diff --git a/src/main/codex/codex-session-backfill-scan-dates.test.ts b/src/main/codex/codex-session-backfill-scan-dates.test.ts
index f9bf4530733..8e7001c636c 100644
--- a/src/main/codex/codex-session-backfill-scan-dates.test.ts
+++ b/src/main/codex/codex-session-backfill-scan-dates.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it } from 'vitest'
+import { describe, expect, it, vi } from 'vitest'
 import {
   compareCodexSessionBackfillDates,
   expandCodexSessionBackfillDatesThroughToday,
@@ -9,6 +9,7 @@ import {
   parseCodexSessionBackfillDates,
   subtractCodexSessionBackfillDates
 } from './codex-session-backfill-scan-dates'
+import type { CodexSessionBackfillDate } from './codex-session-backfill-types'
 
 describe('codex session backfill scan dates', () => {
   it('reads UTC parts so a local evening never lands on the wrong directory', () => {
@@ -100,3 +101,63 @@ describe('codex session backfill scan dates', () => {
     ).toBeNull()
   })
 })
+
+describe('bounded backfill range construction', () => {
+  it('does not allocate rejected dates for a decades-old pending marker', () => {
+    const advance = vi.spyOn(Date.prototype, 'setUTCDate')
+    try {
+      expect(
+        expandCodexSessionBackfillDatesThroughToday(
+          [['2000', '01', '01']],
+          ['2026', '09', '07'],
+          31
+        )
+      ).toBeNull()
+      expect(advance).not.toHaveBeenCalled()
+    } finally {
+      advance.mockRestore()
+    }
+  })
+
+  it('keeps exact, fractional, leap-day and future-clock bounds', () => {
+    const dates = [['2024', '02', '28']] as [string, string, string][]
+    expect(expandCodexSessionBackfillDatesThroughToday(dates, ['2024', '03', '01'], 3)).toEqual([
+      ['2024', '02', '28'],
+      ['2024', '02', '29'],
+      ['2024', '03', '01']
+    ])
+    expect(expandCodexSessionBackfillDatesThroughToday(dates, ['2024', '03', '01'], 2.5)).toBeNull()
+    expect(
+      expandCodexSessionBackfillDatesThroughToday([['2024', '03', '01']], ['2024', '02', '28'], 3)
+    ).toEqual(expandCodexSessionBackfillDatesThroughToday(dates, ['2024', '03', '01'], 3))
+  })
+
+  // The arithmetic cardinality gate must admit and reject exactly what enumerating the range
+  // would, on every calendar edge that has ever broken a day count: leap days, century rules,
+  // year rollover, and the DST switches the UTC-only arithmetic has to stay indifferent to.
+  it.each<[string, CodexSessionBackfillDate, CodexSessionBackfillDate]>([
+    ['leap February', ['2024', '02', '27'], ['2024', '03', '02']],
+    ['non-leap February', ['2023', '02', '27'], ['2023', '03', '02']],
+    ['US spring-forward', ['2024', '03', '09'], ['2024', '03', '11']],
+    ['US fall-back', ['2024', '11', '02'], ['2024', '11', '04']],
+    ['EU spring-forward', ['2025', '03', '29'], ['2025', '03', '31']],
+    ['southern-hemisphere DST', ['2025', '04', '05'], ['2025', '04', '07']],
+    ['year rollover', ['2024', '12', '30'], ['2025', '01', '02']],
+    ['leap century', ['1999', '12', '31'], ['2000', '01', '02']],
+    ['non-leap century', ['2100', '02', '27'], ['2100', '03', '02']],
+    ['30-day month end', ['2026', '04', '29'], ['2026', '05', '02']],
+    ['single day', ['2026', '09', '07'], ['2026', '09', '07']]
+  ])('matches the enumerated range at the %s cap boundary', (_label, from, to) => {
+    const start = new Date(Date.UTC(Number(from[0]), Number(from[1]) - 1, Number(from[2])))
+    const end = new Date(Date.UTC(Number(to[0]), Number(to[1]) - 1, Number(to[2])))
+    const enumerated = getCodexSessionBackfillDatesBetween(start, end)
+    const pending = [from]
+
+    expect(expandCodexSessionBackfillDatesThroughToday(pending, to, enumerated.length)).toEqual(
+      enumerated
+    )
+    expect(
+      expandCodexSessionBackfillDatesThroughToday(pending, to, enumerated.length - 1)
+    ).toBeNull()
+  })
+})
diff --git a/src/main/codex/codex-session-backfill-scan-dates.ts b/src/main/codex/codex-session-backfill-scan-dates.ts
index ca4e1d311f7..ac9a07d3e79 100644
--- a/src/main/codex/codex-session-backfill-scan-dates.ts
+++ b/src/main/codex/codex-session-backfill-scan-dates.ts
@@ -99,8 +99,13 @@ export function expandCodexSessionBackfillDatesThroughToday(
     return []
   }
   const bounds = mergeCodexSessionBackfillDates(dates, [today])
-  const range = getCodexSessionBackfillDatesBetween(toUtcDate(bounds[0]), toUtcDate(bounds.at(-1)!))
-  return range.length > maxDates ? null : range
+  const first = toUtcDate(bounds[0])
+  const last = toUtcDate(bounds.at(-1)!)
+  const dateCount = (last.getTime() - first.getTime()) / 86_400_000 + 1
+  if (dateCount > maxDates) {
+    return null
+  }
+  return getCodexSessionBackfillDatesBetween(first, last)
 }
 
 function toUtcDate([year, month, day]: readonly string[]): Date {