Goa mcp server plugin generation

README.md

@@ -0,0 +1,44 @@
+# goa-mcp-plugin
+
+Goa plugin that adds MCP (Model Context Protocol) as a transport and DSL annotations to expose Goa service methods as MCP tools, resources and prompts.
+
+Status: experimental scaffold. Generates an MCP server wrapper over Goa endpoints using JSON-RPC 2.0 over stdio.
+
+## Install
+
+```
+go install goa.design/goa/v3/cmd/goa@latest
+```
+
+## Usage (preview)
+
+- In your Goa design, import `github.com/workspace/goa-mcp-plugin/dsl/mcp` and annotate methods:
+
+```go
+import mcpdsl "github.com/workspace/goa-mcp-plugin/dsl/mcp"
+
+var _ = Service("calc", func() {
+    Method("add", func() {
+        Payload(func() {
+            Attribute("a", Int)
+            Attribute("b", Int)
+            Required("a", "b")
+        })
+        Result(Int)
+        // Declare as an MCP tool with description
+        mcpdsl.Tool(func() {
+            mcpdsl.Description("Add two integers")
+        })
+    })
+})
+```
+
+- Run `goa gen` as usual. The plugin generates `gen/mcp/server` with a stdio MCP server exposing your annotated methods as tools.
+
+- Start the MCP server:
+
+```
+go run ./gen/mcp/server
+```
+
+Point an MCP client (e.g., Claude Desktop) to the binary with stdio transport.

go.mod

@@ -0,0 +1,21 @@
+module github.com/workspace/goa-mcp-plugin
+
+go 1.23.0
+
+toolchain go1.24.2
+
+require goa.design/goa/v3 v3.21.5
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/dimfeld/httppath v0.0.0-20170720192232-ee938bf73598 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/manveru/faker v0.0.0-20171103152722-9fbc68a78c4d // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/stretchr/testify v1.10.0 // indirect
+	golang.org/x/mod v0.25.0 // indirect
+	golang.org/x/sync v0.15.0 // indirect
+	golang.org/x/text v0.26.0 // indirect
+	golang.org/x/tools v0.34.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

go.sum

@@ -0,0 +1,32 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/dimfeld/httppath v0.0.0-20170720192232-ee938bf73598 h1:MGKhKyiYrvMDZsmLR/+RGffQSXwEkXgfLSA08qDn9AI=
+github.com/dimfeld/httppath v0.0.0-20170720192232-ee938bf73598/go.mod h1:0FpDmbrt36utu8jEmeU05dPC9AB5tsLYVVi+ZHfyuwI=
+github.com/gohugoio/hashstructure v0.5.0 h1:G2fjSBU36RdwEJBWJ+919ERvOVqAg9tfcYp47K9swqg=
+github.com/gohugoio/hashstructure v0.5.0/go.mod h1:Ser0TniXuu/eauYmrwM4o64EBvySxNzITEOLlm4igec=
+github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
+github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/manveru/faker v0.0.0-20171103152722-9fbc68a78c4d h1:Zj+PHjnhRYWBK6RqCDBcAhLXoi3TzC27Zad/Vn+gnVQ=
+github.com/manveru/faker v0.0.0-20171103152722-9fbc68a78c4d/go.mod h1:WZy8Q5coAB1zhY9AOBJP0O6J4BuDfbupUDavKY+I3+s=
+github.com/manveru/gobdd v0.0.0-20131210092515-f1a17fdd710b h1:3E44bLeN8uKYdfQqVQycPnaVviZdBLbizFhU49mtbe4=
+github.com/manveru/gobdd v0.0.0-20131210092515-f1a17fdd710b/go.mod h1:Bj8LjjP0ReT1eKt5QlKjwgi5AFm5mI6O1A2G4ChI0Ag=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.10.0 h1:Xv5erBjTwe/5IxqUQTdXv5kgmIvbHo3QQyRwhJsOfJA=
+github.com/stretchr/testify v1.10.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
+goa.design/goa/v3 v3.21.5 h1:eS6SHJ1KZ5q5bhT/llw0LMTCWbosBwlFX4v8MctYs38=
+goa.design/goa/v3 v3.21.5/go.mod h1:5THVDuChOIctYM+t3xmL4f2fJbFPzzwvrYMj3PQZg9g=
+golang.org/x/mod v0.25.0 h1:n7a+ZbQKQA/Ysbyb0/6IbB1H/X41mKgbhfv7AfG/44w=
+golang.org/x/mod v0.25.0/go.mod h1:IXM97Txy2VM4PJ3gI61r1YEk/gAj6zAHN3AdZt6S9Ww=
+golang.org/x/sync v0.15.0 h1:KWH3jNZsfyT6xfAfKiz6MRNmd46ByHDYaZ7KSkCtdW8=
+golang.org/x/sync v0.15.0/go.mod h1:1dzgHSNfp02xaA81J2MS99Qcpr2w7fw1gpm99rleRqA=
+golang.org/x/text v0.26.0 h1:P42AVeLghgTYr4+xUnTRKDMqpar+PtX7KWuNQL21L8M=
+golang.org/x/text v0.26.0/go.mod h1:QK15LZJUUQVJxhz7wXgxSy/CJaTFjd0G+YLonydOVQA=
+golang.org/x/tools v0.34.0 h1:qIpSLOxeCYGg9TrcJokLBG4KFA6d795g0xkBkiESGlo=
+golang.org/x/tools v0.34.0/go.mod h1:pAP9OwEaY1CAW3HOmg3hLZC5Z0CCmzjAF2UQMSqNARg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

