FA-75916 / Chat ordering and read receipts / Open access

Summarize group delivery and read receipts for one message: late joiner exclusion · case 01

A member who joined after a message was sent can never read it, so the message stays "delivered" forever.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The late joiner exclusion decision evaluates `u != sender` where the contract requires `u != sender and joined_at.get(u, 0) <= sent_seq`.

VERIFIED REPAIR

Use `u != sender and joined_at.get(u, 0) <= sent_seq` for the late joiner exclusion decision and keep every other rule of the model unchanged.

Unsuccessful approach: Using a strict join comparison drops members who joined exactly at the message and could see it. The attempted `u != sender and joined_at.get(u, 0) < sent_seq` still disagrees with a fixture.

Case contract

Recipients are members other than the sender whose joined_at (first visible seq, default 0) is <= sent_seq. A recipient has read the message when read[u] (their read marker) >= sent_seq; reading implies delivery. Status is "sent" with no recipients, "read" when every recipient read, "delivered" when every recipient got it, else "sent". Result {status, read_by (sorted), recipients (count)}.

Why this case matters

Group tick marks summarise many receipts; wrong membership or thresholds show false "read" states.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(sender, members, joined_at, sent_seq, delivered, read):
    recipients = [u for u in members if u != sender]
    has_read = [u for u in recipients if read.get(u, -1) >= sent_seq]
    got = [u for u in recipients if delivered.get(u) or u in has_read]
    if not recipients:
        status = 'sent'
    elif len(has_read) == len(recipients):
        status = 'read'
    elif len(got) == len(recipients):
        status = 'delivered'
    else:
        status = 'sent'
    return {'status': status, 'read_by': sorted(has_read), 'recipients': len(recipients)}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 21}, 11, {'bo': True}, {'bo': 16}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 10}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 5}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 1, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 5, {'bo': True}, {'cy': 5}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 2: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 22}, 12, {'bo': True}, {'bo': 17}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 11}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 6}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 2, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 6, {'bo': True}, {'cy': 6}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 3: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 23}, 13, {'bo': True}, {'bo': 18}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 7}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 3, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 7, {'bo': True}, {'cy': 7}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 4: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 24}, 14, {'bo': True}, {'bo': 19}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 13}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 8}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 4, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 8, {'bo': True}, {'cy': 8}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 5: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 25}, 15, {'bo': True}, {'bo': 20}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 14}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 9}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 5, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 9, {'bo': True}, {'cy': 9}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})]}
for _label, _args, _expected in _CASES[N]:
    check(_label, solve(*_args), _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
late joiner does not block read	`{'read_by': ['bo'], 'recipients': 2, 'status': 'sent'}`	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	Failed
member joined exactly at the message	`{'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}`	`{'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}`	Passed
read marker beyond message	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	Passed
read without delivery ack	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	Passed
everyone read	`{'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}`	`{'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}`	Passed
sender alone	`{'read_by': [], 'recipients': 0, 'status': 'sent'}`	`{'read_by': [], 'recipients': 0, 'status': 'sent'}`	Passed
read exactly without ack	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	Passed
partial delivery	`{'read_by': [], 'recipients': 3, 'status': 'sent'}`	`{'read_by': [], 'recipients': 3, 'status': 'sent'}`	Passed

SHA-256 / 02809b5fffca31de35c6ca9a012a18cae7e1c460b93a11ae41937b0efd3a5103

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(sender, members, joined_at, sent_seq, delivered, read):
    recipients = [u for u in members if u != sender and joined_at.get(u, 0) < sent_seq]
    has_read = [u for u in recipients if read.get(u, -1) >= sent_seq]
    got = [u for u in recipients if delivered.get(u) or u in has_read]
    if not recipients:
        status = 'sent'
    elif len(has_read) == len(recipients):
        status = 'read'
    elif len(got) == len(recipients):
        status = 'delivered'
    else:
        status = 'sent'
    return {'status': status, 'read_by': sorted(has_read), 'recipients': len(recipients)}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 21}, 11, {'bo': True}, {'bo': 16}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 10}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 5}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 1, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 5, {'bo': True}, {'cy': 5}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 2: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 22}, 12, {'bo': True}, {'bo': 17}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 11}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 6}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 2, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 6, {'bo': True}, {'cy': 6}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 3: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 23}, 13, {'bo': True}, {'bo': 18}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 7}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 3, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 7, {'bo': True}, {'cy': 7}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 4: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 24}, 14, {'bo': True}, {'bo': 19}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 13}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 8}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 4, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 8, {'bo': True}, {'cy': 8}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 5: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 25}, 15, {'bo': True}, {'bo': 20}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 14}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 9}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 5, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 9, {'bo': True}, {'cy': 9}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})]}
for _label, _args, _expected in _CASES[N]:
    check(_label, solve(*_args), _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
late joiner does not block read	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	Passed
member joined exactly at the message	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	`{'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}`	Failed
read marker beyond message	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	Passed
read without delivery ack	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	Passed
everyone read	`{'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}`	`{'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}`	Passed
sender alone	`{'read_by': [], 'recipients': 0, 'status': 'sent'}`	`{'read_by': [], 'recipients': 0, 'status': 'sent'}`	Passed
read exactly without ack	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	Passed
partial delivery	`{'read_by': [], 'recipients': 3, 'status': 'sent'}`	`{'read_by': [], 'recipients': 3, 'status': 'sent'}`	Passed

