Before publishing the volfi finding we ran six independent checks designed to catch the most common cause of false "library X has a bug" claims: a misconfigured adapter. All six agreed. Source for each check is in bench/python/ and tests/.
We pulled the top 10 worst volfi rows from the oracle output and called volfi.iv_call(F, K, disc, T, c) directly in a standalone script, with no adapter layer of any kind. All 10 rows reproduced ULP-identical to the oracle run. Maximum disagreement across the 10 rows was 0.0 floats.
For each put-side row we computed σ two ways. Path A: our oracle's parity transform c = p + S - K*disc, then volfi.iv_call(c). Path B: volfi's own bs_call from their published bench_vollib.py evaluated at sigma_true to produce a call price, then volfi.iv_call on that price. Path A and Path B agreed to better than 1e-13 absolute σ on every row tested, and both produced the same catastrophic answer. The error does not depend on which side of parity the row originated on.
We re-ran the worst rows through volfi.iv_otm instead of volfi.iv_call. The error reproduced on both paths. On Row 10 (true σ ≈ 0.786), iv_otm returned 0.4903 and iv_call returned 0.4961; both are off by roughly 0.29-0.30 absolute. The failure is not confined to a single volfi entry point.
To remove our BS-pricer from the loop entirely, we used volfi's own bs_call formula to construct the call price c at sigma_true, then fed that price back into volfi.iv_call. The returned σ was the same wrong value. The failure mode is independent of which Black-Scholes formula produced the input price.
The call shape used in our benchmark, volfi.iv_call(F, K, disc, T, c), is the same pattern volfi's own published benchmark bench_vollib.py uses. The prices we feed volfi agree with what volfi's own bs_call produces to within 3.55e-15 absolute, which is the f64 ULP at the relevant magnitudes.
Volfi's documented OTM-native API is volfi.otm_context(h) returning a context, then ctx.iv(c, t). The v1.0.1 verification used volfi.iv_call; v1.0.2 also exercised volfi.iv_otm. Both produced catastrophic σ on the deep-wing failure regime. For v1.1.0 the 20 worst rows were retested via the precomputed-context API: volfi.otm_context(h) followed by ctx.iv(c, t). Result: identical σ to iv_call within 2.3e-12 (machine precision) on all 20 rows.
Reading volfi's Python binding source (bindings/python/src/volfi_py.cpp lines 95-110) confirms iv_call, ctx.iv, iv_otm, and iv_call_norm are all wrappers over the same volfi::implied_volatility_otm core. The precomputed-context API caches moneyness-dependent quantities; it does not invoke a different numerical algorithm. Volfi's own documented test reproduces ctx.iv([0.05], [1.25]) = 0.43990879.... The defect lives in the core OTM solver, not in any wrapper or parity-adapter convention.
The failure mode is intrinsic to volfi at this regime; 913 of 99,996 rows show err > 1e-3, with floor-ratio (volfi_err / f64_BS_inversion_floor) median 1.34e+11, max 3.04e+15. Reproduced independently on the CLY-3D 2021 standard grid (5,488 catastrophic / 51,321 rows) and the v×Δ wing-saturated stress grid.