Slack invalid_blocks: Every Block Kit Limit We Hit, With the Error

invalid_blocks means Slack rejected your blocks array because one block broke a Block Kit rule, and nothing was posted. The Web API tells you which rule and where, in response_metadata.messages. An incoming webhook returns only the words invalid_blocks with HTTP 400. We sent each case below on 1 October 2026 through chat.postMessage with a bot token and through an incoming webhook, in a test workspace. Both paths enforced the same limits.

Reading the error

From chat.postMessage with 51 section blocks:

{
  "ok": false,
  "error": "invalid_blocks",
  "errors": [
    "no more than 50 items allowed [json-pointer:/blocks]"
  ],
  "response_metadata": {
    "messages": [
      "[ERROR] no more than 50 items allowed [json-pointer:/blocks]"
    ]
  }
}

The json-pointer is the path to the bad part: /blocks/0/text/text means the text field inside the text object of the first block. Slack numbers blocks from 0. slack_sdk raises SlackApiError; print the messages to see them:

import os
from slack_sdk import WebClient
from slack_sdk.errors import SlackApiError

client = WebClient(token=os.environ["SLACK_BOT_TOKEN"])
blocks = [{"type": "header", "text": {"type": "plain_text", "text": "h" * 151}}]
try:
    client.chat_postMessage(channel=os.environ["CHANNEL"], text="Deploy report", blocks=blocks)
except SlackApiError as e:
    print(e.response["error"])
    for line in e.response["response_metadata"]["messages"]:
        print(line)

Output:

invalid_blocks
[ERROR] failed to match all allowed schemas [json-pointer:/blocks/0/text]
[ERROR] must be less than 151 characters [json-pointer:/blocks/0/text/text]

The webhook gives you none of this. When a webhook says invalid_blocks, send the same blocks once through chat.postMessage or paste them into Block Kit Builder to find the field.

The limits we measured

Fifty sections of 3,000 characters each, 150,000 characters in one message, posted without an error. We found no separate total for blocks up to that size. The 40,000-character figure people quote is for the top-level text, and long text behaves differently: our chat.update test found chat.postMessage splits it into messages of 4,000 characters.

Structure errors

A button without an action_id posted fine. Slack fills one in. The duplicate check runs within the message, so give each button its own action_id when you build them in a loop. Headers take only plain_text.

The image error comes from Slack fetching the URL when you post; our test URL returned 404. Use a URL Slack can download without a login.

Fixing it in Block Kit Builder

Builder runs the same checks before you send. Our 152-character header showed this:

The banner only says "failed to match all allowed schemas". The red markers next to lines 5 and 7 point at the header's text, and the preview drops the broken block. The API's error text named the limit; Builder did not.

Text and blocks together

Always send text next to blocks. In our run, chat.postMessage with blocks and no text posted without a warning and copied the section text into text. Slack's docs say notifications use text, so write it yourself instead of relying on the copy. Formatting rules inside mrkdwn text are on the Block Kit markdown page, and chart blocks, which have their own limits, are on Block Kit charts.

FAQ

Why does my webhook return invalid_blocks when the same JSON works in Builder?

Check the shape of the body first. We sent the wrong shapes through a webhook: a bare [...] array returned no_text, and {"blocks": {"blocks": [...]}} returned invalid_blocks_format. The body must be {"blocks": [...]} at the top level, which is the shape Builder's payload tab shows. If the shape is right, a 400 invalid_blocks is one of the limits in the tables above.

What is the limit for a section block's text?

3,000 characters. 3,001 failed in our test, through the API and the webhook.