fb-wash-0004

status: publishedversion: v1corpus: 2026.9key: computedseverity: high

Scenario (all facts stipulated)

{
  "shares_sold": 100,
  "sale_proceeds_per_share": 40,
  "basis_per_share": 55,
  "replacement_shares_in_window": 60,
  "replacement_cost_per_share": 42,
  "facts": "The account sells 100 shares of GHI at a loss and buys 60 replacement shares of GHI at $42 per share ten days later. No other GHI acquisitions occur within the 61-day window."
}

Question posed to the model under test

What is the per-share basis of the 60 replacement shares after the wash-sale basis adjustment of section 1091(d)? Respond with the numeric amount only.

Answer key

57 USD per share (tolerance ±0.01)

Produced by compute washsale.replacement-basis-per-share in src/lib/fiduciarybench/keys/index.ts — the stored value is machine-checked against the compute on every build; it cannot drift from its own code.

Rubric

Replacement basis per share = basis of the matched sold shares adjusted by the price difference (equivalently, replacement cost plus the disallowed loss per matched share).

Cited primary sources (quote-gated)

26 U.S.C. § 1091 · 1091(d)

“then the basis shall be the basis of the stock or securities so sold or disposed of, increased or decreased, as the case may be, by the difference, if any, between the price at which the property was acquired and the price at which such substantially identical stock or securities were sold or otherwise disposed of”

Each quoted passage is exact-matched, after typographic normalization, against the corpus text fetched from the issuing authority (hashes in the corpus manifest). An item whose quote stops matching fails the build.

Verification record

generator: claude-fable (session 2026-09-01)disagreements: 0controls: fb-2026-09-a (5/5 caught)adjudicated by: — (none required or pending)

gemini-3.1-pro-previewagrees the key is correctquote matched

“then the basis shall be the basis of the stock or securities so sold or disposed of, increased or decreased, as the case may be, by the difference, if any, between the price at which the property was acquired and the price at which such substantially identical stock or securities were sold or otherwise disposed of” — usc-1091

Under section 1091(d), the basis of the replacement shares is the basis of the stock sold ($55) increased by the difference between the price at which the replacement property was acquired ($42) and the price at which the stock was sold ($40). The difference is $42 - $40 = $2. Therefore, the new basis is $55 + $2 = $57 per share. The key correctly computes this value.

deepseek-v4-proagrees the key is correctquote matched

“then the basis shall be the basis of the stock or securities so sold or disposed of, increased or decreased, as the case may be, by the difference, if any, between the price at which the property was acquired and the price at which such substantially identical stock or securities were sold or otherwise disposed of” — usc-1091

The statute supports the per-share computation: basis of sold shares is $55 per share, replacement acquisition price is $42 per share, sale price of the original shares is $40 per share. The difference is $42 - $40 = $2, so the replacement basis per share is $55 + $2 = $57. The fact that only 60 replacement shares were acquired does not change the per-share adjustment calculation.

Version history & errata

v1; no errata. Demonstrate a key error and the correction is published here, credited — challenge policy.