diff --git a/config/scripts/session-timeline-benchmark.mjs b/config/scripts/session-timeline-benchmark.mjs
new file mode 100644
index 00000000000..4737b7669a2
--- /dev/null
+++ b/config/scripts/session-timeline-benchmark.mjs
@@ -0,0 +1,182 @@
+import assert from 'node:assert/strict'
+import { readFileSync } from 'node:fs'
+import { dirname, resolve } from 'node:path'
+import { performance } from 'node:perf_hooks'
+import { build } from 'esbuild'
+import { buildCounterbalancedSchedule } from './counterbalanced-benchmark-schedule.mjs'
+import { summarizeBenchmarkSamples } from './benchmark-sample-summary.mjs'
+
+// git show <ref>:src/main/ai-vault/session-scanner-accumulator.ts | node config/scripts/session-timeline-benchmark.mjs
+const target = resolve('src/main/ai-vault/session-scanner-accumulator.ts')
+async function load(source) {
+  const result = await build({
+    stdin: {
+      contents: `
+        export {createAccumulator, cloneSessionAccumulator, updateTimeline, finalizeSession}
+          from './session-scanner-accumulator';
+        export {createClaudeSessionParseState, consumeClaudeSessionLine}
+          from './session-scanner-primary-parsers';`,
+      loader: 'ts',
+      resolveDir: dirname(target)
+    },
+    bundle: true,
+    write: false,
+    platform: 'node',
+    format: 'esm',
+    plugins: [
+      {
+        name: 'timeline-baseline',
+        setup(plugin) {
+          plugin.onLoad({ filter: /session-scanner-accumulator\.ts$/ }, () => ({
+            contents: source,
+            loader: 'ts',
+            resolveDir: dirname(target)
+          }))
+        }
+      }
+    ]
+  })
+  return import(
+    `data:text/javascript;base64,${Buffer.from(result.outputFiles[0].text).toString('base64')}`
+  )
+}
+const implementations = {
+  before: await load(readFileSync(0, 'utf8')),
+  after: await load(readFileSync(target, 'utf8'))
+}
+const file = { path: 'timeline.jsonl', mtimeMs: 0, modifiedAt: '2026-01-01T00:00:00.000Z' }
+const create = (implementation) =>
+  implementation.createAccumulator({ agent: 'claude', sessionId: 'timeline', file })
+const observable = ({ createdAt, updatedAt, latestTimestampMs }) => ({
+  createdAt,
+  updatedAt,
+  latestTimestampMs
+})
+
+let seed = 42
+const random = (max) => {
+  seed = (Math.imul(seed, 1664525) + 1013904223) >>> 0
+  return Math.floor((seed / 2 ** 32) * max)
+}
+const tokens = [
+  null,
+  undefined,
+  '',
+  'bad',
+  0,
+  -1,
+  Infinity,
+  Number.NaN,
+  8_640_000_000_000_001,
+  '1969-12-31T23:59:59.999Z',
+  '-000001-01-01T00:00:00.000Z',
+  '+010000-01-01T00:00:00.000Z',
+  '2026-01-01T01:00:00+01:00',
+  1_700_000_000.0009,
+  1_700_000_000_000.9,
+  1_700_000_000_000.1,
+  1_700_000_000_000 - 0.1
+]
+let differentialUpdates = 0
+for (let trial = 0; trial < 3000; trial += 1) {
+  let before = create(implementations.before)
+  let after = create(implementations.after)
+  for (let index = 0; index < 32; index += 1) {
+    const input = random(2) ? tokens[random(tokens.length)] : 1_700_000_000_000 + random(1000) / 10
+    const update = (implementation, state) => {
+      try {
+        implementation.updateTimeline(state, input)
+      } catch (error) {
+        return String(error)
+      }
+      return null
+    }
+    assert.equal(update(implementations.after, after), update(implementations.before, before))
+    assert.deepEqual(observable(after), observable(before))
+    if (index === 15) {
+      before = implementations.before.cloneSessionAccumulator(before)
+      after = implementations.after.cloneSessionAccumulator(after)
+    }
+    differentialUpdates += 1
+  }
+  assert.deepEqual(
+    implementations.after.finalizeSession(after, 'linux'),
