C# Implementation (#2)

C# implementation of SEP-1763

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

csharp/sdk/README.md

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+# MCP Interceptors C# SDK
+
+This library provides interceptor support for the Model Context Protocol (MCP) .NET SDK. Interceptors enable validation, mutation, and observation of MCP messages without modifying the original server or client implementations.
+
+## Overview
+
+MCP Interceptors (SEP-1763) allow you to:
+
+- **Validate** incoming requests before they reach handlers
+- **Mutate** requests or responses to transform data
+- **Observe** message flow for logging, metrics, or auditing
+
+Interceptors can be deployed as:
+- Sidecars alongside MCP servers
+- Gateway services that proxy MCP traffic
+- Embedded validators within applications
+
+## Installation
+
+```bash
+dotnet add package ModelContextProtocol.Interceptors
+```
+
+## Quick Start
+
+```csharp
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using ModelContextProtocol.Interceptors;
+
+var builder = Host.CreateApplicationBuilder(args);
+
+builder.Services.AddMcpServer()
+    .WithStdioServerTransport()
+    .WithInterceptors<ParameterValidator>();
+
+await builder.Build().RunAsync();
+```
+
+## Creating an Interceptor
+
+```csharp
+using ModelContextProtocol.Interceptors;
+using ModelContextProtocol.Interceptors.Server;
+using System.Text.Json.Nodes;
+
+[McpServerInterceptorType]
+public class ParameterValidator
+{
+    [McpServerInterceptor(
+        Name = "parameter-validator",
+        Description = "Validates tool call parameters",
+        Events = new[] { InterceptorEvents.ToolsCall },
+        Phase = InterceptorPhase.Request)]
+    public ValidationInterceptorResult ValidateToolCall(JsonNode? payload)
+    {
+        if (payload is null)
+        {
+            return new ValidationInterceptorResult
+            {
+                Valid = false,
+                Severity = ValidationSeverity.Error,
+                Messages = [new() { Message = "Payload is required" }]
+            };
+        }
+
+        return new ValidationInterceptorResult { Valid = true };
+    }
+}
+```
+
+## Interceptor Types
+
+### Validation Interceptors
+Validate requests/responses and return pass/fail results with optional error messages.
+
+### Mutation Interceptors
+Transform request or response payloads before they continue through the pipeline.
+
+### Observability Interceptors
+Observe message flow for logging, metrics collection, or auditing without modifying data.
+
+## Configuration Options
+
+### Phase
+- `InterceptorPhase.Request` - Intercept incoming requests
+- `InterceptorPhase.Response` - Intercept outgoing responses
+- `InterceptorPhase.Both` - Intercept both directions
+
+### Events
+Interceptors can target specific MCP events:
+- `InterceptorEvents.ToolsCall` - Tool invocation requests
+- `InterceptorEvents.PromptGet` - Prompt retrieval
+- `InterceptorEvents.ResourceRead` - Resource access
+- And more...
+
+### Priority
+Use `PriorityHint` to control interceptor execution order (lower values run first).
+
+## Sample Projects
+
+See the `samples/InterceptorServiceSample` directory for a complete example of a security-focused validation interceptor.
+
+## Requirements
+
+- .NET 8.0 or later (or .NET Standard 2.0 compatible runtime)
+- ModelContextProtocol SDK 0.1.0-preview.10 or later
+
+## License
+
+MIT License - see LICENSE file for details.
+
+## Contributing
+
+Contributions are welcome! Please see the FSIG CONTRIBUTING.md for guidelines.