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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SEP Conformance
Status of this Go SDK implementation against the SEP-1763 interceptor proposal.
Implemented
| Area | Notes |
|---|---|
| Validation interceptors | Parallel execution, severity-based blocking, fail-open support |
| Mutation interceptors | Sequential execution, priority ordering, payload threading via json.RawMessage |
| Interceptor metadata | Name, version, description, events, phase, priorityHint (polymorphic JSON), compat, configSchema, mode, failOpen |
| Event names | Constants for all standard server-side MCP methods; JSON-RPC method names used directly as event names |
| Protocol methods | interceptors/list for discovery, interceptor/invoke for invocation — both registered as custom JSON-RPC methods |
InterceptorChain (SEP) |
chain.Chain type with chain.ChainEntry objects pairing interceptor descriptors with MCP client sessions |
ChainEntry (SEP) |
chain.ChainEntry struct holds InterceptorInfo + *mcp.ClientSession |
| Chain discovery | chain.Chain.AddMCPServer() calls interceptors/list to discover interceptors from an MCP server |
| Chain execution | chain.Chain.Execute() invokes interceptors via interceptor/invoke on the appropriate server per entry |
InvokeResult envelope |
Per-interceptor result with interceptor name, type, phase, duration, validation/mutation result, mutated payload |
chain.ExecutionResult |
Aggregated chain result with status, results, finalPayload, validationSummary, abortedAt, totalDurationMs |
| JSON-RPC error mapping | Typed error data structs: -32602 for validation, -32603 for mutation, -32000 for timeout |
| Trust-boundary execution order | Request: validate (parallel) then mutate (sequential); Response: mutate (sequential) then validate (parallel) |
| Priority ordering | Mutators sorted by priorityHint.Resolve(phase) ascending, alphabetical tiebreak |
| Fail-open behavior | FailOpen: true interceptors log errors without aborting the chain |
| Audit mode | ModeAudit records results without blocking; mutated payloads not propagated |
| Timeout & context | Per-interceptor timeouts via InvokeParams.TimeoutMs, chain-level context cancellation, InvocationContext with principal/traceId |
| Receiving direction (client → server) | All server-side method calls intercepted via AddReceivingMiddleware |
| Capability declaration | Interceptor metadata injected into initialize response via Capabilities.Experimental |
| First-party (in-process) deployment | Server.LocalChain() creates an in-memory transport chain.Chain; interceptors invoked via JSON-RPC even in-process |
| Third-party and hybrid deployment | Chain entries can point to any *mcp.ClientSession — local (in-memory), stdio, or HTTP transport |
json.RawMessage payloads |
Interceptor handlers receive json.RawMessage via interceptor/invoke, matching the SEP's JSON-level payload model |
Not Implemented
| Area | SEP expects | Notes |
|---|---|---|
| Wildcard event matching | "*/request", "*/response", "*" |
matchesEvent does exact match only; wildcard patterns are planned |
| Server → client interception | Client features as interceptable events: "sampling/createMessage", "elicitation/create", "roots/list" |
Requires Server.AddSendingMiddleware support in the go-sdk |
| Per-interceptor config passthrough | ChainExecutionParams.Config map |
Wired through to InvokeParams.Config but not yet used by middleware |
| Remote interceptor servers | Connecting chain to external MCP servers over stdio/HTTP | Infrastructure is ready (chain.Chain.AddMCPServer accepts any *mcp.ClientSession); no convenience helpers yet |