diff --git a/DECISIONS.md b/DECISIONS.md
index 8baeff6..a815750 100644
--- a/DECISIONS.md
+++ b/DECISIONS.md
@@ -1182,3 +1182,9 @@ Implementation decisions not spelled out in `memory-bank/implementation-plan.md`
 - **Origins are discovered locations with an HQ or an idle ship**, in content order. In-flight ships don't count (they aren't anywhere yet). A player with neither, as in the tutorial, is measured from Earth, as before.
 - **Upgraded ships are listed under the origin they are at**, only when their base model can't make the leg from there but their effective stats can ("your Swift Heron (upgraded)"). Before, they were checked from Earth wherever they were.
 - **Origins with nobody are left out**; if none can reach, the panel says "No ship can reach it from your bases right now."
+
+## Step 10L.9
+
+- **The client now passes discovered ids** to `validateDispatch`, so the sheet agrees with the server (10L.3 left it lenient).
+- **The stranding line replaces the range line, never adds to it**: it is shown only when the leg itself is in range, matching the sim's check order.
+- **`wouldStrand` accepts a bare `ShipModel`** (travel time already falls back to the game-wide handling rate), so the sheet's explainer can use it with the stats it already has.
diff --git a/apps/web/src/screens/DispatchSheet.tsx b/apps/web/src/screens/DispatchSheet.tsx
index b3b4d5d..c860807 100644
--- a/apps/web/src/screens/DispatchSheet.tsx
+++ b/apps/web/src/screens/DispatchSheet.tsx
@@ -13,6 +13,7 @@ import {
   tripFuelCost,
   tripGameHours,
   validateDispatch,
+  wouldStrand,
   type DispatchFailure,
 } from "@deadhead/sim";
 import { useState } from "react";
@@ -60,6 +61,17 @@ export const INELIGIBLE_REASONS: Record<DispatchFailure, string> = {
   "insufficient-credits": "Not enough credits for fuel",
 };
 
+/** The no-strand rule in words (CR-012), for the dispatch sheet and deadhead picker. */
+export function strandedText(destinationId: string): string {
+  const name = locationName(destinationId);
+  return `Would be stranded — can't fly out of ${name} from there`;
+}
+
+/** Locations the player has discovered, for the no-strand rule. */
+export function discoveredIdsOf(state: PlayerState): Set<string> {
+  return new Set(state.locations.filter((l) => l.discovered).map((l) => l.id));
+}
+
 /**
  * Every reason a ship and pilot cannot take a contract, in plain words with the
  * numbers involved, e.g. "Too heavy: 228 t (holds 20 t)". Display only: the
@@ -97,6 +109,18 @@ export function explainProblems(
         ? `Out of range right now (${model.name} reaches ${Math.round(rangeFrom(model, location(contract.originId)))}M km from ${locationName(contract.originId)})`
         : INELIGIBLE_REASONS["cannot-land"],
     );
+  } else if (
+    wouldStrand(
+      model,
+      location(contract.originId),
+      location(contract.destinationId),
+      contract.massTonnes,
+      gameNow,
+      discoveredIdsOf(state),
+      content,
+    )
+  ) {
+    problems.push(strandedText(contract.destinationId));
   }
   // Priority (CR-007): the trip must fit the delivery window.
