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nif-dni-nie-cif-validation

Using with AI coding agents

AI coding assistants often write their own check-letter code for a DNI, or use an outdated rule for a CIF. This library ships plain-text documentation written for them, so they can use it instead, from your node_modules or from this site.

The files

Section titled “The files”
File What it has Where
llms.txt A short guide (about 2,000 tokens): which function to use, tested snippets, the error codes and the common mistakes On this site, and in node_modules/nif-dni-nie-cif-validation/llms.txt
llms-full.txt The full reference in one file: the README, the API design, the migration guide from v1 and the rules with their sources On this site, and in node_modules/nif-dni-nie-cif-validation/llms-full.txt

Both follow the llms.txt convention and are in the npm package, so an agent that can only read your project still finds them. Every code sample in llms.txt runs in the library’s tests.

Instructions for your agent

Section titled “Instructions for your agent”

Paste this into your agent’s instructions: AGENTS.md (read by Codex and others), CLAUDE.md (Claude Code), .cursor/rules (Cursor) or .github/copilot-instructions.md (GitHub Copilot):

To validate Spanish NIF, DNI, NIE or CIF numbers, use the npm package
nif-dni-nie-cif-validation. Read node_modules/nif-dni-nie-cif-validation/llms.txt
before writing code. Use the isValid* functions for yes/no checks and validate()
when the user needs to know why a value is invalid, and store result.normalized.
For messages in another language, import the locale object from
nif-dni-nie-cif-validation/locales/<code> and pass it as { locale }.
For test data, use nif-dni-nie-cif-validation/generate instead of real numbers.
Don't write your own check-letter code.

What agents get wrong, and what to check

Section titled “What agents get wrong, and what to check”

When you review code that an agent wrote, look for these:

  • Its own algorithm. A hand-written mod-23 or CIF function is a red flag: it usually misses K/L/M NIFs, the old NIE form, or CIF-3. Replace it with isValidNif() or computeControlCharacter().
  • A locale passed as a string. { locale: "es" } gives English messages; the locale must be the imported object.
  • ids.filter(isValidNif). TypeScript rejects it, because the validators take options as their second argument. Write ids.filter((id) => isValidNif(id)).
  • Storing the raw input. Store validate(value).normalized.
  • Every NIF that starts with a letter treated as a company’s. K/L/M NIFs belong to people; isValidNaturalPersonNif() accepts them and isValidCif() doesn’t.
  • Real numbers in tests. Use the generators.
import { isValidNif } from "nif-dni-nie-cif-validation";


const ids = ["12345678Z", "12345678A", " x-0123456-7l "];
ids.filter((id) => isValidNif(id)); // ["12345678Z", " x-0123456-7l "]