diff --git a/src/lib/auth/__tests__/refresh-token.test.ts b/src/lib/auth/__tests__/refresh-token.test.ts
index 1ee9835cc..b19d98a4b 100644
--- a/src/lib/auth/__tests__/refresh-token.test.ts
+++ b/src/lib/auth/__tests__/refresh-token.test.ts
@@ -394,21 +394,95 @@ describe("rotateRefreshToken", () => {
     expect(liveDev1).toHaveLength(0);
   });
 
-  it("rejects expired refresh tokens", async () => {
-    await issueAccessAndRefresh({
+  it("rejects an expired refresh token by its own secret", async () => {
+    // The token presented here is the REAL one, so the row is found and the
+    // expiry branch is what refuses it. Presenting an unknown string instead
+    // would pass on `not_found` and prove nothing about expiry at all.
+    const initial = await issueAccessAndRefresh({
       userId: "u1",
       policy: NATIVE_POLICY,
       source: "login.password",
     });
-    // Manually expire
     dbState.refreshTokens[0].expiresAt = new Date(Date.now() - 1000);
+
     const result = await rotateRefreshToken({
-      refreshToken: "hlr_doesnt_matter",
+      refreshToken: initial.refreshToken,
       policy: NATIVE_POLICY,
     });
+
     expect(result.ok).toBe(false);
     if (result.ok) return;
-    expect(result.reason).toBe("not_found");
+    expect(result.reason).toBe("expired");
+    // An expired token must not mint a replacement pair.
+    expect(dbState.refreshTokens).toHaveLength(1);
+  });
+
+  it("accepts the same token one millisecond before it expires", async () => {
+    // Boundary companion: proves the expiry refusal above is driven by the
+    // clock and not by the rotation path refusing everything.
+    const initial = await issueAccessAndRefresh({
+      userId: "u1",
+      policy: NATIVE_POLICY,
+      source: "login.password",
+    });
+    dbState.refreshTokens[0].expiresAt = new Date(Date.now() + 60_000);
+
+    const result = await rotateRefreshToken({
+      refreshToken: initial.refreshToken,
+      policy: NATIVE_POLICY,
+    });
+
+    expect(result.ok).toBe(true);
+  });
+
+  it("refuses a live token presented under a different device id", async () => {
+    // Device A's token replayed from device B. The token is valid and unused,
+    // so only the device binding can refuse it.
+    const dev1 = await issueAccessAndRefresh({
+      userId: "u1",
+      policy: NATIVE_POLICY,
+      source: "login.password",
+      deviceId: "dev-1",
+    });
+
+    const result = await rotateRefreshToken({
+      refreshToken: dev1.refreshToken,
+      policy: NATIVE_POLICY,
+      deviceId: "dev-2",
+    });
+
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.reason).toBe("device_mismatch");
+    // Refused, not consumed: the legitimate device can still rotate it.
+    expect(dbState.refreshTokens[0].usedAt).toBeNull();
+    expect(dbState.refreshTokens).toHaveLength(1);
+
+    const legitimate = await rotateRefreshToken({
+      refreshToken: dev1.refreshToken,
+      policy: NATIVE_POLICY,
+      deviceId: "dev-1",
+    });
+    expect(legitimate.ok).toBe(true);
+  });
+
+  it("still rotates when the caller sends no device id at all", async () => {
+    // An older client that never sends `X-Device-Id` must not be locked out
+    // by the device binding — a presented null is unattributable, not a
+    // mismatch.
+    const dev1 = await issueAccessAndRefresh({
+      userId: "u1",
+      policy: NATIVE_POLICY,
+      source: "login.password",
+      deviceId: "dev-1",
+    });
+
+    const result = await rotateRefreshToken({
+      refreshToken: dev1.refreshToken,
+      policy: NATIVE_POLICY,
+    });
+
+    expect(result.ok).toBe(true);
   });
 
   it("rejects revoked refresh tokens", async () => {
diff --git a/src/lib/auth/refresh-token.ts b/src/lib/auth/refresh-token.ts
index 812588f57..37997964f 100644
--- a/src/lib/auth/refresh-token.ts
+++ b/src/lib/auth/refresh-token.ts
@@ -83,7 +83,7 @@ export async function issueAccessAndRefresh(
 }
 
 export type RotationFailureReason =
-  "not_found" | "expired" | "already_used" | "revoked";
+  "not_found" | "expired" | "already_used" | "revoked" | "device_mismatch";
 
 export type RotationResult =
   | { ok: true; bundle: IssuedRefreshBundle }
@@ -158,6 +158,26 @@ export async function rotateRefreshToken(input: {
     return { ok: false, reason: "already_used" };
   }
 
+  // A live token belongs to the device it was issued to. The M-4 escalation
+  // above already treats a mismatched deviceId on a REPLAY as a spoofing
+  // signal; the same mismatch on a still-live token is refused outright
+  // rather than silently rotated. Without this the reuse-detection scope is
+  // only as honest as the caller: a thief could rotate a stolen token under
+  // their own fabricated device id, and the resulting family — keyed on the
+  // attacker's id, because `issueAccessAndRefresh` below inherits
+  // `row.deviceId ?? input.deviceId` — would sit outside the victim device's
+  // containment blast radius.
+  //
+  // Both ids must be present to compare. A stored null means the row predates
+  // per-device issuance and cannot be attributed; a presented null means the
+  // caller sent no `X-Device-Id` header. Neither is treated as a mismatch, so
+  // this cannot lock out an older client that never sends the header.
+  const presentedId = input.deviceId ?? null;
+  const storedId = row.deviceId ?? null;
+  if (presentedId !== null && storedId !== null && presentedId !== storedId) {
+    return { ok: false, reason: "device_mismatch" };
+  }
+
   // Issue the new pair first, THEN mark old consumed in a transaction.
   // (Race: if two concurrent refreshes hit the same row, both will try to
   // mark `usedAt`. We use updateMany with a `usedAt: null` guard so only