fb-wash-0015

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

Scenario (all facts stipulated)

{
  "shares_sold": 300,
  "sale_proceeds_per_share": 9.85,
  "basis_per_share": 12.6,
  "replacement_shares_in_window": 180,
  "facts": "The account sells 300 shares of EFG at a loss and repurchases 180 shares of EFG eleven days later. No other EFG acquisitions occur within the 61-day window. The taxpayer is not a dealer."
}

Question posed to the model under test

How many dollars of the realized loss are disallowed under section 1091? Respond with the numeric amount only.

Answer key

495 USD (tolerance ±0.01)

Produced by compute washsale.disallowed-loss 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

Only the 180 matched shares' loss is disallowed.

Cited primary sources (quote-gated)

26 U.S.C. § 1091 · 1091(b)

“If the amount of stock or securities acquired (or covered by the contract or option to acquire) is less than the amount of stock or securities sold or otherwise disposed of, then the particular shares of stock or securities the loss from the sale or other disposition of which is not deductible shall be determined under regulations prescribed by the Secretary”

26 CFR § 1.1091-1 · (c)

“The stock or securities acquired will be matched in accordance with the order of their acquisition (beginning with the earliest acquisition) with an equal number of the shares of stock or securities 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-d (3/3 caught)adjudicated by: — (none required or pending)

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

“The stock or securities acquired will be matched in accordance with the order of their acquisition (beginning with the earliest acquisition) with an equal number of the shares of stock or securities sold or otherwise disposed of.” — cfr-1-1091-1

The realized loss per share is $12.60 (basis) - $9.85 (proceeds) = $2.75. Because 180 replacement shares were purchased within the 61-day window, the wash sale rule disallows the loss on exactly 180 of the 300 shares sold. The disallowed loss is therefore 180 shares * $2.75 per share = $495. The key's value and derivation are correct.

deepseek-v4-proagrees the key is correctquote matched

“The stock or securities acquired will be matched in accordance with the order of their acquisition (beginning with the earliest acquisition) with an equal number of the shares of stock or securities sold or otherwise disposed of.” — cfr-1-1091-1

The realized loss per share is $12.60 - $9.85 = $2.75. Because 180 shares were reacquired within the 61-day window and that is less than the 300 shares sold, under Treas. Reg. §1.1091-1(c) the 180 acquired shares are matched with an equal number of the sold shares, so the loss on 180 shares is disallowed: 180 × $2.75 = $495.

Version history & errata

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