FAILURE MAP
← Case archive

FA-76331 / Chat ordering and read receipts / Open access

Compute a member's visible history and initial read marker: initial marker · case 01

The first message after the join is already considered read.

Verified by executionVariant 1 · 6 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The initial marker decision evaluates `'initial_marker': join_seq` where the contract requires `'initial_marker': join_seq - 1`.

VERIFIED REPAIR

Use `'initial_marker': join_seq - 1` for the initial marker decision and keep every other rule of the model unchanged.

Unsuccessful approach: Deriving the marker from the visibility floor marks all shared history as unread. The attempted `'initial_marker': lo - 1` still disagrees with a fixture.

Case contract

messages are seqs (duplicates possible). A member sees seqs >= join_seq, or all seqs from 1 when the room shares history with new members; after leaving at leave_seq they see nothing after leave_seq (their own leave event included). The initial read marker is join_seq - 1, so only messages from the join onward are unread. Result {visible (sorted, unique), initial_marker}.

Why this case matters

Membership changes decide which history a user may read and where their unread count starts.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(messages, join_seq, leave_seq, history_visible):
    lo = 1 if history_visible else join_seq
    vis = [s for s in sorted(set(messages)) if s >= lo and (leave_seq is None or s <= leave_seq)]
    return {'visible': vis, 'initial_marker': join_seq}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('shared history', ([1, 2, 3, 5], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 5]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 9], 7, None, False), {'initial_marker': 6, 'visible': [7, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 4, None, False), {'initial_marker': 3, 'visible': []})], 2: [('shared history', ([1, 2, 3, 6], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 6]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 8], 7, None, False), {'initial_marker': 6, 'visible': [7, 8, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 5, None, False), {'initial_marker': 4, 'visible': []})], 3: [('shared history', ([1, 2, 3, 7], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 7]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 9], 7, None, False), {'initial_marker': 6, 'visible': [7, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 6, None, False), {'initial_marker': 5, 'visible': []})], 4: [('shared history', ([1, 2, 3, 8], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 8]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 8], 7, None, False), {'initial_marker': 6, 'visible': [7, 8, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 7, None, False), {'initial_marker': 6, 'visible': []})], 5: [('shared history', ([1, 2, 3, 9], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 9]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 9], 7, None, False), {'initial_marker': 6, 'visible': [7, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 8, None, False), {'initial_marker': 7, 'visible': []})]}
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 fixtureActualExpectedOutcome
shared history{'initial_marker': 3, 'visible': [1, 2, 3, 5]}{'initial_marker': 2, 'visible': [1, 2, 3, 5]}Failed
join message visible{'initial_marker': 5, 'visible': [5, 6]}{'initial_marker': 4, 'visible': [5, 6]}Failed
leave event visible{'initial_marker': 5, 'visible': [5, 6, 7]}{'initial_marker': 4, 'visible': [5, 6, 7]}Failed
duplicate deliveries{'initial_marker': 7, 'visible': [7, 9]}{'initial_marker': 6, 'visible': [7, 9]}Failed
history visible after leave{'initial_marker': 2, 'visible': [1, 2]}{'initial_marker': 1, 'visible': [1, 2]}Failed
nothing yet{'initial_marker': 4, 'visible': []}{'initial_marker': 3, 'visible': []}Failed

