Return CallToolResult<T> from CallToolAsync<T>, use ToString and FirstOrDefault

- CallToolAsync<T> now returns CallToolResult<T> instead of T
- No longer throws on IsError; caller can inspect IsError on the result
- Use OfType<TextContentBlock>().FirstOrDefault() instead of indexing
- Use ToString() instead of ToJsonString(serializerOptions) in ToCallToolResult
- Updated tests accordingly

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Client/McpClient.Methods.cs

@@ -915,33 +915,27 @@ public ValueTask<CallToolResult> CallToolAsync(
     }
 
     /// <summary>
-    /// Invokes a tool on the server and deserializes the result as a strongly-typed <typeparamref name="T"/>.
+    /// Invokes a tool on the server and deserializes the result as a strongly-typed <see cref="CallToolResult{T}"/>.
     /// </summary>
     /// <typeparam name="T">The type to deserialize the tool's structured content or text content into.</typeparam>
     /// <param name="toolName">The name of the tool to call on the server.</param>
     /// <param name="arguments">An optional dictionary of arguments to pass to the tool.</param>
     /// <param name="progress">An optional progress reporter for server notifications.</param>
     /// <param name="options">Optional request options including metadata, serialization settings, and progress tracking.</param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
-    /// <returns>The deserialized result from the tool execution.</returns>
+    /// <returns>A <see cref="CallToolResult{T}"/> containing the deserialized content, error state, and metadata.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="toolName"/> is <see langword="null"/>.</exception>
-    /// <exception cref="McpException">
-    /// The request failed, the server returned an error response, or <see cref="CallToolResult.IsError"/> was <see langword="true"/>.
-    /// </exception>
+    /// <exception cref="McpException">The request failed or the server returned an error response.</exception>
     /// <exception cref="JsonException">The result content could not be deserialized as <typeparamref name="T"/>.</exception>
     /// <remarks>
     /// <para>
     /// This method calls the existing <see cref="CallToolAsync(string, IReadOnlyDictionary{string, object?}?, IProgress{ProgressNotificationValue}?, RequestOptions?, CancellationToken)"/>
     /// and then deserializes the result. If the result has <see cref="CallToolResult.StructuredContent"/>, that is deserialized
-    /// as <typeparamref name="T"/>. Otherwise, if the result has text content, the text of the last <see cref="TextContentBlock"/>
+    /// as <typeparamref name="T"/>. Otherwise, if the result has text content, the text of the first <see cref="TextContentBlock"/>
     /// is deserialized as <typeparamref name="T"/>.
     /// </para>
-    /// <para>
-    /// If <see cref="CallToolResult.IsError"/> is <see langword="true"/>, an <see cref="McpException"/> is thrown
-    /// with the error content details.
-    /// </para>
     /// </remarks>
-    public async ValueTask<T> CallToolAsync<T>(
+    public async ValueTask<CallToolResult<T>> CallToolAsync<T>(
         string toolName,
         IReadOnlyDictionary<string, object?>? arguments = null,
         IProgress<ProgressNotificationValue>? progress = null,
@@ -950,29 +944,30 @@ public async ValueTask<T> CallToolAsync<T>(
     {
         CallToolResult result = await CallToolAsync(toolName, arguments, progress, options, cancellationToken).ConfigureAwait(false);
 
-        if (result.IsError is true)
-        {
-            string errorMessage = result.Content.Count > 0 && result.Content[^1] is TextContentBlock textBlock
-                ? textBlock.Text
-                : "The tool call returned an error.";
-            throw new McpException(errorMessage);
-        }
-
-        var serializerOptions = options?.JsonSerializerOptions ?? McpJsonUtilities.DefaultOptions;
-        JsonTypeInfo<T> typeInfo = (JsonTypeInfo<T>)serializerOptions.GetTypeInfo(typeof(T));
+        T? content = default;
 
-        // Prefer StructuredContent if available, otherwise fall back to text content
-        if (result.StructuredContent is { } structuredContent)
+        if (result.IsError is not true)
         {
-            return JsonSerializer.Deserialize(structuredContent, typeInfo)!;
-        }
+            var serializerOptions = options?.JsonSerializerOptions ?? McpJsonUtilities.DefaultOptions;
+            JsonTypeInfo<T> typeInfo = (JsonTypeInfo<T>)serializerOptions.GetTypeInfo(typeof(T));
 
