Synthesize return description into tool description when UseStructuredContent is false (#1049)

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -118,7 +118,7 @@ internal sealed partial class AIFunctionMcpServerTool : McpServerTool
         Tool tool = new()
         {
             Name = options?.Name ?? function.Name,
-            Description = options?.Description ?? function.Description,
+            Description = GetToolDescription(function, options),
             InputSchema = function.JsonSchema,
             OutputSchema = CreateOutputSchema(function, options, out bool structuredOutputRequiresWrapping),
             Icons = options?.Icons,
@@ -406,6 +406,57 @@ internal sealed partial class AIFunctionMcpServerTool : McpServerTool
         }
     }
 
+    /// <summary>
+    /// Gets the tool description, synthesizing from both the function description and return description when appropriate.
+    /// </summary>
+    /// <remarks>
+    /// When UseStructuredContent is true, the return description is included in the output schema.
+    /// When UseStructuredContent is false (default), if there's a return description in the ReturnJsonSchema,
+    /// it will be appended to the tool description so the information is still available to consumers.
+    /// </remarks>
+    private static string? GetToolDescription(AIFunction function, McpServerToolCreateOptions? options)
+    {
+        string? description = options?.Description ?? function.Description;
+
+        // If structured content is enabled, the return description will be in the output schema
+        if (options?.UseStructuredContent is true)
+        {
+            return description;
+        }
+
+        // When structured content is disabled, try to extract the return description from ReturnJsonSchema
+        // and append it to the tool description so the information is available to consumers
+        string? returnDescription = GetReturnDescription(function.ReturnJsonSchema);
+        if (string.IsNullOrWhiteSpace(returnDescription))
+        {
+            return description;
+        }
+
+        // Synthesize a combined description
+        if (string.IsNullOrWhiteSpace(description))
+        {
+            return $"Returns: {returnDescription}";
+        }
+
+        return $"{description}\nReturns: {returnDescription}";
+    }
+
+    /// <summary>
+    /// Extracts the description property from a ReturnJsonSchema if present.
+    /// </summary>
+    private static string? GetReturnDescription(JsonElement? returnJsonSchema)
+    {
+        if (returnJsonSchema is not JsonElement schema ||
+            schema.ValueKind is not JsonValueKind.Object ||
+            !schema.TryGetProperty("description", out JsonElement descriptionElement) ||
+            descriptionElement.ValueKind is not JsonValueKind.String)
+        {
+            return null;
+        }
+
+        return descriptionElement.GetString();
+    }
+
     private static JsonElement? CreateOutputSchema(AIFunction function, McpServerToolCreateOptions? toolCreateOptions, out bool structuredOutputRequiresWrapping)
     {
         structuredOutputRequiresWrapping = false;

tests/ModelContextProtocol.Tests/Server/McpServerToolTests.cs

@@ -4,6 +4,7 @@ using Microsoft.Extensions.DependencyInjection;
 using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
 using Moq;
+using System.ComponentModel;
 using System.Reflection;
 using System.Runtime.InteropServices;
 using System.Text.Json;
@@ -743,6 +744,87 @@ public partial class McpServerToolTests
         Assert.Null(tool.ProtocolTool.Icons);
     }
 
+    [Fact]
+    public void ReturnDescription_StructuredOutputDisabled_IncludedInToolDescription()
+    {
+        // When UseStructuredContent is false (default), return description should be appended to tool description
+        McpServerTool tool = McpServerTool.Create(ToolWithReturnDescription);
+
+        Assert.Equal("Tool that returns data.\nReturns: The computed result", tool.ProtocolTool.Description);
+        Assert.Null(tool.ProtocolTool.OutputSchema);
+    }
+
+    [Fact]
+    public void ReturnDescription_StructuredOutputEnabled_NotIncludedInToolDescription()
+    {
+        // When UseStructuredContent is true, return description should be in the output schema, not in tool description
+        McpServerTool tool = McpServerTool.Create(ToolWithReturnDescription, new() { UseStructuredContent = true });
+
+        Assert.Equal("Tool that returns data.", tool.ProtocolTool.Description);
+        Assert.NotNull(tool.ProtocolTool.OutputSchema);
+        // Verify the output schema contains the description
+        Assert.True(tool.ProtocolTool.OutputSchema.Value.TryGetProperty("properties", out var properties));
+        Assert.True(properties.TryGetProperty("result", out var result));
+        Assert.True(result.TryGetProperty("description", out var description));
+        Assert.Equal("The computed result", description.GetString());
+    }
+
+    [Fact]
+    public void ReturnDescription_NoFunctionDescription_OnlyReturnsDescription()
+    {
+        // When there's no function description but there's a return description
+        McpServerTool tool = McpServerTool.Create(ToolWithOnlyReturnDescription);
+
+        Assert.Equal("Returns: The computed result", tool.ProtocolTool.Description);
+        Assert.Null(tool.ProtocolTool.OutputSchema);
+    }
+
+    [Fact]
+    public void ReturnDescription_ExplicitDescriptionOption_SynthesizesWithReturnDescription()
+    {
+        // When Description is explicitly set in options and there's a return description,
+        // the return description should be appended since UseStructuredContent is false
+        McpServerTool tool = McpServerTool.Create(ToolWithReturnDescription, new() { Description = "Custom description" });
+
+        Assert.Equal("Custom description\nReturns: The computed result", tool.ProtocolTool.Description);
+        Assert.Null(tool.ProtocolTool.OutputSchema);
+    }
+
+    [Fact]
+    public void ReturnDescription_NoReturnDescription_NoChange()
+    {
+        // When there's no return description, the tool description should remain unchanged
+        McpServerTool tool = McpServerTool.Create(ToolWithoutReturnDescription);
+
+        Assert.Equal("Tool without return description.", tool.ProtocolTool.Description);
+        Assert.Null(tool.ProtocolTool.OutputSchema);
+    }
+
+    [Fact]
+    public void ReturnDescription_StructuredOutputEnabled_WithExplicitDescription_NoSynthesis()
+    {
+        // When UseStructuredContent is true and Description is set, return description goes to output schema
+        McpServerTool tool = McpServerTool.Create(ToolWithReturnDescription, new() 
+        { 
+            Description = "Custom description", 
+            UseStructuredContent = true 
+        });
+
+        // Description should not have the return description appended
+        Assert.Equal("Custom description", tool.ProtocolTool.Description);
+        Assert.NotNull(tool.ProtocolTool.OutputSchema);
+    }
+
+    [Description("Tool that returns data.")]
+    [return: Description("The computed result")]
+    private static string ToolWithReturnDescription() => "result";
+
+    [return: Description("The computed result")]
+    private static string ToolWithOnlyReturnDescription() => "result";
+
+    [Description("Tool without return description.")]
+    private static string ToolWithoutReturnDescription() => "result";
+
     [JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
     [JsonSerializable(typeof(DisposableToolType))]
     [JsonSerializable(typeof(AsyncDisposableToolType))]