SHA-256 / 7040109dbe88c994ea56fa7753f8db8cbd38c20429fd74f8a52654cb7846b17f

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(messages, join_seq, leave_seq, history_visible):
    lo = 1 if history_visible else join_seq
    vis = [s for s in sorted(set(messages)) if s >= lo and (leave_seq is None or s <= leave_seq)]
    return {'visible': vis, 'initial_marker': lo - 1}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('shared history', ([1, 2, 3, 5], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 5]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 9], 7, None, False), {'initial_marker': 6, 'visible': [7, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 4, None, False), {'initial_marker': 3, 'visible': []})], 2: [('shared history', ([1, 2, 3, 6], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 6]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 8], 7, None, False), {'initial_marker': 6, 'visible': [7, 8, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 5, None, False), {'initial_marker': 4, 'visible': []})], 3: [('shared history', ([1, 2, 3, 7], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 7]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 9], 7, None, False), {'initial_marker': 6, 'visible': [7, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 6, None, False), {'initial_marker': 5, 'visible': []})], 4: [('shared history', ([1, 2, 3, 8], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 8]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 8], 7, None, False), {'initial_marker': 6, 'visible': [7, 8, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 7, None, False), {'initial_marker': 6, 'visible': []})], 5: [('shared history', ([1, 2, 3, 9], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 9]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 9], 7, None, False), {'initial_marker': 6, 'visible': [7, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 8, None, False), {'initial_marker': 7, 'visible': []})]}
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 fixtureActualExpectedOutcome
shared history{'initial_marker': 0, 'visible': [1, 2, 3, 5]}{'initial_marker': 2, 'visible': [1, 2, 3, 5]}Failed
join message visible{'initial_marker': 4, 'visible': [5, 6]}{'initial_marker': 4, 'visible': [5, 6]}Passed
leave event visible{'initial_marker': 4, 'visible': [5, 6, 7]}{'initial_marker': 4, 'visible': [5, 6, 7]}Passed
duplicate deliveries{'initial_marker': 6, 'visible': [7, 9]}{'initial_marker': 6, 'visible': [7, 9]}Passed
history visible after leave{'initial_marker': 0, 'visible': [1, 2]}{'initial_marker': 1, 'visible': [1, 2]}Failed
nothing yet{'initial_marker': 3, 'visible': []}{'initial_marker': 3, 'visible': []}Passed

SHA-256 / bb56fa0346ab19f16df5bbeb06dcab467987ac423a423bf8a2c71d987415f41d

3 / The verified repair

Exit 0
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(messages, join_seq, leave_seq, history_visible):
    lo = 1 if history_visible else join_seq
    vis = [s for s in sorted(set(messages)) if s >= lo and (leave_seq is None or s <= leave_seq)]
    return {'visible': vis, 'initial_marker': join_seq - 1}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('shared history', ([1, 2, 3, 5], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 5]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 9], 7, None, False), {'initial_marker': 6, 'visible': [7, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 4, None, False), {'initial_marker': 3, 'visible': []})], 2: [('shared history', ([1, 2, 3, 6], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 6]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 8], 7, None, False), {'initial_marker': 6, 'visible': [7, 8, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 5, None, False), {'initial_marker': 4, 'visible': []})], 3: [('shared history', ([1, 2, 3, 7], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 7]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 9], 7, None, False), {'initial_marker': 6, 'visible': [7, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 6, None, False), {'initial_marker': 5, 'visible': []})], 4: [('shared history', ([1, 2, 3, 8], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 8]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 8], 7, None, False), {'initial_marker': 6, 'visible': [7, 8, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 7, None, False), {'initial_marker': 6, 'visible': []})], 5: [('shared history', ([1, 2, 3, 9], 3, None, True), {'initial_marker': 2, 'visible': [1, 2, 3, 9]}), ('join message visible', ([4, 5, 6], 5, None, False), {'initial_marker': 4, 'visible': [5, 6]}), ('leave event visible', ([5, 6, 7, 8], 5, 7, False), {'initial_marker': 4, 'visible': [5, 6, 7]}), ('duplicate deliveries', ([9, 7, 7, 9], 7, None, False), {'initial_marker': 6, 'visible': [7, 9]}), ('history visible after leave', ([1, 2, 3], 2, 2, True), {'initial_marker': 1, 'visible': [1, 2]}), ('nothing yet', ([], 8, None, False), {'initial_marker': 7, 'visible': []})]}
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 fixtureActualExpectedOutcome
shared history{'initial_marker': 2, 'visible': [1, 2, 3, 5]}{'initial_marker': 2, 'visible': [1, 2, 3, 5]}Passed
join message visible{'initial_marker': 4, 'visible': [5, 6]}{'initial_marker': 4, 'visible': [5, 6]}Passed
leave event visible{'initial_marker': 4, 'visible': [5, 6, 7]}{'initial_marker': 4, 'visible': [5, 6, 7]}Passed
duplicate deliveries{'initial_marker': 6, 'visible': [7, 9]}{'initial_marker': 6, 'visible': [7, 9]}Passed
history visible after leave{'initial_marker': 1, 'visible': [1, 2]}{'initial_marker': 1, 'visible': [1, 2]}Passed
nothing yet{'initial_marker': 3, 'visible': []}{'initial_marker': 3, 'visible': []}Passed

SHA-256 / 5a9bb6fd562f55e8901de307e491c6008e3b33f4b3a3133441f314027a0bb297

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 2026-09-29T14:49:15.350905+00:00.

Case digest / 33107e9ef68f8c843344f37ff7b7cf4b46c1e00e68b185f1c861cf0da2d34b68