plugins/mcp/dsl/mcp.go

@@ -0,0 +1,70 @@
+//go:build goa_mcp_plugin
+
+package mcp
+
+import (
+	"goa.design/goa/v3/eval"
+	"goa.design/goa/v3/expr"
+)
+
+// Tool marks a method as an MCP tool and allows setting metadata via nested functions.
+func Tool(fn ...func()) {
+	if _, ok := eval.Current().(*expr.MethodExpr); !ok {
+		eval.IncompatibleDSL()
+		return
+	}
+	md := ensureMetadata("mcp:tool")
+	_ = md
+	for _, f := range fn {
+		f()
+	}
+}
+
+// Resource marks a method as providing MCP resources (list/get) semantics.
+func Resource(fn ...func()) {
+	if _, ok := eval.Current().(*expr.MethodExpr); !ok {
+		eval.IncompatibleDSL()
+		return
+	}
+	_ = ensureMetadata("mcp:resource")
+	for _, f := range fn {
+		f()
+	}
+}
+
+// Prompt marks a method as an MCP prompt provider.
+func Prompt(fn ...func()) {
+	if _, ok := eval.Current().(*expr.MethodExpr); !ok {
+		eval.IncompatibleDSL()
+		return
+	}
+	_ = ensureMetadata("mcp:prompt")
+	for _, f := range fn {
+		f()
+	}
+}
+
+// Description sets a short description for the annotated MCP element.
+func Description(text string) {
+	switch cur := eval.Current().(type) {
+	case *expr.MethodExpr:
+		if cur.Meta == nil {
+			cur.Meta = make(expr.MetaExpr)
+		}
+		cur.Meta["mcp:description"] = []string{text}
+	default:
+		eval.IncompatibleDSL()
+	}
+}
+
+func ensureMetadata(key string) expr.MetaExpr {
+	m, _ := eval.Current().(*expr.MethodExpr)
+	if m == nil {
+		return nil
+	}
+	if m.Meta == nil {
+		m.Meta = make(expr.MetaExpr)
+	}
+	m.Meta[key] = []string{"true"}
+	return m.Meta
+}

