diff --git a/.changeset/perp-cancel-oid-safe-integer.md b/.changeset/perp-cancel-oid-safe-integer.md
new file mode 100644
index 00000000..53a6756b
--- /dev/null
+++ b/.changeset/perp-cancel-oid-safe-integer.md
@@ -0,0 +1,5 @@
+---
+"nansen-cli": patch
+---
+
+Reject `--oid` values above 2^53-1 on `perp cancel`: large Hyperliquid uint64 order IDs would be silently rounded by JS Number, potentially cancelling the wrong order.
diff --git a/src/__tests__/perp.test.js b/src/__tests__/perp.test.js
index f5a7d0b4..c8212070 100644
--- a/src/__tests__/perp.test.js
+++ b/src/__tests__/perp.test.js
@@ -260,6 +260,21 @@ describe('perp cancel validation', () => {
       cmds.cancel([], null, {}, { coin: 'ETH', oid: '0', wallet: 'x' }),
     ).rejects.toThrow(/Invalid --oid "0"/);
   });
+
+  it('rejects --oid above 2^53-1 to prevent silent rounding of large uint64 ids', async () => {
+    const unsafeOid = String(Number.MAX_SAFE_INTEGER + 1); // 9007199254740992
+    await expect(
+      cmds.cancel([], null, {}, { coin: 'ETH', oid: unsafeOid, wallet: 'x' }),
+    ).rejects.toThrow(/exceeds safe integer precision/);
+  });
+
+  it('accepts --oid exactly at 2^53-1 (MAX_SAFE_INTEGER)', async () => {
+    const safeOid = String(Number.MAX_SAFE_INTEGER); // 9007199254740991
+    // Should not throw on validation; will fail later on missing wallet — that's fine.
+    await expect(
+      cmds.cancel([], null, {}, { coin: 'ETH', oid: safeOid, wallet: 'x' }),
+    ).rejects.not.toThrow(/exceeds safe integer precision/);
+  });
 });
 
 describe('perp meta listing (L1)', () => {
diff --git a/src/perp.js b/src/perp.js
index 4a601156..ea072afa 100644
--- a/src/perp.js
+++ b/src/perp.js
@@ -466,6 +466,12 @@ function parsePositiveInt(raw, name) {
   if (!Number.isInteger(n) || n <= 0) {
     throw invalid(`Invalid --${name} "${raw}". Must be a positive integer.`);
   }
+  // Hyperliquid order IDs are uint64. JS Number loses precision above 2^53-1,
+  // so parseInt would silently round a large oid and cancel the wrong order.
+  // Refuse here for the same reason the response path withholds unsafe oids.
+  if (!Number.isSafeInteger(n)) {
+    throw invalid(`Invalid --${name} "${raw}". Value exceeds safe integer precision (2^53-1); copy the exact order ID from "nansen perp positions".`);
+  }
   return n;
 }