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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) - #3053

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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD)#3053
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Stacked PR — based on #3050 (quickjs-vm-snapshots) ← #3049 ← #3048. Review only the top commit here until the bases merge.

Summary

PR 4 of the QuickJS VM roadmap: threshold-based VM-memory snapshotting — the middle ground that motivated reviving this effort (see #1298 / #1300 discussion). Instead of snapshotting at every suspension (the original branch's model, which cost ~25% on e2e wall clock), snapshots are taken only once WORKFLOW_SNAPSHOT_THRESHOLD events have been processed since the last one:

  • Short-lived runs never snapshot — they keep PR 1/2's pure replay behavior with zero snapshot overhead.
  • Long/forever runs stop scaling their resume cost with event-log length — a resumption restores the VM heap and replays only the delta events since the snapshot's cursor.

How it works

  • WORKFLOW_SNAPSHOT_THRESHOLD env var (default 0 = disabled) or per-run executionContext.snapshotThreshold, stamped at start() for run affinity like WORKFLOW_VM.
  • Save (suspension exit, threshold met): capture live VM memory (session.snapshot()) → compress (zstd/gzip via the shared serialization pipeline; QuickJS heaps compress ~4×, measured 16.5 MB → 3.9 MB) → encrypt with the run's key when configured → world.snapshots.save with the events cursor at the VM's feed frontier.
  • Restore (subsequent invocation): world.snapshots.load → decrypt → decompress → QuickJS.restore over the cached WASM module, re-register host callbacks, fetch events from the snapshot's cursor and feed only the delta. Runs seamlessly through PR 2's inline continuation loop.
  • Delete on run completion/failure.
  • Fallback is always full replay: missing snapshot, load error, corrupt bytes, or restore failure logs a warning and boots fresh against the full event log — the log remains the source of truth; snapshots are strictly an optimization.

Determinism model (restore + partial replay)

The threshold model's new mechanism vs. the original branch: a resumption may restore a snapshot older than the log head (suspensions since the snapshot weren't persisted) and must deterministically re-derive everything in between:

  • The PRNG seed mixes in the restored snapshot's eventsCursor: the heap already consumed pre-snapshot draws, so re-seeding from the base would replay the first-N draws and collide with recorded correlationIds. The cursor is identical for every resume from the same snapshot (concurrent resumes still collide ids for the world's dedup) and advances only when a newer snapshot is taken.
  • Feeding already-consumed events is harmless by construction (consumed resolvers are gone; hook deliveries are deduped by eventId in the VM heap, which travels with the snapshot), so imprecise cursors only cost redundant scanning.
  • Covered by dedicated unit tests, including restore-from-older-snapshot with multi-suspension partial replay and identical post-restore correlationIds across concurrent resumes.

Validation

  • 135/135 e2e on nextjs-turbopack with WORKFLOW_SNAPSHOT_THRESHOLD=1 (maximum churn: snapshot on every qualifying suspension), wall clock within ~10% of the node baseline
  • Verified via debug diagnostics: restored: true resumptions, save/restore/delete lifecycle, and threshold gating (threshold=100 short run ⇒ zero snapshots, pure replay)
  • Full core unit suite green (1,573 tests); new tests for the config knobs and the snapshot/restore/partial-replay determinism
  • CI: new quickjs-snapshot matrix leg (nextjs-turbopack, threshold=1) across local dev/prod/postgres e2e jobs

Notes / follow-ups

  • Version skew: snapshot bytes are tied to the quickjs-wasi build that produced them. Per the deployment contract (runs continue on the version they started on — free on Vercel), this is a non-issue in production; environments without skew protection are covered by the restore-failure fallback to full replay.
  • An ID-divergence window exists when concurrent invocations resume from different snapshot generations; the world's per-(run, correlation) uniqueness rejects duplicates and the log-consistent invocation drives progress, with full-replay convergence as the backstop. Noted in code comments.
  • Docs: WORKFLOW_SNAPSHOT_THRESHOLD section added to v5 Runtime Tuning.

