FA-100696 / Speculative decoding bookkeeping / Open access

Decoding: Absolute position omits the replacement token after rejection · case 01

Absolute position omits the replacement token after rejection.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The replacement token is omitted from absolute-position advancement.

THE FAILURE

The replacement token is omitted from absolute-position advancement.

Unsuccessful approach: Adding a constant two advances correctly only when exactly one draft token was accepted.

Case contract

Advance absolute position by accepted draft tokens plus one replacement token after rejection.

Why this case matters

A self-contained bookkeeping model makes prefix and token-count semantics observable without a model runtime.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(position, accepted):
    return position+accepted
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('partial advance', solve(10,2),13)
check('zero accepted', solve(4,0),5)
check('single accepted', solve(0,1),2)
check('negative base position', solve(-2,2),1)
check('vary position', solve(N,1),N+2)
check('large accepted', solve(3,4),8)
check('no prefix', solve(9,0),10)
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
partial advance	`12`	`13`	Failed
zero accepted	`4`	`5`	Failed
single accepted	`1`	`2`	Failed
negative base position	`0`	`1`	Failed
vary position	`2`	`3`	Failed
large accepted	`7`	`8`	Failed
no prefix	`9`	`10`	Failed

SHA-256 / 3aecfb9648b780d6ecda39d09ce988d59f2bf104a89d6a3ee2f58e34665301f4

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(position, accepted):
    return position+2
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('partial advance', solve(10,2),13)
check('zero accepted', solve(4,0),5)
check('single accepted', solve(0,1),2)
check('negative base position', solve(-2,2),1)
check('vary position', solve(N,1),N+2)
check('large accepted', solve(3,4),8)
check('no prefix', solve(9,0),10)
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
partial advance	`12`	`13`	Failed
zero accepted	`6`	`5`	Failed
single accepted	`2`	`2`	Passed
negative base position	`0`	`1`	Failed
vary position	`3`	`3`	Passed
large accepted	`5`	`8`	Failed
no prefix	`11`	`10`	Failed

SHA-256 / f44cd12e6ccca251a3ce41052935c3d3cc1b588927c093cdebed10dc415a5be7

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 7 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 are finite toy sequences with explicit contracts; they make no claim about production decoder behavior or model quality. 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 2026-09-30T16:48:38.598986+00:00.

Case digest / 08314f6855dc9b58196603345fa36b620fe88f4e0b5402dfba498e95392fa465