Kurt Degiorgio
c9900f02ab
Implement Go interceptor chain (#4)
## 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>
|
2026-04-29 15:29:25 +01:00 |
|
sdk
|
Implement Go interceptor chain (#4)
|
2026-04-29 15:29:25 +01:00 |