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. In our API test on 1 October 2026, Slack did not cut long text: it split it into separate messages of 4,000 characters. 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.

To check how a long message looks before you post, draft it in the Markdown to Slack converter.

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 (101 returned too_long in our test on 7 October), 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, as two messages when it comes through the API. It is long for a chat, so a thread or a snippet reads better.

What happens when a Slack message is too long

• • In the API or a webhook. We measured this on 1 October 2026 with a bot token and an incoming webhook. chat.postMessage accepted 40,001 characters with ok: true and no warning, and posted the text as ten separate messages of about 4,000 characters each. The webhook also answered ok, but it posted at most four messages of 4,000 characters: from a 20,000-character text, 16,000 characters arrived and the rest was dropped without an error. chat.update refuses anything over 4,000 characters with msg_too_long; the full test is in Slack chat.update, tested.
• • 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. Every limit we hit, with its error text, is on the invalid_blocks page.
• • In the Slack app. We repeated the split on 7 October 2026 with the web app's own session token: a 5,000-character text of numbered lines became two messages of 3,994 and 1,005 characters, cut at the line break before 4,000. Both showed in full in the web app, with no collapse.

How to post a long message in Slack

To check the length before you post, paste the message into the Block Kit checker. It counts the text the way Slack stores it (& becomes &, an emoji becomes its :name:) and shows how many messages it turns into on chat.postMessage or a webhook.

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? Slack's docs say 40,000 in the text field. In our test, chat.postMessage split anything longer than about 4,000 characters into several messages, and an incoming webhook posted at most 16,000 characters.

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 did my long Slack message arrive as two messages? Text over about 4,000 characters sent through the API is split at a line break into several messages. Our 5,000-character test became 3,994 and 1,005 characters.

How long can a Slack status be? 100 characters.