Slack Message Character Limit: 40,000 Max, 4,000 Advised

The Slack message character limit is 40,000 characters. Slack's developer docs say messages with more than 40,000 characters in the text field are truncated, and they advise keeping the text under 4,000 characters for best results. If you use Block Kit, each block has its own limit: 3,000 characters for a section block's text, 150 for a header, and up to 50 blocks per message. For anything longer, Slack suggests a snippet, and a canvas works well for long documents.

We build Slack Green, which keeps your Slack status green from our servers during the hours you set. We also make a free Slack message formatter for drafting a message and checking how it looks before you post.

Slack text limits in one table

The 40,000 limit has applied since August 2018, when Slack's changelog says it lowered the ceiling step by step from 500,000 characters. It applies to every way of posting: chat.postMessage, chat.postEphemeral, incoming webhooks, slash command responses and interactive messages.

A few other limits people search for: a Slack status holds 100 characters, and a channel name 80. Ideas that fit in 100 characters are in Slack status examples.

Is there a 4,000 character limit in Slack?

Not a hard one. 4,000 is the length Slack's docs recommend for the text field, not the point where Slack cuts. Before 2018, Slack's guidance was to keep messages under 4,000 bytes, which is where the number people quote comes from. A 5,000-character message posts fine. It is just long for a chat, and readers see it collapsed, with a link to expand it.

What happens when a Slack message is too long

• • In the API or a webhook. Past 40,000 characters, Slack posts the message with the text cut off and returns a warning in the response, message_truncated, with the note that the text field accepts up to 40,000 characters. Slack's changelog adds that very long messages may also be broken into several messages.
• • In a Block Kit message. A section with more than 3,000 characters, or more than 50 blocks, fails validation, and nothing posts. Split the text across several section blocks.
• • In the Slack app. Long messages post, and Slack shows them collapsed so they do not take over the channel.

The warning in an API response looks like this:

{
  "ok": true,
  "response_metadata": {
    "warnings": ["message_truncated"],
    "messages": ["[WARN] Your message was truncated but still posted. The `text` field accepts up to 40,000 characters."]
  }
}

How to post a long message in Slack


• Put the point first and trim. Two short paragraphs get read. A long message gets skimmed.
• Split it into a thread. Post a summary, then the details as replies. Each reply has its own limit.
• Post a snippet for logs and files. Click + next to the message field, choose Create a text snippet, and paste. If you already typed it, press Cmd+Shift+Enter (Ctrl+Shift+Enter). Snippets also get syntax highlighting. See how to make a code block in Slack.
• Write a canvas for documents. A canvas has headings and tables and does not clutter the channel. See Slack canvas vs lists.

To check the length before you post, paste the text into a character counter, or run wc -m on a file in a terminal.

Split a long message for the Slack API

If a bot posts reports that can run long, split the text at line breaks before you send:

def split_for_slack(text: str, limit: int = 3900) -> list[str]:
    # Split on line breaks so no chunk passes the limit
    chunks, current = [], ""
    for line in text.splitlines(keepends=True):
        while len(line) > limit:  # one very long line
            chunks.append(line[:limit])
            line = line[limit:]
        if len(current) + len(line) > limit:
            chunks.append(current)
            current = ""
        current += line
    if current:
        chunks.append(current)
    return chunks

Post the first chunk, then post the rest with thread_ts set to the first message's ts, so they stay together in one thread. A limit of 3,900 keeps each part under the 4,000 characters Slack recommends. If you send the parts as section blocks, use 3,000 instead. Converting markdown first? The Markdown to Slack converter turns a long markdown report into Slack's format.

FAQ

How many characters can a Slack message have? Up to 40,000 in the text field. Past that, Slack truncates it.

What is the Slack character limit for Block Kit? 3,000 characters for a section's text, 150 for a header, and 50 blocks per message.

Why is my long Slack message collapsed? Slack collapses long messages in the channel. Readers click the link under it to read the rest.

How long can a Slack status be? 100 characters.