task: add _meta field to relevant types (#429)

Author: buraksenn

mcp/prompts.go

@@ -47,6 +47,8 @@ type GetPromptResult struct {
 // that requires argument values to be provided when calling prompts/get.
 // If Arguments is nil or empty, this is a static prompt that takes no arguments.
 type Prompt struct {
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta `json:"_meta,omitempty"`
 	// The name of the prompt or prompt template.
 	Name string `json:"name"`
 	// An optional description of what this prompt provides

mcp/tools.go

@@ -545,6 +545,8 @@ type ToolListChangedNotification struct {
 
 // Tool represents the definition for a tool the client can call.
 type Tool struct {
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta `json:"_meta,omitempty"`
 	// The name of the tool.
 	Name string `json:"name"`
 	// A human-readable description of the tool.

mcp/types.go

@@ -152,6 +152,18 @@ func (m *Meta) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
+func NewMetaFromMap(m map[string]any) *Meta {
+	progressToken := m["progressToken"]
+	if progressToken != nil {
+		delete(m, "progressToken")
+	}
+
+	return &Meta{
+		ProgressToken:    progressToken,
+		AdditionalFields: m,
+	}
+}
+
 type Request struct {
 	Method string        `json:"method"`
 	Params RequestParams `json:"params,omitempty"`
@@ -233,7 +245,7 @@ func (p *NotificationParams) UnmarshalJSON(data []byte) error {
 type Result struct {
 	// This result property is reserved by the protocol to allow clients and
 	// servers to attach additional metadata to their responses.
-	Meta map[string]any `json:"_meta,omitempty"`
+	Meta *Meta `json:"_meta,omitempty"`
 }
 
 // RequestId is a uniquely identifying ID for a request in JSON-RPC.
@@ -644,6 +656,8 @@ type ResourceUpdatedNotificationParams struct {
 // Resource represents a known resource that the server is capable of reading.
 type Resource struct {
 	Annotated
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta `json:"_meta,omitempty"`
 	// The URI of this resource.
 	URI string `json:"uri"`
 	// A human-readable name for this resource.
@@ -668,6 +682,8 @@ func (r Resource) GetName() string {
 // on the server.
 type ResourceTemplate struct {
 	Annotated
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta `json:"_meta,omitempty"`
 	// A URI template (according to RFC 6570) that can be used to construct
 	// resource URIs.
 	URITemplate *URITemplate `json:"uriTemplate"`
@@ -697,6 +713,8 @@ type ResourceContents interface {
 }
 
 type TextResourceContents struct {
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta `json:"_meta,omitempty"`
 	// The URI of this resource.
 	URI string `json:"uri"`
 	// The MIME type of this resource, if known.
@@ -709,6 +727,8 @@ type TextResourceContents struct {
 func (TextResourceContents) isResourceContents() {}
 
 type BlobResourceContents struct {
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta `json:"_meta,omitempty"`
 	// The URI of this resource.
 	URI string `json:"uri"`
 	// The MIME type of this resource, if known.
@@ -867,6 +887,8 @@ type Content interface {
 // It must have Type set to "text".
 type TextContent struct {
 	Annotated
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta  `json:"_meta,omitempty"`
 	Type string `json:"type"` // Must be "text"
 	// The text content of the message.
 	Text string `json:"text"`
@@ -878,6 +900,8 @@ func (TextContent) isContent() {}
 // It must have Type set to "image".
 type ImageContent struct {
 	Annotated
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta  `json:"_meta,omitempty"`
 	Type string `json:"type"` // Must be "image"
 	// The base64-encoded image data.
 	Data string `json:"data"`
@@ -891,6 +915,8 @@ func (ImageContent) isContent() {}
 // It must have Type set to "audio".
 type AudioContent struct {
 	Annotated
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta  `json:"_meta,omitempty"`
 	Type string `json:"type"` // Must be "audio"
 	// The base64-encoded audio data.
 	Data string `json:"data"`
@@ -922,6 +948,8 @@ func (ResourceLink) isContent() {}
 // benefit of the LLM and/or the user.
 type EmbeddedResource struct {
 	Annotated
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta     *Meta            `json:"_meta,omitempty"`
 	Type     string           `json:"type"`
 	Resource ResourceContents `json:"resource"`
 }
@@ -1056,6 +1084,8 @@ type ListRootsResult struct {
 
 // Root represents a root directory or file that the server can operate on.
 type Root struct {
+	// Meta is a metadata object that is reserved by MCP for storing additional information.
+	Meta *Meta `json:"_meta,omitempty"`
 	// The URI identifying the root. This *must* start with file:// for now.
 	// This restriction may be relaxed in future versions of the protocol to allow
 	// other URI schemes.

mcp/utils.go

@@ -567,7 +567,7 @@ func ParseGetPromptResult(rawMessage *json.RawMessage) (*GetPromptResult, error)
 	meta, ok := jsonContent["_meta"]
 	if ok {
 		if metaMap, ok := meta.(map[string]any); ok {
-			result.Meta = metaMap
+			result.Meta = NewMetaFromMap(metaMap)
 		}
 	}
 
@@ -633,7 +633,7 @@ func ParseCallToolResult(rawMessage *json.RawMessage) (*CallToolResult, error) {
 	meta, ok := jsonContent["_meta"]
 	if ok {
 		if metaMap, ok := meta.(map[string]any); ok {
-			result.Meta = metaMap
+			result.Meta = NewMetaFromMap(metaMap)
 		}
 	}
 
@@ -715,7 +715,7 @@ func ParseReadResourceResult(rawMessage *json.RawMessage) (*ReadResourceResult,
 	meta, ok := jsonContent["_meta"]
 	if ok {
 		if metaMap, ok := meta.(map[string]any); ok {
-			result.Meta = metaMap
+			result.Meta = NewMetaFromMap(metaMap)
 		}
 	}
 

server/sse_test.go

@@ -1257,7 +1257,7 @@ func TestSSEServer(t *testing.T) {
 			WithHooks(&Hooks{
 				OnAfterInitialize: []OnAfterInitializeFunc{
 					func(ctx context.Context, id any, message *mcp.InitializeRequest, result *mcp.InitializeResult) {
-						result.Meta = map[string]any{"invalid": func() {}} // marshal will fail
+						result.Meta = mcp.NewMetaFromMap(map[string]any{"invalid": func() {}}) // marshal will fail
 					},
 				},
 			}),