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>
75 lines
1.8 KiB
Go
75 lines
1.8 KiB
Go
// Copyright 2025 The MCP Interceptors Authors. All rights reserved.
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// Use of this source code is governed by an Apache-2.0
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// license that can be found in the LICENSE file.
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package gomiddleware
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import (
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"encoding/json"
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"github.com/modelcontextprotocol/go-sdk/jsonrpc"
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"github.com/modelcontextprotocol/ext-interceptors/go/sdk/interceptors/chain"
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)
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type validationErrorData struct {
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ValidationErrors []chain.AbortInfo `json:"validationErrors"`
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}
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type mutationErrorData struct {
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FailedInterceptor string `json:"failedInterceptor"`
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Reason string `json:"reason"`
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}
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type timeoutErrorData struct {
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Interceptor string `json:"interceptor"`
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TimeoutMs int64 `json:"timeoutMs,omitempty"`
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Phase string `json:"phase"`
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}
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const codeServerError = -32000
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// abortToJSONRPCError converts a list of abort infos into a *jsonrpc.Error.
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func abortToJSONRPCError(aborts []chain.AbortInfo) *jsonrpc.Error {
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first := aborts[0]
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var code int64
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switch first.Type {
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case chain.AbortValidation:
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code = jsonrpc.CodeInvalidParams
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case chain.AbortTimeout:
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code = codeServerError
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default:
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code = jsonrpc.CodeInternalError
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}
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var data json.RawMessage
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switch first.Type {
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case chain.AbortValidation:
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d, _ := json.Marshal(validationErrorData{ValidationErrors: aborts})
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data = d
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case chain.AbortTimeout:
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d, _ := json.Marshal(timeoutErrorData{
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Interceptor: first.Interceptor,
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Phase: first.Phase,
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})
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data = d
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default:
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d, _ := json.Marshal(mutationErrorData{
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FailedInterceptor: first.Interceptor,
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Reason: first.Reason,
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})
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data = d
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}
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msg := "interceptor abort: " + first.Reason
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if len(aborts) > 1 {
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msg = "interceptor abort: multiple validation failures"
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}
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return &jsonrpc.Error{
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Code: code,
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Message: msg,
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Data: data,
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}
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}
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