mcp: ensure required fields are not omitted in ImageContent and AudioContent

ImageContent and AudioContent now use custom MarshalJSON methods with
inline structs to ensure data and mimeType fields are always included
in JSON output, even when empty. This matches the approach used for
TextContent.Text and follows TypeScript schema requirements.

Updated tests to verify empty fields are serialized as empty strings
rather than being omitted.

Author: martinemde

mcp/content.go

@@ -58,13 +58,25 @@ type ImageContent struct {
 }
 
 func (c *ImageContent) MarshalJSON() ([]byte, error) {
-	return json.Marshal(&wireContent{
+	// Custom wire format to ensure required fields are always included, even when empty.
+	data := c.Data
+	if data == nil {
+		data = []byte{}
+	}
+	wire := struct {
+		Type        string       `json:"type"`
+		MIMEType    string       `json:"mimeType"`
+		Data        []byte       `json:"data"`
+		Meta        Meta         `json:"_meta,omitempty"`
+		Annotations *Annotations `json:"annotations,omitempty"`
+	}{
 		Type:        "image",
 		MIMEType:    c.MIMEType,
-		Data:        c.Data,
+		Data:        data,
 		Meta:        c.Meta,
 		Annotations: c.Annotations,
-	})
+	}
+	return json.Marshal(wire)
 }
 
 func (c *ImageContent) fromWire(wire *wireContent) {
@@ -83,13 +95,25 @@ type AudioContent struct {
 }
 
 func (c AudioContent) MarshalJSON() ([]byte, error) {
-	return json.Marshal(&wireContent{
+	// Custom wire format to ensure required fields are always included, even when empty.
+	data := c.Data
+	if data == nil {
+		data = []byte{}
+	}
+	wire := struct {
+		Type        string       `json:"type"`
+		MIMEType    string       `json:"mimeType"`
+		Data        []byte       `json:"data"`
+		Meta        Meta         `json:"_meta,omitempty"`
+		Annotations *Annotations `json:"annotations,omitempty"`
+	}{
 		Type:        "audio",
 		MIMEType:    c.MIMEType,
-		Data:        c.Data,
+		Data:        data,
 		Meta:        c.Meta,
 		Annotations: c.Annotations,
-	})
+	}
+	return json.Marshal(wire)
 }
 
 func (c *AudioContent) fromWire(wire *wireContent) {
@@ -169,7 +193,7 @@ type ResourceContents struct {
 }
 
 func (r ResourceContents) MarshalJSON() ([]byte, error) {
-	// If we could assume Go 1.24, we could use omitzero for Blob and avoid this method.
+	// URI is a required field, so we return an error when it is missing.
 	if r.URI == "" {
 		return nil, errors.New("ResourceContents missing URI")
 	}

mcp/content_test.go

@@ -45,6 +45,14 @@ func TestContent(t *testing.T) {
 			},
 			`{"type":"image","mimeType":"image/png","data":"YTFiMmMz"}`,
 		},
+		{
+			&mcp.ImageContent{MIMEType: "image/png", Data: []byte{}},
+			`{"type":"image","mimeType":"image/png","data":""}`,
+		},
+		{
+			&mcp.ImageContent{Data: []byte("test")},
+			`{"type":"image","mimeType":"","data":"dGVzdA=="}`,
+		},
 		{
 			&mcp.ImageContent{
 				Data:        []byte("a1b2c3"),
@@ -61,6 +69,14 @@ func TestContent(t *testing.T) {
 			},
 			`{"type":"audio","mimeType":"audio/wav","data":"YTFiMmMz"}`,
 		},
+		{
+			&mcp.AudioContent{MIMEType: "audio/wav", Data: []byte{}},
+			`{"type":"audio","mimeType":"audio/wav","data":""}`,
+		},
+		{
+			&mcp.AudioContent{Data: []byte("test")},
+			`{"type":"audio","mimeType":"","data":"dGVzdA=="}`,
+		},
 		{
 			&mcp.AudioContent{
 				Data:        []byte("a1b2c3"),