mcp: avoid "null" in structuredContent

Avoid a typed nil in structuredContent (yet another typed nil), leading
to "null" on the wire.

Fixes #417

mcp/server.go

@@ -267,19 +267,21 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHan
 				outval = elemZero
 			}
 		}
-		outbytes, err := json.Marshal(outval)
-		if err != nil {
-			return nil, fmt.Errorf("marshaling output: %w", err)
-		}
-		res.StructuredContent = json.RawMessage(outbytes) // avoid a second marshal over the wire
+		if outval != nil {
+			outbytes, err := json.Marshal(outval)
+			if err != nil {
+				return nil, fmt.Errorf("marshaling output: %w", err)
+			}
+			res.StructuredContent = json.RawMessage(outbytes) // avoid a second marshal over the wire
 
-		// If the Content field isn't being used, return the serialized JSON in a
-		// TextContent block, as the spec suggests:
-		// https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content.
-		if res.Content == nil {
-			res.Content = []Content{&TextContent{
-				Text: string(outbytes),
-			}}
+			// If the Content field isn't being used, return the serialized JSON in a
+			// TextContent block, as the spec suggests:
+			// https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content.
+			if res.Content == nil {
+				res.Content = []Content{&TextContent{
+					Text: string(outbytes),
+				}}
+			}
 		}
 		return res, nil
 	} // end of handler

mcp/streamable_test.go

@@ -1115,8 +1115,7 @@ func TestStreamableStateless(t *testing.T) {
 			},
 			wantMessages: []jsonrpc.Message{
 				resp(2, &CallToolResult{
-					Content:           []Content{&TextContent{Text: "hi World"}},
-					StructuredContent: json.RawMessage("null"),
+					Content: []Content{&TextContent{Text: "hi World"}},
 				}, nil),
 			},
 		},
@@ -1128,8 +1127,7 @@ func TestStreamableStateless(t *testing.T) {
 			},
 			wantMessages: []jsonrpc.Message{
 				resp(2, &CallToolResult{
-					Content:           []Content{&TextContent{Text: "hi foo"}},
-					StructuredContent: json.RawMessage("null"),
+					Content: []Content{&TextContent{Text: "hi foo"}},
 				}, nil),
 			},
 		},

mcp/testdata/conformance/server/tools.txtar

@@ -21,11 +21,15 @@ structured
     "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
   }
 }
-{"jsonrpc":"2.0", "method": "notifications/initialized"}
+{ "jsonrpc":"2.0", "method": "notifications/initialized" }
 { "jsonrpc": "2.0", "id": 2, "method": "tools/list" }
 { "jsonrpc": "2.0", "id": 3, "method": "resources/list" }
 { "jsonrpc": "2.0", "id": 4, "method": "prompts/list" }
 { "jsonrpc": "2.0", "id": 5, "method": "tools/call" }
+{ "jsonrpc": "2.0", "id": 6, "method": "tools/call", "params": {"name": "greet", "arguments": {"name": "you"} } }
+{ "jsonrpc": "2.0", "id": 1, "result": {} }
+{ "jsonrpc": "2.0", "id": 7, "method": "tools/call", "params": {"name": "structured", "arguments": {"In": "input"} } }
+
 -- server --
 {
 	"jsonrpc": "2.0",
@@ -119,3 +123,35 @@ structured
 		"message": "invalid request: missing required \"params\""
 	}
 }
+{
+	"jsonrpc": "2.0",
+	"id": 1,
+	"method": "ping"
+}
+{
+	"jsonrpc": "2.0",
+	"id": 6,
+	"result": {
+		"content": [
+			{
+				"type": "text",
+				"text": "hi you"
+			}
+		]
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 7,
+	"result": {
+		"content": [
+			{
+				"type": "text",
+				"text": "{\"Out\":\"Ack input\"}"
+			}
+		],
+		"structuredContent": {
+			"Out": "Ack input"
+		}
+	}
+}