# Postlint MCP Server [Health: Active] **Category:** 🌐 Social Media **Repository:** https://github.com/conorbronsdon/postlint-mcp **GitHub Stars:** 0 **Views:** 0 **Installs:** 0 **Upvotes:** 0 **Directory Page:** https://allmcps.com/mcp/postlint-mcp-server ## Description Check a social post against a platform's real character limit: X, Bluesky, LinkedIn, Threads. ## Claude Desktop Quick Installation Install path detected from listing signals. Uses `npx` (confidence: high): ```json "mcpServers": { "postlint-mcp-server": { "command": "npx", "args": ["-y","@conorbronsdon/postlint-mcp"] } } ``` ## Documentation & README
# postlint-mcp 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. [![npm version](https://img.shields.io/npm/v/@conorbronsdon/postlint-mcp?style=flat-square)](https://www.npmjs.com/package/@conorbronsdon/postlint-mcp) [![License: Apache 2.0](https://img.shields.io/badge/License-Apache_2.0-blue.svg?style=flat-square)](LICENSE) [![Node](https://img.shields.io/badge/Node-20.19+-339933?style=flat-square&logo=node.js&logoColor=white)](https://nodejs.org/) [![Glama score](https://glama.ai/mcp/servers/conorbronsdon/postlint-mcp/badges/score.svg)](https://glama.ai/mcp/servers/conorbronsdon/postlint-mcp) [![Podcast](https://img.shields.io/badge/Podcast-Chain_of_Thought-purple?style=flat-square)](https://chainofthought.show/?utm_source=github&utm_medium=referral&utm_campaign=repo-readme&utm_content=postlint-mcp) [![X](https://img.shields.io/badge/X-@ConorBronsdon-black?style=flat-square&logo=x)](https://x.com/ConorBronsdon) Three drafts checked in a terminal: an X post at 308 of 280 with three URLs billed at 23 each, a Bluesky post at 302 of 300, and that same draft with its link still a [URL] placeholder, which a length check reads as 277 but the server prices at exactly 300 of 300 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. ## Tools | Tool | What it returns | |------|-----------------| | `check_post` | Verdict for one platform: counted length, the limit, headroom, and what drove the count | | `check_post_all` | One row per platform, with the breakdown attached only to the rows that fail | | `platform_limits` | 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. ## How each platform counts | Platform | Limit | Unit | The part that surprises people | Source | |----------|-------|------|-------------------------------|--------| | `x` | 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. | [twitter-text v3 config](https://github.com/twitter/twitter-text/blob/master/config/v3.json) | | `x_premium` | 25,000 | weighted characters | Same weighting, higher ceiling. | [X help center](https://help.x.com/en/using-x/types-of-posts) | | `bluesky` | 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. | [atproto lexicon](https://github.com/bluesky-social/atproto/blob/main/lexicons/app/bsky/feed/post.json) | | `linkedin` | 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. | [LinkedIn help](https://www.linkedin.com/help/linkedin/answer/a528176) | | `threads` | 500 | characters | The September 2025 change added a 10,000-character *attachment*. The post body is still 500. | [Meta newsroom](https://about.fb.com/news/2025/09/attach-text-threads-posts-share-longer-perspectives/) | | `mastodon` | 500 | graphemes | URLs cost 23, as on X. On `@user@example.social` only `@user` counts. The limit is per-instance and plenty of servers run higher. | [Mastodon API docs](https://docs.joinmastodon.org/methods/instance/) | | `discord` | 2,000 | characters | 4,000 with Nitro. Embeds have a separate 6,000 total. | [Discord support](https://support.discord.com/hc/en-us/articles/360034632292-Sending-Messages) | 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. ## Setup Published on npm. The config blocks below use `npx`, which fetches it on first run; no clone required. ```bash git clone https://github.com/conorbronsdon/postlint-mcp.git cd postlint-mcp npm install npm run build ``` ### Claude Code Add to your `.mcp.json`: ```json { "mcpServers": { "postlint": { "command": "node", "args": ["/absolute/path/to/postlint-mcp/dist/index.js"] } } } ``` ### Claude Desktop Same block, in `claude_desktop_config.json`. ### Codex Add to `~/.codex/config.toml`: ```toml [mcp_servers.postlint] command = "npx" args = ["-y", "@conorbronsdon/postlint-mcp"] ``` No token, no environment variables, no network access. Once the package is published, `npx -y @conorbronsdon/postlint-mcp` replaces the `node` invocation everywhere above. ### Verify Ask your assistant: "Check this post for X and Bluesky," and paste something with a couple of links in it. ## A worked example The X post that started this, run through `check_post` with `platform: "x"`: ```json { "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`: ```json { "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. ## Draft placeholders 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. ## What it does not do - **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. ## Development ```bash npm install npm run build npm test ``` Tests 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. ## Contributing 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. ## About Built and maintained by [Conor Bronsdon](https://github.com/conorbronsdon). I host the [Chain of Thought](https://chainofthought.show/?utm_source=github&utm_medium=referral&utm_campaign=repo-readme&utm_content=postlint-mcp) 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](https://github.com/conorbronsdon/op3-mcp): podcast analytics through OP3 — downloads, geography, apps, per-episode breakdowns. - [podcastindex-mcp](https://github.com/conorbronsdon/podcastindex-mcp): the Podcast Index MCP server, search by person or topic, trending shows, feed health. - [substack-mcp](https://github.com/conorbronsdon/substack-mcp): read posts and manage drafts on Substack, safe for agent workflows. - [Transistor-MCP](https://github.com/conorbronsdon/Transistor-MCP): the Transistor.fm MCP server. Episodes, transcripts, download counts. - [ai-tools-for-creators](https://github.com/conorbronsdon/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](https://chainofthought.show/?utm_source=github&utm_medium=referral&utm_campaign=repo-readme&utm_content=postlint-mcp) and on [X](https://x.com/ConorBronsdon). --- ## Disclaimer *This is an independent personal project, not affiliated with, sponsored by, or endorsed by any company. All views expressed are my own.* ## License Apache-2.0