"Invalid blocks" when updating ephemeral message via response_url webhook - slack

Question

According to Slack's documentation, you can use blocks to update an entire ephemeral message by simply replacing the entire message by setting replace_original as true when posting to the response_url provided in the interaction payload when using buttons.
However, when I'm sending the POST request to the provided response_url after a button has been pressed and I attempt to use blocks, I get the following error: 404 : [{"ok":false,"error":"invalid_blocks"}]
Strangely enough, the payload I'm including for blocks is generated by the same method that creates the original ephemeral message which works fine without any issues.
I was able to replace the entire message with just a plain text message by using the text field indicated in the documentation and it works fine if I don't include the blocks array. I did find some documentation here for posting messages via incoming webhooks here but it looks the same as what I'm sending now.
Here's the body of the request I'm sending:
{
"blocks":
[
{
"type": "image",
"imageUrl": "https://example.com/image.jpeg",
"altText": "alt-text",
"title":
{
"type": "plain_text",
"text": "some text"
}
},
{
"type": "actions",
"elements":
[
{
"type": "button",
"text":
{
"type": "plain_text",
"text": "Send",
"emoji": false
},
"value": "send",
"style": "primary"
},
{
"type": "button",
"text":
{
"type": "plain_text",
"text": "Find new",
"emoji": false
},
"value": "some text"
},
{
"type": "button",
"text":
{
"type": "plain_text",
"text": "Cancel",
"emoji": false
},
"value": "cancel"
}
]
}
],
"replace_original": "true",
"text": "some fallback text"
}

Answer 1

You have two typos in your blocks: imageUrl and altText. Slack expects these to be snake case, i.e. image_url and alt_text.
You can use Slack's Block Kit Builder to debug the blocks payload.