FAILURE MAP
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FA-98591 / KV cache position mapping / Open access

Sink tokens survive rolling-window eviction · case 01

Eviction removes the initial sink positions along with stale middle positions.

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

ROOT CAUSE

The retained region is modeled as one contiguous suffix.

THE FAILURE

The retained region is modeled as one contiguous suffix.

Unsuccessful approach: Adding the sink prefix but leaving the recent suffix unbounded also admits positions beyond the current position.

Case contract

Given ordered (position,value) entries, retain positions below sink_count or in the inclusive window [current-window+1,current].

Why this case matters

A small in-memory model makes the cache address contract explicit for a serving component.

1 / The failure

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

N = 1
observations = []
def solve(entries, sink_count, current_position, window):
    return [(p,v) for p,v in entries if p>=current_position-window+1]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
entries=[(p,p) for p in range(N+6)]
check('sink and recent',solve(entries,2,N+4,2),[(0,0),(1,1),(N+3,N+3),(N+4,N+4)])
check('middle evicted',solve(entries,1,N+4,1),[(0,0),(N+4,N+4)])
check('recent starts at zero',solve(entries,2,1,3),[(0,0),(1,1)])
check('future clipped',solve([(0,'s'),(4,'f')],1,2,3),[(0,'s')])
check('no sink',solve(entries,0,N+2,1),[(N+2,N+2)])
check('empty',solve([],2,N,2),[])
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
sink and recent[[4, 4], [5, 5], [6, 6]][[0, 0], [1, 1], [4, 4], [5, 5]]Failed
middle evicted[[5, 5], [6, 6]][[0, 0], [5, 5]]Failed
recent starts at zero[[0, 0], [1, 1], [2, 2], [3, 3], [4, 4], [5, 5], [6, 6]][[0, 0], [1, 1]]Failed
future clipped[[0, 's'], [4, 'f']][[0, 's']]Failed
no sink[[3, 3], [4, 4], [5, 5], [6, 6]][[3, 3]]Failed
empty[][]Passed

SHA-256 / cc694b45c2af1dad542f3c849870c45f598b7094228e66c92964c4987049fe6c

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(entries, sink_count, current_position, window):
    return [(p,v) for p,v in entries if p<sink_count or p>=current_position-window+1]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
entries=[(p,p) for p in range(N+6)]
check('sink and recent',solve(entries,2,N+4,2),[(0,0),(1,1),(N+3,N+3),(N+4,N+4)])
check('middle evicted',solve(entries,1,N+4,1),[(0,0),(N+4,N+4)])
check('recent starts at zero',solve(entries,2,1,3),[(0,0),(1,1)])
check('future clipped',solve([(0,'s'),(4,'f')],1,2,3),[(0,'s')])
check('no sink',solve(entries,0,N+2,1),[(N+2,N+2)])
check('empty',solve([],2,N,2),[])
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
sink and recent[[0, 0], [1, 1], [4, 4], [5, 5], [6, 6]][[0, 0], [1, 1], [4, 4], [5, 5]]Failed
middle evicted[[0, 0], [5, 5], [6, 6]][[0, 0], [5, 5]]Failed
recent starts at zero[[0, 0], [1, 1], [2, 2], [3, 3], [4, 4], [5, 5], [6, 6]][[0, 0], [1, 1]]Failed
future clipped[[0, 's'], [4, 'f']][[0, 's']]Failed
no sink[[3, 3], [4, 4], [5, 5], [6, 6]][[3, 3]]Failed
empty[][]Passed

SHA-256 / 18bef65e4597df97c758ae8ef1ac35337828bccc29d2ce0314d0f68181376785

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 6 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.

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Verification & scope

These toy integer maps omit tensor layout, concurrency, device memory, and model-specific cache formats. 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 2026-09-30T16:48:14.544863+00:00.

Case digest / a81a2898a46c1e5320c7576eee51c93f2250e4799773e62fee38481281456d1c