diff --git a/CHANGELOG.md b/CHANGELOG.md
index 590a1a0..a3decac 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -76,6 +76,19 @@ who reopens it with "see the full results" comes back to.
 the end of a timetable. Same shape as the three attempts before it: the thing that was verified was
 not the thing the player uses.
 
+### `configWith` let the day count and the Revenue floor disagree
+
+`minCombinedRevenue` fell back to `SOLO_CONFIG`'s constant — the floor for a *five*-Day game —
+whatever `days` said. So `configWith({ days: 1 })` asked a one-Day game to clear **15**, a figure a
+full five-Day game averages barely half of, and `configWith({ days: 10 })` asked for the same 15 a
+five-Day game does. It derives from the days it was actually given now.
+
+Not a live fault: `createLocalSession` is the only caller, and the page always writes the floor
+itself, so no dealt game was ever wrong. It was found by a throwaway probe written to reproduce the
+extension bug above, which passed only `days` — which is exactly how the next caller would reach for
+it. At the default day count the answer is unchanged, since `SOLO_CONFIG`'s own floor is this same
+formula at `DEFAULT_DAYS`; the three new tests pin that as well as the derivation.
+
 ### Three wording and layout fixes
 
 - **The collision entries** on all three screens now read "The game ends immediately and results in
@@ -90,7 +103,7 @@ not the thing the player uses.
   cost, not a danger, and red would outrank the actual rules above it. The buttons say **Continue
   Existing Saved Game** and **Deal New Game** rather than "Continue saved game" and "Deal".
 
-870 tests pass, three of them new; one existing test asserted the opposite of the collision ruling
+873 tests pass, six of them new; one existing test asserted the opposite of the collision ruling
 above and says so where it was reversed.
 
 ---
diff --git a/src/web/game.ts b/src/web/game.ts
index 66505b5..c7c656d 100644
--- a/src/web/game.ts
+++ b/src/web/game.ts
@@ -134,10 +134,25 @@ export type NewGameOptions = {
 
 /** The same config with the New Game dialog's answers in it. */
 export function configWith(opts: NewGameOptions): GameConfig {
+  const days = opts.days ?? SOLO_CONFIG.days;
   return {
     ...SOLO_CONFIG,
-    days: opts.days ?? SOLO_CONFIG.days,
-    minCombinedRevenue: opts.minCombinedRevenue ?? SOLO_CONFIG.minCombinedRevenue,
+    days,
+    /**
+     * DERIVED FROM THE DAYS ACTUALLY IN PLAY, not from `SOLO_CONFIG`'s five-Day constant.
+     *
+     * It fell back to the constant until 2026-08-30, so `configWith({ days: 1 })` asked a one-Day
+     * game to clear **15** — a floor a five-Day game averages barely half of — and
+     * `configWith({ days: 10 })` asked for the same 15 a five-Day game does. The two fields silently
+     * disagreed, which is the one thing a "build me a config" helper must not let happen.
+     *
+     * Not a live fault when it was found: `createLocalSession` is the only caller, and the page
+     * always writes `minCombinedRevenue` itself (`solitaireDefaults` re-derives it from the preset).
+     * Found by a throwaway probe that passed only `days` — which is exactly how the next caller
+     * would use this. At the default day count the answer is unchanged, since `SOLO_CONFIG`'s own
+     * floor is this same formula at `DEFAULT_DAYS`.
+     */
+    minCombinedRevenue: opts.minCombinedRevenue ?? collectiveRevenueFloor(1, days),
     maxCollisionsPerDay: opts.maxCollisionsPerDay ?? SOLO_CONFIG.maxCollisionsPerDay,
     maxCollisionsTotal: opts.maxCollisionsTotal ?? SOLO_CONFIG.maxCollisionsTotal,
     optionalRules: { ...SOLO_CONFIG.optionalRules, ...(opts.optionalRules ?? {}) },
diff --git a/test/web.test.ts b/test/web.test.ts
index 32177a6..4b4c294 100644
--- a/test/web.test.ts
+++ b/test/web.test.ts
@@ -32,6 +32,8 @@ import { createGame as createEngineGame } from '../src/engine/setup.ts';
 import { advance as advanceEngine } from '../src/engine/advance.ts';
 import type { GameConfig } from '../src/engine/state.ts';
 import {
+  configWith,
+  SOLO_CONFIG,
   overHandLimit,
   actionGroups,
   actionMenu,
@@ -3339,6 +3341,34 @@ describe('the Day rolling over says so (Gitea#10)', () => {
 
 // ---------------------------------------------------------------------------
 
+describe('configWith', () => {
+  /**
+   * The Revenue floor is a function of how long the game is, so a helper that takes `days` and
+   * defaults the floor has to derive it. It fell back to `SOLO_CONFIG`'s five-Day constant until
+   * 2026-08-30, which let the two fields disagree: a one-Day game was asked to clear 15, a floor a
+   * five-Day game averages barely half of. Found by a probe that passed only `days` — which is how
+   * the next caller would reach for it.
+   */
+  it('derives the Revenue floor from the days it was given', () => {
+    assert.equal(configWith({ days: 1 }).minCombinedRevenue, 3, 'a one-Day game kept the five-Day floor');
+    assert.equal(configWith({ days: 10 }).minCombinedRevenue, 30, 'a ten-Day game kept the five-Day floor');
+  });
+
+  it('leaves the default day count exactly where it was', () => {
+    // The fix must not move the floor anybody is actually playing against: SOLO_CONFIG's own floor
+    // is this same formula at DEFAULT_DAYS, so the default path is unchanged.
+    assert.equal(configWith({}).minCombinedRevenue, SOLO_CONFIG.minCombinedRevenue);
+    assert.equal(configWith({ days: SOLO_CONFIG.days }).minCombinedRevenue, SOLO_CONFIG.minCombinedRevenue);
+  });
+
+  it('still lets a caller name a floor that has nothing to do with the length', () => {
+    // Deriving is the DEFAULT, not a rule — the New Game dialog and the lobby both let a player set
+    // a floor directly, and that has to survive being passed through here.
+    assert.equal(configWith({ days: 10, minCombinedRevenue: 4 }).minCombinedRevenue, 4);
+    assert.equal(configWith({ days: 3, minCombinedRevenue: 0 }).minCombinedRevenue, 0, 'an off floor was re-derived');
+  });
+});
+
 describe('the end-of-game results screen (Gitea#16)', () => {
   /**
    * A finished game, built by running the clock off the end of a real one rather than by hand — the