diff --git a/config/scripts/terminal-shutdown-byte-limit-benchmark.mjs b/config/scripts/terminal-shutdown-byte-limit-benchmark.mjs
new file mode 100644
index 00000000000..366015c984f
--- /dev/null
+++ b/config/scripts/terminal-shutdown-byte-limit-benchmark.mjs
@@ -0,0 +1,271 @@
+import assert from 'node:assert/strict'
+import { readFileSync } from 'node:fs'
+import path from 'node:path'
+import { performance } from 'node:perf_hooks'
+import ts from 'typescript-api'
+import { build } from 'esbuild'
+
+// Pipe the baseline capture module on stdin. No Electron or visible terminal is launched.
+const renderer = path.resolve('src/renderer/src')
+const entry = path.join(renderer, 'components/terminal-pane/terminal-shutdown-layout-capture.ts')
+const sources = [readFileSync(0, 'utf8'), readFileSync(entry, 'utf8')]
+assert(sources.every((source) => source.includes('export function captureTerminalShutdownLayout')))
+const layoutFile = path.join(renderer, 'components/terminal-pane/layout-serialization.ts')
+const layoutSource = readFileSync(layoutFile, 'utf8')
+const layoutAst = ts.createSourceFile(layoutFile, layoutSource, ts.ScriptTarget.Latest, true)
+const layoutNames = ['getLayoutChildNodes', 'serializePaneTree', 'serializeTerminalLayout']
+const layoutFunctions = layoutAst.statements.filter(
+  (node) => ts.isFunctionDeclaration(node) && layoutNames.includes(node.name?.text)
+)
+assert.equal(layoutFunctions.length, layoutNames.length)
+const layoutSubset = `import { isTerminalLeafId } from '../../../../shared/stable-pane-id';
+${layoutFunctions.map((node) => node.getText(layoutAst)).join('\n')}`
+
+async function load(source) {
+  const result = await build({
+    entryPoints: [entry],
+    bundle: true,
+    platform: 'node',
+    format: 'esm',
+    write: false,
+    alias: { '@': renderer },
+    plugins: [
+      {
+        name: 'capture-benchmark',
+        setup(builder) {
+          builder.onLoad({ filter: /terminal-shutdown-layout-capture\.ts$/ }, () => ({
+            contents: `${source}\nexport { fitsSessionScrollbackByteLimit, MAX_BUFFER_BYTES };`,
+            loader: 'ts',
+            resolveDir: path.dirname(entry)
+          }))
+          // Select unchanged layout declarations to exclude unrelated renderer module side effects.
+          builder.onLoad({ filter: /[/\\]layout-serialization\.ts$/ }, () => ({
+            contents: layoutSubset,
+            loader: 'ts',
+            resolveDir: path.dirname(layoutFile)
+          }))
+          builder.onResolve({ filter: /pane-terminal-output-scheduler$/ }, () => ({
+            path: 'capture-scheduler',
+            namespace: 'fixture'
+          }))
+          builder.onLoad({ filter: /.*/, namespace: 'fixture' }, () => ({
+            contents: 'export function flushTerminalOutput(terminal) { terminal.recordFlush?.(); }',
+            loader: 'js'
+          }))
+        }
+      }
+    ]
+  })
+  const bundled = `${result.outputFiles[0].text}\n//# sourceURL=shutdown-byte-limit-benchmark-bundle.js`
+  return import(`data:text/javascript;base64,${Buffer.from(bundled).toString('base64')}`)
+}
+const modules = await Promise.all(sources.map(load))
+const captures = modules.map((module) => module.captureTerminalShutdownLayout)
+const predicates = modules.map((module) => module.fitsSessionScrollbackByteLimit)
+
+class FixtureElement {
+  constructor({ classes = [], dataset = {}, children = [], firstElementChild = null } = {}) {
+    this.classList = { contains: (name) => classes.includes(name) }
+    this.dataset = dataset
+    this.children = children
+    this.firstElementChild = firstElementChild
+    this.style = { flex: '1' }
+  }
+}
+globalThis.HTMLElement = FixtureElement
+
+function leafId(index) {
+  return `${String(index + 1).padStart(8, '0')}-1111-4111-8111-111111111111`
