diff --git a/platform/completion/common/src/protocol/RpcLookupElementEvent.kt b/platform/completion/common/src/protocol/RpcLookupElementEvent.kt
index 35fb75cc8bfd..1f7f0f720aab 100644
--- a/platform/completion/common/src/protocol/RpcLookupElementEvent.kt
+++ b/platform/completion/common/src/protocol/RpcLookupElementEvent.kt
@@ -49,17 +49,28 @@ sealed interface RpcLookupElementEvent {
    * older request when this event is handled (IJPL-252099 — the frontend swaps a stale-seeded request for a fresh one
    * right before the accept). Resolving against the arranger's own session instead would fail to find the item and
    * insert one carrying a shorter prefix matcher, leaving the leading typed characters in the document.
+   *
+   * [itemPattern] is the prefix the *frontend* lookup matched the chosen element against
+   * (`Lookup.itemPattern`), and it is the authoritative measure of the range the insertion must replace: the frontend
+   * owns the lookup, so it owns the lookup start offset. It can be **longer** than the matcher the backend stored for
+   * the item — a stale-seeded placeholder shows an older request's items re-matched against the prefix typed since
+   * (`FrontendCompletionRequestSessionImpl.seedStaleAndSwap`), and accepting one before the swap-restart lands would
+   * otherwise measure the replaced range against the older, shorter matcher and duplicate the leading typed characters
+   * (IJPL-252099). `null` means "no pattern reported" (nothing chosen, or an older frontend), in which case the backend
+   * keeps using the item's own matcher.
    */
   @Serializable
   data class ItemSelected(
     val projectId: ProjectId,
     val requestId: RpcCompletionRequestId,
     val selectedItemId: RpcCompletionItemId? = null,
+    val itemPattern: String? = null,
   ) : RpcLookupElementEvent {
     override fun toString(): String = buildToString("ItemSelected") {
       field("projectId", projectId)
       field("requestId", requestId)
       fieldWithNullDefault("selectedItemId", selectedItemId)
+      fieldWithNullDefault("itemPattern", itemPattern)
     }
   }