mcp: validate required fields in ImageContent and AudioContent

ImageContent and AudioContent now return errors when required fields
are missing during JSON marshaling, matching TypeScript schema requirements.
ImageContent requires data and mimeType fields. AudioContent requires
data and mimeType fields.

Added comprehensive test coverage for validation error cases.

Author: martinemde

mcp/content.go

@@ -48,6 +48,12 @@ type ImageContent struct {
 }
 
 func (c *ImageContent) MarshalJSON() ([]byte, error) {
+	if len(c.Data) == 0 {
+		return nil, errors.New("ImageContent missing data")
+	}
+	if c.MIMEType == "" {
+		return nil, errors.New("ImageContent missing mimeType")
+	}
 	return json.Marshal(&wireContent{
 		Type:        "image",
 		MIMEType:    c.MIMEType,
@@ -73,6 +79,12 @@ type AudioContent struct {
 }
 
 func (c AudioContent) MarshalJSON() ([]byte, error) {
+	if len(c.Data) == 0 {
+		return nil, errors.New("AudioContent missing data")
+	}
+	if c.MIMEType == "" {
+		return nil, errors.New("AudioContent missing mimeType")
+	}
 	return json.Marshal(&wireContent{
 		Type:        "audio",
 		MIMEType:    c.MIMEType,
@@ -159,7 +171,7 @@ 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

@@ -164,3 +164,44 @@ func TestEmbeddedResource(t *testing.T) {
 		}
 	}
 }
+
+func TestContentMarshalErrors(t *testing.T) {
+	tests := []struct {
+		name    string
+		content mcp.Content
+		wantErr string
+	}{
+		{
+			name:    "ImageContent missing data",
+			content: &mcp.ImageContent{MIMEType: "image/png"},
+			wantErr: "json: error calling MarshalJSON for type *mcp.ImageContent: ImageContent missing data",
+		},
+		{
+			name:    "ImageContent missing mimeType",
+			content: &mcp.ImageContent{Data: []byte("test")},
+			wantErr: "json: error calling MarshalJSON for type *mcp.ImageContent: ImageContent missing mimeType",
+		},
+		{
+			name:    "AudioContent missing data",
+			content: &mcp.AudioContent{MIMEType: "audio/wav"},
+			wantErr: "json: error calling MarshalJSON for type *mcp.AudioContent: AudioContent missing data",
+		},
+		{
+			name:    "AudioContent missing mimeType",
+			content: &mcp.AudioContent{Data: []byte("test")},
+			wantErr: "json: error calling MarshalJSON for type *mcp.AudioContent: AudioContent missing mimeType",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := json.Marshal(tt.content)
+			if err == nil {
+				t.Fatal("expected error, got nil")
+			}
+			if got := err.Error(); got != tt.wantErr {
+				t.Errorf("got error %q, want %q", got, tt.wantErr)
+			}
+		})
+	}
+}