+}
+
+function captureArgs(panes, captureBuffers = true, cleared = false) {
+  const leaves = panes.map(
+    (pane) =>
+      new FixtureElement({
+        classes: ['pane'],
+        dataset: { leafId: pane.leafId }
+      })
+  )
+  let root = leaves[0] ?? null
+  for (const leaf of leaves.slice(1)) {
+    root = new FixtureElement({ classes: ['pane-split', 'is-horizontal'], children: [root, leaf] })
+  }
+  const prior = Object.fromEntries(panes.map((pane) => [pane.leafId, 'prior-buffer']))
+  return {
+    manager: { getPanes: () => panes, getActivePane: () => panes[0] ?? null },
+    container: new FixtureElement({ firstElementChild: root }),
+    expandedPaneId: panes[1]?.id ?? null,
+    paneTransports: new Map(
+      panes.map((pane, index) => [
+        pane.id,
+        {
+          getPtyId: () => (index % 2 ? null : `ssh:target@@pty-${index}`)
+        }
+      ])
+    ),
+    paneTitlesByPaneId: Object.fromEntries(panes.map((pane) => [pane.id, `title-${pane.id}`])),
+    existingLayout: Object.freeze({
+      root: null,
+      activeLeafId: null,
+      expandedLeafId: null,
+      buffersByLeafId: Object.freeze(prior),
+      scrollbackRefsByLeafId: Object.freeze({ [leafId(0)]: 'v1-prior' }),
+      ptyIdsByLeafId: Object.freeze({ [leafId(1)]: 'local-prior' })
+    }),
+    captureBuffers,
+    clearedScrollbackLeafIds: new Set(cleared ? [leafId(0)] : [])
+  }
+}
+
+function syntheticFixture(config) {
+  const events = []
+  const panes = config.units.map((unit, index) =>
+    Object.freeze({
+      id: index + 1,
+      leafId: leafId(index),
+      terminal: Object.freeze({
+        options: { scrollback: config.rows },
+        cols: 120,
+        rows: 40,
+        buffer: { active: { cursorX: index, cursorY: index } },
+        recordFlush: () => events.push(['flush', index])
+      }),
+      serializeAddon: {
+        serialize(options) {
+          const rows = options?.scrollback ?? 0
+          events.push(['serialize', index, rows])
+          if (config.fail && index === 0) {
+            throw new Error('fixture serializer failure')
+          }
+          return unit.repeat(Math.max(0, rows))
+        }
+      }
+    })
+  )
+  return { args: captureArgs(panes, config.capture, config.cleared), events }
+}
+
+let seed = 0x17c0ffee
+function random(max) {
+  seed = (Math.imul(seed, 1664525) + 1013904223) >>> 0
+  return (seed >>> 8) % max
+}
+const limit = modules[0].MAX_BUFFER_BYTES
+assert.equal(modules[1].MAX_BUFFER_BYTES, limit)
+const units = ['x', 'é', '界', '😀', '\ud83d', '\udc00', '\x1b[31mtext\x1b[0m\r\n', '']
+let comparisons = 0
+function compareFixture(config) {
+  const results = captures.map((capture) => {
+    const fixture = syntheticFixture(config)
+    const layout = capture(fixture.args)
+    return { layout, events: fixture.events }
+  })
+  assert.deepEqual(results[1], results[0])
+  comparisons++
+}
+for (const unit of units.filter(Boolean)) {
+  for (let delta = -2; delta <= 2; delta++) {
+    const rows = Math.floor((limit - 7) / Buffer.byteLength(unit)) + delta
+    compareFixture({ units: [unit], rows, capture: true, cleared: false, fail: false })
+  }
+}
+for (let iteration = 0; iteration < 1500; iteration++) {
+  compareFixture({
+    units: Array.from({ length: random(4) }, () => units[random(units.length)].repeat(random(50))),
+    rows: random(2000),
+    capture: random(5) !== 0,
+    cleared: random(3) === 0,
+    fail: random(9) === 0
+  })
+}
+for (const unit of units) {
+  for (const length of [0, 1, 64, limit - 1, limit, limit + 1]) {
+    const input = unit.repeat(length)
+    assert.equal(predicates[1](input), predicates[0](input))
+  }
+}
+console.log(`${comparisons} full capture layouts/event traces and 48 byte-fit cases match`)
+
