test(stream): direct coverage + comment for splitConcatenatedToolCallArguments (#11043 followup) (#11135)

Validated on the combined batch board over release/v3.8.50 tip d91238b7: static gates clean, typecheck:core clean, focused tests green.

Test-only followup to #11043 — 3 direct tests for splitConcatenatedToolCallArguments plus the index-normalization comment, exactly the two review nits. Thank you @maxmad64bis!

open-sse/utils/streamPayloadCollector.ts

@@ -337,6 +337,7 @@ function createOpenAIReducer(fallbackModel?: string | null): SummaryReducer {
       // same-name tool_calls) into its own separate tool_calls entries.
       const finalToolCalls: ToolCall[] = [];
       let nextIndex = 0;
+      // Normalize tool_call indexes to contiguous 0-based (OpenAI contract).
       for (const tc of mergedToolCalls) {
         const splitArgs = splitConcatenatedToolCallArguments(tc.function.arguments);
         if (!splitArgs) {

tests/unit/stream-payload-collector.test.ts

@@ -2,6 +2,7 @@ import test from "node:test";
 import assert from "node:assert/strict";
 
 const collector = await import("../../open-sse/utils/streamPayloadCollector.ts");
+import { splitConcatenatedToolCallArguments } from "../../open-sse/utils/streamPayloadCollector.ts";
 
 test("compactStructuredStreamPayload returns null for null input", () => {
   assert.equal(collector.compactStructuredStreamPayload(null), null);
@@ -413,3 +414,33 @@ test("#9315: getSummary() returns undefined when no format was configured (unaff
   c.push({ choices: [{ index: 0, delta: { content: "hi" } }] });
   assert.equal(c.getSummary(), undefined);
 });
+
+test("splitConcatenatedToolCallArguments — two back-to-back JSON objects", () => {
+  const a = JSON.stringify({ tool: "x", args: "1" });
+  const b = JSON.stringify({ tool: "y", args: "2" });
+  const out = splitConcatenatedToolCallArguments(a + b);
+  assert.deepEqual(out, [a, b]); // >=2 valid values -> split (array of parts)
+});
+
+test("splitConcatenatedToolCallArguments — nested object + escaped quotes stay single JSON", () => {
+  const a = JSON.stringify({ a: 'he said "hi"', b: { c: 1 } });
+  const single = a; // a is ONE valid JSON object -> no split
+  const out = splitConcatenatedToolCallArguments(single);
+  assert.equal(out, null); // single valid JSON -> untouched (null)
+});
+
+test("splitConcatenatedToolCallArguments — braces/quotes inside strings exercise escaped scanner", () => {
+  // Two valid JSON values whose string bodies contain braces and escaped quotes.
+  // Concatenated they reach the inString/escaped state machine (not the JSON.parse
+  // fast path), so this covers the case the owner asked about.
+  const a = JSON.stringify({ cmd: 'echo "}{" ; x' });
+  const b = JSON.stringify({ cmd: "{[not json]}" });
+  const out = splitConcatenatedToolCallArguments(a + b);
+  assert.deepEqual(out, [a, b]); // >=2 valid values -> split into parts
+});
+
+test("splitConcatenatedToolCallArguments — top-level array is single value", () => {
+  const arr = JSON.stringify([{ tool: "x" }, { tool: "y" }]);
+  const out = splitConcatenatedToolCallArguments(arr);
+  assert.equal(out, null); // one value boundary (array) -> not split
+});