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🦋 Changeset detected

Latest commit: f1b1e10

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 16 packages
NameType
@workflow/coreMinor
workflowMinor
@workflow/buildersPatch
@workflow/cliPatch
@workflow/nextPatch
@workflow/nitroPatch
@workflow/vitestPatch
@workflow/web-sharedPatch
@workflow/webPatch
@workflow/world-testingPatch
@workflow/astroPatch
@workflow/nestPatch
@workflow/rollupPatch
@workflow/sveltekitPatch
@workflow/vitePatch
@workflow/nuxtPatch

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example-nextjs-workflow-turbopackReadyReadyPreviewJul 31, 2026 1:13am
example-nextjs-workflow-webpackReadyReadyPreviewJul 31, 2026 1:13am
example-workflowReadyReadyPreviewJul 31, 2026 1:13am
workbench-astro-workflowReadyReadyPreviewJul 31, 2026 1:13am
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🧪 E2E Test Results

✅ All tests passed

E2E Test Summary

Summary
PassedFailedSkippedTotal
✅ ▲ Vercel Production145502391694
✅ 💻 Local Development312604163542
✅ 📦 Local Production339604543850
✅ 🐘 Local Postgres339604543850
✅ 🪟 Windows30800308
✅ 📋 Other178803682156
✅ vercel-multi-region270027
Total134960193115427
Details by Category

✅ ▲ Vercel Production

AppPassedFailedSkipped
✅ astro-node126028
✅ example-node126028
✅ express-node126028
✅ fastify-node126028
✅ hono-node126028
✅ nextjs-turbopack-node15103
✅ nextjs-webpack-node15103
✅ nitro-node126028
✅ nuxt-node126028
✅ sveltekit-node14509
✅ vite-node126028

✅ 💻 Local Development

AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026

✅ 📦 Local Production

AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-canary-node135019
✅ nextjs-webpack-canary-quickjs135019
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026

✅ 🐘 Local Postgres

AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-canary-node135019
✅ nextjs-webpack-canary-quickjs135019
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026

✅ 🪟 Windows

AppPassedFailedSkipped
✅ nextjs-turbopack-node15400
✅ nextjs-turbopack-quickjs15400

✅ 📋 Other

AppPassedFailedSkipped
✅ e2e-local-dev-nest-stable-node128026
✅ e2e-local-dev-nest-stable-quickjs128026
✅ e2e-local-dev-tanstack-start-node128026
✅ e2e-local-dev-tanstack-start-quickjs128026
✅ e2e-local-postgres-nest-stable-node128026
✅ e2e-local-postgres-nest-stable-quickjs128026
✅ e2e-local-postgres-tanstack-start-node128026
✅ e2e-local-postgres-tanstack-start-quickjs128026
✅ e2e-local-prod-nest-stable-node128026
✅ e2e-local-prod-nest-stable-quickjs128026
✅ e2e-local-prod-tanstack-start-node128026
✅ e2e-local-prod-tanstack-start-quickjs128026
✅ e2e-vercel-prod-nest-node126028
✅ e2e-vercel-prod-tanstack-start-node126028

✅ vercel-multi-region

AppPassedFailedSkipped
✅ nextjs-turbopack2700

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@TooTallNate
TooTallNateforce-pushed the quickjs-vm-snapshots branch from 1e0b6e6 to f43ee63CompareJuly 31, 2026 01:07
@TooTallNate
TooTallNateforce-pushed the quickjs-vm-threshold-snapshots branch from 64f40c3 to f1b1e10CompareJuly 31, 2026 01:09
@TooTallNate
TooTallNate merged commit f64932e into quickjs-vm-snapshotsJul 31, 2026
@TooTallNate
TooTallNateforce-pushed the quickjs-vm-snapshots branch from f43ee63 to f64932eCompareJuly 31, 2026 02:15
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TooTallNate deleted the quickjs-vm-threshold-snapshots branch July 31, 2026 02:15
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) by TooTallNate · Pull Request #3053 · vercel/workflow · GitHub
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) - #3053
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quickjs-vm-snapshotsfrom 
quickjs-vm-threshold-snapshots
Jul 31, 2026
Merged
QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD)#3053
TooTallNate merged 0 commit into
quickjs-vm-snapshotsfrom 
quickjs-vm-threshold-snapshots
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Jul 22, 2026

