chore: import upstream snapshot with attribution

open-sse/utils/toolCallArguments.ts

@@ -0,0 +1,32 @@
+/**
+ * Accumulate streamed tool-call `arguments` fragments without corrupting them.
+ *
+ * Providers stream tool-call arguments in one of two shapes:
+ *   - Incremental deltas: each chunk carries only the NEW fragment. These must
+ *     be concatenated verbatim — even when a fragment's leading bytes repeat the
+ *     tail of what we already have (e.g. the doubled `l` in `ls -ll`).
+ *   - Full snapshots: each chunk re-sends the ENTIRE accumulated arguments so
+ *     far. Concatenating those would duplicate the payload (issue #3701).
+ *
+ * We only dedup the snapshot case when it is UNAMBIGUOUS: an identical repeat,
+ * or a growing superset that still starts with everything seen so far. Every
+ * other fragment is treated as an incremental delta and appended as-is.
+ *
+ * A fuzzy suffix/prefix-overlap heuristic must NOT be used here: it silently
+ * drops bytes from legitimate incremental deltas (turning `ll` into `l`, `xx`
+ * into `x`), which trades a visible duplication bug for a silent truncation bug.
+ */
+export function appendToolCallArgumentDelta(current: unknown, incoming: unknown): string {
+  const existing = typeof current === "string" ? current : "";
+  const next = typeof incoming === "string" ? incoming : "";
+
+  if (!existing) return next;
+  if (!next) return existing;
+
+  // Unambiguous snapshot repeat / growth — replace instead of concatenating.
+  if (next === existing) return existing;
+  if (next.startsWith(existing)) return next;
+
+  // Incremental delta fragment — append verbatim (preserves repeated chars).
+  return existing + next;
+}