Check whether a social post fits a platform's real character limit before publishing.
check_post: Check one post against a single platform (X, X Premium, Bluesky, LinkedIn, Threads, Mastodon, Discord) and get the counted length, limit, headroom, and what drove the count.
check_post_all: Check one post against every supported platform at once, with a row per platform showing length, limit, and pass/fail; detailed breakdowns are included only for failing platforms.
platform_limits: List each platform's limit, counting unit, why it's not a plain character count, and the published source; optionally filter to one platform.
Handles platform-specific rules: URL weightings, grapheme vs. character counting, emoji sequences, CJK weighting, and draft [URL] placeholders priced as real links.
Pure local computation: no API keys, no network calls, no posting.
Checks a social post against Bluesky's 300 grapheme limit, correctly counting extended grapheme clusters (emoji, flags, combining accents, etc.) and URLs in full.
Checks a social post against Discord's 2,000 character limit (4,000 with Nitro), using standard character counting.
Checks a social post against Mastodon's default 500 grapheme limit, accounting for URLs billed at 23 characters and remote mentions where only the local username counts.
Checks a social post against Threads' 500 character limit for the post body, distinct from the 10,000-character attachment.
Maintenance mode. This project is stable and receives security fixes. New features aren't planned, but issues and pull requests are still welcome.
Check a social post against a platform's real character limit before it ships. X, Bluesky, LinkedIn, Threads, Mastodon, Discord. Pure compute — no API, no auth, no network.
Recorded from docs/demo.tape with vhs. The posts and counts come from scripts/fixtures.mjs, which the regression tests import too.
An MCP server that answers one question: does this post fit?
A language model cannot count characters by inspection, and on these platforms neither can you. The limits are not what they look like. X bills every URL at 23 characters through t.co whether the link is 12 characters or 200. Bluesky counts extended grapheme clusters, so a four-person family emoji is 1 and not 11. Mastodon charges nothing for the domain on a remote mention. Getting any of that wrong shows up as a rejected post, or a truncated one, at publish time.
Counting is what a tool call is for. The model cannot do it by inspection, and a deterministic function can do it exactly.
Why this exists. Two posts went out of a podcast promo workflow over the limit. A Bluesky post shipped at 302 against 300, with the line "Under 300 graphemes. Audit clean." sitting directly beneath it. An X post was drafted at 308 against 280 and would have been rejected on launch morning. Both were invisible to eyeballing, because in both cases the count was a claim and not a measurement. Both are regression tests in this repo.
Tool | What it returns |
| Verdict for one platform: counted length, the limit, headroom, and what drove the count |
| One row per platform, with the breakdown attached only to the rows that fail |
| Each platform's limit, its counting unit, why that unit is not a character count, and the source |
Responses are small on purpose. check_post_all omits the arithmetic on passing rows because agents pay tokens per response.
Related MCP server: Twitter MCP Server
Platform | Limit | Unit | The part that surprises people | Source |
| 280 | weighted characters | Every URL costs exactly 23. CJK, Hangul, and emoji cost 2 each; Latin, Greek, Cyrillic, Hebrew, and Arabic cost 1. An emoji sequence is one unit of 2, not 2 per code point. | |
| 25,000 | weighted characters | Same weighting, higher ceiling. | |
| 300 | graphemes | Flags, ZWJ emoji, skin-tone modifiers, and combining accents each count as 1. URLs count in full. A second cap of 3,000 UTF-8 bytes can bind first on ZWJ-heavy text. | |
| 3,000 | characters | The 3,000 is generous; the fold is the real constraint. The feed collapses the post behind "see more" after a few lines. | |
| 500 | characters | The September 2025 change added a 10,000-character attachment. The post body is still 500. | |
| 500 | graphemes | URLs cost 23, as on X. On | |
| 2,000 | characters | 4,000 with Nitro. Embeds have a separate 6,000 total. |
Every number above traces to a published source. Widely repeated figures that no primary source states — the Facebook post limit, the YouTube community post limit, Reddit's title cap, Instagram's organic caption cap — are deliberately absent. A limit that cannot be defended makes a passing check worth nothing.
Published on npm. The config blocks below use npx, which fetches it on first run; no clone required.
git clone https://github.com/conorbronsdon/postlint-mcp.git
cd postlint-mcp
npm install
npm run buildAdd to your .mcp.json:
{
"mcpServers": {
"postlint": {
"command": "npx",
"args": ["-y", "@conorbronsdon/postlint-mcp"]
}
}
}Same block, in claude_desktop_config.json.
Add to ~/.codex/config.toml:
[mcp_servers.postlint]
command = "npx"
args = ["-y", "@conorbronsdon/postlint-mcp"]No token, no environment variables, no network access. To run a local build instead, set command to node and args to the absolute path of dist/index.js in your clone.
Ask your assistant: "Check this post for X and Bluesky," and paste something with a couple of links in it.
The X post that started this, run through check_post with platform: "x":
{
"platform": "x",
"limit": 280,
"unit": "weighted characters",
"length": 308,
"over": true,
"remaining": -28,
"drivers": [
"3 URLs counted as 23 each = 69",
"239 other characters counted as 1 each"
],
"warnings": []
}The drivers line is the useful part. 69 of the budget went to links before a word was written, which tells you to move two of them into a reply rather than trimming prose.