plugins/mcp/plugin/generator.go

@@ -0,0 +1,80 @@
+package plugin
+
+import (
+	"bytes"
+	"fmt"
+	"sort"
+
+	"goa.design/goa/v3/codegen"
+	"goa.design/goa/v3/eval"
+	"goa.design/goa/v3/expr"
+)
+
+type Generator struct {
+	genpkg string
+	roots  []eval.Root
+	files  []*codegen.File
+}
+
+type tool struct {
+	Service string
+	Method  string
+	Desc    string
+}
+
+func NewGenerator(genpkg string, roots []eval.Root, files []*codegen.File) *Generator {
+	return &Generator{genpkg: genpkg, roots: roots, files: files}
+}
+
+func (g *Generator) Run() ([]*codegen.File, error) {
+	tools := collectTools()
+	if len(tools) == 0 {
+		return g.files, nil
+	}
+	var buf bytes.Buffer
+	buf.WriteString("package server\n\n")
+	buf.WriteString("import (\ncontext \"context\"\njson \"encoding/json\"\nfmt \"fmt\"\nos \"os\"\nbufio \"bufio\"\nstrings \"strings\"\n)\n\n")
+	buf.WriteString("type rpcRequest struct{ JSONRPC string `json:\"jsonrpc\"` ID any `json:\"id\"` Method string `json:\"method\"` Params json.RawMessage `json:\"params\"`}\n")
+	buf.WriteString("type rpcResponse struct{ JSONRPC string `json:\"jsonrpc\"` ID any `json:\"id\"` Result any `json:\"result,omitempty\"` Error *rpcError `json:\"error,omitempty\"`}\n")
+	buf.WriteString("type rpcError struct{ Code int `json:\"code\"` Message string `json:\"message\"`}\n")
+	buf.WriteString("\nfunc main(){ if err := run(); err!=nil { fmt.Fprintln(os.Stderr, err); os.Exit(1)}}\n")
+	buf.WriteString("func run() error {\n")
+	buf.WriteString("in := bufio.NewScanner(os.Stdin)\n")
+	buf.WriteString("for in.Scan(){ line := strings.TrimSpace(in.Text()); if line==\"\" {continue}; var req rpcRequest; if err := json.Unmarshal([]byte(line), &req); err!=nil { continue } ; resp := handle(req); b,_ := json.Marshal(resp); fmt.Println(string(b)) }\n")
+	buf.WriteString("return in.Err()}\n")
+	buf.WriteString("\nfunc handle(req rpcRequest) rpcResponse {\nresp := rpcResponse{JSONRPC:\"2.0\", ID:req.ID}\nctx := context.Background()\n")
+
+	sort.Slice(tools, func(i, j int) bool {
+		if tools[i].Service == tools[j].Service {
+			return tools[i].Method < tools[j].Method
+		}
+		return tools[i].Service < tools[j].Service
+	})
+	for _, t := range tools {
+		buf.WriteString(fmt.Sprintf("if req.Method==\"%s.%s\" {\n\tvar p gen_%s_%s_Payload\n\t_ = json.Unmarshal(req.Params, &p)\n\tres, err := impl_%s_%s(ctx, &p)\n\tif err!=nil { resp.Error=&rpcError{Code:-32000, Message:err.Error()} } else { resp.Result = res }\n\treturn resp\n}\n", t.Service, t.Method, t.Service, codegen.Goify(t.Method, true), t.Service, codegen.Goify(t.Method, true)))
+	}
+	buf.WriteString("resp.Error=&rpcError{Code:-32601, Message:\"method not found\"}; return resp}\n")
+
+	sf := &codegen.SectionTemplate{Name: "source", Source: buf.String()}
+	file := &codegen.File{Path: "gen/mcp/server/server.go", SectionTemplates: []*codegen.SectionTemplate{sf}}
+	g.files = append(g.files, file)
+	return g.files, nil
+}
+
+func collectTools() []tool {
+	var out []tool
+	for _, s := range expr.Root.Services {
+		for _, m := range s.Methods {
+			if m.Meta != nil {
+				if _, ok := m.Meta["mcp:tool"]; ok {
+					d := ""
+					if v, ok2 := m.Meta["mcp:description"]; ok2 && len(v) > 0 {
+						d = v[0]
+					}
+					out = append(out, tool{Service: s.Name, Method: m.Name, Desc: d})
+				}
+			}
+		}
+	}
+	return out
+}

plugins/mcp/plugin/init.go

@@ -0,0 +1,19 @@
+package plugin
+
+import (
+	"goa.design/goa/v3/codegen"
+	"goa.design/goa/v3/eval"
+)
+
+func init() {
+	codegen.RegisterPlugin("gen", "mcp", Prepare, Generate)
+}
+
+// Prepare runs before generation. No-op for now.
+func Prepare(_ string, _ []eval.Root) error { return nil }
+
+// Generate inspects the design roots and augments generated files with MCP server code.
+func Generate(genpkg string, roots []eval.Root, files []*codegen.File) ([]*codegen.File, error) {
+	g := NewGenerator(genpkg, roots, files)
+	return g.Run()
+}