FA-96576 / Phone number formatting / Open access

As-you-type ten-digit formatter: a typed national prefix becomes part of the area code · case 01

"1555" renders as "(155) 5".

Verified by executionVariant 1 · 11 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The leading 1 is not separated before formatting.

VERIFIED REPAIR

Split off a leading 1 as the prefix before formatting the rest.

Unsuccessful approach: Splitting it off only for complete eleven-digit input misformats partial input.

Case contract

Input the digits typed so far. A leading 1 becomes the prefix "1 ". With more than ten remaining digits the raw input is returned. Up to 3 digits are shown bare; 4..6 as "(AAA) E.."; 7..10 as "(AAA) EEE-S...". The result is stripped of surrounding spaces.

Why this case matters

Input masks re-render on every keystroke; premature punctuation or a lost prefix makes the caret jump and corrupts what users type.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    d = x
    prefix = ''
    if len(d) > 10:
        return x
    if len(d) <= 3:
        body = d
    elif len(d) <= 6:
        body = '(' + d[:3] + ') ' + d[3:]
    else:
        body = '(' + d[:3] + ') ' + d[3:6] + '-' + d[6:]
    return (prefix + body).strip()
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['first digit', '5', '5'], ['three digits stay bare', '555', '555'], ['fourth digit opens the area', '5550', '(555) 0'], ['six digits without hyphen', '555011', '(555) 011'], ['seventh digit adds the hyphen', '5550113', '(555) 011-3'], ['complete number', '5550113456', '(555) 011-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1555', '1 555'], ['national prefix with full number', '15550113456', '1 (555) 011-3456'], ['too many digits after the prefix', '155501134561', '155501134561'], ['too many digits without prefix', '55501134561', '55501134561']], [['first digit', '6', '6'], ['three digits stay bare', '655', '655'], ['fourth digit opens the area', '6550', '(655) 0'], ['six digits without hyphen', '655012', '(655) 012'], ['seventh digit adds the hyphen', '6550123', '(655) 012-3'], ['complete number', '6550123456', '(655) 012-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1655', '1 655'], ['national prefix with full number', '16550123456', '1 (655) 012-3456'], ['too many digits after the prefix', '165501234562', '165501234562'], ['too many digits without prefix', '65501234562', '65501234562']], [['first digit', '7', '7'], ['three digits stay bare', '755', '755'], ['fourth digit opens the area', '7550', '(755) 0'], ['six digits without hyphen', '755013', '(755) 013'], ['seventh digit adds the hyphen', '7550133', '(755) 013-3'], ['complete number', '7550133456', '(755) 013-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1755', '1 755'], ['national prefix with full number', '17550133456', '1 (755) 013-3456'], ['too many digits after the prefix', '175501334563', '175501334563'], ['too many digits without prefix', '75501334563', '75501334563']], [['first digit', '8', '8'], ['three digits stay bare', '855', '855'], ['fourth digit opens the area', '8550', '(855) 0'], ['six digits without hyphen', '855014', '(855) 014'], ['seventh digit adds the hyphen', '8550143', '(855) 014-3'], ['complete number', '8550143456', '(855) 014-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1855', '1 855'], ['national prefix with full number', '18550143456', '1 (855) 014-3456'], ['too many digits after the prefix', '185501434564', '185501434564'], ['too many digits without prefix', '85501434564', '85501434564']], [['first digit', '9', '9'], ['three digits stay bare', '955', '955'], ['fourth digit opens the area', '9550', '(955) 0'], ['six digits without hyphen', '955015', '(955) 015'], ['seventh digit adds the hyphen', '9550153', '(955) 015-3'], ['complete number', '9550153456', '(955) 015-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1955', '1 955'], ['national prefix with full number', '19550153456', '1 (955) 015-3456'], ['too many digits after the prefix', '195501534565', '195501534565'], ['too many digits without prefix', '95501534565', '95501534565']]]
for label, arg, expected in fixtures[N - 1]:
    check(label, solve(arg), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
first digit	`5`	`5`	Passed
three digits stay bare	`555`	`555`	Passed
fourth digit opens the area	`(555) 0`	`(555) 0`	Passed
six digits without hyphen	`(555) 011`	`(555) 011`	Passed
seventh digit adds the hyphen	`(555) 011-3`	`(555) 011-3`	Passed
complete number	`(555) 011-3456`	`(555) 011-3456`	Passed
lone national prefix	`1`	`1`	Passed
national prefix with three digits	`(155) 5`	`1 555`	Failed
national prefix with full number	`15550113456`	`1 (555) 011-3456`	Failed
too many digits after the prefix	`155501134561`	`155501134561`	Passed
too many digits without prefix	`55501134561`	`55501134561`	Passed

