Thread splitter

What it does, in order

1. Finds sentence boundaries. A boundary is a full stop, question mark, exclamation mark or ellipsis followed by whitespace — unless the word before it is an abbreviation (Dr., e.g., etc.) or a single initial. A single newline is also a boundary, so bullet lists and line-broken drafts are not glued into run-on posts, and a blank line is preserved as a paragraph break.
2. Packs sentences greedily into posts. A sentence is added to the current post if the whole post still fits; otherwise it starts the next one.
3. Measures with X's weighted count. A link is budgeted at 23, a CJK character at 2, an emoji sequence at 2 — the same code the character counter uses, so the two agree by construction rather than by coincidence.
4. Reserves room for the marker. With 1/7-style numbering the prefix is part of the post, so it comes out of the 280.

The 1/7 problem, and why it is not a rounding error

You cannot know the total until you have finished splitting, and you cannot finish splitting until you know how much room the marker takes. Nine posts numbered 1/9 need four characters; ten need six for 10/10. Solve it naively and the tenth post pushes every marker a character wider, which can push a post that fitted exactly over the limit — silently, because by then the numbers have been assigned. This tool budgets every post against the widest marker the current total could produce and then iterates until the total stops changing, so no post can be over the limit no matter how the count lands.

When a clean split is impossible

Sometimes one sentence is longer than a whole post. There is no correct answer, so the tool picks the least-bad one and tells you: it splits that sentence at word boundaries, never inside a word, and marks the affected posts mid-sentence with a warning above the results. That is a prompt to rewrite the sentence, which is almost always better than the break.

The one case where a word itself is broken is a single token longer than an entire post — a 200-character identifier, or a very long word in a language that does not use spaces. That gets its own mid-word mark, because it is a different and worse problem.

Caveats

• Sentence detection is heuristic. The abbreviation list is short and deliberately so; a missed abbreviation costs you an early break, not a broken post. No abbreviation list is complete in any language.
• The per-post limit is configurable, but 280 is X's. Raise it only if you know your account has a longer limit, and remember that X validates the real post.
• Posting is still manual. This tool has no access to your account, so it copies text for you to paste. That is a consequence of it running entirely in your browser, and the same reason it can promise nothing is uploaded.
• Nothing is rewritten. Your words come out in the order you wrote them, with whitespace normalized between posts and nothing else changed.