From 70221a84ad92961f77cc1d2d65eb0b111b78999a Mon Sep 17 00:00:00 2001
From: Martin Emde <me@martinemde.com>
Date: Wed, 2 Jul 2025 19:52:34 -0700
Subject: [PATCH] mcp: render blank text field in TextContent

The text field is required. Some clients fail to parse the response if
the text field is omitted and a blank text field is allowed.
Return the field even when it is blank by removing omitempty.
---
 mcp/content.go      | 14 ++++++++++++--
 mcp/content_test.go |  8 ++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/mcp/content.go b/mcp/content.go
index ed7f6f99..61111a99 100644
--- a/mcp/content.go
+++ b/mcp/content.go
@@ -2,6 +2,9 @@
 // Use of this source code is governed by an MIT-style
 // license that can be found in the LICENSE file.
 
+// TODO(findleyr): update JSON marshalling of all content types to preserve required fields.
+// (See [TextContent.MarshalJSON], which handles this for text content).
+
 package mcp
 
 import (
@@ -25,12 +28,19 @@ type TextContent struct {
 }
 
 func (c *TextContent) MarshalJSON() ([]byte, error) {
-	return json.Marshal(&wireContent{
+	// Custom wire format to ensure the required "text" field is always included, even when empty.
+	wire := struct {
+		Type        string       `json:"type"`
+		Text        string       `json:"text"`
+		Meta        Meta         `json:"_meta,omitempty"`
+		Annotations *Annotations `json:"annotations,omitempty"`
+	}{
 		Type:        "text",
 		Text:        c.Text,
 		Meta:        c.Meta,
 		Annotations: c.Annotations,
-	})
+	}
+	return json.Marshal(wire)
 }
 
 func (c *TextContent) fromWire(wire *wireContent) {
diff --git a/mcp/content_test.go b/mcp/content_test.go
index 5ee6f66c..8c4e5306 100644
--- a/mcp/content_test.go
+++ b/mcp/content_test.go
@@ -22,6 +22,14 @@ func TestContent(t *testing.T) {
 			&mcp.TextContent{Text: "hello"},
 			`{"type":"text","text":"hello"}`,
 		},
+		{
+			&mcp.TextContent{Text: ""},
+			`{"type":"text","text":""}`,
+		},
+		{
+			&mcp.TextContent{},
+			`{"type":"text","text":""}`,
+		},
 		{
 			&mcp.TextContent{
 				Text:        "hello",