diff --git a/src/components/medications/wizard/__tests__/wizard-payload.test.ts b/src/components/medications/wizard/__tests__/wizard-payload.test.ts
index caa0854aa..24f920607 100644
--- a/src/components/medications/wizard/__tests__/wizard-payload.test.ts
+++ b/src/components/medications/wizard/__tests__/wizard-payload.test.ts
@@ -5,6 +5,7 @@ import {
   buildCreateBody,
   commitActiveDraft,
   emptyWizardPayload,
+  firstInvalidIndex,
   hydrateWizardPayload,
   landingStepForEdit,
   type MedicationPayload,
@@ -193,6 +194,102 @@ describe("validateStep", () => {
   });
 });
 
+/**
+ * v1.8.6 W4b — `firstInvalidIndex` is the multi-step forward lookahead
+ * that powers the dot stepper's jump gate (`goToStep`) and its
+ * `reachableUntil` ceiling. The dialog's rule:
+ *   - backward jumps are always allowed;
+ *   - a forward jump to path-index `j` is allowed iff every gate from
+ *     the active slot up to (but not including) `j` validates, which is
+ *     exactly `firstInvalidIndex(payload, stepList, from) >= j`.
+ */
+describe("firstInvalidIndex — forward-jump lookahead", () => {
+  /** A daily-path payload with every gate satisfied. */
+  function completeDaily(): WizardPayload {
+    return {
+      ...withCadence("daily"),
+      startsOn: new Date(Date.UTC(2026, 5, 1)),
+      endsOn: null,
+      timesOfDay: ["08:00"],
+    };
+  }
+
+  it("returns stepList.length when every slot from the index validates", () => {
+    const payload = completeDaily();
+    const list = progressIndices(payload.mode, payload.cadence.kind);
+    expect(firstInvalidIndex(payload, list, 0)).toBe(list.length);
+  });
+
+  it("stops at the first failing slot — blank name gates the path at 0", () => {
+    const payload = { ...completeDaily(), name: "   " };
+    const list = progressIndices(payload.mode, payload.cadence.kind);
+    // Step 1 (index 0) fails its gate, so no forward jump is allowed.
+    expect(firstInvalidIndex(payload, list, 0)).toBe(0);
+  });
+
+  it("a missing dose gates the path at Step 3 (path-index 2)", () => {
+    const payload = { ...completeDaily(), doseAmount: "" };
+    const list = progressIndices(payload.mode, payload.cadence.kind);
+    // [1,2,3,...] — Step 3 sits at index 2; it's the first to fail.
+    expect(firstInvalidIndex(payload, list, 0)).toBe(2);
+  });
+
+  it("blocks a forward jump past an invalid intervening step", () => {
+    // Missing times (Step 7) — daily path is [1,2,3,4,5,7,8]; Step 7
+    // sits at index 5. A jump from Step 1 (index 0) to the review slot
+    // (Step 8, index 6) must be refused because index 5 fails.
+    const payload = { ...completeDaily(), timesOfDay: [] };
+    const list = progressIndices(payload.mode, payload.cadence.kind);
+    const firstInvalid = firstInvalidIndex(payload, list, 0);
+    expect(firstInvalid).toBe(5);
+    const targetIndex = list.indexOf(8);
+    // Gate rule: forward jump allowed iff firstInvalid >= targetIndex.
+    expect(firstInvalid >= targetIndex).toBe(false);
+  });
+
+  it("allows a forward jump when all intervening gates pass", () => {
+    const payload = completeDaily();
+    const list = progressIndices(payload.mode, payload.cadence.kind);
+    const fromIndex = 0;
+    const targetIndex = list.indexOf(8);
+    expect(firstInvalidIndex(payload, list, fromIndex) >= targetIndex).toBe(
+      true,
+    );
+  });
+
+  it("a fully-hydrated edit payload makes jump-to-last (review) reachable", () => {
+    // Edit hydration populates every field, so on the recurring path
+    // every gate validates and the whole path is reachable — including
+    // the final review slot, so jump-to-last is one tap.
+    const initial: MedicationPayload = {
+      id: "med-1",
+      name: "Ramipril",
+      dose: "5 mg",
+      category: "BLOOD_PRESSURE",
+      treatmentClass: "GENERIC",
+      deliveryForm: "ORAL",
+      notificationsEnabled: true,
+      startsOn: new Date(Date.UTC(2026, 5, 1)),
+      endsOn: null,
+      oneShot: false,
+      schedules: [
+        {
+          windowStart: "06:00",
+          windowEnd: "22:00",
+          timesOfDay: ["08:00"],
+          rrule: "FREQ=DAILY",
+          rollingIntervalDays: null,
+        },
+      ],
+    };
+    const payload = hydrateWizardPayload(initial);
+    const list = progressIndices(payload.mode, payload.cadence.kind);
+    expect(firstInvalidIndex(payload, list, 0)).toBe(list.length);
+    // Whole-path validity → jump-to-last enabled.
+    expect(firstInvalidIndex(payload, list, 0) >= list.length).toBe(true);
+  });
+});
+
 describe("WIZARD_TREATMENT_MAPPING — Step 2 row → request body", () => {
   it("maps Blutdruck → (GENERIC, BLOOD_PRESSURE)", () => {
     expect(WIZARD_TREATMENT_MAPPING.bloodPressure).toEqual({
diff --git a/src/components/medications/wizard/wizard-payload.ts b/src/components/medications/wizard/wizard-payload.ts
index 984a05707..16ec63843 100644
--- a/src/components/medications/wizard/wizard-payload.ts
+++ b/src/components/medications/wizard/wizard-payload.ts
@@ -465,6 +465,36 @@ export function validateStep(payload: WizardPayload, step: number): boolean {
   }
 }
 
+// ────────────────────────────────────────────────────────────────────
+// firstInvalidIndex — multi-step forward lookahead
+// ────────────────────────────────────────────────────────────────────
+
+/**
+ * Walks `stepList` from `fromIndex` (inclusive) and returns the index of
+ * the first slot whose `validateStep` gate fails, or `stepList.length`
+ * when every slot from `fromIndex` onward validates.
+ *
+ * This generalises the single-step `canContinue` gate
+ * (`validateStep(payload, step)`) into the forward-jump lookahead the
+ * dot stepper uses: a forward jump to path-index `j` is allowed iff
+ * `firstInvalidIndex(payload, stepList, i) >= j` — i.e. every gate
+ * between the current slot `i` and the target `j` passes. The dot
+ * stepper reuses the same value as its `reachableUntil` ceiling so the
+ * gate and the visual reachability never drift.
+ *
+ * Pure: reuses `validateStep` verbatim, no new validation logic.
+ */
+export function firstInvalidIndex(
+  payload: WizardPayload,
+  stepList: readonly number[],
+  fromIndex: number,
+): number {
+  for (let k = Math.max(0, fromIndex); k < stepList.length; k++) {
+    if (!validateStep(payload, stepList[k])) return k;
+  }
+  return stepList.length;
+}
+
 // ────────────────────────────────────────────────────────────────────
 // buildCreateBody — POST /api/medications request body
 // ────────────────────────────────────────────────────────────────────