
+    }
+
+    [Test]
+    public async Task Assigning_a_beat_to_a_stage_moves_it_out_of_the_characters_other_stage()
+    {
+        var chapter = await Chapters.CreateAsync(_projectId, new CreateChapterRequest("Landfall"));
+        var beat = await Beats.CreateAsync(chapter.Id, new CreateBeatRequest("Gets hurt", CharacterIds: [_characterId]));
+        var early = await Arcs.CreateAsync(_characterId, new CreateArcStageRequest("Spoiled noble"));
+        var later = await Arcs.CreateAsync(_characterId, new CreateArcStageRequest("Humbled"));
+
+        await Arcs.SetBeatsAsync(early.Id, new SetArcStageBeatsRequest([beat!.Id]));
+        await Arcs.SetBeatsAsync(later.Id, new SetArcStageBeatsRequest([beat.Id]));
+
+        var earlyAfter = (await Arcs.GetAsync(early.Id))!;
+        var laterAfter = (await Arcs.GetAsync(later.Id))!;
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(earlyAfter.Beats, Is.Empty);
+            Assert.That(laterAfter.Beats.Select(b => b.Id), Is.EqualTo(new[] { beat.Id }));
+        });
+    }
+
+    [Test]
+    public async Task A_beat_can_only_be_grouped_into_a_stage_for_a_character_who_appears_in_it()
+    {
+        var chapter = await Chapters.CreateAsync(_projectId, new CreateChapterRequest("Landfall"));
+        var mara = await Characters.CreateAsync(_projectId, new CreateCharacterRequest("Mara"));
+        var beat = await Beats.CreateAsync(chapter.Id, new CreateBeatRequest("Mara alone", CharacterIds: [mara.Id]));
+        var stage = await Arcs.CreateAsync(_characterId, new CreateArcStageRequest("Spoiled noble"));
+
+        Assert.That(
+            async () => await Arcs.SetBeatsAsync(stage.Id, new SetArcStageBeatsRequest([beat!.Id])),
+            Throws.TypeOf<InvalidOperationException>().With.Message.Contains("appears in it"));
+    }
+
+    [Test]
+    public async Task Setting_beats_on_an_unknown_stage_returns_null_rather_than_throwing() =>
+        Assert.That(
+            await Arcs.SetBeatsAsync(Guid.NewGuid(), new SetArcStageBeatsRequest([])),
+            Is.Null);
+
+    [Test]
+    public async Task Clearing_a_stages_beats_with_an_empty_list_ungroups_them()
+    {
+        var chapter = await Chapters.CreateAsync(_projectId, new CreateChapterRequest("Landfall"));
+        var beat = await Beats.CreateAsync(chapter.Id, new CreateBeatRequest("Snaps at the crew", CharacterIds: [_characterId]));
+        var stage = await Arcs.CreateAsync(_characterId, new CreateArcStageRequest("Spoiled noble"));
+        await Arcs.SetBeatsAsync(stage.Id, new SetArcStageBeatsRequest([beat!.Id]));
+
+        var cleared = await Arcs.SetBeatsAsync(stage.Id, new SetArcStageBeatsRequest([]));
+
+        Assert.That(cleared!.Beats, Is.Empty);
+    }
 }

tests/Novelly.Api.Tests/CharacterServiceTests.cs

@@ -101,6 +101,58 @@ public class CharacterServiceTests : ServiceTestFixture
         });
     }
 
+    [Test]
+    public async Task Adding_a_relationship_records_it_on_both_characters()
+    {
+        var ines = await Characters.CreateAsync(_projectId, new CreateCharacterRequest("Ines"));
+        var mara = await Characters.CreateAsync(_projectId, new CreateCharacterRequest("Mara"));
+
+        await Characters.AddRelationshipAsync(
+            ines.Id, new CreateRelationshipRequest(mara.Id, "sister", ReciprocalRelationshipType: "brother"));
+
+        var inesAfter = (await Characters.GetAsync(ines.Id))!;
+        var maraAfter = (await Characters.GetAsync(mara.Id))!;
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(inesAfter.Relationships, Has.Count.EqualTo(1));
+            Assert.That(inesAfter.Relationships[0].RelationshipType, Is.EqualTo("sister"));
+            Assert.That(inesAfter.Relationships[0].RelatedCharacterId, Is.EqualTo(mara.Id));
+            Assert.That(maraAfter.Relationships, Has.Count.EqualTo(1));
+            Assert.That(maraAfter.Relationships[0].RelationshipType, Is.EqualTo("brother"));
+            Assert.That(maraAfter.Relationships[0].RelatedCharacterId, Is.EqualTo(ines.Id));
+        });
+    }
+
+    [Test]
+    public async Task A_relationship_with_no_reciprocal_type_mirrors_the_same_type_both_ways()
+    {
+        var ines = await Characters.CreateAsync(_projectId, new CreateCharacterRequest("Ines"));
+        var mara = await Characters.CreateAsync(_projectId, new CreateCharacterRequest("Mara"));
+
+        await Characters.AddRelationshipAsync(ines.Id, new CreateRelationshipRequest(mara.Id, "rival"));
+
+        var maraAfter = (await Characters.GetAsync(mara.Id))!;
+
+        Assert.That(maraAfter.Relationships[0].RelationshipType, Is.EqualTo("rival"));
+    }
+
+    [Test]
+    public async Task Removing_a_relationship_removes_the_reciprocal_side_too()
+    {
+        var ines = await Characters.CreateAsync(_projectId, new CreateCharacterRequest("Ines"));
+        var mara = await Characters.CreateAsync(_projectId, new CreateCharacterRequest("Mara"));
+
+        var withRelationship = (await Characters.AddRelationshipAsync(
+            ines.Id, new CreateRelationshipRequest(mara.Id, "sister", ReciprocalRelationshipType: "brother")))!;
+        var relationshipId = withRelationship.Relationships[0].Id;
+
+        await Characters.RemoveRelationshipAsync(relationshipId);
+        var maraAfter = (await Characters.GetAsync(mara.Id))!;
+
+        Assert.That(maraAfter.Relationships, Is.Empty);
+    }
+
     [Test]
     public async Task Deleting_a_character_detaches_it_from_beats_rather_than_deleting_them()
     {

tests/Novelly.Api.Tests/ServiceTestFixture.cs

@@ -72,7 +72,8 @@ public abstract class ServiceTestFixture
             new AssignCharacterToBeatsRequestValidator(), new MoveBeatsRequestValidator());
         Arcs = new CharacterArcService(
             Db.Context, Access, ArcLogs,
-            new CreateArcStageRequestValidator(), new UpdateArcStageRequestValidator(), new ReorderArcStagesRequestValidator());
+            new CreateArcStageRequestValidator(), new UpdateArcStageRequestValidator(), new ReorderArcStagesRequestValidator(),
+            new SetArcStageBeatsRequestValidator());
         Questions = new OpenQuestionService(
             Db.Context, Access, QuestionLogs,
             new CreateOpenQuestionRequestValidator(), new UpdateOpenQuestionRequestValidator(), new ResolveOpenQuestionRequestValidator());