7cf90c9ccbAddresses the first two items of #15 (the chain-orchestrator multi-server item follows in a separate PR). ## `mode`: keep `"active"`, add `"audit"` round-trip coverage > **Update:** an earlier revision of this PR renamed `mode` from `"active"` to `"enforce"`, following a SEP **draft**. The now-current revised SEP-1763 ([PR #2624](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2624)) defines `mode?: "active" | "audit"` with default `"active"` — so `"active"` was correct all along. @jeongukjae flagged this; the rename has been reverted (commit 58edf2c). The C# SDK's wire value `"active"` matches the SEP and the TypeScript SDK. `InterceptorMode` keeps members `Active` (wire `"active"`, default) and `Audit` (wire `"audit"`). Added a deserialization test alongside the existing serialization test so both wire forms are covered. ## `priorityHint`: support `number | { request?, response? }` The SDK only supported a plain `int`. This adds a `PriorityHint` protocol type whose converter accepts both wire forms and preserves them on round-trip (a number stays a number, an object stays an object). Chain ordering now resolves the effective priority per phase exactly as the SEP's `resolvePriority` algorithm (unset → 0, alphabetical tie-break unchanged). - `Interceptor.PriorityHint` is now `PriorityHint?`; an implicit conversion from `int` keeps existing code compiling. - `[McpServerInterceptor]` gains `RequestPriorityHint` / `ResponsePriorityHint` (mutually exclusive with the scalar `PriorityHint`), following the official SDK's backing-field pattern for optional attribute values. - Behavior note: attribute-discovered interceptors with no priority set now omit `priorityHint` from the wire instead of emitting `0` — semantically identical per the SEP default, and consistent with the existing default-skipping for `Mode`/`FailOpen`. ## Testing `dotnet test` from `csharp/sdk/`: all passing, including new coverage for both `priorityHint` wire forms, partial objects, unknown-key tolerance, phase-dependent mutation ordering (the SEP's pii-redactor/compressor example), the attribute conflict error, and both `mode` wire values. Part of #15. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
98 lines
8.2 KiB
Markdown
98 lines
8.2 KiB
Markdown
# MCP Interceptors - C# SDK
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## What this is
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C# implementation of gateway-level interceptors from [SEP-2624](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/2624). NuGet package additive to the official [C# MCP SDK](https://github.com/modelcontextprotocol/csharp-sdk) (v1.1.0). Focus is on the protocol-level extension (client → interceptor server → server), NOT in-process middleware.
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## Build & test
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```
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dotnet build # from csharp/sdk/
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dotnet test # 87 tests across the interceptor test project
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```
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## Key architectural constraints
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**Why message filter, not handlers**: `McpServerHandlers` and `McpServerImpl` are `internal` in the SDK. We can't register handlers for new JSON-RPC methods from outside. Instead we use `McpServerOptions.Filters.Message.IncomingFilters` to intercept `interceptors/list` and `interceptor/invoke`, handle them, send `JsonRpcResponse` via `context.Server.SendMessageAsync()`, and skip calling `next`. See `InterceptorMessageFilter.cs`. Chain execution is no longer a wire method — it's SDK orchestration in `Client/InterceptorChainOrchestrator.cs`.
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**Why `ServerCapabilities.Extensions`**: The SDK's intended mechanism for protocol extensions. Requires `#pragma warning disable MCPEXP001`. We advertise `InterceptorsCapability { SupportedEvents }` under `Extensions["io.modelcontextprotocol/interceptors"]`.
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**Client `SendRequestAsync`**: The public overload (`McpSession.Methods.cs:24`) takes `JsonSerializerOptions`. We pass `InterceptorJsonUtilities.DefaultOptions` which chains `McpJsonUtilities.DefaultOptions` + our `InterceptorJsonContext`. The internal overload takes `JsonTypeInfo<T>` — we can't use it.
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**`InterceptingMcpClient` is composition**: `McpClient` has an internal constructor; subclassing is `[Experimental]`. We wrap it as a concrete class exposing `.Inner` for direct access.
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## Chain execution order (SEP-2624)
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- **Request phase (sending)**: Mutations (sequential by priority ↑) → Validations (parallel) → Sinks (fire-and-forget)
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- **Response phase (receiving)**: Validations (parallel) → Sinks (fire-and-forget) → Mutations (sequential by priority ↑)
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- Lower `PriorityHint` executes first; scalar (both phases) or per-phase `{request, response}` form, unset phase resolves to 0; ties broken alphabetically by name
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## JSON-RPC methods
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| Method | Params → Result |
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|--------|----------------|
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| `interceptors/list` | `ListInterceptorsRequestParams` → `ListInterceptorsResult` |
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| `interceptor/invoke` | `InvokeInterceptorRequestParams` → `InterceptorResult` (polymorphic) |
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## `InterceptorResult` polymorphism
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Uses `[JsonPolymorphic(TypeDiscriminatorPropertyName = "type")]` with `"validation"`, `"mutation"`, `"sink"` discriminators. Serialization/deserialization handles this automatically via STJ source-gen in `InterceptorJsonContext`.