SHA-256 / 6f9804145e6931f9838adc7552cc318e27ac3993175e2b5b844fa93bd88c3da6

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    d = x
    prefix = ''
    if d.startswith('1') and len(d) == 11:
        prefix, d = '1 ', d[1:]
    if len(d) > 10:
        return x
    if len(d) <= 3:
        body = d
    elif len(d) <= 6:
        body = '(' + d[:3] + ') ' + d[3:]
    else:
        body = '(' + d[:3] + ') ' + d[3:6] + '-' + d[6:]
    return (prefix + body).strip()
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['first digit', '5', '5'], ['three digits stay bare', '555', '555'], ['fourth digit opens the area', '5550', '(555) 0'], ['six digits without hyphen', '555011', '(555) 011'], ['seventh digit adds the hyphen', '5550113', '(555) 011-3'], ['complete number', '5550113456', '(555) 011-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1555', '1 555'], ['national prefix with full number', '15550113456', '1 (555) 011-3456'], ['too many digits after the prefix', '155501134561', '155501134561'], ['too many digits without prefix', '55501134561', '55501134561']], [['first digit', '6', '6'], ['three digits stay bare', '655', '655'], ['fourth digit opens the area', '6550', '(655) 0'], ['six digits without hyphen', '655012', '(655) 012'], ['seventh digit adds the hyphen', '6550123', '(655) 012-3'], ['complete number', '6550123456', '(655) 012-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1655', '1 655'], ['national prefix with full number', '16550123456', '1 (655) 012-3456'], ['too many digits after the prefix', '165501234562', '165501234562'], ['too many digits without prefix', '65501234562', '65501234562']], [['first digit', '7', '7'], ['three digits stay bare', '755', '755'], ['fourth digit opens the area', '7550', '(755) 0'], ['six digits without hyphen', '755013', '(755) 013'], ['seventh digit adds the hyphen', '7550133', '(755) 013-3'], ['complete number', '7550133456', '(755) 013-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1755', '1 755'], ['national prefix with full number', '17550133456', '1 (755) 013-3456'], ['too many digits after the prefix', '175501334563', '175501334563'], ['too many digits without prefix', '75501334563', '75501334563']], [['first digit', '8', '8'], ['three digits stay bare', '855', '855'], ['fourth digit opens the area', '8550', '(855) 0'], ['six digits without hyphen', '855014', '(855) 014'], ['seventh digit adds the hyphen', '8550143', '(855) 014-3'], ['complete number', '8550143456', '(855) 014-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1855', '1 855'], ['national prefix with full number', '18550143456', '1 (855) 014-3456'], ['too many digits after the prefix', '185501434564', '185501434564'], ['too many digits without prefix', '85501434564', '85501434564']], [['first digit', '9', '9'], ['three digits stay bare', '955', '955'], ['fourth digit opens the area', '9550', '(955) 0'], ['six digits without hyphen', '955015', '(955) 015'], ['seventh digit adds the hyphen', '9550153', '(955) 015-3'], ['complete number', '9550153456', '(955) 015-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1955', '1 955'], ['national prefix with full number', '19550153456', '1 (955) 015-3456'], ['too many digits after the prefix', '195501534565', '195501534565'], ['too many digits without prefix', '95501534565', '95501534565']]]
for label, arg, expected in fixtures[N - 1]:
    check(label, solve(arg), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
first digit	`5`	`5`	Passed
three digits stay bare	`555`	`555`	Passed
fourth digit opens the area	`(555) 0`	`(555) 0`	Passed
six digits without hyphen	`(555) 011`	`(555) 011`	Passed
seventh digit adds the hyphen	`(555) 011-3`	`(555) 011-3`	Passed
complete number	`(555) 011-3456`	`(555) 011-3456`	Passed
lone national prefix	`1`	`1`	Passed
national prefix with three digits	`(155) 5`	`1 555`	Failed
national prefix with full number	`1 (555) 011-3456`	`1 (555) 011-3456`	Passed
too many digits after the prefix	`155501134561`	`155501134561`	Passed
too many digits without prefix	`55501134561`	`55501134561`	Passed

