From b136041461809888b2e4637d74ff595bf642f93b Mon Sep 17 00:00:00 2001
From: Jeremy Schoemaker <jeremy@shoemoney.com>
Date: Wed, 5 Aug 2026 23:33:55 -0500
Subject: [PATCH] mcp: preserve required text field when marshaling empty
 ResourceContents

ResourceContents.Text carries json:"text,omitempty", so a text resource
with empty content marshals without a text key and matches neither branch
of the spec's anyOf[TextResourceContents, BlobResourceContents] union.
Strict clients reject the response even though it round-trips fine between
two Go peers, since the Go unmarshaler is lenient about the missing field.

Add MarshalJSON on ResourceContents, branching on Blob so nil Blob always
emits text (even empty) and omits blob, while a non-nil Blob (including
an empty one) emits blob and omits text entirely. _meta is carried on
both branches. This mirrors TextContent.MarshalJSON and finishes the
series from the TODO at the top of content.go: TextContent (#91) and
ImageContent/AudioContent (#95) already got this treatment; ResourceContents
was the one left.
---
 mcp/content.go      | 33 +++++++++++++++++++++++++++++++++
 mcp/content_test.go |  4 +++-
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/mcp/content.go b/mcp/content.go
index 7af90fe7..adb2048e 100644
--- a/mcp/content.go
+++ b/mcp/content.go
@@ -307,6 +307,39 @@ type ResourceContents struct {
 	Meta     Meta   `json:"_meta,omitempty"`
 }
 
+func (r *ResourceContents) MarshalJSON() ([]byte, error) {
+	// Custom wire format to ensure the required "text" or "blob" field is
+	// always included, even when empty, so a text resource with empty
+	// content still matches the TextResourceContents branch of the spec's
+	// anyOf union instead of matching neither branch.
+	if r.Blob != nil {
+		wire := struct {
+			URI      string `json:"uri"`
+			MIMEType string `json:"mimeType,omitempty"`
+			Blob     []byte `json:"blob"`
+			Meta     Meta   `json:"_meta,omitempty"`
+		}{
+			URI:      r.URI,
+			MIMEType: r.MIMEType,
+			Blob:     r.Blob,
+			Meta:     r.Meta,
+		}
+		return json.Marshal(wire)
+	}
+	wire := struct {
+		URI      string `json:"uri"`
+		MIMEType string `json:"mimeType,omitempty"`
+		Text     string `json:"text"`
+		Meta     Meta   `json:"_meta,omitempty"`
+	}{
+		URI:      r.URI,
+		MIMEType: r.MIMEType,
+		Text:     r.Text,
+		Meta:     r.Meta,
+	}
+	return json.Marshal(wire)
+}
+
 // wireContent is the wire format for content.
 // It represents the protocol types TextContent, ImageContent, AudioContent,
 // ResourceLink, EmbeddedResource, ToolUseContent, and ToolResultContent.
diff --git a/mcp/content_test.go b/mcp/content_test.go
index 094950eb..39af5e20 100644
--- a/mcp/content_test.go
+++ b/mcp/content_test.go
@@ -160,8 +160,10 @@ func TestEmbeddedResource(t *testing.T) {
 			`{"uri":"u","mimeType":"m","text":"t","_meta":{"key":"value"}}`,
 		},
 		{
+			// Text is required by TextResourceContents in the spec, so it must
+			// always be present, even when empty.
 			&mcp.ResourceContents{URI: "u"},
-			`{"uri":"u"}`,
+			`{"uri":"u","text":""}`,
 		},
 		{
 			&mcp.ResourceContents{URI: "u", Blob: []byte{}},