FA-526 / Consensus / Open access

Choose a value after prepare responses: A proposer proceeds with too few distinct promises · case 01

The proposal adoption operation is admitted even though a proposer proceeds with too few distinct promises.

Verified by executionVariant 1 · 9 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The admission path omits the promise quorum invariant while validating the other operation preconditions.

THE FAILURE

The admission path omits the promise quorum invariant while validating the other operation preconditions.

Unsuccessful approach: Adding the promise quorum check repairs the reported defect, but replacing the adjacent highest accepted check loses that independent invariant.

Case contract

Return a Boolean admission decision for choose a value after prepare responses. The record r must satisfy all of: len(set(r['promise_quorum'][0])) >= r['promise_quorum'][1]; r['highest_accepted'][0] == max(r['highest_accepted'][1]); r['accepted_value'][0] == r['accepted_value'][1]; all(x == r['responses_ballot'][0] for x in r['responses_ballot'][1]); len(r['ballot_identity']) == len(set(tuple(x) for x in r['ballot_identity'])). Extra tracing fields are ignored; validation does not mutate the record.

Why this case matters

A deterministic local contract for consensus. Each negative fixture violates exactly one invariant. No transport timing, persistence, cryptographic verification, or full protocol implementation is claimed.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(r):
    return (r['highest_accepted'][0] == max(r['highest_accepted'][1])) and (r['accepted_value'][0] == r['accepted_value'][1]) and (all(x == r['responses_ballot'][0] for x in r['responses_ballot'][1])) and (len(r['ballot_identity']) == len(set(tuple(x) for x in r['ballot_identity'])))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'promise_quorum': [[1, 2], 2], 'highest_accepted': [7, [2, 7, 5]], 'accepted_value': ['old', 'old'], 'responses_ballot': [8, [8, 8]], 'ballot_identity': [[8, 'a'], [8, 'b']]}
check('valid operation', solve(r), True)
check('A proposer proceeds with too few distinct promises', solve(dict(r, **{'promise_quorum': [[1, 1], 2]})), False)
check('A proposer ignores the highest previously accepted ballot', solve(dict(r, **{'highest_accepted': [5, [2, 7, 5]]})), False)
check('A proposer replaces the adopted value with its client value', solve(dict(r, **{'accepted_value': ['new', 'old']})), False)
check('Prepare responses from different ballots are combined', solve(dict(r, **{'responses_ballot': [8, [7, 8]]})), False)
check('Two proposers reuse one ballot identity', solve(dict(r, **{'ballot_identity': [[8, 'a'], [8, 'a']]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'promise_quorum': [[1, 1], 2], 'highest_accepted': [5, [2, 7, 5]], 'accepted_value': ['new', 'old'], 'responses_ballot': [8, [7, 8]], 'ballot_identity': [[8, 'a'], [8, 'a']]}
keys = list(invalid)
pair = {keys[N % len(keys)]: invalid[keys[N % len(keys)]], keys[(N+1) % len(keys)]: invalid[keys[(N+1) % len(keys)]]}
check('two independent violations in variant', solve(dict(r, **pair)), False)
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
valid operation	`True`	`True`	Passed
A proposer proceeds with too few distinct promises	`True`	`False`	Failed
A proposer ignores the highest previously accepted ballot	`False`	`False`	Passed
A proposer replaces the adopted value with its client value	`False`	`False`	Passed
Prepare responses from different ballots are combined	`False`	`False`	Passed
Two proposers reuse one ballot identity	`False`	`False`	Passed
unrelated tracing metadata	`True`	`True`	Passed
repeat validation is pure	`True`	`True`	Passed
two independent violations in variant	`False`	`False`	Passed

SHA-256 / 6085bdbd881336882776485e356ea1ab839edd3151b33322e90b01f7a868f2f7

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(r):
    return (len(set(r['promise_quorum'][0])) >= r['promise_quorum'][1]) and (r['accepted_value'][0] == r['accepted_value'][1]) and (all(x == r['responses_ballot'][0] for x in r['responses_ballot'][1])) and (len(r['ballot_identity']) == len(set(tuple(x) for x in r['ballot_identity'])))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'promise_quorum': [[1, 2], 2], 'highest_accepted': [7, [2, 7, 5]], 'accepted_value': ['old', 'old'], 'responses_ballot': [8, [8, 8]], 'ballot_identity': [[8, 'a'], [8, 'b']]}
check('valid operation', solve(r), True)
check('A proposer proceeds with too few distinct promises', solve(dict(r, **{'promise_quorum': [[1, 1], 2]})), False)
check('A proposer ignores the highest previously accepted ballot', solve(dict(r, **{'highest_accepted': [5, [2, 7, 5]]})), False)
check('A proposer replaces the adopted value with its client value', solve(dict(r, **{'accepted_value': ['new', 'old']})), False)
check('Prepare responses from different ballots are combined', solve(dict(r, **{'responses_ballot': [8, [7, 8]]})), False)
check('Two proposers reuse one ballot identity', solve(dict(r, **{'ballot_identity': [[8, 'a'], [8, 'a']]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'promise_quorum': [[1, 1], 2], 'highest_accepted': [5, [2, 7, 5]], 'accepted_value': ['new', 'old'], 'responses_ballot': [8, [7, 8]], 'ballot_identity': [[8, 'a'], [8, 'a']]}
keys = list(invalid)
pair = {keys[N % len(keys)]: invalid[keys[N % len(keys)]], keys[(N+1) % len(keys)]: invalid[keys[(N+1) % len(keys)]]}
check('two independent violations in variant', solve(dict(r, **pair)), False)
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
valid operation	`True`	`True`	Passed
A proposer proceeds with too few distinct promises	`False`	`False`	Passed
A proposer ignores the highest previously accepted ballot	`True`	`False`	Failed
A proposer replaces the adopted value with its client value	`False`	`False`	Passed
Prepare responses from different ballots are combined	`False`	`False`	Passed
Two proposers reuse one ballot identity	`False`	`False`	Passed
unrelated tracing metadata	`True`	`True`	Passed
repeat validation is pure	`True`	`True`	Passed
two independent violations in variant	`False`	`False`	Passed

SHA-256 / d37c62e25fe976956fd4cc540d4e71137b666832b7c4b857df764cc4b8ece289

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 9 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

Verification & scope

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 2026-09-29T14:36:53.808351+00:00.

Case digest / a934917604e226d934bc3042cf719715969484b7d9d98b05378ce9a0de968532