diff --git a/src/app/api/medications/[id]/schedule-revisions/__tests__/route.test.ts b/src/app/api/medications/[id]/schedule-revisions/__tests__/route.test.ts
index d9774e10d..d2c02ed12 100644
--- a/src/app/api/medications/[id]/schedule-revisions/__tests__/route.test.ts
+++ b/src/app/api/medications/[id]/schedule-revisions/__tests__/route.test.ts
@@ -233,6 +233,25 @@ describe("POST /api/medications/[id]/schedule-revisions", () => {
     expect(json.error).toMatch(/current plan/i);
   });
 
+  it("422s when no revision exists and the era reaches past createdAt", async () => {
+    // With zero archived revisions the live plan has covered everything
+    // since `createdAt` (2026-01-10); a manual era ending inside the
+    // tracked window would re-score live compliance against the manual
+    // snapshot. The boundary is createdAt, not "now".
+    const res = await POST(
+      postReq({
+        ...VALID_BODY,
+        validFrom: "2026-01-01T00:00:00.000Z",
+        validUntil: "2026-02-01T00:00:00.000Z",
+      }),
+      ROUTE_CTX,
+    );
+    expect(res.status).toBe(422);
+    const json = await res.json();
+    expect(json.error).toMatch(/current plan/i);
+    expect(prisma.medicationScheduleRevision.create).not.toHaveBeenCalled();
+  });
+
   it("creates a MANUAL revision with a daily snapshot entry", async () => {
     vi.mocked(prisma.medicationScheduleRevision.create).mockResolvedValue({
       id: "rev-new",
diff --git a/src/app/api/medications/[id]/schedule-revisions/route.ts b/src/app/api/medications/[id]/schedule-revisions/route.ts
index 0a4ca4259..c7cd2075e 100644
--- a/src/app/api/medications/[id]/schedule-revisions/route.ts
+++ b/src/app/api/medications/[id]/schedule-revisions/route.ts
@@ -137,6 +137,14 @@ export const POST = apiHandler(
     const guard = await assertMedicationOwnership(id, user.id);
     if (guard) return guard;
 
+    const med = await prisma.medication.findUnique({
+      where: { id },
+      select: { createdAt: true },
+    });
+    if (!med) {
+      return apiError("Medication not found", 404);
+    }
+
     const { data: body, error: jsonError } = await safeJson(request, {
       maxBytes: 16 * 1024,
     });
@@ -162,13 +170,18 @@ export const POST = apiHandler(
     });
 
     // "Before the current plan": the live era began at the newest
-    // archived `validUntil`; with no archive it spans up to now, so a
-    // manual era may cover anything up to this instant.
+    // archived `validUntil`; with no archive the live plan has covered
+    // everything since the medication was created (the era splitter
+    // reads `[newest validUntil, ∞)` — or the whole range when no
+    // revision exists), so a manual era must end at or before
+    // `createdAt`. Allowing it to reach "now" would let the manual
+    // snapshot swallow tracked live history and re-score compliance
+    // against the wrong plan.
     const liveStart = existing.reduce(
       (latest, r) => (r.validUntil > latest ? r.validUntil : latest),
       new Date(0),
     );
-    const liveBoundary = existing.length > 0 ? liveStart : new Date();
+    const liveBoundary = existing.length > 0 ? liveStart : med.createdAt;
     if (validUntil.getTime() > liveBoundary.getTime()) {
       return apiError(
         "A manual era must end before the current plan begins",