Align error handling documentation with SEP-1303

src/ModelContextProtocol.Core/McpErrorCode.cs

@@ -30,12 +30,22 @@ public enum McpErrorCode
     MethodNotFound = -32601,
 
     /// <summary>
-    /// Indicates that one or more parameters provided in the request are invalid.
+    /// Indicates that the request parameters are invalid at the protocol level.
     /// </summary>
     /// <remarks>
-    /// This error is returned when the parameters do not match the expected method signature or constraints.
-    /// This includes cases where required parameters are missing or not understood, such as when a name for
-    /// a tool or prompt is not recognized.
+    /// <para>
+    /// This error is returned for protocol-level parameter issues, such as:
+    /// </para>
+    /// <list type="bullet">
+    /// <item><description>Malformed requests that fail to satisfy the request schema (e.g., CallToolRequest)</description></item>
+    /// <item><description>Unknown or unrecognized primitive names (e.g., tool, prompt, or resource names)</description></item>
+    /// <item><description>Missing required protocol-level parameters</description></item>
+    /// </list>
+    /// <para>
+    /// Note: Input validation errors within tool/prompt/resource arguments should be reported as execution errors
+    /// (e.g., via <see cref="Protocol.CallToolResult.IsError"/>) rather than as protocol errors, allowing language
+    /// models to receive error feedback and self-correct.
+    /// </para>
     /// </remarks>
     InvalidParams = -32602,
 

src/ModelContextProtocol.Core/McpProtocolException.cs

@@ -1,14 +1,25 @@
 namespace ModelContextProtocol;
 
 /// <summary>
-/// Represents an exception that is thrown when an Model Context Protocol (MCP) error occurs.
+/// Represents an exception that is thrown when a Model Context Protocol (MCP) protocol-level error occurs.
 /// </summary>
 /// <remarks>
-/// This exception is used to represent failures to do with protocol-level concerns, such as invalid JSON-RPC requests,
-/// invalid parameters, or internal errors. It is not intended to be used for application-level errors.
+/// <para>
+/// This exception is used to represent failures related to protocol-level concerns, such as malformed
+/// JSON-RPC requests, unknown methods, unknown primitive names (tools/prompts/resources), or internal
+/// server errors. It is not intended to be used for tool execution errors, including input validation failures.
+/// </para>
+/// <para>
+/// Tool execution errors (including input validation errors, API failures, and business logic errors)
+/// should be returned in the result object with <c>IsError</c> set to <see langword="true"/>, allowing
+/// language models to see error details and self-correct. Only protocol-level issues should throw
+/// <see cref="McpProtocolException"/>.
+/// </para>
+/// <para>
 /// <see cref="Exception.Message"/> or <see cref="ErrorCode"/> from a <see cref="McpProtocolException"/> may be 
 /// propagated to the remote endpoint; sensitive information should not be included. If sensitive details need
 /// to be included, a different exception type should be used.
+/// </para>
 /// </remarks>
 public sealed class McpProtocolException : McpException
 {
@@ -65,7 +76,7 @@ public McpProtocolException(string message, Exception? innerException, McpErrorC
     /// <item><description>-32700: Parse error - Invalid JSON received</description></item>
     /// <item><description>-32600: Invalid request - The JSON is not a valid Request object</description></item>
     /// <item><description>-32601: Method not found - The method does not exist or is not available</description></item>
-    /// <item><description>-32602: Invalid params - Invalid method parameters</description></item>
+    /// <item><description>-32602: Invalid params - Malformed request or unknown primitive name (tool/prompt/resource)</description></item>
     /// <item><description>-32603: Internal error - Internal JSON-RPC error</description></item>
     /// </list>
     /// </remarks>

src/ModelContextProtocol.Core/Protocol/CallToolResult.cs

@@ -8,13 +8,14 @@ namespace ModelContextProtocol.Protocol;
 /// </summary>
 /// <remarks>
 /// <para>
-/// Any errors that originate from the tool should be reported inside the result
-/// object, with <see cref="IsError"/> set to true, rather than as a <see cref="JsonRpcError"/>.
+/// Tool execution errors (including input validation errors, API failures, and business logic errors)
+/// should be reported inside the result object with <see cref="IsError"/> set to <see langword="true"/>,
+/// rather than as a <see cref="JsonRpcError"/>. This allows language models to see error details
+/// and potentially self-correct in subsequent requests.
 /// </para>
 /// <para>
-/// However, any errors in finding the tool, an error indicating that the
-/// server does not support tool calls, or any other exceptional conditions,
-/// should be reported as an MCP error response.
+/// Protocol-level errors (such as unknown tool names, malformed requests that fail schema validation,
+/// or server errors) should be reported as MCP protocol error responses using <see cref="McpErrorCode"/>.
 /// </para>
 /// <para>
 /// See the <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">schema</see> for details.
@@ -38,10 +39,18 @@ public sealed class CallToolResult : Result
     /// Gets or sets an indication of whether the tool call was unsuccessful.
     /// </summary>
     /// <remarks>
+    /// <para>
     /// When set to <see langword="true"/>, it signifies that the tool execution failed.
-    /// Tool errors are reported with this property set to <see langword="true"/> and details in the <see cref="Content"/>
-    /// property, rather than as protocol-level errors. This allows LLMs to see that an error occurred
-    /// and potentially self-correct in subsequent requests.
+    /// Tool execution errors (including input validation errors, API failures, and business logic errors)
+    /// are reported with this property set to <see langword="true"/> and details in the <see cref="Content"/>
+    /// property, rather than as protocol-level errors.
+    /// </para>
+    /// <para>
+    /// This allows language models to receive detailed error feedback and potentially self-correct
+    /// in subsequent requests. For example, if a date parameter is in the wrong format or out of range,
+    /// the error message in <see cref="Content"/> can explain the issue, enabling the model to retry
+    /// with corrected parameters.
+    /// </para>
     /// </remarks>
     [JsonPropertyName("isError")]
     public bool? IsError { get; set; }

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -591,6 +591,41 @@ await Can_Handle_Requests(
             });
     }
 
+    [Fact]
+    public async Task Can_Handle_Call_Tool_Requests_With_InputValidationException()
+    {
+        // Test that input validation errors (like ArgumentException from JSON deserialization)
+        // are returned as tool execution errors (IsError=true) rather than protocol errors, per SEP-1303.
+        const string errorMessage = "Input validation failed: invalid date format";
+
+        await Can_Handle_Requests(
+            new ServerCapabilities
+            {
+                Tools = new()
+            },
+            method: RequestMethods.ToolsCall,
+            configureOptions: options =>
+            {
+                options.Handlers.CallToolHandler = async (request, ct) =>
+                {
+                    // Simulate an input validation error (like what would happen with wrong argument types)
+                    throw new ArgumentException(errorMessage);
+                };
+                options.Handlers.ListToolsHandler = (request, ct) => throw new NotImplementedException();
+            },
+            assertResult: (_, response) =>
+            {
+                var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
+                Assert.NotNull(result);
+                Assert.True(result.IsError, "Input validation errors should be returned as tool execution errors (IsError=true), not protocol errors");
+                Assert.NotEmpty(result.Content);
+                var textContent = Assert.IsType<TextContentBlock>(result.Content[0]);
+                // ArgumentException should result in a generic error message that doesn't expose the exception details
+                Assert.DoesNotContain(errorMessage, textContent.Text);
+                Assert.Contains("An error occurred", textContent.Text);
+            });
+    }
+
     [Fact]
     public async Task Can_Handle_Call_Tool_Requests_With_McpProtocolException()
     {