---
title: "Slack API: post and schedule from code"
url: "https://get1post.com/developers/slack.md"
canonical_url: "https://get1post.com/developers/slack"
last_updated: "2026-10-26"
type: "reference"
summary: "Post and schedule to Slack with the 1Post API and MCP server: the request, the reply, every rule it is checked against and a dry run."
related: ["https://get1post.com/platforms/slack.md", "https://get1post.com/platforms/slack/specs.md", "https://get1post.com/docs/api-posts.md", "https://get1post.com/docs/mcp.md"]
---

# Slack API: post and schedule from code

Post and schedule to Slack with the 1Post API and MCP server: the request, the reply, every rule it is checked against and a dry run.

Post to Slack from code with one request: POST /v1/posts with your channel id, the text and a schedule_at time. Slack text is limited to 4,000 characters, counted by code points. 1Post allows up to 100 posts a day for each connected channel, and a dry run checks a post against every rule first.

## Connect Slack first

1. In Slack, create an incoming webhook for the channel.
2. Copy the webhook URL.
3. Open Accounts in 1Post and choose Connect, then Slack, and paste the URL.
4. Choose the channel in the composer.

Then GET /v1/accounts returns the id of the Slack channel to use in the request.

## Send a post to Slack

**Schedule a post**

```bash
curl https://api.get1post.com/v1/posts \
  -H "Authorization: Bearer $ONEPOST_API_KEY" \
  -H "Idempotency-Key: slack-first-post" \
  -d '{"accounts":["ACCOUNT_ID"],"text":"The weekly report is ready.","schedule_at":"2026-11-02T15:00:00Z"}'
```

The reply is 202 with the post (status scheduled) and one destination for Slack, which moves from queued to published or failed.

## What you can send to Slack

- Text

## What Slack publishes

- Slack suggests keeping message text to 4,000 characters for best results and truncates messages over 40,000.

## Slack rules the request is checked against

| Rule | What 1Post checks |
| --- | --- |
| Text limit | 4,000 characters, counted by code points |
| Media accepted | Text only |
| Threads | No |
| First comment | No |
| Posts per day, per connected account (1Post) | 100 |

The same table with sources is on the [Slack specs page](https://get1post.com/platforms/slack/specs.md).

## Good to know for Slack

A Slack post is text only and goes to the one channel its incoming webhook was made for. Connect one account for each channel you post to.

## Check the post before it is queued

Add "dry_run": true to test a post: nothing is kept, a valid one answers 200 and a broken one answers 422 naming the rule.

**A refused request, abridged**

```json
{
  "error": {
    "code": "invalid_for_platform",
    "message": "slack refuses this post: Slack counts 4,032 of 4,000 characters in part 1. (text_too_long).",
    "fix": "Fix the rule named in the message and send the post again.",
    "retryable": false,
    "request_id": "REQUEST_ID"
  }
}
```

An assistant connected to the [MCP server](https://get1post.com/docs/mcp.md) sends the same fields to create_post; [set one up](https://get1post.com/assistants.md).

## Questions

**Can I post to Slack with an API key?**

Yes. A key with write scope can create, change and cancel Slack posts for the workspace it belongs to, with the role of the member who made it. See [API authentication](https://get1post.com/docs/api-authentication.md).

**What happens if a Slack post breaks a rule?**

The request is refused with invalid_for_platform and the failing rule is named. Fix it and send the request again, or send it with dry_run to test the fix first.

**How many Slack posts can I send in a day?**

Up to 100 for each connected channel. A request that would pass that answers daily_cap with the time the day frees up.

## Sources

- [Slack developer docs: chat.postMessage](https://docs.slack.dev/reference/methods/chat.postMessage), read 2026-10-26

## Related

- [Schedule Slack posts: formats, limits, setup](https://get1post.com/platforms/slack.md)
- [Slack specs and limits: text, media, daily cap](https://get1post.com/platforms/slack/specs.md)
- [Creating and changing posts with the API: 1Post docs](https://get1post.com/docs/api-posts.md)
- [The MCP server: 1Post docs](https://get1post.com/docs/mcp.md)