diff --git a/scripts/whatsapp-bridge/outbound_ids.js b/scripts/whatsapp-bridge/outbound_ids.js
index cb2b95fdcbb..56bf9b34b2e 100644
--- a/scripts/whatsapp-bridge/outbound_ids.js
+++ b/scripts/whatsapp-bridge/outbound_ids.js
@@ -1,10 +1,13 @@
 /**
- * Bounded LRU set of outbound message IDs.
+ * Bounded FIFO set of outbound message IDs.
  *
  * Used by the WhatsApp bridge to distinguish "echo of our own /send" from
  * "owner-typed message on the linked device" when forwarding `fromMe`
  * inbound events back to the Python adapter.
  *
+ * Eviction drops the oldest insertion-order entry when the cap is exceeded.
+ * Re-remembering an existing id is a no-op for ordering (not LRU refresh).
+ *
  * Heuristic limitation (intentional, documented for future debugging):
  * the set is in-memory only.  On bridge restart it is empty, so for the
  * brief window between restart and the first new outbound, any in-flight
@@ -15,6 +18,9 @@
  */
 
 export function createOutboundIdTracker(maxSize = 512) {
+  if (!Number.isInteger(maxSize) || maxSize < 1) {
+    throw new RangeError('createOutboundIdTracker: maxSize must be a positive integer');
+  }
   const ids = new Set();
 
   function remember(id) {
diff --git a/scripts/whatsapp-bridge/outbound_ids.test.mjs b/scripts/whatsapp-bridge/outbound_ids.test.mjs
index 7c88d5e58ae..54db18df390 100644
--- a/scripts/whatsapp-bridge/outbound_ids.test.mjs
+++ b/scripts/whatsapp-bridge/outbound_ids.test.mjs
@@ -46,11 +46,11 @@ test('cap holds across many inserts (bounded memory)', () => {
   assert.equal(tracker.has('id-99'), true);
 });
 
-test('re-remembering an existing id refreshes its position', () => {
+test('re-remembering an existing id does not promote it (FIFO, not LRU)', () => {
   // Insertion-order semantics: re-adding doesn't move it forward in
   // Set iteration order. This is intentional — we don't need recency,
   // just bounded membership.  Pin the actual behaviour so future
-  // refactors don't accidentally introduce LRU semantics.
+  // refactors don't accidentally introduce LRU refresh semantics.
   const tracker = createOutboundIdTracker(2);
   tracker.remember('a');
   tracker.remember('b');
@@ -60,3 +60,9 @@ test('re-remembering an existing id refreshes its position', () => {
   assert.equal(tracker.has('b'), true);
   assert.equal(tracker.has('c'), true);
 });
+
+test('rejects non-positive maxSize', () => {
+  assert.throws(() => createOutboundIdTracker(0), RangeError);
+  assert.throws(() => createOutboundIdTracker(-1), RangeError);
+  assert.throws(() => createOutboundIdTracker(1.5), RangeError);
+});