+    implementations.before.finalizeSession(before, 'linux')
+  )
+}
+
+const results = []
+for (const records of [100, 10_000, 100_000]) {
+  for (const workload of [
+    'numeric-timeline',
+    'iso-timeline',
+    'out-of-order-timeline',
+    'claude-record-fold'
+  ]) {
+    const timestamps = Array.from({ length: records }, (_, index) => {
+      const ms =
+        1_700_000_000_000 + (workload === 'out-of-order-timeline' ? random(records) : index)
+      return workload === 'numeric-timeline' ? ms : new Date(ms).toISOString()
+    })
+    const lines =
+      workload === 'claude-record-fold'
+        ? timestamps.map((timestamp, index) =>
+            JSON.stringify({
+              type: index % 2 ? 'assistant' : 'user',
+              sessionId: 'timeline',
+              timestamp,
+              message: {
+                role: index % 2 ? 'assistant' : 'user',
+                content: 'Example transcript message'
+              }
+            })
+          )
+        : []
+    function run(arm) {
+      const implementation = implementations[arm]
+      const parser = implementation.createClaudeSessionParseState(file)
+      const state = workload === 'claude-record-fold' ? parser.accumulator : create(implementation)
+      const started = performance.now()
+      if (workload === 'claude-record-fold') {
+        for (const line of lines) {
+          implementation.consumeClaudeSessionLine(parser, line)
+        }
+      } else {
+        for (const timestamp of timestamps) {
+          implementation.updateTimeline(state, timestamp)
+        }
+      }
+      const ms = performance.now() - started
+      return {
+        ms,
+        result: implementation.finalizeSession(state, 'linux'),
+        timeline: observable(state)
+      }
+    }
+    const expected = run('before')
+    assert.deepEqual(run('after').result, expected.result)
+    /** @type {{ before: number[], after: number[] }} */
+    const samples = { before: [], after: [] }
+    for (const pair of buildCounterbalancedSchedule(8, 'before', 'after')) {
+      for (const arm of pair) {
+        const actual = run(arm)
+        samples[arm].push(actual.ms)
+        assert.deepEqual(actual.result, expected.result)
+        assert.deepEqual(actual.timeline, expected.timeline)
+      }
+    }
+    results.push({
+      records,
+      workload,
+      before: summarizeBenchmarkSamples(samples.before),
+      after: summarizeBenchmarkSamples(samples.after)
+    })
+  }
+}
+console.log(
+  JSON.stringify(
+    { node: process.version, platform: process.platform, differentialUpdates, results },
+    null,
+    2
+  )
+)
diff --git a/src/main/ai-vault/session-scanner-accumulator.ts b/src/main/ai-vault/session-scanner-accumulator.ts
index 18f273d6be3..34f1134f270 100644
--- a/src/main/ai-vault/session-scanner-accumulator.ts
+++ b/src/main/ai-vault/session-scanner-accumulator.ts
@@ -64,6 +64,7 @@ export function createAccumulator(args: {
     lastUserPrompt: null,
     queuedMessageCount: 0,
     subagentTranscriptCount: 0,
+    earliestTimestampMs: 0,
     latestTimestampMs: 0
   }
 }
@@ -191,10 +192,12 @@ export function updateTimeline(accumulator: SessionAccumulator, timestamp: unkno
     return
   }
   const iso = new Date(parsed).toISOString()
-  if (!accumulator.createdAt || parsed < Date.parse(accumulator.createdAt)) {
+  if (!accumulator.createdAt || parsed < accumulator.earliestTimestampMs) {
     accumulator.createdAt = iso
+    accumulator.earliestTimestampMs = Math.trunc(parsed)
   }
-  if (!accumulator.updatedAt || parsed >= Date.parse(accumulator.updatedAt)) {
+  // ISO serialization truncates fractional milliseconds; latestTimestampMs retains them.
