Implement .md link preservation for internal markdown links in LLM output

- Modified LlmLinkInlineRenderer to detect internal markdown links
- Internal markdown links now preserve .md extension
- External links and cross-links continue to use absolute URLs
- Added comprehensive test coverage for link handling
- Verified functionality with CLI tool output

Co-authored-by: reakaleek <[email protected]>

Author: Copilot

src/Elastic.Markdown/Myst/Renderers/LlmMarkdown/LlmInlineRenderers.cs

@@ -2,6 +2,7 @@
 // Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information
 
+using Elastic.Markdown.IO;
 using Elastic.Markdown.Myst.InlineParsers.Substitution;
 using Elastic.Markdown.Myst.Roles;
 using Elastic.Markdown.Myst.Roles.Kbd;
@@ -32,8 +33,23 @@ protected override void Write(LlmMarkdownRenderer renderer, LinkInline obj)
 			renderer.WriteChildren(obj);
 			renderer.Writer.Write("](");
 			var url = obj.GetDynamicUrl?.Invoke() ?? obj.Url;
-			var absoluteUrl = LlmRenderingHelpers.MakeAbsoluteUrl(renderer, url);
-			renderer.Writer.Write(absoluteUrl ?? string.Empty);
+
+			// Check if this is an internal link to a markdown page
+			var isCrossLink = (obj.GetData("isCrossLink") as bool?) == true;
+			var hasTargetNavigationRoot = obj.GetData($"Target{nameof(MarkdownFile.NavigationRoot)}") != null;
+			var isInternalMarkdownLink = !isCrossLink && hasTargetNavigationRoot;
+
+			if (isInternalMarkdownLink)
+			{
+				// For internal markdown links, preserve the .md extension
+				renderer.Writer.Write(EnsureMarkdownExtension(url) ?? string.Empty);
+			}
+			else
+			{
+				// For external links and cross-links, make absolute
+				var absoluteUrl = LlmRenderingHelpers.MakeAbsoluteUrl(renderer, url);
+				renderer.Writer.Write(absoluteUrl ?? string.Empty);
+			}
 		}
 		if (!string.IsNullOrEmpty(obj.Title))
 		{
@@ -43,6 +59,25 @@ protected override void Write(LlmMarkdownRenderer renderer, LinkInline obj)
 		}
 		renderer.Writer.Write(")");
 	}
+
+	/// <summary>
+	/// Ensures the URL ends with .md extension for markdown links
+	/// </summary>
+	private static string? EnsureMarkdownExtension(string? url)
+	{
+		if (string.IsNullOrEmpty(url))
+			return url;
+
+		// If it already has .md extension, return as-is
+		if (url.EndsWith(".md", StringComparison.OrdinalIgnoreCase))
+			return url;
+
+		// Convert absolute paths to relative paths for markdown links
+		var processedUrl = url.StartsWith('/') ? url.TrimStart('/') : url;
+
+		// Add .md extension to internal markdown links
+		return processedUrl + ".md";
+	}
 }
 
 public class LlmEmphasisInlineRenderer : MarkdownObjectRenderer<LlmMarkdownRenderer, EmphasisInline>

tests/authoring/LlmMarkdown/LlmMarkdownOutput.fs

@@ -538,3 +538,27 @@ sub:
         markdown |> convertsToNewLLM """
 ## Hello, World!
 """
+
+type ``links`` () =
+    static let generator = Setup.Generate [
+        Index """
+This is a [link to another page](another-page.md).
+
+This is an [external link](https://example.com).
+
+This is a [cross-link](https://docs.elastic.co/some-page).
+"""
+        Markdown "another-page.md" """
+# Another Page
+
+This is another page for testing internal links.
+"""
+    ]
+
+    [<Fact>]
+    let ``internal markdown links preserve .md extension while external links become absolute`` () =
+        generator |> convertsToNewLLM """
+This is a [link to another page](another-page.md).
+This is an [external link](https://example.com).
+This is a [cross-link](https://docs.elastic.co/some-page).
+"""