Subject Line + Spam Signal Checker

Scores an email subject line and preheader for display and deliverability risk.

THE SILENT FAILURE. Subject lines fail invisibly. They are truncated at different lengths by every client — mobile Gmail cuts far shorter than desktop — so the punchline lands outside the visible window. Spam-trigger words do not bounce the mail, they lower placement. All-caps, repeated punctuation and confusable unicode raise risk with no feedback loop at all. The sender sees a successful delivery and a poor open rate, and cannot tell which cause it was.

Every point deducted NAMES its reason. An unexplained score cannot be acted on and is therefore worthless.

WHAT IT CANNOT KNOW, STATED PLAINLY: deliverability depends on sender reputation, domain authentication and recipient engagement, none of which a string checker can see. This does not predict inbox placement and the README says so rather than implying otherwise — overclaiming is the failure mode of every competing tool.

Client widths are cited to their sources, and where sources disagree the disagreement is recorded rather than silently resolved.

VERIFIED: 217 tests, 10/10 mutations caught. Every mutation was observed FAILING before the source was restored — a test never seen to fail is a decoration.

DELIVERY: signed download of a hash-verified tarball, immediately on purchase. Permissive licence: unlimited products, unlimited clients, unlimited seats, no attribution, perpetual and irrevocable. One restriction, do not republish the source as source.

Interface

What you call, and what comes back. Types and signatures only — the implementation ships with the source.

  export function scoreSubjectLine( subject: string, preheader?: string, options: ScoreOptions = {}, ): SubjectLineReport;
  export function detectHighValueTokens(subject: string): HighValueToken[];
  export function checkTruncation( subject: string, clients: readonly ClientWidth[] = CLIENT_WIDTHS, ): TruncationResult[];
  export function detectInvisibleChars(text: string): InvisibleCharHit[];
  export function detectMixedScript(text: string): MixedScriptHit[];

  export type TokenType = "currency" | "percent" | "number" | "date";