+function sample(arm, input, repeats) {
+  const start = performance.now()
+  let output
+  for (let i = 0; i < repeats; i++) {
+    output = arm(input)
+  }
+  return { elapsed: (performance.now() - start) / repeats, output }
+}
+
+function benchmark(name, input, arms, minimumRepeats = 1) {
+  const expected = arms[0](input)
+  assert.deepEqual(arms[1](input), expected)
+  for (const arm of arms) {
+    const until = performance.now() + 250
+    while (performance.now() < until) {
+      sample(arm, input, 1)
+    }
+  }
+  const repeats = Math.max(
+    minimumRepeats,
+    Math.min(100000, Math.ceil(50 / sample(arms[0], input, 1).elapsed))
+  )
+  /** @type {number[][]} */
+  const times = [[], []]
+  for (let pair = 0; pair < 8; pair++) {
+    for (const index of pair % 2 ? [1, 0] : [0, 1]) {
+      const result = sample(arms[index], input, repeats)
+      times[index].push(result.elapsed)
+      assert.deepEqual(result.output, expected)
+    }
+  }
+  const median = times.map((values) => {
+    values.sort((a, b) => a - b)
+    return (values[3] + values[4]) / 2
+  })
+  console.log(JSON.stringify({ name, repeats, median, times }))
+}
+
+console.log(
+  JSON.stringify({
+    node: process.version,
+    platform: process.platform,
+    arch: process.arch,
+    pairs: 8,
+    unit: 'ms'
+  })
+)
+for (const size of [64, 32768, limit, 2 * 1024 * 1024]) {
+  benchmark(`fit ASCII ${size} code units`, 'x'.repeat(size), predicates)
+  benchmark(`fit Unicode ${size} code units`, '界'.repeat(size), predicates)
+}
+
+const xterm = await import('@xterm/headless')
+const addon = await import('@xterm/addon-serialize')
+const Terminal = xterm.Terminal ?? xterm.default.Terminal
+const SerializeAddon = addon.SerializeAddon ?? addon.default.SerializeAddon
+for (const [rows, unicode, paneCount] of [
+  [50, false, 1],
+  [500, false, 1],
+  [5000, false, 1],
+  [5000, true, 1],
+  [500, false, 8]
+]) {
+  const panes = []
+  for (let index = 0; index < paneCount; index++) {
+    const terminal = new Terminal({ cols: 120, rows: 40, scrollback: 5000, allowProposedApi: true })
+    const serializeAddon = new SerializeAddon()
+    terminal.loadAddon(serializeAddon)
+    const row = unicode ? '界é😀'.repeat(15) : 'line x'.repeat(18)
+    await new Promise((resolve) => terminal.write(`${row}\r\n`.repeat(rows), resolve))
+    panes.push({ id: index + 1, leafId: leafId(index), terminal, serializeAddon })
+  }
+  try {
+    benchmark(
+      `full capture ${paneCount} panes / ${rows} ${unicode ? 'Unicode' : 'ASCII'} lines`,
+      captureArgs(panes),
+      captures,
+      4
+    )
+  } finally {
+    panes.forEach((pane) => pane.terminal.dispose())
+  }
+}
diff --git a/src/renderer/src/components/terminal-pane/terminal-shutdown-layout-capture.test.ts b/src/renderer/src/components/terminal-pane/terminal-shutdown-layout-capture.test.ts
index 9e2b375939c..066aadfdb46 100644
--- a/src/renderer/src/components/terminal-pane/terminal-shutdown-layout-capture.test.ts
+++ b/src/renderer/src/components/terminal-pane/terminal-shutdown-layout-capture.test.ts
@@ -91,6 +91,70 @@ function mockRootForSplit(firstPaneId = 1, secondPaneId = 2): HTMLDivElement {
 }
 
 describe('captureTerminalShutdownLayout', () => {
+  it('uses the bounded native fit check without a separate JS byte scan', async () => {
+    const byteLimits = await import('../../../../shared/utf8-byte-limits')
+    const { captureTerminalShutdownLayout } = await import('./terminal-shutdown-layout-capture')
+    const measure = vi.spyOn(byteLimits, 'measureUtf8ByteLength')
+    const contents = 'x'.repeat(32 * 1024)
+    const pane = {
+      id: 1,
+      leafId: LEAF_ID,
+      terminal: mockTerminal(5_000),
+      serializeAddon: { serialize: vi.fn(() => contents) }
+    }
+    try {
+      const layout = captureTerminalShutdownLayout({