+  if (!accumulator.updatedAt || parsed >= Math.trunc(accumulator.latestTimestampMs)) {
     accumulator.updatedAt = iso
     accumulator.latestTimestampMs = parsed
   }
diff --git a/src/main/ai-vault/session-scanner-timeline.test.ts b/src/main/ai-vault/session-scanner-timeline.test.ts
new file mode 100644
index 00000000000..cd488347a7f
--- /dev/null
+++ b/src/main/ai-vault/session-scanner-timeline.test.ts
@@ -0,0 +1,96 @@
+import { describe, expect, it, vi } from 'vitest'
+import {
+  cloneSessionAccumulator,
+  createAccumulator,
+  finalizeSession,
+  updateTimeline
+} from './session-scanner-accumulator'
+
+function accumulator() {
+  return createAccumulator({
+    agent: 'claude',
+    sessionId: 'timeline-test',
+    file: { path: 'transcript.jsonl', mtimeMs: 0, modifiedAt: '2026-01-01T00:00:00.000Z' }
+  })
+}
+
+describe('session timeline bounds', () => {
+  it('retains earliest and latest timestamps despite duplicates and out-of-order records', () => {
+    const state = accumulator()
+    for (const timestamp of [
+      '2026-01-03T01:00:00+01:00',
+      '2026-01-01T00:00:00Z',
+      '2026-01-04T00:00:00Z',
+      '2026-01-02T00:00:00Z',
+      '2026-01-04T00:00:00Z'
+    ]) {
+      updateTimeline(state, timestamp)
+    }
+    expect(finalizeSession(state, 'linux')).toMatchObject({
+      createdAt: '2026-01-01T00:00:00.000Z',
+      updatedAt: '2026-01-04T00:00:00.000Z'
+    })
+    expect(state.latestTimestampMs).toBe(Date.parse('2026-01-04T00:00:00Z'))
+  })
+
+  it('compares fractional numeric timestamps against the rounded ISO bound', () => {
+    const state = accumulator()
+    const base = 1_700_000_000_000
+    updateTimeline(state, base + 0.9)
+    updateTimeline(state, base + 0.1)
+    expect(state.latestTimestampMs).toBe(base + 0.1)
+    expect(state.createdAt).toBe(new Date(base).toISOString())
+    updateTimeline(state, base - 0.1)
+    expect(state.createdAt).toBe(new Date(base - 1).toISOString())
+    expect(state.latestTimestampMs).toBe(base + 0.1)
+  })
+
+  it('preserves pre-epoch and extended-year ISO timestamps', () => {
+    const state = accumulator()
+    updateTimeline(state, '+010000-01-01T00:00:00.000Z')
+    updateTimeline(state, '-000001-01-01T00:00:00.000Z')
+    updateTimeline(state, '1969-12-31T23:59:59.999Z')
+    expect(state.createdAt).toBe('-000001-01-01T00:00:00.000Z')
+    expect(state.updatedAt).toBe('+010000-01-01T00:00:00.000Z')
+  })
+
+  it('ignores invalid timestamps and retains the existing out-of-range error', () => {
+    const state = accumulator()
+    for (const timestamp of [null, undefined, '', 'bad-date', 0, -1, Number.NaN, Infinity]) {
+      updateTimeline(state, timestamp)
+    }
+    expect(state.createdAt).toBeNull()
+    expect(state.updatedAt).toBeNull()
+    expect(() => updateTimeline(state, 8_640_000_000_000_001)).toThrow(RangeError)
+    expect(state.createdAt).toBeNull()
+    expect(state.updatedAt).toBeNull()
+  })
+
+  it('keeps cloned parse-state bounds independent', () => {
+    const state = accumulator()
+    updateTimeline(state, '2026-01-02T00:00:00Z')
+    const clone = cloneSessionAccumulator(state)
+    updateTimeline(clone, '2026-01-01T00:00:00Z')
+    updateTimeline(clone, '2026-01-03T00:00:00Z')
+    expect(state.createdAt).toBe('2026-01-02T00:00:00.000Z')
+    expect(state.updatedAt).toBe('2026-01-02T00:00:00.000Z')
+    expect(clone.createdAt).toBe('2026-01-01T00:00:00.000Z')
+    expect(clone.updatedAt).toBe('2026-01-03T00:00:00.000Z')
+  })
+
+  it('does not reparse accumulated bounds for every numeric record', () => {
+    const state = accumulator()
+    const spy = vi.spyOn(Date, 'parse')
+    let parseCalls: number
+    try {
+      for (let index = 0; index < 1000; index += 1) {
+        updateTimeline(state, 1_700_000_000_000 + index)
+      }
+      parseCalls = spy.mock.calls.length
+    } finally {
+      spy.mockRestore()
+    }
+    expect(state.latestTimestampMs).toBe(1_700_000_000_999)
+    expect(parseCalls).toBe(0)
+  })
+})
diff --git a/src/main/ai-vault/session-scanner-types.ts b/src/main/ai-vault/session-scanner-types.ts
index f4b60270544..6845ec5b6b7 100644
--- a/src/main/ai-vault/session-scanner-types.ts
+++ b/src/main/ai-vault/session-scanner-types.ts
@@ -144,6 +144,7 @@ export type SessionAccumulator = {
   // Recoverable signal for a zero-turn transcript (see AiVaultSession).
   queuedMessageCount: number
   subagentTranscriptCount: number
+  earliestTimestampMs: number
   latestTimestampMs: number
 }