fb-wash-0021

status: verifiedversion: v1corpus: 2026.9key: computedseverity: high

Scenario (all facts stipulated)

{
  "facts": "On October 6, 2026, a taxable account sells 100 shares of QRS common stock for $33.75 per share; the basis is $40.50 per share. On October 27, 2026 it buys 40 shares of QRS for $35.20 per share. There are no other QRS acquisitions within 30 days before or after the sale.",
  "shares_sold": 100,
  "sale_proceeds_per_share": 33.75,
  "basis_per_share": 40.5,
  "replacement_shares_in_window": 40,
  "replacement_cost_per_share": 35.2
}

Question posed to the model under test

What is the per-share basis of the 40 replacement shares after the wash-sale basis adjustment of section 1091(d)? Respond with the numeric amount only.

Answer key

41.95 USD (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

The 40 replacement shares wash 40 sold shares; each carries its $35.20 cost plus the $6.75-per-share disallowed loss, a basis of $41.95 per share.

Cited primary sources (quote-gated)

26 U.S.C. § 1091 · 1091(a)

“within a period beginning 30 days before the date of such sale or disposition and ending 30 days after such date, the taxpayer has acquired (by purchase or by an exchange on which the entire amount of gain or loss was recognized by law), or has entered into a contract or option so to acquire, substantially identical stock or securities, then no deduction shall be allowed”

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-e (4/4 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

The final per-share basis is 41.95 USD. Initial logic and parameters are validated. Applying the statutory adjustment directly: 40.50 + (35.20 - 33.75) = 41.95.

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 replacement shares were acquired within 30 days after the sale, so the loss on 40 of the 100 shares is nondeductible under §1091. Under §1091(d), the replacement basis per share is the basis of the sold shares ($40.50) adjusted by the difference between replacement cost ($35.20) and sale proceeds ($33.75). That difference is +$1.45, so basis = $40.50 + $1.45 = $41.95.

Version history & errata

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