   const window = contract.priority ? contract.deliveryWindowGameHours : null;
@@ -204,6 +228,7 @@ export function pairOptions(state: PlayerState, contract: Contract, now: number)
             tutorial: false,
             credits: state.company.credits,
             now,
+            discoveredIds: discoveredIdsOf(state),
           },
           content,
         );
diff --git a/apps/web/src/screens/Fleet.tsx b/apps/web/src/screens/Fleet.tsx
index 142a105..64df130 100644
--- a/apps/web/src/screens/Fleet.tsx
+++ b/apps/web/src/screens/Fleet.tsx
@@ -12,6 +12,7 @@ import { Link } from "react-router";
 import { useClock } from "../clock/clock";
 import { Countdown } from "../components/Countdown";
 import { ShipUpgrades } from "./ShipUpgrades";
+import { strandedText } from "./DispatchSheet";
 import { formatClock, formatCredits, formatDuration, locationName, shipModelName } from "../format";
 import { location, shipModel, tripRealMs, shipStatsOf, simShip } from "../game";
 import { errorMessage, useGameAction, useLoadedPlayer } from "../state/actions";
@@ -234,7 +235,11 @@ export function DeadheadPicker({ ship, onClose }: { ship: Ship; onClose: () => v
       );
       return {
         id,
-        problem: check.ok ? null : (DEADHEAD_REASONS[check.reason] ?? check.reason),
+        problem: check.ok
+          ? null
+          : check.reason === "would-strand"
+            ? strandedText(id)
+            : (DEADHEAD_REASONS[check.reason] ?? check.reason),
         fuelCost: tripFuelCost(model, location(origin), location(id), gameNow),
         tripMs: tripRealMs(model, origin, id, 0, now),
       };
diff --git a/apps/web/test/stranding-reason.test.tsx b/apps/web/test/stranding-reason.test.tsx
new file mode 100644
index 0000000..fda4dcc
--- /dev/null
+++ b/apps/web/test/stranding-reason.test.tsx
@@ -0,0 +1,99 @@
+import { content } from "@deadhead/content";
+import type { Contract, PlayerState } from "@deadhead/schema";
+import { checkRoute, gameToReal, realToGame, wouldStrand } from "@deadhead/sim";
+import { screen, within } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { useClock } from "../src/clock/clock";
+import { pairOptions } from "../src/screens/DispatchSheet";
+import { fakeApi, renderApp, samplePlayerState, shipExtras } from "./render";
+
+const DAY = 24 * 3_600_000;
+const gameStart = realToGame(Date.parse(content.config.epoch.realStart), content.config);
+const clipper = content.shipModel("clipper")!;
+const [moon, mars] = [content.location("moon")!, content.location("mars")!];
+const discovered = new Set(["earth", "moon", "mars"]);
+
+/** A date on which a Clipper can hop Moon→Mars but couldn't leave Mars (CR-012). */
+const NOW = (() => {
+  for (let d = 0; d < 16 * 365; d += 5) {
+    const t = gameStart + d * DAY;
+    if (
+      checkRoute(clipper, moon, mars, t).ok &&
+      wouldStrand(clipper, moon, mars, 10, t, discovered, content)
+    ) {
+      return gameToReal(t, content.config);
+    }
+  }
+  throw new Error("no stranding date");
+})();
+const STRANDED = "Would be stranded — can't fly out of Mars from there";
+
+/** A crewed Clipper at the Moon, with Mars discovered. */
+function state(): PlayerState {
+  const base = samplePlayerState({ tutorial: null });
+  return {
+    ...base,
+    company: { ...base.company, credits: 100_000 },
+    bays: [{ id: "b1", locationId: "moon" }],
+    locations: base.locations.map((l) => ({ ...l, discovered: discovered.has(l.id) })),
+    ships: [
+      {
+        id: "s1",
+        modelId: "clipper",
+        name: "Swift Heron",
+        status: "idle",
+        locationId: "moon",
+        dispatchId: null,
+        lastArrivalAt: 0,
+        ...shipExtras("clipper"),
+        pilotId: "avatar",
+      },
+    ],
+    pilots: [{ ...base.pilots[0]!, assignedShipId: "s1", locationId: "moon" }],
+  };
+}
+
+const toMars: Contract = {
+  id: "c1",
+  originId: "moon",
+  destinationId: "mars",
+  cargoTypeId: "standard",
+  goodId: "consumer-goods",
+  shipper: "Pacific Orbital Logistics",
+  priority: false,
+  deliveryWindowGameHours: null,
+  tier: 0,
+  massTonnes: 10,
+  payout: 50_000,
+  requiredSkill: null,
+  requiredLongHaulLevel: 0,
+  offeredAt: NOW,
+  expiresAt: NOW + 3_600_000,
+  firstContact: false,
+};
+
+describe("stranding reason (Step 10L.9)", () => {
+  beforeEach(() => {
