@@ -0,0 +1,36 @@
+import { describe, expect, it } from "vite-plus/test";
+import { parseMarkdownWithMath } from "@t3tools/shared/markdownMath";
+import { remarkThoughtPreview, thoughtPreviewSource } from "./TimelineWorkEntryRow.logic";
+
+describe("thoughtPreviewSource", () => {
+  it("keeps short traces intact", () => {
+    expect(thoughtPreviewSource("Checking the parser.")).toBe("Checking the parser.");
+  });
+
+  it("cuts long traces at a word boundary", () => {
+    const source = thoughtPreviewSource(`${"word ".repeat(1_000)}tail`);
+    expect(source.length).toBeLessThanOrEqual(2_000);
+    expect(source.endsWith("word")).toBe(true);
+  });
+
+  it("never cuts inside an emoji when the trace has no whitespace", () => {
+    expect(thoughtPreviewSource(`${"a".repeat(1_999)}🙂${"b".repeat(10)}`)).toBe("a".repeat(1_999));
+  });
+});
+
+describe("remarkThoughtPreview", () => {
+  it("flattens a long trace into one collapsed text run", () => {
+    const tree = parseMarkdownWithMath(`**Plan**\n\n${"Read `a.ts`  and *b*.\n\n".repeat(2_000)}`);
+    remarkThoughtPreview("Thought")(tree);
+    expect(tree.children).toHaveLength(1);
+    const [text] = tree.children;
+    expect(text?.type === "text" && text.value.startsWith("Plan Read a.ts and b. Read")).toBe(true);
+    expect(text?.type === "text" && /\s{2}/.test(text.value)).toBe(false);
+  });
+
+  it("falls back when the trace has no visible text", () => {
+    const tree = parseMarkdownWithMath("<b></b>");
+    remarkThoughtPreview("Thought")(tree);
+    expect(tree.children).toEqual([{ type: "text", value: "Thought" }]);
+  });
+});
@@ -196,7 +196,8 @@ export function thoughtPreviewSource(text: string): string {
   if (text.length <= THOUGHT_PREVIEW_MAX_CHARS) return text;
   const head = text.slice(0, THOUGHT_PREVIEW_MAX_CHARS);
   const boundary = head.search(/\s\S*$/);
-  return boundary > 0 ? head.slice(0, boundary) : head;
+  // Without whitespace, drop a lone high surrogate so the cut never splits an emoji.
+  return boundary > 0 ? head.slice(0, boundary) : head.replace(/[\uD800-\uDBFF]$/, "");
 }
 
 /** Compact reasoning keeps formulas while flattening other Markdown into label text. */
 
@@ -115,12 +115,15 @@ const toolCallExpandedBodyClassName =
 
 const stopRowToggle = (e: { stopPropagation: () => void }) => e.stopPropagation();
 
+const NO_REASONING_ENTRIES: ReadonlyArray<TimelineWorkEntry> = [];
+
 export function ReasoningTraceContent({ entries }: { entries: ReadonlyArray<TimelineWorkEntry> }) {
   const ctx = use(TimelineRowCtx);
   const { isWorking, latestRunId } = use(TimelineRowActivityCtx);
   // Long traces re-parse on every delta; deferring lets newer deltas interrupt a stale render.
   // Defer the whole list so the commit that grows the text also reruns live-follow scrolling.
-  const deferredEntries = useDeferredValue(entries);
+  // The empty initial value moves the first full parse out of the expanding click's commit.
+  const deferredEntries = useDeferredValue(entries, NO_REASONING_ENTRIES);
   const isStreaming = (entry: TimelineWorkEntry) =>
     isWorking && entry.runId === latestRunId && entry.toolLifecycleStatus === "inProgress";
   return (
Original file line number	Diff line number	Diff line change
`@@ -196,7 +196,8 @@ export function thoughtPreviewSource(text: string): string {`
`196`	`196`	`if (text.length <= THOUGHT_PREVIEW_MAX_CHARS) return text;`
`197`	`197`	`const head = text.slice(0, THOUGHT_PREVIEW_MAX_CHARS);`
`198`	`198`	`const boundary = head.search(/\s\S*$/);`
`199`		`- return boundary > 0 ? head.slice(0, boundary) : head;`
	`199`	`+ // Without whitespace, drop a lone high surrogate so the cut never splits an emoji.`
	`200`	`+ return boundary > 0 ? head.slice(0, boundary) : head.replace(/[\uD800-\uDBFF]$/, "");`
`200`	`201`	`}`
`201`	`202`
`202`	`203`	`/** Compact reasoning keeps formulas while flattening other Markdown into label text. */`