feat(mcp): map MCP tool results to core tool results

The truncation note follows the kept text after a single newline, as the built-in read/grep tools do (plan snippet updated).
2026-09-26 20:19:03 +02:00
parent b30c71d30a
commit da6068c5dc
2 changed files with 222 additions and 0 deletions
--- a/extensions/Volcanic.AgenticCore.Extensions.Mcp/McpResultMapper.cs
+++ b/extensions/Volcanic.AgenticCore.Extensions.Mcp/McpResultMapper.cs
@@ -0,0 +1,98 @@
+using System.Text;
+using ModelContextProtocol.Protocol;
+using Volcanic.AgenticCore.Abstractions.Json;
+using Volcanic.AgenticCore.Abstractions.Messages;
+using Volcanic.AgenticCore.Abstractions.Tools;
+using CoreBlock = Volcanic.AgenticCore.Abstractions.Messages.ContentBlock;
+using McpBlock = ModelContextProtocol.Protocol.ContentBlock;
+using TextContent = Volcanic.AgenticCore.Abstractions.Messages.TextContent;
+
+namespace Volcanic.AgenticCore.Extensions.Mcp;
+
+/// <summary>Maps an MCP <c>tools/call</c> result to a core <see cref="ToolResult"/> (spec §7).</summary>
+internal static class McpResultMapper
+{
+    public const int MaxImageBytes = 5 * 1024 * 1024;
+    public const int MaxLines = 2000;
+    public const int MaxBytes = 50 * 1024;
+
+    /// <summary>
+    /// One <see cref="TextContent"/> (the joined, truncated text) followed by the images in order.
+    /// Throws <see cref="ToolExecutionException"/> carrying the mapped result when the server set <c>isError</c>.
+    /// </summary>
+    public static ToolResult Map(string server, string tool, CallToolResult result)
+    {
+        var text = new List<string>();
+        var images = new List<ImageContent>();
+        foreach (var block in result.Content ?? []) Add(block, text, images);
+        if (text.Count == 0 && images.Count == 0 && result.StructuredContent is { } structured)
+            text.Add(structured.GetRawText());
+
+        var (shown, truncated) = Truncate(string.Join('\n', text));
+        var content = new List<CoreBlock>();
+        if (shown.Length > 0) content.Add(new TextContent(shown));
+        else if (images.Count == 0) content.Add(new TextContent("(no output)"));
+        content.AddRange(images);
+
+        var isError = result.IsError == true;
+        var mapped = new ToolResult(content, VolcanicJson.ToElement(new { server, tool, isError, truncated, images = images.Count }));
+        if (isError) throw new ToolExecutionException(mapped);
+        return mapped;
+    }
+
+    private static void Add(McpBlock block, List<string> text, List<ImageContent> images)
+    {
+        switch (block)
+        {
+            case TextContentBlock t:
+                text.Add(t.Text);
+                break;
+            case ImageContentBlock image:
+                var size = image.DecodedData.Length;
+                if (size > MaxImageBytes) text.Add($"[image omitted: {image.MimeType}, {size} bytes]");
+                else images.Add(new ImageContent(Encoding.UTF8.GetString(image.Data.Span), image.MimeType));
+                break;
+            case AudioContentBlock audio:
+                text.Add($"[audio omitted: {audio.MimeType}, {audio.DecodedData.Length} bytes]");
+                break;
+            case EmbeddedResourceBlock { Resource: TextResourceContents resource }:
+                text.Add($"Resource {resource.Uri}:\n{resource.Text}");
+                break;
+            case EmbeddedResourceBlock { Resource: BlobResourceContents blob }:
+                text.Add($"[resource {blob.Uri}: {blob.MimeType ?? "application/octet-stream"}, {blob.DecodedData.Length} bytes omitted]");
+                break;
+            case ResourceLinkBlock link:
+                text.Add($"Resource link: {link.Uri} ({link.Name})");
+                break;
+            default:
+                text.Add($"[{block.Type} content omitted]");
+                break;
+        }
+    }
+
+    /// <summary>
+    /// Head truncation with the core <c>OutputTruncation.Head</c> limits and note wording, reimplemented here because
+    /// that type lives in the runtime. The note follows the kept text on its own line, as in the built-in tools.
+    /// </summary>
+    private static (string Text, bool Truncated) Truncate(string text)
+    {
+        var lines = text.Split('\n');
+        var totalBytes = Encoding.UTF8.GetByteCount(text);
+        if (lines.Length <= MaxLines && totalBytes <= MaxBytes) return (text, false);
+
+        var sb = new StringBuilder();
+        var bytes = 0;
+        var shown = 0;
+        foreach (var line in lines)
+        {
+            var lineBytes = Encoding.UTF8.GetByteCount(line) + 1;
+            if (shown >= MaxLines || bytes + lineBytes > MaxBytes) break;
+            if (shown > 0) sb.Append('\n');
+            sb.Append(line);
+            bytes += lineBytes;
+            shown++;
+        }
+        var note = $"[Output truncated: showing {shown} of {lines.Length} lines ({totalBytes} bytes). Use offset/limit or a narrower query to see more.]";
+        return ($"{sb}\n{note}", true);
+    }
+}