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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github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
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github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
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github.com/golang-jwt/jwt/v5 v5.3.0 h1:pv4AsKCKKZuqlgs5sUmn4x8UlGa0kEVt/puTpKx9vvo=
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github.com/golang-jwt/jwt/v5 v5.3.0/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
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github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
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github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
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github.com/google/jsonschema-go v0.4.2 h1:tmrUohrwoLZZS/P3x7ex0WAVknEkBZM46iALbcqoRA8=
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github.com/google/jsonschema-go v0.4.2/go.mod h1:r5quNTdLOYEz95Ru18zA0ydNbBuYoo9tgaYcxEYhJVE=
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github.com/modelcontextprotocol/go-sdk v1.4.0 h1:u0kr8lbJc1oBcawK7Df+/ajNMpIDFE41OEPxdeTLOn8=
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github.com/modelcontextprotocol/go-sdk v1.4.0/go.mod h1:Nxc2n+n/GdCebUaqCOhTetptS17SXXNu9IfNTaLDi1E=
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github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
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github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
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github.com/segmentio/asm v1.1.3 h1:WM03sfUOENvvKexOLp+pCqgb/WDjsi7EK8gIsICtzhc=
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github.com/segmentio/asm v1.1.3/go.mod h1:Ld3L4ZXGNcSLRg4JBsZ3//1+f/TjYl0Mzen/DQy1EJg=
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github.com/segmentio/encoding v0.5.3 h1:OjMgICtcSFuNvQCdwqMCv9Tg7lEOXGwm1J5RPQccx6w=
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github.com/segmentio/encoding v0.5.3/go.mod h1:HS1ZKa3kSN32ZHVZ7ZLPLXWvOVIiZtyJnO1gPH1sKt0=
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github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
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github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
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github.com/yosida95/uritemplate/v3 v3.0.2 h1:Ed3Oyj9yrmi9087+NczuL5BwkIc4wvTb5zIM+UJPGz4=
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github.com/yosida95/uritemplate/v3 v3.0.2/go.mod h1:ILOh0sOhIJR3+L/8afwt/kE++YT040gmv5BQTMR2HP4=
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golang.org/x/oauth2 v0.34.0 h1:hqK/t4AKgbqWkdkcAeI8XLmbK+4m4G5YeQRrmiotGlw=
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golang.org/x/oauth2 v0.34.0/go.mod h1:lzm5WQJQwKZ3nwavOZ3IS5Aulzxi68dUSgRHujetwEA=
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golang.org/x/sys v0.40.0 h1:DBZZqJ2Rkml6QMQsZywtnjnnGvHza6BTfYFWY9kjEWQ=
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golang.org/x/sys v0.40.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
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golang.org/x/tools v0.41.0 h1:a9b8iMweWG+S0OBnlU36rzLp20z1Rp10w+IY2czHTQc=
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golang.org/x/tools v0.41.0/go.mod h1:XSY6eDqxVNiYgezAVqqCeihT4j1U2CCsqvH3WhQpnlg=
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gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
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gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
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gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
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gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
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