Discussion on "Unicode Bugs That Look Like Business Logic Bugs"

Patrick Kearns · 2026-10-14T07:30:39.604Z

A customer import creates a duplicate that looks identical to an existing record. A display name fails a length check even though the user can count fewer characters than the form allows. A reference

One common source of the decomposed form: file names from macOS. HFS+ stored names decomposed, so when an import key is derived from a file or folder name created on a Mac, it can arrive as "Jose\u0301" while the same name typed into a browser form is usually precomposed. Same NFC step fixes it, but it helps to know where the second variant comes from when you go looking.

On the length section: if the display-name limit is also checked in the browser, JavaScript's string.length counts UTF-16 units too, so client and server can agree with each other and still disagree with the user. Intl.Segmenter with granularity "grapheme" gives the client the same unit as StringInfo.

If you use Grapheme dont forget that counting depends on runtime versions. Modern .NETs StringInfo handles extended grapheme clusters, but versions before .NET 5 behaved differently. Browser and server Unicode versions can also differ, so test shared examples especially emoji sequences and keep server validation authoritative

Good point on the runtime versions. Older StringInfo split ZWJ sequences like the family emoji into several elements, so a shared list of test strings checked on both sides (a ZWJ family, a flag, a skin-tone modifier, a combining accent) catches the drift early. And agreed, the browser count is only a hint for the user; the server decides.

One migration edge case to add: moving existing identifiers to NFC/OrdinalIgnoreCase can collapse rows that were distinct under the old database collation. Before adding a unique index, report collisions under the proposed canonical key, resolve them, then verify the new key during a staged migration. Disclosure: BOMTOBE AI employee; we build Orke for coding agents.

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Unicode Bugs That Look Like Business Logic Bugs

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