+    vi.useFakeTimers({ toFake: ["Date", "setInterval", "clearInterval"] });
+    vi.setSystemTime(NOW);
+    useClock.setState({ offsetMs: 0, synced: true, now: NOW });
+  });
+  afterEach(() => vi.useRealTimers());
+
+  it("marks a would-strand ship and pilot pair with the reason", () => {
+    const [pair] = pairOptions(state(), toMars, NOW);
+    expect(pair!.problems).toEqual([STRANDED]);
+  });
+
+  it("disables a would-strand deadhead destination with the reason", async () => {
+    renderApp(fakeApi({ getPlayerState: state }), "/fleet");
+    const card = (await screen.findByText("Swift Heron")).closest("li")!;
+    await userEvent.click(within(card).getByRole("button", { name: /Deadhead/ }));
+    const dialog = screen.getByRole("dialog", { name: "Deadhead Swift Heron" });
+    const option = within(dialog).getByRole("button", { name: "Mars" });
+    expect(option).toBeDisabled();
+    expect(option).toHaveTextContent(STRANDED);
+    expect(within(dialog).getByRole("button", { name: "Earth" })).toBeEnabled();
+  });
+});
diff --git a/memory-bank/progress.md b/memory-bank/progress.md
index e10083c..832a8ca 100644
--- a/memory-bank/progress.md
+++ b/memory-bank/progress.md
@@ -277,6 +277,6 @@ Spec: `memory-bank/change-requests/CR-012-lighter-clipper-range.md` (received as
 - [x] **Step 10L.6 — Schema and server.** `would-strand` is in `ERROR_CODES` (it comes from the sim's failure lists) with a server message; buy-ship already accepts any purchasable model. Tests (`apps/server/test/lighter-clipper.test.ts`): buying a Lighter and a Clipper places them idle at the chosen HQ; a Clipper at the Moon deadheading to Mars on a stranding date gets 409 `would-strand` and credits, ships and flights are unchanged.
 - [x] **Step 10L.7 — Client: Market shows roles and reach.** Each Market card shows the model's role and a Reach line from `shipReach` (`apps/web/src/game.ts`, sim `checkRoute`): "From Earth now: Moon, Mars · Other dates: … · By hopping: Ceres Station", with one "From X now" part per HQ location. "Other dates" lists places reachable directly from an HQ but not today; "By hopping" lists places reachable only through other locations (each leg in range on some monthly date over 16 game years). Tests (`market-reach.test.tsx`): roles; the Clipper lists Mars from Earth only on a date when it is in range (otherwise under Other dates) and always lists Ceres by hopping; the Lighter lists only the Moon.
 - [x] **Step 10L.8 — Client: reachability from where ships actually are.** An undiscovered location's map panel now reads "From Mars: Clipper, Freighter · From Earth: Freighter": one part per discovered location with an HQ or idle ship (`playerOrigins`, Earth when there are none yet), listing purchasable models in range from there now plus the player's ships there that reach it only thanks to upgrades (`reachersByOrigin` in `map/reach.ts`, replacing the Earth-only helpers). Tests: `map-reach-origins.test.tsx` (a Clipper at Mars on a date Mars→Ceres is in range: Ceres lists Clipper from Mars, not from Earth); `map-locked.test.tsx` now expects "From Earth: …" for a tutorial player.
-- [ ] Step 10L.9 — Client: stranding reason
+- [x] **Step 10L.9 — Client: stranding reason.** The dispatch sheet passes the player's discovered ids to `validateDispatch` and explains a stranding pair as "Would be stranded — can't fly out of Mars from there" (`strandedText`); the deadhead picker disables such a destination with the same text. `wouldStrand` now takes a bare model or effective stats. Test (`stranding-reason.test.tsx`, a Clipper at the Moon on a date it could hop to Mars but not leave): the pair's only problem is the stranding text; the Mars deadhead is disabled with it while Earth stays enabled.
 - [ ] Step 10L.10 — Art
 - [ ] Step 10L.11 — Plan amendments and regression
diff --git a/packages/sim/src/dispatch.ts b/packages/sim/src/dispatch.ts
index 004b0c3..455ae70 100644
--- a/packages/sim/src/dispatch.ts
+++ b/packages/sim/src/dispatch.ts
@@ -119,7 +119,8 @@ export function contractTripGameHours(
  * location. `discoveredIds` absent means every location counts.
  */
 export function wouldStrand(
-  model: EffectiveStats,
+  /** Effective stats, or a bare model (handling falls back to the game-wide base). */
+  model: ShipModel,
   origin: Location,
   destination: Location,
   massTonnes: number,