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c9900f02ab## Summary Implements the Go SDK for MCP interceptors — validation and mutation middleware for any MCP server. The framework treats interceptors as first-class MCP primitives: they are registered on an MCP server, discoverable via `interceptors/list`, and invocable via `interceptor/invoke` — the same JSON-RPC pattern as tools, prompts, and resources. A chain engine orchestrates execution across one or more interceptor servers, and an optional go-sdk middleware integration applies interceptors automatically on every matching request/response. ## Architecture **Three-layer design:** 1. **`interceptors`** — Protocol-agnostic core. Defines interceptor types (validators, mutators), metadata, invocation, wire types, and results. Zero MCP dependencies. 2. **`interceptors/mcpserver`** — MCP server integration. Wraps `mcp.Server` to register `interceptors/list` and `interceptor/invoke` as custom JSON-RPC methods and advertise interceptor capabilities during `initialize`. 3. **`interceptors/chain`** — Chain engine. Orchestrates execution across N interceptor servers with trust-boundary-aware ordering, priority-based mutation sequencing, parallel validation, audit-mode isolation, and fail-open/fail-closed semantics. 4. **`interceptors/integrations/gomiddleware`** — Optional go-sdk middleware. Installs on `mcp.Server` via `AddReceivingMiddleware()` to automatically run the interceptor chain on every matching request/response. **Key design decisions:** - **Trust-boundary ordering** — request phase: validate → mutate; response phase: mutate → validate. Validation always guards the boundary crossing. - **Wire-level invocation** — the chain calls `interceptor/invoke` over the MCP transport (even in-process via `LocalChain()`), so local and remote interceptor servers use the same interface. - **Audit-mode isolation** — audit-mode mutators compute transformations without applying them. Only audit mutators pay the serialization cost. - **Concurrency** — validators run in parallel; mutators run sequentially in priority order. ### Package Structure | Package | File | Responsibility | |---------|------|----------------| | `interceptors` | `interceptor.go` | Enums, Metadata, Interceptor interface, Validator/Mutator structs, handler types, Invocation | | `interceptors` | `wire.go` | Wire types: ListParams/Result, InvokeParams/Result, InterceptorInfo, event/method constants | | `interceptors` | `priority.go` | Priority with phase-aware resolution and JSON marshaling | | `interceptors` | `doc.go` | Package documentation with examples | | `interceptors/chain` | `chain.go` | Chain engine: discovery, filtering, sorting, execution orchestration | | `interceptors/chain` | `result.go` | ExecutionResult, AbortInfo | | `interceptors/chain` | `doc.go` | Sub-package documentation | | `interceptors/mcpserver` | `server.go` | MCP server wrapper, interceptor registry, capability advertisement, `LocalChain()` | | `interceptors/mcpserver` | `rpc.go` | `interceptors/list` and `interceptor/invoke` JSON-RPC handlers | | `interceptors/mcpserver` | `doc.go` | Sub-package documentation | | `integrations/gomiddleware` | `middleware.go` | go-sdk `Middleware` that runs the chain on every request/response | | `integrations/gomiddleware` | `errors.go` | Abort-to-JSONRPC error conversion | | `integrations/gomiddleware` | `doc.go` | Sub-package documentation | ### Documentation - `doc/DESIGN.md` — architecture, execution model, integration with go-sdk - `doc/CONFORMANCE.md` — SEP conformance status - `doc/PERFORMANCE.md` — per-request cost model ### Examples - `examples/validator` — blocks dangerous tool calls, demonstrates `WithContextProvider` - `examples/mutator` — tags tool responses with audit trail ## Vendor: go-sdk v1.4.0 with custom method support Vendors `go-sdk` with minimal extensions for custom JSON-RPC methods: - `Server.AddCustomMethod` — registers a handler that bypasses the typed pipeline - `ClientSession.CallCustom` — sends raw JSON-RPC calls for custom methods These changes are generic (not interceptor-specific) and enable any MCP extension to define its own JSON-RPC methods. ## TODO - More integration tests - Unit tests - Support wildcards for event interception (separate PR) --------- Signed-off-by: Kurt Degiorgio <kdegiorgio@bloomberg.net> |
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MCP Interceptors - Go Implementation
Go implementation of the MCP Interceptor Extension based on SEP-1763.
Note: Currently the MCP SDK is vendored, in-order to add the Protocol Methods needed for interceptors.
Quick Start
mcpServer := mcp.NewServer(&mcp.Implementation{
Name: "my-server",
Version: "0.1.0",
}, nil)
// Wrap with interceptor support.
srv := mcpserver.NewServer(mcpServer)
// Register a validator that blocks dangerous tool calls.
srv.AddInterceptor(&interceptors.Validator{
Metadata: interceptors.Metadata{
Name: "block-dangerous",
Hook: interceptors.Hook{
Events: []string{interceptors.EventToolsCall},
Phase: interceptors.PhaseRequest,
},
Mode: interceptors.ModeEnforce,
},
Handler: func(_ context.Context, inv *interceptors.Invocation) (*interceptors.ValidationResult, error) {
raw := inv.Payload.(json.RawMessage)
var params struct{ Name string `json:"name"` }
json.Unmarshal(raw, ¶ms)
// validate the request...
return &interceptors.ValidationResult{Valid: true}, nil
},
})
// Create a chain and install middleware for automatic execution.
chain, err := srv.LocalChain(ctx)
mcpServer.AddReceivingMiddleware(gomiddleware.Middleware(chain))
srv.Run(context.Background(), &mcp.StdioTransport{})
See examples/ for complete working examples.
Documentation
- DESIGN.md — architecture, execution model, integration with the go-sdk.
- PERFORMANCE.md — per-request cost model, allocation summary, and optimization notes.
- CONFORMANCE.md — SEP conformance status.
Package API documentation is available via go doc:
go doc github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors