internal/mcp: pointerize all the things

Use pointers for the protocol's struct types, to future-proof against
them getting large.

Change-Id: I55b204080a41c8140539697ba264e502e912e613
Reviewed-on: https://go-review.googlesource.com/c/tools/+/672575
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

examples/hello/main.go

@@ -20,8 +20,8 @@ type HiParams struct {
 	Name string `json:"name"`
 }
 
-func SayHi(ctx context.Context, cc *mcp.ServerConnection, params *HiParams) ([]mcp.Content, error) {
+func SayHi(ctx context.Context, cc *mcp.ServerConnection, params *HiParams) ([]*mcp.Content, error) {
-	return []mcp.Content{
+	return []*mcp.Content{
 		mcp.NewTextContent("Hi " + params.Name),
 	}, nil
 }
@@ -29,7 +29,7 @@ func SayHi(ctx context.Context, cc *mcp.ServerConnection, params *HiParams) ([]m
 func PromptHi(ctx context.Context, cc *mcp.ServerConnection, params *HiParams) (*mcp.GetPromptResult, error) {
 	return &mcp.GetPromptResult{
 		Description: "Code review prompt",
-		Messages: []mcp.PromptMessage{
+		Messages: []*mcp.PromptMessage{
 			{Role: "user", Content: mcp.NewTextContent("Say hi to " + params.Name)},
 		},
 	}, nil

examples/sse/main.go

@@ -19,8 +19,8 @@ type SayHiParams struct {
 	Name string `json:"name" mcp:"the name to say hi to"`
 }
 
-func SayHi(ctx context.Context, cc *mcp.ServerConnection, params *SayHiParams) ([]mcp.Content, error) {
+func SayHi(ctx context.Context, cc *mcp.ServerConnection, params *SayHiParams) ([]*mcp.Content, error) {
-	return []mcp.Content{
+	return []*mcp.Content{
 		mcp.NewTextContent("Hi " + params.Name),
 	}, nil
 }

jsonschema/schema.go

@@ -138,6 +138,14 @@ func (s *Schema) String() string {
 	return "<anonymous schema>"
 }
 
+// ResolvedRef returns the Schema to which this schema's $ref keyword
+// refers, or nil if it doesn't have a $ref.
+// It returns nil if this schema has not been resolved, meaning that
+// [Schema.Resolve] was called on it or one of its ancestors.
+func (s *Schema) ResolvedRef() *Schema {
+	return s.resolvedRef
+}
+
 // json returns the schema in json format.
 func (s *Schema) json() string {
 	data, err := json.MarshalIndent(s, "", "  ")

mcp/client.go

@@ -24,7 +24,7 @@ type Client struct {
 	opts             ClientOptions
 	mu               sync.Mutex
 	conn             *jsonrpc2.Connection
-	roots            *featureSet[Root]
+	roots            *featureSet[*Root]
 	initializeResult *initializeResult
 }
 
@@ -38,7 +38,7 @@ func NewClient(name, version string, t Transport, opts *ClientOptions) *Client {
 		name:      name,
 		version:   version,
 		transport: t,
-		roots:     newFeatureSet(func(r Root) string { return r.URI }),
+		roots:     newFeatureSet(func(r *Root) string { return r.URI }),
 	}
 	if opts != nil {
 		c.opts = *opts
@@ -84,7 +84,7 @@ func (c *Client) Start(ctx context.Context) (err error) {
 		return err
 	}
 	params := &initializeParams{
-		ClientInfo: implementation{Name: c.name, Version: c.version},
+		ClientInfo: &implementation{Name: c.name, Version: c.version},
 	}
 	if err := call(ctx, c.conn, "initialize", params, &c.initializeResult); err != nil {
 		return err
@@ -112,7 +112,7 @@ func (c *Client) Wait() error {
 // replacing any with the same URIs,
 // and notifies any connected servers.
 // TODO: notification
-func (c *Client) AddRoots(roots ...Root) {
+func (c *Client) AddRoots(roots ...*Root) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	c.roots.add(roots...)
@@ -159,7 +159,7 @@ func (c *Client) Ping(ctx context.Context) error {
 }
 
 // ListPrompts lists prompts that are currently available on the server.
-func (c *Client) ListPrompts(ctx context.Context) ([]Prompt, error) {
+func (c *Client) ListPrompts(ctx context.Context) ([]*Prompt, error) {
 	var (
 		params = &ListPromptsParams{}
 		result ListPromptsResult
@@ -186,7 +186,7 @@ func (c *Client) GetPrompt(ctx context.Context, name string, args map[string]str
 }
 
 // ListTools lists tools that are currently available on the server.
-func (c *Client) ListTools(ctx context.Context) ([]Tool, error) {
+func (c *Client) ListTools(ctx context.Context) ([]*Tool, error) {
 	var (
 		params = &ListToolsParams{}
 		result ListToolsResult

mcp/cmd_test.go

@@ -57,7 +57,7 @@ func TestCmdTransport(t *testing.T) {
 		log.Fatal(err)
 	}
 	want := &mcp.CallToolResult{
-		Content: []mcp.Content{{Type: "text", Text: "Hi user"}},
+		Content: []*mcp.Content{{Type: "text", Text: "Hi user"}},
 	}
 	if diff := cmp.Diff(want, got); diff != "" {
 		t.Errorf("greet returned unexpected content (-want +got):\n%s", diff)

mcp/content.go

@@ -45,23 +45,23 @@ func (c *Content) UnmarshalJSON(data []byte) error {
 }
 
 // NewTextContent creates a [Content] with text.
-func NewTextContent(text string) Content {
+func NewTextContent(text string) *Content {
-	return Content{Type: "text", Text: text}
+	return &Content{Type: "text", Text: text}
 }
 
 // NewImageContent creates a [Content] with image data.
-func NewImageContent(data []byte, mimeType string) Content {
+func NewImageContent(data []byte, mimeType string) *Content {
-	return Content{Type: "image", Data: data, MIMEType: mimeType}
+	return &Content{Type: "image", Data: data, MIMEType: mimeType}
 }
 
 // NewAudioContent creates a [Content] with audio data.
-func NewAudioContent(data []byte, mimeType string) Content {
+func NewAudioContent(data []byte, mimeType string) *Content {
-	return Content{Type: "audio", Data: data, MIMEType: mimeType}
+	return &Content{Type: "audio", Data: data, MIMEType: mimeType}
 }
 
 // NewResourceContent creates a [Content] with an embedded resource.
-func NewResourceContent(resource *ResourceContents) Content {
+func NewResourceContent(resource *ResourceContents) *Content {
-	return Content{Type: "resource", Resource: resource}
+	return &Content{Type: "resource", Resource: resource}
 }
 
 // ResourceContents represents the union of the spec's {Text,Blob}ResourceContents types.

mcp/content_test.go

@@ -14,7 +14,7 @@ import (
 
 func TestContent(t *testing.T) {
 	tests := []struct {
-		in   mcp.Content
+		in   *mcp.Content
 		want string // json serialization
 	}{
 		{mcp.NewTextContent("hello"), `{"type":"text","text":"hello"}`},
@@ -48,7 +48,7 @@ func TestContent(t *testing.T) {
 		if diff := cmp.Diff(test.want, string(got)); diff != "" {
 			t.Errorf("json.Marshal(%v) mismatch (-want +got):\n%s", test.in, diff)
 		}
-		var out mcp.Content
+		var out *mcp.Content
 		if err := json.Unmarshal(got, &out); err != nil {
 			t.Fatal(err)
 		}

mcp/generate.go

@@ -139,6 +139,10 @@ func main() {
 	if err := json.Unmarshal(data, &schema); err != nil {
 		log.Fatal(err)
 	}
+	// Resolve the schema so we have the referents of all the Refs.
+	if _, err := schema.Resolve("", nil); err != nil {
+		log.Fatal(err)
+	}
 
 	// Collect named types. Since we may create new type definitions while
 	// writing types, we collect definitions and concatenate them later. This
@@ -253,7 +257,8 @@ func writeDecl(configName string, config typeConfig, def *jsonschema.Schema, nam
 // be added during writeType, if they are extracted from inner fields.
 func writeType(w io.Writer, config *typeConfig, def *jsonschema.Schema, named map[string]*bytes.Buffer) error {
 	// Use type names for Named types.
-	if name := strings.TrimPrefix(def.Ref, "#/definitions/"); name != "" {
+	name, resolved := deref(def)
+	if name != "" {
 		// TODO: this check is not quite right: we should really panic if the
 		// definition is missing, *but only if w is not io.Discard*. That's not a
 		// great API: see if we can do something more explicit than io.Discard.
@@ -266,6 +271,9 @@ func writeType(w io.Writer, config *typeConfig, def *jsonschema.Schema, named ma
 			} else if cfg.Name != "" {
 				name = cfg.Name
 			}
+			if isStruct(resolved) {
+				w.Write([]byte{'*'})
+			}
 		}
 		w.Write([]byte(name))
 		return nil
@@ -288,7 +296,7 @@ func writeType(w io.Writer, config *typeConfig, def *jsonschema.Schema, named ma
 		if slices.ContainsFunc(def.AnyOf, func(s *jsonschema.Schema) bool {
 			return s.Ref == "#/definitions/TextContent"
 		}) {
-			fmt.Fprintf(w, "Content")
+			fmt.Fprintf(w, "*Content")
 		} else {
 			// E.g. union types.
 			fmt.Fprintf(w, "json.RawMessage")
@@ -322,14 +330,15 @@ func writeType(w io.Writer, config *typeConfig, def *jsonschema.Schema, named ma
 
 				required := slices.Contains(def.Required, name)
 
-				// If the field is not required, and is a struct type, indirect with a
+				// If the field is a struct type, indirect with a
 				// pointer so that it can be empty as defined by encoding/json.
-				//
+				// This also future-proofs against the struct getting large.
-				// TODO: use omitzero when available.
+				fieldTypeSchema := fieldDef
-				needPointer := !required &&
+				// If the schema is a reference, dereference it.
-					(strings.HasPrefix(fieldDef.Ref, "#/definitions/") ||
+				if _, rs := deref(fieldDef); rs != nil {
-						fieldDef.Type == "object" && !canHaveAdditionalProperties(fieldDef))
+					fieldTypeSchema = rs
-
+				}
+				needPointer := isStruct(fieldTypeSchema)
 				if config != nil && config.Fields[export] != nil {
 					r := config.Fields[export]
 					if r.Substitute != "" {
@@ -348,13 +357,8 @@ func writeType(w io.Writer, config *typeConfig, def *jsonschema.Schema, named ma
 						}
 						fmt.Fprintf(w, typename)
 					}
-				} else {
+				} else if err := writeType(w, nil, fieldDef, named); err != nil {
-					if needPointer {
+					return fmt.Errorf("failed to write type for field %s: %v", export, err)
-						fmt.Fprintf(w, "*")
-					}
-					if err := writeType(w, nil, fieldDef, named); err != nil {
-						return fmt.Errorf("failed to write type for field %s: %v", export, err)
-					}
 				}
 				fmt.Fprintf(w, " `json:\"%s", name)
 				if !required {
@@ -452,6 +456,33 @@ func exportName(s string) string {
 	return s
 }
 
+// deref dereferences s.Ref.
+// If s.Ref refers to a schema in the Definitions section, deref
+// returns the definition name and the associated schema.
+// Otherwise, deref returns "", nil.
+func deref(s *jsonschema.Schema) (name string, _ *jsonschema.Schema) {
+	name, ok := strings.CutPrefix(s.Ref, "#/definitions/")
+	if !ok {
+		return "", nil
+	}
+	return name, s.ResolvedRef()
+}
+
+// isStruct reports whether s should be translated to a struct.
+func isStruct(s *jsonschema.Schema) bool {
+	return s.Type == "object" && s.Properties != nil && !canHaveAdditionalProperties(s)
+}
+
+// schemaJSON returns the JSON for s.
+// For debugging.
+func schemaJSON(s *jsonschema.Schema) string {
+	data, err := json.Marshal(s)
+	if err != nil {
+		return fmt.Sprintf("<jsonschema.Schema:%v>", err)
+	}
+	return string(data)
+}
+
 // Map from initialism to the regexp that matches it.
 var initialisms = map[string]*regexp.Regexp{
 	"Id":   nil,

mcp/mcp_test.go

@@ -24,11 +24,11 @@ type hiParams struct {
 	Name string
 }
 
-func sayHi(ctx context.Context, cc *ServerConnection, v hiParams) ([]Content, error) {
+func sayHi(ctx context.Context, cc *ServerConnection, v hiParams) ([]*Content, error) {
 	if err := cc.Ping(ctx); err != nil {
 		return nil, fmt.Errorf("ping failed: %v", err)
 	}
-	return []Content{NewTextContent("hi " + v.Name)}, nil
+	return []*Content{NewTextContent("hi " + v.Name)}, nil
 }
 
 func TestEndToEnd(t *testing.T) {
@@ -43,7 +43,7 @@ func TestEndToEnd(t *testing.T) {
 	// The 'fail' tool returns this error.
 	failure := errors.New("mcp failure")
 	s.AddTools(
-		NewTool("fail", "just fail", func(context.Context, *ServerConnection, struct{}) ([]Content, error) {
+		NewTool("fail", "just fail", func(context.Context, *ServerConnection, struct{}) ([]*Content, error) {
 			return nil, failure
 		}),
 	)
@@ -52,7 +52,7 @@ func TestEndToEnd(t *testing.T) {
 		NewPrompt("code_review", "do a code review", func(_ context.Context, _ *ServerConnection, params struct{ Code string }) (*GetPromptResult, error) {
 			return &GetPromptResult{
 				Description: "Code review prompt",
-				Messages: []PromptMessage{
+				Messages: []*PromptMessage{
 					{Role: "user", Content: NewTextContent("Please review the following code: " + params.Code)},
 				},
 			}, nil
@@ -82,7 +82,7 @@ func TestEndToEnd(t *testing.T) {
 	}()
 
 	c := NewClient("testClient", "v1.0.0", ct, nil)
-	c.AddRoots(Root{URI: "file:///root"})
+	c.AddRoots(&Root{URI: "file:///root"})
 
 	// Connect the client.
 	if err := c.Start(ctx); err != nil {
@@ -97,11 +97,11 @@ func TestEndToEnd(t *testing.T) {
 		if err != nil {
 			t.Errorf("prompts/list failed: %v", err)
 		}
-		wantPrompts := []Prompt{
+		wantPrompts := []*Prompt{
 			{
 				Name:        "code_review",
 				Description: "do a code review",
-				Arguments:   []PromptArgument{{Name: "Code", Required: true}},
+				Arguments:   []*PromptArgument{{Name: "Code", Required: true}},
 			},
 			{Name: "fail"},
 		}
@@ -115,7 +115,7 @@ func TestEndToEnd(t *testing.T) {
 		}
 		wantReview := &GetPromptResult{
 			Description: "Code review prompt",
-			Messages: []PromptMessage{{
+			Messages: []*PromptMessage{{
 				Content: NewTextContent("Please review the following code: 1+1"),
 				Role:    "user",
 			}},
@@ -134,7 +134,7 @@ func TestEndToEnd(t *testing.T) {
 		if err != nil {
 			t.Errorf("tools/list failed: %v", err)
 		}
-		wantTools := []Tool{
+		wantTools := []*Tool{
 			{
 				Name:        "fail",
 				Description: "just fail",
@@ -165,7 +165,7 @@ func TestEndToEnd(t *testing.T) {
 			t.Fatal(err)
 		}
 		wantHi := &CallToolResult{
-			Content: []Content{{Type: "text", Text: "hi user"}},
+			Content: []*Content{{Type: "text", Text: "hi user"}},
 		}
 		if diff := cmp.Diff(wantHi, gotHi); diff != "" {
 			t.Errorf("tools/call 'greet' mismatch (-want +got):\n%s", diff)
@@ -179,7 +179,7 @@ func TestEndToEnd(t *testing.T) {
 		}
 		wantFail := &CallToolResult{
 			IsError: true,
-			Content: []Content{{Type: "text", Text: failure.Error()}},
+			Content: []*Content{{Type: "text", Text: failure.Error()}},
 		}
 		if diff := cmp.Diff(wantFail, gotFail); diff != "" {
 			t.Errorf("tools/call 'fail' mismatch (-want +got):\n%s", diff)
@@ -187,18 +187,18 @@ func TestEndToEnd(t *testing.T) {
 	})
 
 	t.Run("resources", func(t *testing.T) {
-		resource1 := Resource{
+		resource1 := &Resource{
 			Name:     "public",
 			MIMEType: "text/plain",
 			URI:      "file:///file1.txt",
 		}
-		resource2 := Resource{
+		resource2 := &Resource{
 			Name:     "public", // names are not unique IDs
 			MIMEType: "text/plain",
 			URI:      "file:///nonexistent.txt",
 		}
 
-		readHandler := func(_ context.Context, r Resource, _ *ReadResourceParams) (*ReadResourceResult, error) {
+		readHandler := func(_ context.Context, r *Resource, _ *ReadResourceParams) (*ReadResourceResult, error) {
 			if r.URI == "file:///file1.txt" {
 				return &ReadResourceResult{
 					Contents: &ResourceContents{
@@ -216,7 +216,7 @@ func TestEndToEnd(t *testing.T) {
 		if err != nil {
 			t.Fatal(err)
 		}
-		if diff := cmp.Diff([]Resource{resource1, resource2}, lrres.Resources); diff != "" {
+		if diff := cmp.Diff([]*Resource{resource1, resource2}, lrres.Resources); diff != "" {
 			t.Errorf("resources/list mismatch (-want, +got):\n%s", diff)
 		}
 
@@ -370,7 +370,7 @@ func TestCancellation(t *testing.T) {
 		cancelled = make(chan struct{}, 1) // don't block the request
 	)
 
-	slowRequest := func(ctx context.Context, cc *ServerConnection, v struct{}) ([]Content, error) {
+	slowRequest := func(ctx context.Context, cc *ServerConnection, v struct{}) ([]*Content, error) {
 		start <- struct{}{}
 		select {
 		case <-ctx.Done():

mcp/prompt.go

@@ -20,7 +20,7 @@ type PromptHandler func(context.Context, *ServerConnection, map[string]string) (
 
 // A Prompt is a prompt definition bound to a prompt handler.
 type ServerPrompt struct {
-	Definition Prompt
+	Definition *Prompt
 	Handler    PromptHandler
 }
 
@@ -41,7 +41,7 @@ func NewPrompt[TReq any](name, description string, handler func(context.Context,
 		panic(fmt.Sprintf("handler request type must be a struct"))
 	}
 	prompt := &ServerPrompt{
-		Definition: Prompt{
+		Definition: &Prompt{
 			Name:        name,
 			Description: description,
 		},
@@ -54,7 +54,7 @@ func NewPrompt[TReq any](name, description string, handler func(context.Context,
 		if prop.Type != "string" {
 			panic(fmt.Sprintf("handler type must consist only of string fields"))
 		}
-		prompt.Definition.Arguments = append(prompt.Definition.Arguments, PromptArgument{
+		prompt.Definition.Arguments = append(prompt.Definition.Arguments, &PromptArgument{
 			Name:        name,
 			Description: prop.Description,
 			Required:    required[name],
@@ -95,13 +95,13 @@ func (s promptSetter) set(p *ServerPrompt) { s(p) }
 // Required and Description, and panics when encountering any other option.
 func Argument(name string, opts ...SchemaOption) PromptOption {
 	return promptSetter(func(p *ServerPrompt) {
-		i := slices.IndexFunc(p.Definition.Arguments, func(arg PromptArgument) bool {
+		i := slices.IndexFunc(p.Definition.Arguments, func(arg *PromptArgument) bool {
 			return arg.Name == name
 		})
-		var arg PromptArgument
+		var arg *PromptArgument
 		if i < 0 {
 			i = len(p.Definition.Arguments)
-			arg = PromptArgument{Name: name}
+			arg = &PromptArgument{Name: name}
 			p.Definition.Arguments = append(p.Definition.Arguments, arg)
 		} else {
 			arg = p.Definition.Arguments[i]

mcp/prompt_test.go

@@ -20,7 +20,7 @@ func testPromptHandler[T any](context.Context, *mcp.ServerConnection, T) (*mcp.G
 func TestNewPrompt(t *testing.T) {
 	tests := []struct {
 		prompt *mcp.ServerPrompt
-		want   []mcp.PromptArgument
+		want   []*mcp.PromptArgument
 	}{
 		{
 			mcp.NewPrompt("empty", "", testPromptHandler[struct{}]),
@@ -28,7 +28,7 @@ func TestNewPrompt(t *testing.T) {
 		},
 		{
 			mcp.NewPrompt("add_arg", "", testPromptHandler[struct{}], mcp.Argument("x")),
-			[]mcp.PromptArgument{{Name: "x"}},
+			[]*mcp.PromptArgument{{Name: "x"}},
 		},
 		{
 			mcp.NewPrompt("combo", "", testPromptHandler[struct {
@@ -38,7 +38,7 @@ func TestNewPrompt(t *testing.T) {
 			}],
 				mcp.Argument("name", mcp.Description("the person's name")),
 				mcp.Argument("State", mcp.Required(false))),
-			[]mcp.PromptArgument{
+			[]*mcp.PromptArgument{
 				{Name: "State"},
 				{Name: "country"},
 				{Name: "name", Required: true, Description: "the person's name"},

mcp/protocol.go

@@ -47,7 +47,7 @@ type CallToolResult struct {
 	// This result property is reserved by the protocol to allow clients and servers
 	// to attach additional metadata to their responses.
 	Meta    map[string]json.RawMessage `json:"_meta,omitempty"`
-	Content []Content                  `json:"content"`
+	Content []*Content                 `json:"content"`
 	// Whether the tool call ended in an error.
 	//
 	// If not set, this is assumed to be false (the call was successful).
@@ -73,7 +73,7 @@ type ClientCapabilities struct {
 	Experimental map[string]struct {
 	} `json:"experimental,omitempty"`
 	// Present if the client supports listing roots.
-	Roots *struct {
+	Roots struct {
 		// Whether the client supports notifications for changes to the roots list.
 		ListChanged bool `json:"listChanged,omitempty"`
 	} `json:"roots,omitempty"`
@@ -95,8 +95,8 @@ type GetPromptResult struct {
 	// to attach additional metadata to their responses.
 	Meta map[string]json.RawMessage `json:"_meta,omitempty"`
 	// An optional description for the prompt.
-	Description string          `json:"description,omitempty"`
+	Description string           `json:"description,omitempty"`
-	Messages    []PromptMessage `json:"messages"`
+	Messages    []*PromptMessage `json:"messages"`
 }
 
 type ListPromptsParams struct {
@@ -112,8 +112,8 @@ type ListPromptsResult struct {
 	Meta map[string]json.RawMessage `json:"_meta,omitempty"`
 	// An opaque token representing the pagination position after the last returned
 	// result. If present, there may be more results available.
-	NextCursor string   `json:"nextCursor,omitempty"`
+	NextCursor string    `json:"nextCursor,omitempty"`
-	Prompts    []Prompt `json:"prompts"`
+	Prompts    []*Prompt `json:"prompts"`
 }
 
 type ListResourcesParams struct {
@@ -129,17 +129,17 @@ type ListResourcesResult struct {
 	Meta map[string]json.RawMessage `json:"_meta,omitempty"`
 	// An opaque token representing the pagination position after the last returned
 	// result. If present, there may be more results available.
-	NextCursor string     `json:"nextCursor,omitempty"`
+	NextCursor string      `json:"nextCursor,omitempty"`
-	Resources  []Resource `json:"resources"`
+	Resources  []*Resource `json:"resources"`
 }
 
 type ListRootsParams struct {
-	Meta *struct {
+	Meta struct {
 		// If specified, the caller is requesting out-of-band progress notifications for
 		// this request (as represented by notifications/progress). The value of this
 		// parameter is an opaque token that will be attached to any subsequent
 		// notifications. The receiver is not obligated to provide these notifications.
-		ProgressToken *any `json:"progressToken,omitempty"`
+		ProgressToken any `json:"progressToken,omitempty"`
 	} `json:"_meta,omitempty"`
 }
 
@@ -150,7 +150,7 @@ type ListRootsResult struct {
 	// This result property is reserved by the protocol to allow clients and servers
 	// to attach additional metadata to their responses.
 	Meta  map[string]json.RawMessage `json:"_meta,omitempty"`
-	Roots []Root                     `json:"roots"`
+	Roots []*Root                    `json:"roots"`
 }
 
 type ListToolsParams struct {
@@ -166,14 +166,14 @@ type ListToolsResult struct {
 	Meta map[string]json.RawMessage `json:"_meta,omitempty"`
 	// An opaque token representing the pagination position after the last returned
 	// result. If present, there may be more results available.
-	NextCursor string `json:"nextCursor,omitempty"`
+	NextCursor string  `json:"nextCursor,omitempty"`
-	Tools      []Tool `json:"tools"`
+	Tools      []*Tool `json:"tools"`
 }
 
 // A prompt or prompt template that the server offers.
 type Prompt struct {
 	// A list of arguments to use for templating the prompt.
-	Arguments []PromptArgument `json:"arguments,omitempty"`
+	Arguments []*PromptArgument `json:"arguments,omitempty"`
 	// An optional description of what this prompt provides
 	Description string `json:"description,omitempty"`
 	// The name of the prompt or prompt template.
@@ -195,8 +195,8 @@ type PromptArgument struct {
 // This is similar to `SamplingMessage`, but also supports the embedding of
 // resources from the MCP server.
 type PromptMessage struct {
-	Content Content `json:"content"`
+	Content *Content `json:"content"`
-	Role    Role    `json:"role"`
+	Role    Role     `json:"role"`
 }
 
 type ReadResourceParams struct {
@@ -312,8 +312,8 @@ type implementation struct {
 }
 
 type initializeParams struct {
-	Capabilities ClientCapabilities `json:"capabilities"`
+	Capabilities *ClientCapabilities `json:"capabilities"`
-	ClientInfo   implementation     `json:"clientInfo"`
+	ClientInfo   *implementation     `json:"clientInfo"`
 	// The latest version of the Model Context Protocol that the client supports.
 	// The client MAY decide to support older versions as well.
 	ProtocolVersion string `json:"protocolVersion"`
@@ -325,7 +325,7 @@ type initializeResult struct {
 	// This result property is reserved by the protocol to allow clients and servers
 	// to attach additional metadata to their responses.
 	Meta         map[string]json.RawMessage `json:"_meta,omitempty"`
-	Capabilities serverCapabilities         `json:"capabilities"`
+	Capabilities *serverCapabilities        `json:"capabilities"`
 	// Instructions describing how to use the server and its features.
 	//
 	// This can be used by clients to improve the LLM's understanding of available
@@ -335,8 +335,8 @@ type initializeResult struct {
 	// The version of the Model Context Protocol that the server wants to use. This
 	// may not match the version that the client requested. If the client cannot
 	// support this version, it MUST disconnect.
-	ProtocolVersion string         `json:"protocolVersion"`
+	ProtocolVersion string          `json:"protocolVersion"`
-	ServerInfo      implementation `json:"serverInfo"`
+	ServerInfo      *implementation `json:"serverInfo"`
 }
 
 type initializedParams struct {

mcp/server.go

@@ -121,11 +121,11 @@ const codeResourceNotFound = -31002
 
 // A ReadResourceHandler is a function that reads a resource.
 // If it cannot find the resource, it should return the result of calling [ResourceNotFoundError].
-type ReadResourceHandler func(context.Context, Resource, *ReadResourceParams) (*ReadResourceResult, error)
+type ReadResourceHandler func(context.Context, *Resource, *ReadResourceParams) (*ReadResourceResult, error)
 
 // A ServerResource associates a Resource with its handler.
 type ServerResource struct {
-	Resource Resource
+	Resource *Resource
 	Handler  ReadResourceHandler
 }
 
@@ -381,7 +381,7 @@ func (cc *ServerConnection) initialize(ctx context.Context, _ *ServerConnection,
 	return &initializeResult{
 		// TODO(rfindley): support multiple protocol versions.
 		ProtocolVersion: "2024-11-05",
-		Capabilities: serverCapabilities{
+		Capabilities: &serverCapabilities{
 			Prompts: &promptCapabilities{
 				ListChanged: false, // not yet supported
 			},
@@ -390,7 +390,7 @@ func (cc *ServerConnection) initialize(ctx context.Context, _ *ServerConnection,
 			},
 		},
 		Instructions: cc.server.opts.Instructions,
-		ServerInfo: implementation{
+		ServerInfo: &implementation{
 			Name:    cc.server.name,
 			Version: cc.server.version,
 		},

mcp/server_example_test.go

@@ -16,8 +16,8 @@ type SayHiParams struct {
 	Name string `json:"name" mcp:"the name to say hi to"`
 }
 
-func SayHi(ctx context.Context, cc *mcp.ServerConnection, params *SayHiParams) ([]mcp.Content, error) {
+func SayHi(ctx context.Context, cc *mcp.ServerConnection, params *SayHiParams) ([]*mcp.Content, error) {
-	return []mcp.Content{
+	return []*mcp.Content{
 		mcp.NewTextContent("Hi " + params.Name),
 	}, nil
 }

mcp/sse_example_test.go

@@ -18,8 +18,8 @@ type AddParams struct {
 	X, Y int
 }
 
-func Add(ctx context.Context, cc *mcp.ServerConnection, params *AddParams) ([]mcp.Content, error) {
+func Add(ctx context.Context, cc *mcp.ServerConnection, params *AddParams) ([]*mcp.Content, error) {
-	return []mcp.Content{
+	return []*mcp.Content{
 		mcp.NewTextContent(fmt.Sprintf("%d", params.X+params.Y)),
 	}, nil
 }

mcp/sse_test.go

@@ -48,7 +48,7 @@ func TestSSEServer(t *testing.T) {
 				t.Fatal(err)
 			}
 			wantHi := &CallToolResult{
-				Content: []Content{{Type: "text", Text: "hi user"}},
+				Content: []*Content{{Type: "text", Text: "hi user"}},
 			}
 			if diff := cmp.Diff(wantHi, gotHi); diff != "" {
 				t.Errorf("tools/call 'greet' mismatch (-want +got):\n%s", diff)

mcp/tool.go

@@ -17,7 +17,7 @@ type ToolHandler func(context.Context, *ServerConnection, map[string]json.RawMes
 
 // A Tool is a tool definition that is bound to a tool handler.
 type ServerTool struct {
-	Definition Tool
+	Definition *Tool
 	Handler    ToolHandler
 }
 
@@ -34,7 +34,7 @@ type ServerTool struct {
 //
 // TODO: just have the handler return a CallToolResult: returning []Content is
 // going to be inconsistent with other server features.
-func NewTool[TReq any](name, description string, handler func(context.Context, *ServerConnection, TReq) ([]Content, error), opts ...ToolOption) *ServerTool {
+func NewTool[TReq any](name, description string, handler func(context.Context, *ServerConnection, TReq) ([]*Content, error), opts ...ToolOption) *ServerTool {
 	schema, err := jsonschema.For[TReq]()
 	if err != nil {
 		panic(err)
@@ -55,7 +55,7 @@ func NewTool[TReq any](name, description string, handler func(context.Context, *
 		// rather than returned as jsonrpc2 server errors.
 		if err != nil {
 			return &CallToolResult{
-				Content: []Content{NewTextContent(err.Error())},
+				Content: []*Content{NewTextContent(err.Error())},
 				IsError: true,
 			}, nil
 		}
@@ -65,7 +65,7 @@ func NewTool[TReq any](name, description string, handler func(context.Context, *
 		return res, nil
 	}
 	t := &ServerTool{
-		Definition: Tool{
+		Definition: &Tool{
 			Name:        name,
 			Description: description,
 			InputSchema: schema,

mcp/tool_test.go

@@ -15,7 +15,7 @@ import (
 )
 
 // testToolHandler is used for type inference in TestNewTool.
-func testToolHandler[T any](context.Context, *mcp.ServerConnection, T) ([]mcp.Content, error) {
+func testToolHandler[T any](context.Context, *mcp.ServerConnection, T) ([]*mcp.Content, error) {
 	panic("not implemented")
 }