SHA-256 / 8152d5c3bebd1e08e552540e3ddfa14a974966c8a415b588de47ea94a9b29136

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    d = x
    prefix = ''
    if d.startswith('1'):
        prefix, d = '1 ', d[1:]
    if len(d) > 10:
        return x
    if len(d) <= 3:
        body = d
    elif len(d) <= 6:
        body = '(' + d[:3] + ') ' + d[3:]
    else:
        body = '(' + d[:3] + ') ' + d[3:6] + '-' + d[6:]
    return (prefix + body).strip()
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['first digit', '5', '5'], ['three digits stay bare', '555', '555'], ['fourth digit opens the area', '5550', '(555) 0'], ['six digits without hyphen', '555011', '(555) 011'], ['seventh digit adds the hyphen', '5550113', '(555) 011-3'], ['complete number', '5550113456', '(555) 011-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1555', '1 555'], ['national prefix with full number', '15550113456', '1 (555) 011-3456'], ['too many digits after the prefix', '155501134561', '155501134561'], ['too many digits without prefix', '55501134561', '55501134561']], [['first digit', '6', '6'], ['three digits stay bare', '655', '655'], ['fourth digit opens the area', '6550', '(655) 0'], ['six digits without hyphen', '655012', '(655) 012'], ['seventh digit adds the hyphen', '6550123', '(655) 012-3'], ['complete number', '6550123456', '(655) 012-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1655', '1 655'], ['national prefix with full number', '16550123456', '1 (655) 012-3456'], ['too many digits after the prefix', '165501234562', '165501234562'], ['too many digits without prefix', '65501234562', '65501234562']], [['first digit', '7', '7'], ['three digits stay bare', '755', '755'], ['fourth digit opens the area', '7550', '(755) 0'], ['six digits without hyphen', '755013', '(755) 013'], ['seventh digit adds the hyphen', '7550133', '(755) 013-3'], ['complete number', '7550133456', '(755) 013-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1755', '1 755'], ['national prefix with full number', '17550133456', '1 (755) 013-3456'], ['too many digits after the prefix', '175501334563', '175501334563'], ['too many digits without prefix', '75501334563', '75501334563']], [['first digit', '8', '8'], ['three digits stay bare', '855', '855'], ['fourth digit opens the area', '8550', '(855) 0'], ['six digits without hyphen', '855014', '(855) 014'], ['seventh digit adds the hyphen', '8550143', '(855) 014-3'], ['complete number', '8550143456', '(855) 014-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1855', '1 855'], ['national prefix with full number', '18550143456', '1 (855) 014-3456'], ['too many digits after the prefix', '185501434564', '185501434564'], ['too many digits without prefix', '85501434564', '85501434564']], [['first digit', '9', '9'], ['three digits stay bare', '955', '955'], ['fourth digit opens the area', '9550', '(955) 0'], ['six digits without hyphen', '955015', '(955) 015'], ['seventh digit adds the hyphen', '9550153', '(955) 015-3'], ['complete number', '9550153456', '(955) 015-3456'], ['lone national prefix', '1', '1'], ['national prefix with three digits', '1955', '1 955'], ['national prefix with full number', '19550153456', '1 (955) 015-3456'], ['too many digits after the prefix', '195501534565', '195501534565'], ['too many digits without prefix', '95501534565', '95501534565']]]
for label, arg, expected in fixtures[N - 1]:
    check(label, solve(arg), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
first digit	`5`	`5`	Passed
three digits stay bare	`555`	`555`	Passed
fourth digit opens the area	`(555) 0`	`(555) 0`	Passed
six digits without hyphen	`(555) 011`	`(555) 011`	Passed
seventh digit adds the hyphen	`(555) 011-3`	`(555) 011-3`	Passed
complete number	`(555) 011-3456`	`(555) 011-3456`	Passed
lone national prefix	`1`	`1`	Passed
national prefix with three digits	`1 555`	`1 555`	Passed
national prefix with full number	`1 (555) 011-3456`	`1 (555) 011-3456`	Passed
too many digits after the prefix	`155501134561`	`155501134561`	Passed
too many digits without prefix	`55501134561`	`55501134561`	Passed

SHA-256 / ac6622f773a2e9947f491bbfc7a69c3c2dc6bb870ce0daad0980f363b70853e0

Verification & scope

A deterministic toy contract over synthetic, obviously fictitious telephone numbers and toy country tables. The rules are stipulated for this model only and make no claim of conformance with any numbering plan or telecommunications standard. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:52:23.952457+00:00.

Case digest / 3ab992f226a074fee89528b6a6c2061f6c12b49e5cd3d1a781f0762a463edcc7