diff --git a/src/Wiki.Application/Encyclopedia/ArticlePlan.cs b/src/Wiki.Application/Encyclopedia/ArticlePlan.cs
index af0905e..dc42939 100644
--- a/src/Wiki.Application/Encyclopedia/ArticlePlan.cs
+++ b/src/Wiki.Application/Encyclopedia/ArticlePlan.cs
@@ -19,19 +19,24 @@ public static class ArticlePlanJson
 {
     private static readonly JsonSerializerOptions Options = new(JsonSerializerDefaults.Web) { WriteIndented = true };
 
-    public static string Write(ArticlePlan p)
+    private static readonly JsonSerializerOptions CompactOptions = new(JsonSerializerDefaults.Web);
+
+    public static string Write(ArticlePlan p) =>
+        JsonSerializer.Serialize(new { Articles = Shape(p), p.CoveredNotes }, Options);
+
+    /// <summary>Only the articles array, compact: the form the model sees and answers in.</summary>
+    public static string WriteArticlesCompact(ArticlePlan p) =>
+        JsonSerializer.Serialize(Shape(p), CompactOptions);
+
+    private static IEnumerable<object> Shape(ArticlePlan p) => p.Articles.Select(a => new
     {
-        var articles = p.Articles.Select(a => new
-        {
-            a.Slug,
-            a.TitleDe,
-            a.TitleEn,
-            Category = a.Category.ToString().ToLowerInvariant(),
-            a.Sources,
-            a.Related,
-        });
-        return JsonSerializer.Serialize(new { Articles = articles, p.CoveredNotes }, Options);
-    }
+        a.Slug,
+        a.TitleDe,
+        a.TitleEn,
+        Category = a.Category.ToString().ToLowerInvariant(),
+        a.Sources,
+        a.Related,
+    });
 
     /// <summary>
     /// The plan in <paramref name="json"/>, which may be wrapped in a code fence; null when it is not a plan.
@@ -47,7 +52,9 @@ public static class ArticlePlanJson
             {
                 if (item is not JsonObject o) continue;
                 var slug = Text(o["slug"]);
-                if (slug is null || !Enum.TryParse<ArticleCategory>(Text(o["category"]), ignoreCase: true, out var category)) continue;
+                var categoryText = Text(o["category"]);
+                if (slug is null || categoryText is null || !categoryText.All(char.IsAsciiLetter)
+                    || !Enum.TryParse<ArticleCategory>(categoryText, ignoreCase: true, out var category)) continue;
                 articles.Add(new PlannedArticle(slug, Text(o["titleDe"]) ?? slug, Text(o["titleEn"]) ?? slug, category,
                     Strings(o["sources"]), Strings(o["related"])));
             }
diff --git a/src/Wiki.Application/Encyclopedia/EncyclopediaState.cs b/src/Wiki.Application/Encyclopedia/EncyclopediaState.cs
index 8faa1d4..4705da7 100644
--- a/src/Wiki.Application/Encyclopedia/EncyclopediaState.cs
+++ b/src/Wiki.Application/Encyclopedia/EncyclopediaState.cs
@@ -24,7 +24,7 @@ public sealed record PlanAttempt(string NoteSetFingerprint, DateTimeOffset At);
 /// </summary>
 public sealed record EncyclopediaState(IReadOnlyDictionary<string, SubjectAttempt> Subjects, PlanAttempt? FailedPlan)
 {
-    public static EncyclopediaState Empty { get; } = new(new Dictionary<string, SubjectAttempt>(), null);
+    public static EncyclopediaState Empty => new(new Dictionary<string, SubjectAttempt>(), null);
 
     public static string NoteSetFingerprint(IReadOnlyList<Document> notes) =>
         Hash(Prompts.Version.ToString(), notes.Select(n => n.Slug).Order(StringComparer.Ordinal));
@@ -60,7 +60,9 @@ public static class EncyclopediaStateJson
         try
         {
             var s = JsonSerializer.Deserialize<EncyclopediaState>(json, Options);
-            return s?.Subjects is null ? EncyclopediaState.Empty : s;
+            if (s?.Subjects is null) return EncyclopediaState.Empty;
+            var subjects = s.Subjects.Where(kv => kv.Value is not null).ToDictionary(kv => kv.Key, kv => kv.Value);
+            return new EncyclopediaState(subjects, s.FailedPlan);
         }
         catch (JsonException)
         {
diff --git a/src/Wiki.Application/Encyclopedia/PlanArticlesUseCase.cs b/src/Wiki.Application/Encyclopedia/PlanArticlesUseCase.cs
index ece421b..b1aaf70 100644
--- a/src/Wiki.Application/Encyclopedia/PlanArticlesUseCase.cs
+++ b/src/Wiki.Application/Encyclopedia/PlanArticlesUseCase.cs
@@ -120,11 +120,14 @@ public sealed partial class PlanArticlesUseCase(ITextModel model, EncyclopediaSe
     {
         var sb = new StringBuilder();
         if (current is not null)
-            sb.Append("Current plan:\n").Append(ArticlePlanJson.Write(current)).Append("\n\n");
+            sb.Append("Current plan:\n").Append(ArticlePlanJson.WriteArticlesCompact(current)).Append("\n\n");
         sb.Append("Notes (slug | title | tags | body length in chars):\n");
         foreach (var n in notes.OrderBy(n => n.Slug, StringComparer.Ordinal))
-            sb.Append(n.Slug).Append(" | ").Append(n.Title).Append(" | ").Append(string.Join(", ", n.Tags))
+            sb.Append(n.Slug).Append(" | ").Append(Flatten(n.Title)).Append(" | ").Append(Flatten(string.Join(", ", n.Tags)))
               .Append(" | ").Append(n.Body.Length).Append('\n');
         return sb.ToString();
     }
+
+    /// <summary>Keeps a field on its line and inside its column.</summary>
+    private static string Flatten(string s) => s.Replace('\r', ' ').Replace('\n', ' ').Replace('|', ' ');
 }
diff --git a/src/Wiki.Application/Encyclopedia/Prompts.cs b/src/Wiki.Application/Encyclopedia/Prompts.cs
index 4880ecc..65edb03 100644
--- a/src/Wiki.Application/Encyclopedia/Prompts.cs
+++ b/src/Wiki.Application/Encyclopedia/Prompts.cs
@@ -6,7 +6,7 @@ namespace Wiki.Application.Encyclopedia;
 /// </summary>
 public static class Prompts
 {
-    public const int Version = 1;
+    public const int Version = 2;
 
