Validate credit card numbers, compute Luhn check digits, and complete partial numbers via @mukundakatta/luhn-mcp

@mukundakatta/luhn-mcp v0.1.0 — Luhn algorithm verification, check digit computation, and number completion

Install: npm install @mukundakatta/luhn-mcp @modelcontextprotocol/sdk Entry: dist/server.js (stdio) 3 tools: verify, checksum, complete

Tool schemas

• verify({input: string}) → {valid: boolean} — runs the Luhn check on a number string. Dashes and spaces are stripped.
• checksum({input: string}) → {check_digit: number} — computes the Luhn check digit that would make the input valid.
• complete({input: string}) → {complete: string} — appends the computed check digit to the input.

Key observations

• Dashes and spaces ignored: 4539-5787-6362-1486 and 4539 5787 6362 1486 both validate correctly
• Works for credit cards, IMEI, and any Luhn-checksummed ID: Visa 4539578763621486 ✓, Mastercard 5500000000000004 ✓, IMEI 490154203237518 ✓
• Perfect round-trip: complete("453957876362148") → "4539578763621486", then verify("4539578763621486") → true
• Single-digit "0" is invalid (Luhn spec: length must be ≥2)
• All-zeros "0000000000" is valid (Luhn check passes)
• Graceful errors: empty string and non-numeric input return tool error "input must be digits (spaces and dashes allowed)" — not crash
• Sub-millisecond after JIT: p50 = 1ms, first call 2ms

Gotchas

• Param is `input` NOT `number` — using number will fail
• `verify("0")` is FALSE — single digit 0 is not Luhn-valid (need ≥2 digits)
• `verify("18")` is TRUE, `verify("19")` is FALSE — demonstrates the checksum mechanism on minimal inputs
• No card type detection — the tool only validates the Luhn checksum, not the BIN/IIN prefix
• `checksum` returns a NUMBER (0-9), not a string — {check_digit: 6}
• `complete` returns a STRING — {complete: "4539578763621486"}

Verified traces

verify({input: "4539578763621486"}) => {valid: true}       // Visa test number
verify({input: "5500000000000004"}) => {valid: true}       // Mastercard test
verify({input: "4539578763621487"}) => {valid: false}      // one digit off
verify({input: "4539-5787-6362-1486"}) => {valid: true}    // dashes stripped
verify({input: "4539 5787 6362 1486"}) => {valid: true}    // spaces stripped
verify({input: "0"}) => {valid: false}                     // single digit
verify({input: "18"}) => {valid: true}                     // minimal valid
verify({input: "490154203237518"}) => {valid: true}        // IMEI
verify({input: "0000000000"}) => {valid: true}             // all zeros
checksum({input: "453957876362148"}) => {check_digit: 6}
checksum({input: "7992739871"}) => {check_digit: 3}
checksum({input: "1"}) => {check_digit: 8}
complete({input: "453957876362148"}) => {complete: "4539578763621486"}
complete({input: "550000000000000"}) => {complete: "5500000000000004"}
complete({input: "123456789012345"}) => {complete: "1234567890123452"}
verify({input: "1234567890123452"}) => {valid: true}       // round-trip ✓
verify({input: ""}) => ERROR: "input must be digits"       // graceful
verify({input: "abc"}) => ERROR: "input must be digits"    // graceful