+        manager: { getPanes: () => [pane], getActivePane: () => pane } as never,
+        container: mockRootForPane(1),
+        expandedPaneId: null,
+        paneTransports: new Map(),
+        paneTitlesByPaneId: {},
+        existingLayout: undefined
+      })
+      expect(layout.buffersByLeafId).toEqual({ [LEAF_ID]: `${contents}${CURSOR_HOME}` })
+      expect(pane.serializeAddon.serialize).toHaveBeenCalledExactlyOnceWith({ scrollback: 5_000 })
+      expect(measure.mock.calls.length).toBe(0)
+    } finally {
+      measure.mockRestore()
+    }
+  })
+
+  it.each(['x', 'é', '界', '😀', '\ud83d', '\udc00'])(
+    'preserves exact-cap decisions and probe options for %j',
+    async (unit) => {
+      const { captureTerminalShutdownLayout } = await import('./terminal-shutdown-layout-capture')
+      for (const delta of [-1, 0, 1]) {
+        const payloadBytes =
+          TERMINAL_SCROLLBACK_SESSION_BUFFER_BYTE_LIMIT + delta - CURSOR_HOME.length
+        const unitBytes = Buffer.byteLength(unit)
+        const contents =
+          unit.repeat(Math.floor(payloadBytes / unitBytes)) + 'x'.repeat(payloadBytes % unitBytes)
+        const serialize = vi.fn((options?: { scrollback?: number }) =>
+          options?.scrollback === 5_000 ? contents : 'short'
+        )
+        const pane = Object.freeze({
+          id: 1,
+          leafId: LEAF_ID,
+          terminal: mockTerminal(5_000),
+          serializeAddon: { serialize }
+        })
+        const layout = captureTerminalShutdownLayout({
+          manager: { getPanes: () => [pane], getActivePane: () => pane } as never,
+          container: mockRootForPane(1),
+          expandedPaneId: null,
+          paneTransports: new Map(),
+          paneTitlesByPaneId: {},
+          existingLayout: undefined
+        })
+        const expected = delta <= 0 ? contents : 'short'
+        expect(layout.buffersByLeafId).toEqual({ [LEAF_ID]: `${expected}${CURSOR_HOME}` })
+        expect(serialize.mock.calls.map(([options]) => options?.scrollback)).toEqual(
+          delta <= 0 ? [5_000] : [5_000, 2_500, 3_750, 4_375, 4_687]
+        )
+      }
+    }
+  )
+
   it('flushes queued terminal output before serializing shutdown scrollback', async () => {
     const { captureTerminalShutdownLayout } = await import('./terminal-shutdown-layout-capture')
     const order: string[] = []
diff --git a/src/renderer/src/components/terminal-pane/terminal-shutdown-layout-capture.ts b/src/renderer/src/components/terminal-pane/terminal-shutdown-layout-capture.ts
index f43d8026bd0..9e4be85da55 100644
--- a/src/renderer/src/components/terminal-pane/terminal-shutdown-layout-capture.ts
+++ b/src/renderer/src/components/terminal-pane/terminal-shutdown-layout-capture.ts
@@ -7,7 +7,7 @@ import { mergeCapturedLeafState } from './merge-captured-leaf-state'
 import { resolveTerminalLayoutActiveLeafId } from './terminal-layout-leaf-ids'
 import { TERMINAL_SCROLLBACK_SESSION_BUFFER_BYTE_LIMIT } from '../../../../shared/terminal-scrollback-limits'
 import { serializeWithAbsoluteCursor } from '../../../../shared/terminal-serialize-absolute-cursor'
-import { getUtf8ByteLength, measureUtf8ByteLength } from '../../../../shared/utf8-byte-limits'
+import { getUtf8ByteLength, isUtf8ByteLengthWithinLimit } from '../../../../shared/utf8-byte-limits'
 
 const MAX_BUFFER_BYTES = TERMINAL_SCROLLBACK_SESSION_BUFFER_BYTE_LIMIT
 
@@ -43,7 +43,7 @@ function omitClearedLeafState(
 }
 
 function fitsSessionScrollbackByteLimit(serialized: string): boolean {
-  return !measureUtf8ByteLength(serialized, { stopAfterBytes: MAX_BUFFER_BYTES }).exceededLimit
+  return isUtf8ByteLengthWithinLimit(serialized, MAX_BUFFER_BYTES)
 }
 
 // Why bounded: a plain row bisection costs ~13 full serializes per over-limit pane (~250ms at the