diff --git a/src/main/runtime/orchestration/worker-output-archive-bounding.test.ts b/src/main/runtime/orchestration/worker-output-archive-bounding.test.ts
new file mode 100644
index 00000000000..a45cc6c0531
--- /dev/null
+++ b/src/main/runtime/orchestration/worker-output-archive-bounding.test.ts
@@ -0,0 +1,41 @@
+import { describe, expect, it } from 'vitest'
+import { boundArchiveLines } from './worker-output-archive'
+
+const TERMINAL_ARCHIVE_MAX_CHARS = 262_144
+
+function totalCost(lines: string[]): number {
+  return lines.reduce((sum, line) => sum + line.length + 1, 0)
+}
+
+describe('boundArchiveLines', () => {
+  it('returns the original array untouched when the tail already fits', () => {
+    const lines = ['one', 'two', 'three']
+    const bounded = boundArchiveLines(lines)
+    expect(bounded.truncated).toBe(false)
+    expect(bounded.lines).toBe(lines)
+  })
+
+  it('keeps the newest lines in order and reports truncation', () => {
+    const lines = Array.from({ length: 40_000 }, (_, index) => `line ${index}`)
+    const bounded = boundArchiveLines(lines)
+    expect(bounded.truncated).toBe(true)
+    expect(totalCost(bounded.lines)).toBeLessThanOrEqual(TERMINAL_ARCHIVE_MAX_CHARS)
+    expect(bounded.lines.at(-1)).toBe(lines.at(-1))
+    expect(bounded.lines).toEqual(lines.slice(lines.length - bounded.lines.length))
+  })
+
+  it('truncates a single oversized line from its tail', () => {
+    const bounded = boundArchiveLines(['x'.repeat(TERMINAL_ARCHIVE_MAX_CHARS * 2)])
+    expect(bounded.truncated).toBe(true)
+    expect(bounded.lines).toHaveLength(1)
+    expect(bounded.lines[0]).toHaveLength(TERMINAL_ARCHIVE_MAX_CHARS - 1)
+  })
+
+  it('bounds a blank-line flood in linear time', () => {
+    // The char budget admits ~262k blank lines; an unshift-per-line build was ~4.3s here.
+    const startedAt = performance.now()
+    const bounded = boundArchiveLines(Array.from({ length: 300_000 }, () => ''))
+    expect(bounded.lines).toHaveLength(TERMINAL_ARCHIVE_MAX_CHARS)
+    expect(performance.now() - startedAt).toBeLessThan(500)
+  })
+})
diff --git a/src/main/runtime/orchestration/worker-output-archive.ts b/src/main/runtime/orchestration/worker-output-archive.ts
index f6f2b52ecb1..55d16467269 100644
--- a/src/main/runtime/orchestration/worker-output-archive.ts
+++ b/src/main/runtime/orchestration/worker-output-archive.ts
@@ -114,7 +114,7 @@ export async function captureWorkerOutputArchive(args: {
   }
 }
 
-function boundArchiveLines(lines: string[]): { lines: string[]; truncated: boolean } {
+export function boundArchiveLines(lines: string[]): { lines: string[]; truncated: boolean } {
   let total = 0
   for (const line of lines) {
     total += line.length + 1
@@ -122,18 +122,21 @@ function boundArchiveLines(lines: string[]): { lines: string[]; truncated: boole
   if (total <= TERMINAL_ARCHIVE_MAX_CHARS) {
     return { lines, truncated: false }
   }
-  const kept: string[] = []
+  // Collected newest-first and reversed once: unshift per line is O(n^2) and the
+  // char budget admits ~260k blank lines.
+  const keptReversed: string[] = []
   let budget = TERMINAL_ARCHIVE_MAX_CHARS
   for (let index = lines.length - 1; index >= 0; index -= 1) {
     const cost = lines[index].length + 1
     if (cost > budget) {
-      if (kept.length === 0 && budget > 1) {
-        kept.unshift(lines[index].slice(-(budget - 1)))
+      if (keptReversed.length === 0 && budget > 1) {
+        keptReversed.push(lines[index].slice(-(budget - 1)))
       }
       break
     }
-    kept.unshift(lines[index])
+    keptReversed.push(lines[index])
     budget -= cost
   }
-  return { lines: kept, truncated: true }
+  keptReversed.reverse()
+  return { lines: keptReversed, truncated: true }
 }