diff --git a/src/shared/agent-hook-spool-read.test.ts b/src/shared/agent-hook-spool-read.test.ts
new file mode 100644
index 00000000000..d589f5b2274
--- /dev/null
+++ b/src/shared/agent-hook-spool-read.test.ts
@@ -0,0 +1,65 @@
+import { mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { afterEach, beforeEach, describe, expect, it } from 'vitest'
+import { readSpoolFile } from './agent-hook-spool'
+
+let dir: string
+
+beforeEach(() => {
+  dir = mkdtempSync(join(tmpdir(), 'orca-spool-read-'))
+})
+
+afterEach(() => {
+  rmSync(dir, { recursive: true, force: true })
+})
+
+function write(contents: string): string {
+  const file = join(dir, 'spool.jsonl')
+  writeFileSync(file, contents)
+  return file
+}
+
+function record(paneKey: string): string {
+  return JSON.stringify({ paneKey, source: 'PreToolUse', payload: {}, receivedAt: Date.now() })
+}
+
+describe('readSpoolFile', () => {
+  it('leaves a trailing line without its newline unconsumed', () => {
+    const complete = `${record('a')}\n`
+    const result = readSpoolFile(write(`${complete}${record('b')}`))
+
+    expect(result.records.map((entry) => entry.paneKey)).toEqual(['a'])
+    // The in-flight final record must stay replayable: consumed stops at the last newline.
+    expect(result.consumed).toBe(Buffer.byteLength(complete))
+  })
+
+  it('consumes through the final newline when every line is complete', () => {
+    const contents = `${record('a')}\n${record('b')}\n`
+    const result = readSpoolFile(write(contents))
+
+    expect(result.records.map((entry) => entry.paneKey)).toEqual(['a', 'b'])
+    expect(result.consumed).toBe(Buffer.byteLength(contents))
+  })
+
+  it('skips blank lines without consuming less than the bytes they occupy', () => {
+    const contents = `${record('a')}\n\n\n${record('b')}\n`
+    const result = readSpoolFile(write(contents))
+
+    expect(result.records.map((entry) => entry.paneKey)).toEqual(['a', 'b'])
+    expect(result.consumed).toBe(Buffer.byteLength(contents))
+  })
+
+  it('returns nothing for an empty file', () => {
+    expect(readSpoolFile(write(''))).toEqual({ records: [], consumed: 0 })
+  })
+
+  it('returns nothing for a file that is one torn line', () => {
+    expect(readSpoolFile(write(record('a'))).records).toEqual([])
+    expect(readSpoolFile(write(record('a'))).consumed).toBe(0)
+  })
+
+  it('returns nothing for a missing file', () => {
+    expect(readSpoolFile(join(dir, 'absent.jsonl'))).toEqual({ records: [], consumed: 0 })
+  })
+})
diff --git a/src/shared/agent-hook-spool.ts b/src/shared/agent-hook-spool.ts
index 3ad478ed8a5..275e6f5ce52 100644
--- a/src/shared/agent-hook-spool.ts
+++ b/src/shared/agent-hook-spool.ts
@@ -67,19 +67,17 @@ export function readSpoolFile(
   const records: SpoolRecord[] = []
   let consumed = 0
   let start = 0
-  for (let end = 0; end <= bytes.length; end += 1) {
-    if (end !== bytes.length && bytes[end] !== 0x0a) {
-      continue
-    }
+  // indexOf, not a per-byte loop: this runs over every spooled file before the hook listener binds,
+  // and Buffer.indexOf finds the newline with memchr instead of an interpreted scan.
+  for (;;) {
+    const end = bytes.indexOf(0x0a, start)
     // A final line without its newline may still be in flight from a hook writer.
     // Leave it untouched until the writer terminates the record explicitly.
-    if (end === bytes.length && (end === 0 || bytes[end - 1] !== 0x0a)) {
+    if (end === -1) {
       break
     }
     const lineBytes = bytes.subarray(start, end)
-    if (end !== bytes.length) {
-      consumed = end + 1
-    }
+    consumed = end + 1
     start = end + 1
     if (lineBytes.length === 0) {
       continue