C# Implementation (#2)

C# implementation of SEP-1763

---------

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

csharp/sdk/samples/InterceptorLlmSample/LlmMutationInterceptors.cs

@@ -0,0 +1,175 @@
+using ModelContextProtocol.Interceptors;
+using ModelContextProtocol.Interceptors.Client;
+using ModelContextProtocol.Interceptors.Protocol.Llm;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using System.Text.RegularExpressions;
+
+/// <summary>
+/// Mutation interceptors for llm/completion events.
+/// These interceptors transform requests and responses.
+/// </summary>
+[McpClientInterceptorType]
+public partial class LlmMutationInterceptors
+{
+    // Patterns for sensitive data redaction
+    private static readonly Regex ApiKeyPattern = new(@"\b(sk_live_|api_key[=:]\s*)[a-zA-Z0-9_-]+\b", RegexOptions.Compiled | RegexOptions.IgnoreCase);
+    private static readonly Regex PasswordPattern = new(@"\b(password[=:]\s*|secret[=:]\s*)[^\s]+\b", RegexOptions.Compiled | RegexOptions.IgnoreCase);
+    private static readonly Regex BearerTokenPattern = new(@"Bearer\s+[a-zA-Z0-9._-]+", RegexOptions.Compiled);
+
+    /// <summary>
+    /// Normalizes prompt whitespace by trimming message content.
+    /// </summary>
+    [McpClientInterceptor(
+        Name = "prompt-normalizer",
+        Description = "Normalizes whitespace in prompts",
+        Type = InterceptorType.Mutation,
+        Events = [InterceptorEvents.LlmCompletion],
+        Phase = InterceptorPhase.Request,
+        PriorityHint = -100)] // Run early
+    public static MutationInterceptorResult NormalizePrompts(JsonNode? payload)
+    {
+        if (payload is null)
+        {
+            return MutationInterceptorResult.Unchanged(payload);
+        }
+
+        var request = payload.Deserialize<LlmCompletionRequest>();
+        if (request is null)
+        {
+            return MutationInterceptorResult.Unchanged(payload);
+        }
+
+        var modified = false;
+        foreach (var message in request.Messages)
+        {
+            if (message.Content is not null)
+            {
+                var trimmed = message.Content.Trim();
+                if (trimmed != message.Content)
+                {
+                    message.Content = trimmed;
+                    modified = true;
+                }
+            }
+        }
+
+        if (modified)
+        {
+            var mutatedPayload = JsonSerializer.SerializeToNode(request);
+            return MutationInterceptorResult.Mutated(mutatedPayload);
+        }
+
+        return MutationInterceptorResult.Unchanged(payload);
+    }
+
+    /// <summary>
+    /// Redacts sensitive information from LLM responses.
+    /// </summary>
+    [McpClientInterceptor(
+        Name = "response-redactor",
+        Description = "Redacts sensitive information from LLM responses",
+        Type = InterceptorType.Mutation,
+        Events = [InterceptorEvents.LlmCompletion],
+        Phase = InterceptorPhase.Response,
+        PriorityHint = 100)] // Run late in response chain
+    public static MutationInterceptorResult RedactResponse(JsonNode? payload)
+    {
+        if (payload is null)
+        {
+            return MutationInterceptorResult.Unchanged(payload);
+        }
+
+        var response = payload.Deserialize<LlmCompletionResponse>();
+        if (response is null)
+        {
+            return MutationInterceptorResult.Unchanged(payload);
+        }
+
+        var modified = false;
+        var redactions = new List<string>();
+
+        foreach (var choice in response.Choices)
+        {
+            if (choice.Message.Content is not null)
+            {
+                var content = choice.Message.Content;
+
+                // Redact API keys
+                if (ApiKeyPattern.IsMatch(content))
+                {
+                    content = ApiKeyPattern.Replace(content, "[REDACTED_API_KEY]");
+                    redactions.Add("api_key");
+                    modified = true;
+                }
+
+                // Redact passwords
+                if (PasswordPattern.IsMatch(content))
+                {
+                    content = PasswordPattern.Replace(content, "[REDACTED_SECRET]");
+                    redactions.Add("password");
+                    modified = true;
+                }
+
+                // Redact bearer tokens
+                if (BearerTokenPattern.IsMatch(content))
+                {
+                    content = BearerTokenPattern.Replace(content, "Bearer [REDACTED_TOKEN]");
+                    redactions.Add("bearer_token");
+                    modified = true;
+                }
+
+                choice.Message.Content = content;
+            }
+        }
+
+        if (modified)
+        {
+            var mutatedPayload = JsonSerializer.SerializeToNode(response);
+            var result = MutationInterceptorResult.Mutated(mutatedPayload);
+            result.Info = new JsonObject
+            {
+                ["action"] = "redacted sensitive data",
+                ["types"] = JsonNode.Parse($"[\"{string.Join("\", \"", redactions.Distinct())}\"]")
+            };
+            return result;
+        }
+
+        return MutationInterceptorResult.Unchanged(payload);
+    }
+
+    /// <summary>
+    /// Adds metadata to track request origin.
+    /// </summary>
+    [McpClientInterceptor(
+        Name = "metadata-injector",
+        Description = "Adds tracking metadata to requests",
+        Type = InterceptorType.Mutation,
+        Events = [InterceptorEvents.LlmCompletion],
+        Phase = InterceptorPhase.Request,
+        PriorityHint = 100)] // Run late
+    public static MutationInterceptorResult InjectMetadata(JsonNode? payload)
+    {
+        if (payload is null)
+        {
+            return MutationInterceptorResult.Unchanged(payload);
+        }
+
+        var request = payload.Deserialize<LlmCompletionRequest>();
+        if (request is null)
+        {
+            return MutationInterceptorResult.Unchanged(payload);
+        }
+
+        // Add tracking metadata
+        request.Meta ??= new JsonObject();
+        request.Meta["interceptor_processed"] = true;
+        request.Meta["interceptor_timestamp"] = DateTimeOffset.UtcNow.ToString("O");
+        request.Meta["interceptor_version"] = "1.0.0";
+
+        var mutatedPayload = JsonSerializer.SerializeToNode(request);
+        var result = MutationInterceptorResult.Mutated(mutatedPayload);
+        result.Info = new JsonObject { ["action"] = "added tracking metadata" };
+        return result;
+    }
+}