mcp: add an example customizing a nested type schema

Add an example that demonstrates how to customize a nested type schema,
which is a recurring problem for our users (see #467).

Fixes #467
Updates #368

Author: findleyr

mcp/tool_example_test.go

@@ -0,0 +1,93 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package mcp_test
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"log"
+	"time"
+
+	"github.com/google/jsonschema-go/jsonschema"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+func ExampleAddTool_customTypeSchema() {
+	// Sometimes when you want to customize the input or output schema for a
+	// tool, you need to customize the schema of a single helper type that's used
+	// in several places.
+	//
+	// For example, suppose you had a type that marshals/unmarshals like a
+	// time.Time, and that type was used multiple times in your tool input.
+	type MyDate struct {
+		time.Time
+	}
+	type Input struct {
+		Query string `json:"query,omitempty"`
+		Start MyDate `json:"start,omitempty"`
+		End   MyDate `json:"end,omitempty"`
+	}
+
+	// In this case, you can use jsonschema.For along with jsonschema.ForOptions
+	// to customize the schema inference for your custom type.
+	inputSchema, err := jsonschema.For[Input](&jsonschema.ForOptions{
+		TypeSchemas: map[any]*jsonschema.Schema{
+			MyDate{}: {Type: "string"},
+		},
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	server := mcp.NewServer(&mcp.Implementation{Name: "server", Version: "v0.0.1"}, nil)
+	toolHandler := func(context.Context, *mcp.CallToolRequest, Input) (*mcp.CallToolResult, any, error) {
+		panic("not implemented")
+	}
+	mcp.AddTool(server, &mcp.Tool{Name: "my_tool", InputSchema: inputSchema}, toolHandler)
+
+	ctx := context.Background()
+	session, err := connect(ctx, server) // create an in-memory connection
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer session.Close()
+
+	for t, err := range session.Tools(ctx, nil) {
+		if err != nil {
+			log.Fatal(err)
+		}
+		schemaJSON, err := json.MarshalIndent(t.InputSchema, "", "\t")
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Println(t.Name, string(schemaJSON))
+	}
+	// Output:
+	// my_tool {
+	// 	"type": "object",
+	// 	"properties": {
+	// 		"end": {
+	// 			"type": "string"
+	// 		},
+	// 		"query": {
+	// 			"type": "string"
+	// 		},
+	// 		"start": {
+	// 			"type": "string"
+	// 		}
+	// 	},
+	// 	"additionalProperties": false
+	// }
+}
+
+func connect(ctx context.Context, server *mcp.Server) (*mcp.ClientSession, error) {
+	t1, t2 := mcp.NewInMemoryTransports()
+	if _, err := server.Connect(ctx, t1, nil); err != nil {
+		return nil, err
+	}
+	client := mcp.NewClient(&mcp.Implementation{Name: "client", Version: "v0.0.1"}, nil)
+	return client.Connect(ctx, t2, nil)
+}