experimental-ext-interceptors/.github/workflows/go.yml
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

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YAML

name: Go CI
on:
push:
branches: [main]
paths:
- 'go/**'
- '.github/workflows/go.yml'
pull_request:
paths:
- 'go/**'
- '.github/workflows/go.yml'
workflow_dispatch:
permissions:
contents: read
jobs:
go-lint:
name: "Linting"
runs-on: ubuntu-latest
steps:
- name: Check out code
uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
- name: Set up Go
uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
with:
go-version: "^1.25"
- name: Check formatting
run: |
cd go/sdk
unformatted=$(gofmt -l .)
if [ -n "$unformatted" ]; then
echo "The following files are not properly formatted:"
echo "$unformatted"
exit 1
fi
echo "All Go files are properly formatted"
- name: Run Go vet
run: |
cd go/sdk
go vet ./...
- name: Run staticcheck
uses: dominikh/staticcheck-action@288b4e28bae83c59f35f73651aeb5cab746a06fc # v1.4.0
with:
install-go: false
version: "v0.6.1"
working-directory: go/sdk
go-test:
name: "Unit Tests"
runs-on: ubuntu-latest
strategy:
matrix:
go: ["1.24", "1.25"]
steps:
- name: Check out code
uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
- name: Set up Go
uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
with:
go-version: ${{ matrix.go }}
- name: Test
run: |
cd go/sdk
go test -v ./...
go-race-test:
name: "Race Detection"
runs-on: ubuntu-latest
steps:
- name: Check out code
uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
- name: Set up Go
uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
with:
go-version: "1.24"
- name: Test with -race
run: |
cd go/sdk
go test -v -race ./...