-        if (result.Content.Count > 0 && result.Content[^1] is TextContentBlock textContent)
-        {
-            return JsonSerializer.Deserialize(textContent.Text, typeInfo)!;
+            // Prefer StructuredContent if available, otherwise fall back to text content
+            if (result.StructuredContent is { } structuredContent)
+            {
+                content = JsonSerializer.Deserialize(structuredContent, typeInfo);
+            }
+            else if (result.Content.OfType<TextContentBlock>().FirstOrDefault() is { } textContent)
+            {
+                content = JsonSerializer.Deserialize(textContent.Text, typeInfo);
+            }
         }
 
-        throw new McpException("The tool call did not return any content that could be deserialized.");
+        return new()
+        {
+            Content = content,
+            IsError = result.IsError,
+            Meta = result.Meta,
+        };
     }
 
     /// <summary>

src/ModelContextProtocol.Core/Protocol/CallToolResultOfT.cs

@@ -64,7 +64,7 @@ CallToolResult ICallToolResultTyped.ToCallToolResult(JsonSerializerOptions seria
 
         return new()
         {
-            Content = [new TextContentBlock { Text = structuredContent?.ToJsonString(serializerOptions) ?? "null" }],
+            Content = [new TextContentBlock { Text = structuredContent?.ToString() ?? "null" }],
             StructuredContent = structuredContent,
             IsError = IsError,
             Meta = Meta,

tests/ModelContextProtocol.Tests/Server/CallToolResultOfTTests.cs

@@ -41,13 +41,13 @@ public async Task CallToolAsyncOfT_DeserializesStructuredContent()
             options: new() { JsonSerializerOptions = serOpts },
             cancellationToken: TestContext.Current.CancellationToken);
 
-        Assert.NotNull(result);
-        Assert.Equal("Alice", result.Name);
-        Assert.Equal(30, result.Age);
+        Assert.NotNull(result.Content);
+        Assert.Equal("Alice", result.Content.Name);
+        Assert.Equal(30, result.Content.Age);
     }
 
     [Fact]
-    public async Task CallToolAsyncOfT_ThrowsOnIsError()
+    public async Task CallToolAsyncOfT_PropagatesIsError()
     {
         JsonSerializerOptions serOpts = new() { TypeInfoResolver = new DefaultJsonTypeInfoResolver() };
         _toolCollection.Add(McpServerTool.Create(
@@ -57,13 +57,12 @@ public async Task CallToolAsyncOfT_ThrowsOnIsError()
         StartServer();
         var client = await CreateMcpClientForServer();
 
-        var ex = await Assert.ThrowsAsync<McpException>(
-            () => client.CallToolAsync<string>(
-                "error_tool",
-                options: new() { JsonSerializerOptions = serOpts },
-                cancellationToken: TestContext.Current.CancellationToken).AsTask());
+        var result = await client.CallToolAsync<string>(
+            "error_tool",
+            options: new() { JsonSerializerOptions = serOpts },
+            cancellationToken: TestContext.Current.CancellationToken);
 
-        Assert.Contains("something went wrong", ex.Message);
+        Assert.True(result.IsError);
     }
 
     [Fact]
@@ -83,9 +82,9 @@ public async Task CallToolAsyncOfT_FallsBackToTextContent()
             options: new() { JsonSerializerOptions = serOpts },
             cancellationToken: TestContext.Current.CancellationToken);
 
-        Assert.NotNull(result);
-        Assert.Equal("Bob", result.Name);
-        Assert.Equal(25, result.Age);
+        Assert.NotNull(result.Content);
+        Assert.Equal("Bob", result.Content.Name);
+        Assert.Equal(25, result.Content.Age);
     }
 
     [Fact]
@@ -129,9 +128,9 @@ public async Task CallToolAsyncOfT_WithAsyncTool_Works()
             options: new() { JsonSerializerOptions = serOpts },
             cancellationToken: TestContext.Current.CancellationToken);
 
-        Assert.NotNull(result);
-        Assert.Equal("Charlie", result.Name);
-        Assert.Equal(35, result.Age);
+        Assert.NotNull(result.Content);
+        Assert.Equal("Charlie", result.Content.Name);
+        Assert.Equal(35, result.Content.Age);
     }
 
     [Fact]
@@ -154,9 +153,9 @@ public async Task CallToolAsyncOfT_WithArguments_Works()
             options: new() { JsonSerializerOptions = serOpts },
             cancellationToken: TestContext.Current.CancellationToken);
 
-        Assert.NotNull(result);
-        Assert.Equal("Diana", result.Name);
-        Assert.Equal(28, result.Age);
+        Assert.NotNull(result.Content);
+        Assert.Equal("Diana", result.Content.Name);
+        Assert.Equal(28, result.Content.Age);
     }
 
     private class PersonData