diff --git a/webview-ui/src/utils/__tests__/markdown.spec.ts b/webview-ui/src/utils/__tests__/markdown.spec.ts
index 97b3fdaaf2f..542505a009f 100644
--- a/webview-ui/src/utils/__tests__/markdown.spec.ts
+++ b/webview-ui/src/utils/__tests__/markdown.spec.ts
@@ -8,6 +8,14 @@ describe("markdown heading helpers", () => {
 		expect(countMarkdownHeadings("")).toBe(0)
 	})
 
+	it("returns 0 for non-string truthy values", () => {
+		// When subtasks return to main task, message.text can be a non-string value
+		expect(countMarkdownHeadings(["# heading"] as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings({ text: "# heading" } as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings(42 as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings(true as unknown as string)).toBe(0)
+	})
+
 	it("counts single and multiple headings", () => {
 		expect(countMarkdownHeadings("# One")).toBe(1)
 		expect(countMarkdownHeadings("# One\nContent")).toBe(1)
diff --git a/webview-ui/src/utils/markdown.ts b/webview-ui/src/utils/markdown.ts
index 7a77b9866db..107a248b22a 100644
--- a/webview-ui/src/utils/markdown.ts
+++ b/webview-ui/src/utils/markdown.ts
@@ -4,7 +4,7 @@
  * Code fences are stripped before matching to avoid false positives.
  */
 export function countMarkdownHeadings(text: string | undefined): number {
-	if (!text) return 0
+	if (!text || typeof text !== "string") return 0
 
 	// Remove fenced code blocks to avoid counting headings inside code
 	const withoutCodeBlocks = text.replace(/```[\s\S]*?```/g, "")