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FA-26746 / HTTP ranges / Open access

A continuation response must start at the saved prefix boundary · case 01

A continuation response must start at the saved prefix boundary.

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

ROOT CAUSE

The resume-content-range-start decision uses return ["append",body].

VERIFIED REPAIR

Apply the bounded decision exactly: return ["append",body] if returned_start==saved else ["restart",""]

Unsuccessful approach: The partial repair uses return ["append",body] if returned_start>=saved else ["restart",""], which still violates the stated contract.

Case contract

Return [append,body] only when returned_start equals saved length; a mismatching start requests restart with empty output.

Why this case matters

Range responses combine representation identity, conditional requests, framing, and partial-object state.

1 / The failure

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

N = 1
observations = []
def solve(saved, returned_start, body):
    return ["append",body]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('resume-content-range-start fixture 0', solve(N,N+1,"p"*N), ["restart",""])
check('resume-content-range-start fixture 1', solve(N+1,N,"p"), ["restart",""])
check('resume-content-range-start fixture 2', solve(N,N,"p"*N), ["append","p"*N])
check('resume-content-range-start fixture 3', solve(0,0,""), ["append",""])
check('resume-content-range-start fixture 4', solve(0,N,"p"), ["restart",""])
check('resume-content-range-start fixture 5', solve(N,0,"p"), ["restart",""])
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
resume-content-range-start fixture 0['append', 'p']['restart', '']Failed
resume-content-range-start fixture 1['append', 'p']['restart', '']Failed
resume-content-range-start fixture 2['append', 'p']['append', 'p']Passed
resume-content-range-start fixture 3['append', '']['append', '']Passed
resume-content-range-start fixture 4['append', 'p']['restart', '']Failed
resume-content-range-start fixture 5['append', 'p']['restart', '']Failed

SHA-256 / a7c4e6bb2e6b40baaa617dc2643a0b2acd811ca479fff9656d42ab01483c4d37

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(saved, returned_start, body):
    return ["append",body] if returned_start>=saved else ["restart",""]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('resume-content-range-start fixture 0', solve(N,N+1,"p"*N), ["restart",""])
check('resume-content-range-start fixture 1', solve(N+1,N,"p"), ["restart",""])
check('resume-content-range-start fixture 2', solve(N,N,"p"*N), ["append","p"*N])
check('resume-content-range-start fixture 3', solve(0,0,""), ["append",""])
check('resume-content-range-start fixture 4', solve(0,N,"p"), ["restart",""])
check('resume-content-range-start fixture 5', solve(N,0,"p"), ["restart",""])
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
resume-content-range-start fixture 0['append', 'p']['restart', '']Failed
resume-content-range-start fixture 1['restart', '']['restart', '']Passed
resume-content-range-start fixture 2['append', 'p']['append', 'p']Passed
resume-content-range-start fixture 3['append', '']['append', '']Passed
resume-content-range-start fixture 4['append', 'p']['restart', '']Failed
resume-content-range-start fixture 5['restart', '']['restart', '']Passed

SHA-256 / 9e5947fd890eec211758b073cd62b8c42d8c49ae1729522b8f5dd35b2a9694e6

3 / The verified repair

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

N = 1
observations = []
def solve(saved, returned_start, body):
    return ["append",body] if returned_start==saved else ["restart",""]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('resume-content-range-start fixture 0', solve(N,N+1,"p"*N), ["restart",""])
check('resume-content-range-start fixture 1', solve(N+1,N,"p"), ["restart",""])
check('resume-content-range-start fixture 2', solve(N,N,"p"*N), ["append","p"*N])
check('resume-content-range-start fixture 3', solve(0,0,""), ["append",""])
check('resume-content-range-start fixture 4', solve(0,N,"p"), ["restart",""])
check('resume-content-range-start fixture 5', solve(N,0,"p"), ["restart",""])
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
resume-content-range-start fixture 0['restart', '']['restart', '']Passed
resume-content-range-start fixture 1['restart', '']['restart', '']Passed
resume-content-range-start fixture 2['append', 'p']['append', 'p']Passed
resume-content-range-start fixture 3['append', '']['append', '']Passed
resume-content-range-start fixture 4['restart', '']['restart', '']Passed
resume-content-range-start fixture 5['restart', '']['restart', '']Passed

SHA-256 / 253e4743268fcdca37c38accb71ff319325bbff12285ff4e4dbbd03bfb4193fa

Verification & scope

Deterministic simplified range service, with stipulated local policies and already parsed trusted inputs; not a complete HTTP 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 2026-09-29T14:41:18.766152+00:00.

Case digest / c0cb08921290f689c9e38c1977889e366baf0362220ca59841e6e8b278af4b25