perf: add schema caching to avoid repeated reflection

This change adds a global schema cache that dramatically reduces the cost
of registering tools in stateless server patterns (new server per request).

Key improvements:
- Cache schemas by reflect.Type for typed handlers
- Cache resolved schemas by pointer for pre-defined schemas
- 132x faster tool registration after first call
- 51x fewer allocations per AddTool call
- 32x less memory per AddTool call

Benchmarks:
- BenchmarkAddTool_TypedHandler: 1,223 ns/op vs 161,463 ns/op (no cache)
- BenchmarkAddTool_TypedHandler: 21 allocs vs 1,072 allocs (no cache)

This benefits integrators like github-mcp-server automatically without
any code changes required.

Author: SamMorrowDrums

mcp/schema_cache.go

@@ -0,0 +1,76 @@
+// 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
+
+import (
+	"reflect"
+	"sync"
+
+	"github.com/google/jsonschema-go/jsonschema"
+)
+
+// schemaCache provides concurrent-safe caching for JSON schemas.
+// It caches both by reflect.Type (for auto-generated schemas) and
+// by schema pointer (for pre-defined schemas).
+//
+// This cache significantly improves performance for stateless server deployments
+// where tools are re-registered on every request. Without caching, each AddTool
+// call would trigger expensive reflection-based schema generation and resolution.
+type schemaCache struct {
+	// byType caches schemas generated from Go types via jsonschema.ForType.
+	// Key: reflect.Type, Value: *cachedSchema
+	byType sync.Map
+
+	// bySchema caches resolved schemas for pre-defined Schema objects.
+	// Key: *jsonschema.Schema (pointer identity), Value: *jsonschema.Resolved
+	// This uses pointer identity because integrators typically reuse the same
+	// Tool objects across requests, so the schema pointer remains stable.
+	bySchema sync.Map
+}
+
+// cachedSchema holds both the generated schema and its resolved form.
+type cachedSchema struct {
+	schema   *jsonschema.Schema
+	resolved *jsonschema.Resolved
+}
+
+// globalSchemaCache is the package-level cache used by setSchema.
+// It is unbounded since typical MCP servers have <100 tools.
+var globalSchemaCache = &schemaCache{}
+
+// getByType retrieves a cached schema by Go type.
+// Returns the schema, resolved schema, and whether the cache hit.
+func (c *schemaCache) getByType(t reflect.Type) (*jsonschema.Schema, *jsonschema.Resolved, bool) {
+	if v, ok := c.byType.Load(t); ok {
+		cs := v.(*cachedSchema)
+		return cs.schema, cs.resolved, true
+	}
+	return nil, nil, false
+}
+
+// setByType caches a schema by Go type.
+func (c *schemaCache) setByType(t reflect.Type, schema *jsonschema.Schema, resolved *jsonschema.Resolved) {
+	c.byType.Store(t, &cachedSchema{schema: schema, resolved: resolved})
+}
+
+// getBySchema retrieves a cached resolved schema by the original schema pointer.
+// This is used when integrators provide pre-defined schemas (e.g., github-mcp-server pattern).
+func (c *schemaCache) getBySchema(schema *jsonschema.Schema) (*jsonschema.Resolved, bool) {
+	if v, ok := c.bySchema.Load(schema); ok {
+		return v.(*jsonschema.Resolved), true
+	}
+	return nil, false
+}
+
+// setBySchema caches a resolved schema by the original schema pointer.
+func (c *schemaCache) setBySchema(schema *jsonschema.Schema, resolved *jsonschema.Resolved) {
+	c.bySchema.Store(schema, resolved)
+}
+
+// resetForTesting clears the cache. Only for use in tests.
+func (c *schemaCache) resetForTesting() {
+	c.byType = sync.Map{}
+	c.bySchema = sync.Map{}
+}

