From 2f765d0515ba3ef338fd7ffeac9f8d84c6fe3e97 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Wed, 30 Sep 2026 02:24:30 +0000
Subject: [PATCH] Keep a lone list property's key when merging properties
 [patch]

MergeArrayValues always wrote the list under its canonical key, even when
the group held a single key and nothing was merged, so with
FrontmatterNaming.AsIs a lone 'section' list still became 'categories'
(and 'keywords' became 'tags'). A lone list now passes through under its
own key, the same as a scalar; lists that really are merged still land
under the canonical key.

Fixes #147

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AFCuHRLN6NWMQ4DJNZApHa
---
 Frontmatter.Test/ListKeyNamingTests.cs | 50 ++++++++++++++++++++++++++
 Frontmatter/PropertyMerger.cs          |  5 +--
 2 files changed, 53 insertions(+), 2 deletions(-)
 create mode 100644 Frontmatter.Test/ListKeyNamingTests.cs

diff --git a/Frontmatter.Test/ListKeyNamingTests.cs b/Frontmatter.Test/ListKeyNamingTests.cs
new file mode 100644
index 0000000..d0e84a4
--- /dev/null
+++ b/Frontmatter.Test/ListKeyNamingTests.cs
@@ -0,0 +1,50 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Frontmatter.Test;
+
+using System.Collections.Generic;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Regression tests for #147: merging must not rename a list property, since renaming is the job of
+/// <see cref="FrontmatterNaming.Standard"/>, and <see cref="FrontmatterNaming.AsIs"/> turns it off.
+/// </summary>
+[TestClass]
+public class ListKeyNamingTests
+{
+	private static readonly string Nl = Environment.NewLine;
+
+	[TestMethod]
+	[DataRow(FrontmatterMergeStrategy.None)]
+	[DataRow(FrontmatterMergeStrategy.Conservative)]
+	[DataRow(FrontmatterMergeStrategy.Aggressive)]
+	[DataRow(FrontmatterMergeStrategy.Maximum)]
+	public void CombineFrontmatter_AsIsNaming_LoneListPropertyKeepsItsKey(FrontmatterMergeStrategy strategy)
+	{
+		string input = $"---{Nl}section:{Nl}  - news{Nl}summary: hi{Nl}---{Nl}Body{Nl}";
+
+		string result = Frontmatter.CombineFrontmatter(input, FrontmatterNaming.AsIs, FrontmatterOrder.AsIs, strategy);
+		Dictionary<string, object>? frontmatter = Frontmatter.ExtractFrontmatter(result);
+
+		Assert.IsNotNull(frontmatter);
+		Assert.IsTrue(frontmatter.ContainsKey("section"), $"section was renamed: {result}");
+		Assert.IsFalse(frontmatter.ContainsKey("categories"), $"section was renamed: {result}");
+	}
+
+	[TestMethod]
+	[DataRow("keywords")]
+	[DataRow("category")]
+	public void MergeSimilarProperties_LoneListProperty_KeepsItsKey(string key)
+	{
+		Dictionary<string, object> source = new()
+		{
+			[key] = new List<object> { "a", "b" },
+		};
+
+		Dictionary<string, object> result = PropertyMerger.MergeSimilarProperties(source, FrontmatterMergeStrategy.Conservative);
+
+		Assert.IsTrue(result.ContainsKey(key), $"{key} was renamed to {string.Join(", ", result.Keys)}");
+		Assert.AreEqual(1, result.Count);
+	}
+}
diff --git a/Frontmatter/PropertyMerger.cs b/Frontmatter/PropertyMerger.cs
index baa50d2..0667664 100644
--- a/Frontmatter/PropertyMerger.cs
+++ b/Frontmatter/PropertyMerger.cs
@@ -144,8 +144,9 @@ private static void MergePropertyGroup(
 			return;
 		}
 
-		// Handle array/list types specially
-		if (firstValue is IList<object> || firstValue is object[])
+		// Handle array/list types specially. A lone list has nothing to merge with, so it keeps its own
+		// key like a scalar does: renaming is NameStandardizer's job, which FrontmatterNaming.AsIs turns off.
+		if (originalKeys.Count > 1 && (firstValue is IList<object> || firstValue is object[]))
 		{
 			MergeArrayValues(source, target, canonicalKey, originalKeys);
 		}