     /// <summary>Placeholder in <see cref="Plan"/> replaced by <c>MaxSourceCharsPerArticle</c>.</summary>
     public const string MaxSourceCharsToken = "{maxSourceChars}";
@@ -25,7 +25,7 @@ public static class Prompts
         - Slugs are lowercase ASCII words joined by single hyphens. Categories are room, system, device or topic.
         - related lists the slugs of other articles in this plan that a reader of the article should continue with.
 
-        Reply with JSON only, no prose and no code fence, in this shape:
+        Reply with JSON only, no prose and no code fence. Write it compact on a single line, with no indentation, no line breaks and no spaces between tokens, because the reply is size-limited. Shape:
         {"articles":[{"slug":"...","titleDe":"...","titleEn":"...","category":"room|system|device|topic","sources":["note-slug"],"related":["article-slug"]}]}
         """;
 }
diff --git a/test/Wiki.Application.Tests/EncyclopediaStateTests.cs b/test/Wiki.Application.Tests/EncyclopediaStateTests.cs
index 5a5268a..e0561f4 100644
--- a/test/Wiki.Application.Tests/EncyclopediaStateTests.cs
+++ b/test/Wiki.Application.Tests/EncyclopediaStateTests.cs
@@ -32,6 +32,20 @@ public class EncyclopediaStateTests
         Assert.Null(read.FailedPlan);
     }
 
+    [Fact]
+    public void EncyclopediaStateJson_NullEntry_Dropped()
+    {
+        var read = EncyclopediaStateJson.Read("""{"subjects":{"a":null,"b":{"fingerprint":"f","outcome":"written","at":"2026-10-24T03:00:00+00:00"}},"failedPlan":null}""");
+        Assert.Equal(["b"], read.Subjects.Keys);
+    }
+
+    [Fact]
+    public void Empty_DoesNotShareMutableState()
+    {
+        ((Dictionary<string, SubjectAttempt>)EncyclopediaState.Empty.Subjects)["x"] = new("f", AttemptOutcome.Written, At);
+        Assert.Empty(EncyclopediaState.Empty.Subjects);
+    }
+
     [Fact]
     public void SubjectFingerprint_ChangesWithSourceHashOrTitle()
     {
diff --git a/test/Wiki.Application.Tests/PlanArticlesUseCaseTests.cs b/test/Wiki.Application.Tests/PlanArticlesUseCaseTests.cs
index 939c8ab..3e7ccee 100644
--- a/test/Wiki.Application.Tests/PlanArticlesUseCaseTests.cs
+++ b/test/Wiki.Application.Tests/PlanArticlesUseCaseTests.cs
@@ -133,6 +133,42 @@ public class PlanArticlesUseCaseTests
         Assert.Equal(Settings.PlanMaxTokens, call.MaxTokens);
     }
 
+    [Fact]
+    public async Task Plan_PromptAsksForCompactJson_AndOmitsCoveredNotes()
+    {
+        var model = new FakeTextModel().Reply("nope");
+        var current = new ArticlePlan([new PlannedArticle("heating-valves", "H", "H", ArticleCategory.Device, ["a"], [])], ["a", "covered-only"]);
+
+        await UseCase(model).PlanAsync(current, [Note("a")], CancellationToken.None);
+
+        var call = Assert.Single(model.Calls);
+        Assert.Contains("compact", call.System);
+        Assert.DoesNotContain("coveredNotes", call.User);
+        Assert.DoesNotContain("covered-only", call.User);
+        var jsonLine = call.User.Split('\n').Single(l => l.StartsWith("[{"));
+        Assert.Equal("""[{"slug":"heating-valves","titleDe":"H","titleEn":"H","category":"device","sources":["a"],"related":[]}]""", jsonLine);
+    }
+
+    [Fact]
+    public async Task Plan_TitleWithPipeOrNewline_Sanitized()
+    {
+        var model = new FakeTextModel().Reply("nope");
+        var note = Note("a", title: "Left | Right\nSecond line");
+
+        await UseCase(model).PlanAsync(null, [note], CancellationToken.None);
+
+        var line = Assert.Single(model.Calls[0].User.Split('\n'), l => l.StartsWith("a | "));
+        Assert.Equal(4, line.Split(" | ").Length);
+        Assert.Contains("Left   Right Second line", line);
+    }
+
+    [Fact]
+    public void ArticlePlanJson_NumericCategory_ArticleSkipped()
+    {
+        var plan = ArticlePlanJson.Read("""{"articles":[{"slug":"a","category":"1","sources":["x"]},{"slug":"b","category":"room","sources":["x"]}]}""");
+        Assert.Equal(["b"], plan!.Articles.Select(a => a.Slug));
+    }
+
     [Fact]
     public void ArticlePlanJson_Malformed_ReadsNull()
     {