mcp/schema_cache_benchmark_test.go

@@ -0,0 +1,157 @@
+package mcp
+
+import (
+	"context"
+	"testing"
+
+	"github.com/google/jsonschema-go/jsonschema"
+)
+
+// BenchmarkAddTool_TypedHandler measures performance of AddTool with typed handlers.
+// This simulates the stateless server pattern where new servers are created per request.
+func BenchmarkAddTool_TypedHandler(b *testing.B) {
+	type SearchInput struct {
+		Query   string `json:"query" jsonschema:"required"`
+		Page    int    `json:"page"`
+		PerPage int    `json:"per_page"`
+	}
+
+	type SearchOutput struct {
+		Results []string `json:"results"`
+		Total   int      `json:"total"`
+	}
+
+	handler := func(ctx context.Context, req *CallToolRequest, in SearchInput) (*CallToolResult, SearchOutput, error) {
+		return &CallToolResult{}, SearchOutput{}, nil
+	}
+
+	tool := &Tool{
+		Name:        "search",
+		Description: "Search for items",
+	}
+
+	// Reset cache to simulate cold start for first iteration
+	globalSchemaCache.resetForTesting()
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		s := NewServer(&Implementation{Name: "test", Version: "1.0"}, nil)
+		AddTool(s, tool, handler)
+	}
+}
+
+// BenchmarkAddTool_PreDefinedSchema measures performance with pre-defined schemas.
+// This simulates how github-mcp-server registers tools with manual InputSchema.
+func BenchmarkAddTool_PreDefinedSchema(b *testing.B) {
+	schema := &jsonschema.Schema{
+		Type: "object",
+		Properties: map[string]*jsonschema.Schema{
+			"query":    {Type: "string", Description: "Search query"},
+			"page":     {Type: "integer", Description: "Page number"},
+			"per_page": {Type: "integer", Description: "Results per page"},
+		},
+		Required: []string{"query"},
+	}
+
+	handler := func(ctx context.Context, req *CallToolRequest) (*CallToolResult, error) {
+		return &CallToolResult{}, nil
+	}
+
+	tool := &Tool{
+		Name:        "search",
+		Description: "Search for items",
+		InputSchema: schema, // Pre-defined schema like github-mcp-server
+	}
+
+	// Reset cache to simulate cold start for first iteration
+	globalSchemaCache.resetForTesting()
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		s := NewServer(&Implementation{Name: "test", Version: "1.0"}, nil)
+		s.AddTool(tool, handler)
+	}
+}
+
+// BenchmarkAddTool_TypedHandler_NoCache measures performance without caching.
+// Used to compare before/after performance.
+func BenchmarkAddTool_TypedHandler_NoCache(b *testing.B) {
+	type SearchInput struct {
+		Query   string `json:"query" jsonschema:"required"`
+		Page    int    `json:"page"`
+		PerPage int    `json:"per_page"`
+	}
+
+	type SearchOutput struct {
+		Results []string `json:"results"`
+		Total   int      `json:"total"`
+	}
+
+	handler := func(ctx context.Context, req *CallToolRequest, in SearchInput) (*CallToolResult, SearchOutput, error) {
+		return &CallToolResult{}, SearchOutput{}, nil
+	}
+
+	tool := &Tool{
+		Name:        "search",
+		Description: "Search for items",
+	}
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		// Reset cache every iteration to simulate no caching
+		globalSchemaCache.resetForTesting()
+
+		s := NewServer(&Implementation{Name: "test", Version: "1.0"}, nil)
+		AddTool(s, tool, handler)
+	}
+}
+
+// BenchmarkAddTool_MultipleTools simulates registering multiple tools like github-mcp-server.
+func BenchmarkAddTool_MultipleTools(b *testing.B) {
+	type Input1 struct {
+		Query string `json:"query"`
+	}
+	type Input2 struct {
+		ID int `json:"id"`
+	}
+	type Input3 struct {
+		Name  string `json:"name"`
+		Value string `json:"value"`
+	}
+	type Output struct {
+		Success bool `json:"success"`
+	}
+
+	handler1 := func(ctx context.Context, req *CallToolRequest, in Input1) (*CallToolResult, Output, error) {
+		return &CallToolResult{}, Output{}, nil
+	}
+	handler2 := func(ctx context.Context, req *CallToolRequest, in Input2) (*CallToolResult, Output, error) {
+		return &CallToolResult{}, Output{}, nil
+	}
+	handler3 := func(ctx context.Context, req *CallToolRequest, in Input3) (*CallToolResult, Output, error) {
+		return &CallToolResult{}, Output{}, nil
+	}
+
+	tool1 := &Tool{Name: "tool1", Description: "Tool 1"}
+	tool2 := &Tool{Name: "tool2", Description: "Tool 2"}
+	tool3 := &Tool{Name: "tool3", Description: "Tool 3"}
+
+	// Reset cache before benchmark
+	globalSchemaCache.resetForTesting()
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		s := NewServer(&Implementation{Name: "test", Version: "1.0"}, nil)
+		AddTool(s, tool1, handler1)
+		AddTool(s, tool2, handler2)
+		AddTool(s, tool3, handler3)
+	}
+}