SHA-256 / 0d697dfae724083b97cfb0440eda5e26859fe924bdc461e9e62f9732737b7824

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(sender, members, joined_at, sent_seq, delivered, read):
    recipients = [u for u in members if u != sender and joined_at.get(u, 0) <= sent_seq]
    has_read = [u for u in recipients if read.get(u, -1) >= sent_seq]
    got = [u for u in recipients if delivered.get(u) or u in has_read]
    if not recipients:
        status = 'sent'
    elif len(has_read) == len(recipients):
        status = 'read'
    elif len(got) == len(recipients):
        status = 'delivered'
    else:
        status = 'sent'
    return {'status': status, 'read_by': sorted(has_read), 'recipients': len(recipients)}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 21}, 11, {'bo': True}, {'bo': 16}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 10}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 5}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 1, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 5, {'bo': True}, {'cy': 5}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 2: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 22}, 12, {'bo': True}, {'bo': 17}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 11}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 6}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 2, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 6, {'bo': True}, {'cy': 6}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 3: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 23}, 13, {'bo': True}, {'bo': 18}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 7}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 3, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 7, {'bo': True}, {'cy': 7}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 4: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 24}, 14, {'bo': True}, {'bo': 19}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 13}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 8}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 4, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 8, {'bo': True}, {'cy': 8}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})], 5: [('late joiner does not block read', ('al', ['al', 'bo', 'cy'], {'cy': 25}, 15, {'bo': True}, {'bo': 20}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('member joined exactly at the message', ('al', ['al', 'bo', 'cy'], {'cy': 12}, 12, {'bo': True, 'cy': True}, {'bo': 12}), {'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}), ('read marker beyond message', ('al', ['al', 'bo'], {}, 7, {}, {'bo': 14}), {'read_by': ['bo'], 'recipients': 1, 'status': 'read'}), ('read without delivery ack', ('al', ['al', 'bo', 'cy'], {}, 4, {'bo': True}, {'cy': 9}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('everyone read', ('al', ['al', 'bo', 'cy'], {}, 3, {'bo': True, 'cy': True}, {'bo': 3, 'cy': 5}), {'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}), ('sender alone', ('al', ['al'], {}, 5, {}, {}), {'read_by': [], 'recipients': 0, 'status': 'sent'}), ('read exactly without ack', ('al', ['al', 'bo', 'cy'], {}, 9, {'bo': True}, {'cy': 9}), {'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}), ('partial delivery', ('al', ['al', 'bo', 'cy', 'di'], {}, 2, {'bo': True, 'cy': True}, {'bo': 1}), {'read_by': [], 'recipients': 3, 'status': 'sent'})]}
for _label, _args, _expected in _CASES[N]:
    check(_label, solve(*_args), _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
late joiner does not block read	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	Passed
member joined exactly at the message	`{'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}`	`{'read_by': ['bo'], 'recipients': 2, 'status': 'delivered'}`	Passed
read marker beyond message	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	`{'read_by': ['bo'], 'recipients': 1, 'status': 'read'}`	Passed
read without delivery ack	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	Passed
everyone read	`{'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}`	`{'read_by': ['bo', 'cy'], 'recipients': 2, 'status': 'read'}`	Passed
sender alone	`{'read_by': [], 'recipients': 0, 'status': 'sent'}`	`{'read_by': [], 'recipients': 0, 'status': 'sent'}`	Passed
read exactly without ack	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	`{'read_by': ['cy'], 'recipients': 2, 'status': 'delivered'}`	Passed
partial delivery	`{'read_by': [], 'recipients': 3, 'status': 'sent'}`	`{'read_by': [], 'recipients': 3, 'status': 'sent'}`	Passed

SHA-256 / aeaa22a0b07474d2a6583ab95130f52f4b30e34fd4251e0ecfc3c01b301740eb

Verification & scope

Stipulated offline chat model; not a complete messaging protocol, client or server implementation. 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 2026-09-29T14:49:11.412408+00:00.

Case digest / ff0eea2b288160741d61e973ea67f2a957748f2d90a125419735726782e2365d