perf(terminal): serialize checkpoints with one payload walk

Co-authored-by: Orca <help@stably.ai>

src/main/daemon/terminal-checkpoint-serializer.test.ts

@@ -0,0 +1,60 @@
+import { describe, expect, it } from 'vitest'
+import type { TerminalSnapshot } from './types'
+import { serializeTerminalCheckpointWithinLimit } from './terminal-checkpoint-serializer'
+
+function snapshotWithCountedLinks(
+  linkCount: number,
+  uri: string
+): {
+  snapshot: TerminalSnapshot
+  uriReads: () => number
+} {
+  let reads = 0
+  const oscLinks = Array.from({ length: linkCount }, (_, row) => ({
+    row,
+    startCol: 0,
+    endCol: 1,
+    get uri(): string {
+      reads += 1
+      return uri
+    }
+  }))
+  return {
+    snapshot: {
+      snapshotAnsi: 'visible',
+      scrollbackAnsi: '',
+      oscLinks,
+      rehydrateSequences: '',
+      cwd: '/workspace',
+      modes: {
+        bracketedPaste: false,
+        mouseTracking: false,
+        applicationCursor: false,
+        alternateScreen: false
+      },
+      cols: 80,
+      rows: 24,
+      scrollbackLines: 0
+    },
+    uriReads: () => reads
+  }
+}
+
+describe('terminal checkpoint serializer', () => {
+  it('traverses a large checkpoint payload once', async () => {
+    const linkCount = 4_096
+    const { snapshot, uriReads } = snapshotWithCountedLinks(
+      linkCount,
+      `https://example.com/${'x'.repeat(4_096)}`
+    )
+
+    const json = await serializeTerminalCheckpointWithinLimit(
+      snapshot,
+      { cwd: snapshot.cwd, generation: 1, checkpointedAt: '2026-07-29T00:00:00.000Z' },
+      20 * 1024 * 1024
+    )
+
+    expect(Buffer.byteLength(json, 'utf8')).toBeLessThanOrEqual(20 * 1024 * 1024)
+    expect(uriReads()).toBe(linkCount)
+  })
+})

src/main/daemon/terminal-checkpoint-serializer.ts

@@ -1,7 +1,6 @@
 import type { TerminalCheckpointFile, TerminalSnapshot } from './types'
 import { ColdRestoreReplayWriter } from './cold-restore-replay-writer'
 import { HeadlessEmulator } from './headless-emulator'
-import { jsonUtf8ByteLength } from './json-utf8-byte-length'
 
 type CheckpointMetadata = {
   cwd: string | null
@@ -33,14 +32,8 @@ function checkpointFile(
 }
 
 function stringifyWithinLimit(checkpoint: TerminalCheckpointFile, maxBytes: number): string | null {
-  if (jsonUtf8ByteLength(checkpoint) > maxBytes) {
-    return null
-  }
   const json = JSON.stringify(checkpoint)
-  if (Buffer.byteLength(json, 'utf8') > maxBytes) {
-    throw new Error('Terminal checkpoint size estimator mismatch')
-  }
-  return json
+  return Buffer.byteLength(json, 'utf8') <= maxBytes ? json : null
 }
 
 async function replaySnapshot(snapshot: TerminalSnapshot): Promise<HeadlessEmulator> {