Kitchensink example 'flex json'

[datoslabs]

System Information

• OS: Windows 11
• Go Version [e.g. 1.24.3] 1.25.1
• line-bot-sdk-go version(s) [e.g. 8.13.1] 8.18.0? (github main branch as of 1/13/2026)

Expected Behavior

Display the Brown Cafe flex message.

Current Behavior

Server error:
unexpected status code: 400, {"message":"A message (messages[0]) in the request body is invalid","details":[{"message":"invalid property","property":"/footer/contents/2"}]}

Steps to Reproduce

1. git clone https://github.com/line/line-bot-sdk-go.git
2. cd line-bot-sdk-go\examples\kitchensink
3. go run server.go (after setting the env variables)
4. send flex json message inside LINE App chat (with the bot)

Logs

2026/01/13 13:15:26 unexpected status code: 400, {"message":"A message (messages[0]) in the request body is invalid","details":[{"message":"invalid property","property":"/footer/contents/2"}]}

Additional Context (Optional)

I copied and pasted the json content into the FLEX MESSAGE SIMULATOR and it appears to be valid. UnmarshalFlexContainer call:

line-bot-sdk-go/examples/kitchensink/server.go

Line 843 in 63e224e

contents, err := messaging_api.UnmarshalFlexContainer([]byte(jsonString))

also completed without error. The error is returned from the following:

line-bot-sdk-go/examples/kitchensink/server.go

Lines 847 to 859 in 63e224e

	if _, err := app.bot.ReplyMessage(
	&messaging_api.ReplyMessageRequest{
	ReplyToken: replyToken,
	Messages: []messaging_api.MessageInterface{
	&messaging_api.FlexMessage{
	AltText: "Flex message alt text",
	Contents: contents,
	},
	},
	},
	); err != nil {
	return err
	}

The error message seems to suggest there is an invalid property in /footer/contents/2 which appears to be a spacer according to the JSON string. If the spacer, ie:

line-bot-sdk-go/examples/kitchensink/server.go

Lines 835 to 838 in 63e224e

	{
	"type": "spacer",
	"size": "sm"
	}

is removed, the error disappears.

[datoslabs]

Update 1. It appears that the spacer block in github is different from the spacer block in the Flex Message Simulator Brown Cafe showcase so I swapped out the JSON string with the JSON from the simulator; this resulted a slightly different error from the server:

unexpected status code: 400, {"message":"A message (messages[0]) in the request body is invalid","details":[{"message":"invalid property","property":"/footer/contents/2/contents"}]}

However, using the same example, if I sent it via the simulator (to my line app target), it works without any error.

[datoslabs]

Update 2. I copied the JSON string from the example and send it via the Flex Message Simulator and it works without any problem.

[datoslabs]

Update 3.

Based on Flex Message Simulator, "type":"spacer" has been deprecated; therefore, I believe the code is simply out of date.

After many trials and errors, I have a strong suspicion that the root cause of this error is due to how "contents": [] is unmarshalled and marshalled. The example from Brown Cafe in Flex Message Simulator used an empty messaging_api.FlexBox as spacer and after messaging_api.UnmarshalFlexContainer([]byte(jsonString)) call, contents.(messaging_api.FlexBubble).Footer.Contents[2].(messaging_api.FlexBox) == nil, NOT, an empty array. Next, when contents object is marshalled back to JSON string to be sent to the LINE Messaging API endpoint, it's converted to "contents": null instead of "contents": [] and triggered an error on the LINE server side because contents is a required field. I tried another example - Transit, also from the Flex Message Simulator, which contained several empty FlexBox and as expected, the server returned an error for the first empty FlexBox. To further demonstrate, below is the JSON of Brown Cafe example from Flex Message Simulator:

{
  ...,
  "footer": {
    "type": "box",
    "layout": "vertical",
    "spacing": "sm",
    "contents": [
      {
        "type": "button",
        "style": "link",
        "height": "sm",
        "action": {
          "type": "uri",
          "label": "CALL",
          "uri": "https://line.me/"
        }
      },
      {
        "type": "button",
        "style": "link",
        "height": "sm",
        "action": {
          "type": "uri",
          "label": "WEBSITE",
          "uri": "https://line.me/"
        }
      },
      {
        "type": "box",
        "layout": "vertical",
        "contents": [],
        "margin": "sm"
      }
    ],
    "flex": 0
  }
}

I replaced the jsonString with the one above and added debugging code to the handleFlexJson function in server.go as follows:

func handleFlexJson(app *KitchenSink, replyToken string) error {
...

	contents, err := messaging_api.UnmarshalFlexContainer([]byte(jsonString))
	if err != nil {
		log.Printf("Error unmarshaling flex container: %v %v", jsonString, err)
		return err
	}

	jsonData, err := json.MarshalIndent(contents, "", "  ")
	if err != nil {
		log.Printf("Error marshaling flex container: %v %v", jsonString, err)
		return err
	}

	fmt.Println("Marshalled JSON:", string(jsonData))
...
}

and produced the following output:

Marshalled JSON: {
...,
"footer": {
    "layout": "vertical",
    "flex": 0,
    "spacing": "sm",
    "type": "box",
    "contents": [
      {
        "type": "button",
        "flex": 0,
        "style": "link",
        "action": {
          "type": "uri",
          "label": "CALL",
          "uri": "https://line.me/"
        },
        "height": "sm",
        "scaling": false
      },
      {
        "type": "button",
        "flex": 0,
        "style": "link",
        "action": {
          "type": "uri",
          "label": "WEBSITE",
          "uri": "https://line.me/"
        },
        "height": "sm",
        "scaling": false
      },
      {
        "type": "box",
        "layout": "vertical",
        "flex": 0,
        "contents": null,
        "margin": "sm"
      }
    ]
  }
}

The last FlexBox, ie spacer, marshalled as "contents": null, not "contents": [].

[datoslabs]

Just realized similar/same issue has been reported in #468.