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## Parameter binding (ReflectionMcpServerInterceptor)
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Interceptor methods auto-bind from `InvokeInterceptorRequestParams`:
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- `JsonNode payload` → `.Payload`
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- `JsonNode config` → `.Config`
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- `string event` / `string eventName` → `.Event`
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- `InterceptorPhase phase` → `.Phase`
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- `InvokeInterceptorContext` → `.Context`
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- `CancellationToken`, `McpServer`, `IServiceProvider` → framework
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- Return `bool` → wrapped as `ValidationInterceptorResult { Valid = result }`
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## SDK reference paths (local at /mnt/d/code/ai/mcp/csharp-sdk)
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- `src/ModelContextProtocol.Core/Server/McpServerTool.cs` — pattern we follow
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- `src/ModelContextProtocol.Core/Server/McpMessageFilter.cs` — our hook point
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- `src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs` — Extensions dict
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- `src/ModelContextProtocol.Core/McpSession.Methods.cs` — public SendRequestAsync
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- `src/ModelContextProtocol/McpServerBuilderExtensions.cs` — builder pattern
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- `src/ModelContextProtocol.Core/McpJsonUtilities.cs` — JSON context chaining pattern
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## Transparent gateway/proxy (`Gateway/`)
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**`McpInterceptorGateway`**: Configures an `McpServer` as a transparent proxy. Reads backend `ServerCapabilities`, registers handler delegates (`CallToolHandler`, `ListToolsHandler`, etc.) that route through interceptor chains before forwarding to the backend. By default the gateway is transparent-only; SEP passthrough is opt-in via `ExposeInterceptorProtocol = true`. To connecting clients, the proxy appears to be the backend server.
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**`InterceptorChainRunner`** (internal): Shared interception logic used by both `InterceptingMcpClient` and `McpInterceptorGateway`. Supports multiple interceptor clients executed sequentially — each client's `ExecuteChainAsync` (SDK-level orchestration via list + invoke) receives the mutated payload from the previous one.
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**Gateway split**: `GatewayProxyConfigurator` owns transparent MCP proxy wiring, `GatewayInterceptorProtocolBridge` owns optional SEP passthrough wiring, and `GatewayConnectionForwardingRegistrar` owns connection-bound notification forwarding registration.
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**`McpInterceptorGatewayOptions`**: Configuration — `BackendClient`, optional preconnected `InterceptorClients`, optional external `InterceptorServerConnections`, optional dynamic `InterceptorServerConnectionResolver`, `Events` filter, `TimeoutMs`, `DefaultContext`, optional `ServerInfo` override, and `ExposeInterceptorProtocol`.
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**External interceptor connections**: `McpInterceptorGateway.CreateAsync(...)` can create interceptor clients from standard SDK transports (`IClientTransport`) via `McpInterceptorServerConnectionOptions`. Dynamic per-request transparent resolution is supported via `InterceptorServerConnectionResolver` using the SDK's `MessageContext` plus SEP event name.
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**Builder extension**: `IMcpServerBuilder.WithInterceptorGateway(options)` and `WithInterceptorGateway(Func<IServiceProvider, McpInterceptorGatewayOptions>)` for DI/builder scenarios. Builder-based wiring still expects already-connected interceptor clients; async external transport setup should use `McpInterceptorGateway.CreateAsync(...)`.
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**Notification forwarding**: `gateway.RegisterNotificationForwarding(proxyServer)` subscribes to backend `tools/list_changed`, `prompts/list_changed`, `resources/list_changed` and re-sends through the proxy.
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**Why handler delegates (not message filters) for the proxy**: The SDK's `With*Handler` methods automatically set `ServerCapabilities`, are type-safe, and are the intended extension point. Message filters are still used for interceptor protocol passthrough (`interceptors/list`, `interceptor/invoke`).
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**Tool call error handling note**: When an interceptor validation aborts a `tools/call`, the gateway throws `McpInterceptorValidationException`. The SDK catches this and returns `CallToolResult { IsError = true }` (not a JSON-RPC error), since tool execution errors are returned as results by design.
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## `InterceptingMcpClient` wrapped methods
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- `CallToolAsync` — `tools/call`
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- `ListToolsAsync` — `tools/list`
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- `ListPromptsAsync` — `prompts/list`
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- `GetPromptAsync` — `prompts/get`
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- `ListResourcesAsync` — `resources/list`
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- `ReadResourceAsync` — `resources/read`
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- `SubscribeToResourceAsync` — `resources/subscribe`
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- `ListInterceptorsAsync` — direct passthrough to interceptor client
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## LLM completion payloads (`Protocol/LlmCompletionPayload.cs`)
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- `LlmCompletionRequestPayload` — model, messages, maxTokens, temperature, metadata
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- `LlmCompletionResponsePayload` — model, message, stopReason, usage, metadata
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- `LlmMessage` — role + content
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- `LlmUsage` — inputTokens + outputTokens
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- Registered in `InterceptorJsonContext` for source-gen serialization
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- Not wired into `InterceptingMcpClient` — these are for custom gateway use
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## Samples
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- `InterceptorServerSample` — stdio server hosting 3 interceptors
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- `GatewaySample` — single gateway: client → interceptor → everything server
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- `InterceptorClientSample` — client API: list, invoke, execute chain directly
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- `GatewayChainSample` — chained gateways: security layer → logging layer → server
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- `TransparentProxySample` — stdio proxy server: clients connect to it as if it were the backend, all requests routed through interceptors
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- `ConfigDrivenGatewaySample` — sample-only config-driven host composed from the gateway primitives; demonstrates static and dynamic external interceptor resolution with transport-configured backend/interceptor clients
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