From c5b9f0681ce7905949c2c1afba84955ada25db42 Mon Sep 17 00:00:00 2001
From: Martin Emde <me@martinemde.com>
Date: Fri, 4 Jul 2025 08:57:04 -0700
Subject: [PATCH] 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.
---
 mcp/content.go      | 36 ++++++++++++++++++++++++++++++------
 mcp/content_test.go | 16 ++++++++++++++++
 2 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/mcp/content.go b/mcp/content.go
index fd027cf..04888de 100644
--- a/mcp/content.go
+++ b/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) {
diff --git a/mcp/content_test.go b/mcp/content_test.go
index 7a549be..9366b0d 100644
--- a/mcp/content_test.go
+++ b/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"),