Tellmark is a prose inspector built in Bristol. Paste a draft and it marks the tells: patterns that turn up far more often in machine-written text than in anything a person writes unaided. Each mark carries a name, a count and a published rule. The score at the top, the Tell Density Index, is nothing more mysterious than weighted instances per thousand words.
Tellmark is not an AI detector. Detectors claim to know who wrote a text, and their false accusations have damaged students and professionals who wrote honestly. We refuse the entire category. A Tellmark report states what was found and where, then leaves the judgement with the reader. The standing line appears on every report and it means what it says: Tellmark reports observable patterns and makes no claim about who or what wrote this text.
The rules live in a public, versioned catalogue. Anyone can read what gets counted, argue with a weighting, or watch an entry change between versions. Determinism is the other half of that promise. The same text against the same catalogue always produces the same score, so a report from March can be checked in September.
Who uses it? Editors reviewing submissions. Content teams whose copy has started to sound like everyone else's. Agencies that need client pages to read as credibly human, because readers have learned the patterns too and trust drains away when they spot them.
Everything runs in your browser. The assay never uploads your text; saving a report to an account is a separate, deliberate act, encrypted at rest.
One note on the name. A tell is the poker word for an involuntary giveaway, and a mark is what a proofreader leaves in a margin. Put together they describe the product exactly. We are aware of, and unrelated to, Telemark: the Norwegian county and the skiing turn. Different vowel, different trade.
The catalogue grows as the models change. When it does, the version number moves, the changelog says why, and every article on this site is re-assayed against the new rules. The tool disciplines its own publisher first. That is the whole idea.