The same post through check_post_all:
{
"fits": ["x_premium", "linkedin", "threads", "mastodon", "discord"],
"over": ["x", "bluesky"],
"rows": [
{ "platform": "x", "length": 308, "limit": 280, "over": true, "drivers": ["3 URLs counted as 23 each = 69", "239 other characters counted as 1 each"] },
{ "platform": "bluesky", "length": 330, "limit": 300, "over": true, "drivers": ["3 URLs counted in full = 91 (Bluesky does not shorten links)", "239 other graphemes"] },
{ "platform": "mastodon", "length": 308, "limit": 500, "over": false, "remaining": 192 }
]
}One post, three different lengths — 308, 330, and 308 again — from the same 330 characters of text. That gap is the whole reason this exists.
Drafts carry link placeholders, and [URL] is five characters while a real link is not. A post measured with the placeholder in place and posted with the link filled in is a post measured wrong; one draft came in at 264 that way and posted at 282.
So [URL], [LINK], [YOUTUBE URL], [SUBSTACK URL], and similar are priced as a real link (a 28-character YouTube short link, the shortest thing normally posted) and the response carries a warning saying the count is a floor.
It does not post anything. There is no write path, no credential, and no network call of any kind. That last one is enforced rather than asserted: a test replaces fetch, XMLHttpRequest, and WebSocket with throws and drives every tool, so a call added later fails CI instead of quietly making this sentence false.
It does not check an instance's actual limit. Mastodon servers configure their own; this reports the 500 default and tells you to read configuration.statuses.max_characters from the target server yourself.
It does not truncate. A truncate_to helper was considered and left out. Cutting a post at a character offset splits URLs, breaks grapheme clusters, and lands mid-sentence, and cutting it at a "safe" boundary silently drops whichever clause happened to be last. Either way the tool would be deciding what the post says. It reports the number and leaves the edit to you.
It does not detect every URL a platform would. Links with a scheme and www.-prefixed hosts always match. A bare domain matches only on a common TLD (src/count.ts holds the list), where the real twitter-text implementation carries the full IANA registry. Write https:// in front of a link and the count is exact.
It does not count media, polls, quote posts, or link cards. Those have their own rules and this measures text.
It does not know about content warnings. On Mastodon a CW counts toward the same 500. This checks the body alone.
It does not carry limits it cannot source. See the platform table.
npm install
npm run build
npm testTests make no network calls, because the server makes none. The two historical over-limit posts are regression fixtures in src/__tests__/lint.test.ts, alongside grapheme cases for ZWJ family emoji, regional-indicator flags, skin-tone modifiers, combining accents, and CJK.
Issues and pull requests are welcome. A new platform needs three things: the limit, the unit it is measured in, and a published source. A new counting rule needs a test that fails without it. Numbers repeated by third parties are not sources.
Built and maintained by Conor Bronsdon. I host the Chain of Thought podcast, which covers AI infrastructure, developer tools, and how practitioners actually use this stuff. I built this after shipping two over-limit posts in a workflow that was supposed to catch them.
Companion tools:
op3-mcp: podcast analytics through OP3 — downloads, geography, apps, per-episode breakdowns.
podcastindex-mcp: the Podcast Index MCP server, search by person or topic, trending shows, feed health.
substack-mcp: read posts and manage drafts on Substack, safe for agent workflows.
Transistor MCP: Transistor.fm's official MCP server. Episodes, publishing, and analytics.
ai-tools-for-creators: a curated list of AI skills and MCP servers for people who ship ideas for a living.
More at chainofthought.show and on X.
This is an independent personal project, not affiliated with, sponsored by, or endorsed by any company. All views expressed are my own.
Apache-2.0
Check a social post against one platform's real character limit and report whether it fits. Returns the counted length, the limit, and what drove the count (URLs billed at a fixed width, non-Latin characters billed double, emoji sequences collapsed to one grapheme). Use this before publishing anything with a hard limit — the counting rules are per-platform and are not text.length.
Inputs: text (required), platform (required).
Check one post against every platform at once and get a row per platform: counted length, limit, and whether it fits. Use this when deciding where a draft can go as-is. The per-platform arithmetic is included only for the platforms it fails, to keep the response small — call check_post for a single platform's full breakdown.
Inputs: text (required).
List the platforms this server knows, with each one's limit, the unit it is measured in, why that unit is not a plain character count, and the source the number came from. Use it to explain a result or to see what is covered.
Inputs: platform.
Recent tool additions, removals, and schema changes observed during successful MCP inspections.
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Scored across 3 tools
Each tool has a distinct purpose: check_post for a single platform's detailed breakdown, check_post_all for a summary across platforms, and platform_limits for reference data. There is no ambiguity between them, as the scope and output format are clearly differentiated.
The main actions follow a verb_noun pattern: check_post and check_post_all. The third tool, platform_limits, is a noun phrase rather than an action, but it fits the resource-based naming convention and is still intuitive. Minor deviation, but overall the naming is predictable and readable.
Three tools perfectly cover the server's narrow scope of checking social post limits. Each tool is essential and non-redundant, and the count feels neither too thin nor excessive for a linting utility.
The set covers the full workflow: check one platform, check all platforms, and query platform metadata. The only minor gap is the lack of a tool to add or modify platform definitions, but for a linting server this is a reasonable boundary and not a practical dead end.
Check captions and videos against each social platform's limits; specs, image sizes. No account.
Draft, check and schedule posts to your social accounts from Claude, ChatGPT, Cursor or VS Code.
Draft, preview, schedule and publish posts to X, LinkedIn, Bluesky and 5 more social networks.
Check, format and preview LinkedIn posts: limits, see-more cut, hooks, AI-sounding phrases.
