diff --git a/packages/opencode/src/cli/cmd/tui/util/collapse.ts b/packages/opencode/src/cli/cmd/tui/util/collapse.ts
index c7c84c3a4..c4430b7b4 100644
--- a/packages/opencode/src/cli/cmd/tui/util/collapse.ts
+++ b/packages/opencode/src/cli/cmd/tui/util/collapse.ts
@@ -8,8 +8,8 @@
 // therefore an approximate ceiling, not a hard bound — see the follow-up note in
 // docs/compose/spec/exec-tool-view.md.
 
-export function lines(content: string) {
-  if (!content) return []
+export function lines(content: unknown) {
+  if (typeof content !== "string" || content === "") return []
   return content.replace(/\n$/, "").split("\n")
 }
 
diff --git a/packages/opencode/test/cli/tui/collapse.test.ts b/packages/opencode/test/cli/tui/collapse.test.ts
index 58fa03f1d..e47cdcf4e 100644
--- a/packages/opencode/test/cli/tui/collapse.test.ts
+++ b/packages/opencode/test/cli/tui/collapse.test.ts
@@ -1,6 +1,24 @@
 import { describe, expect, test } from "bun:test"
 import * as Collapse from "../../../src/cli/cmd/tui/util/collapse"
 
+describe("collapse.lines", () => {
+  test("returns [] for non-string truthy values", () => {
+    expect(Collapse.lines(42 as any)).toEqual([])
+    expect(Collapse.lines({} as any)).toEqual([])
+  })
+
+  test("returns [] for undefined / null / empty string", () => {
+    expect(Collapse.lines(undefined as any)).toEqual([])
+    expect(Collapse.lines(null as any)).toEqual([])
+    expect(Collapse.lines("")).toEqual([])
+  })
+
+  test("still splits strings", () => {
+    expect(Collapse.lines("a\nb\n")).toEqual(["a", "b"])
+    expect(Collapse.lines("x")).toEqual(["x"])
+  })
+})
+
 describe("collapse.rows", () => {
   test("counts wrapped height, not source lines", () => {
     expect(Collapse.rows("a".repeat(250), 100)).toBe(3)