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Stacked PR — based on #3050 (quickjs-vm-snapshots) ← #3049 ← #3048. Review only the top commit here until the bases merge.
Summary
PR 4 of the QuickJS VM roadmap: threshold-based VM-memory snapshotting — the middle ground that motivated reviving this effort (see #1298 / #1300 discussion). Instead of snapshotting at every suspension (the original branch's model, which cost ~25% on e2e wall clock), snapshots are taken only once WORKFLOW_SNAPSHOT_THRESHOLD events have been processed since the last one:
  • Short-lived runs never snapshot — they keep PR 1/2's pure replay behavior with zero snapshot overhead.
  • Long/forever runs stop scaling their resume cost with event-log length — a resumption restores the VM heap and replays only the delta events since the snapshot's cursor.
How it works
  • WORKFLOW_SNAPSHOT_THRESHOLD env var (default 0 = disabled) or per-run executionContext.snapshotThreshold, stamped at start() for run affinity like WORKFLOW_VM.
  • Save (suspension exit, threshold met): capture live VM memory (session.snapshot()) → compress (zstd/gzip via the shared serialization pipeline; QuickJS heaps compress ~4×, measured 16.5 MB → 3.9 MB) → encrypt with the run's key when configured → world.snapshots.save with the events cursor at the VM's feed frontier.
  • Restore (subsequent invocation): world.snapshots.load → decrypt → decompress → QuickJS.restore over the cached WASM module, re-register host callbacks, fetch events from the snapshot's cursor and feed only the delta. Runs seamlessly through PR 2's inline continuation loop.
  • Delete on run completion/failure.
  • Fallback is always full replay: missing snapshot, load error, corrupt bytes, or restore failure logs a warning and boots fresh against the full event log — the log remains the source of truth; snapshots are strictly an optimization.
Determinism model (restore + partial replay)
The threshold model's new mechanism vs. the original branch: a resumption may restore a snapshot older than the log head (suspensions since the snapshot weren't persisted) and must deterministically re-derive everything in between:
  • The PRNG seed mixes in the restored snapshot's eventsCursor: the heap already consumed pre-snapshot draws, so re-seeding from the base would replay the first-N draws and collide with recorded correlationIds. The cursor is identical for every resume from the same snapshot (concurrent resumes still collide ids for the world's dedup) and advances only when a newer snapshot is taken.
  • Feeding already-consumed events is harmless by construction (consumed resolvers are gone; hook deliveries are deduped by eventId in the VM heap, which travels with the snapshot), so imprecise cursors only cost redundant scanning.
  • Covered by dedicated unit tests, including restore-from-older-snapshot with multi-suspension partial replay and identical post-restore correlationIds across concurrent resumes.
Validation
  • 135/135 e2e on nextjs-turbopack with WORKFLOW_SNAPSHOT_THRESHOLD=1 (maximum churn: snapshot on every qualifying suspension), wall clock within ~10% of the node baseline
  • Verified via debug diagnostics: restored: true resumptions, save/restore/delete lifecycle, and threshold gating (threshold=100 short run ⇒ zero snapshots, pure replay)
  • Full core unit suite green (1,573 tests); new tests for the config knobs and the snapshot/restore/partial-replay determinism
  • CI: new quickjs-snapshot matrix leg (nextjs-turbopack, threshold=1) across local dev/prod/postgres e2e jobs
Notes / follow-ups
  • Version skew: snapshot bytes are tied to the quickjs-wasi build that produced them. Per the deployment contract (runs continue on the version they started on — free on Vercel), this is a non-issue in production; environments without skew protection are covered by the restore-failure fallback to full replay.
  • An ID-divergence window exists when concurrent invocations resume from different snapshot generations; the world's per-(run, correlation) uniqueness rejects duplicates and the log-consistent invocation drives progress, with full-replay convergence as the backstop. Noted in code comments.
  • Docs: WORKFLOW_SNAPSHOT_THRESHOLD section added to v5 Runtime Tuning.
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🦋 Changeset detected
Latest commit: f1b1e10
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 16 packages
NameType
@workflow/coreMinor
workflowMinor
@workflow/buildersPatch
@workflow/cliPatch
@workflow/nextPatch
@workflow/nitroPatch
@workflow/vitestPatch
@workflow/web-sharedPatch
@workflow/webPatch
@workflow/world-testingPatch
@workflow/astroPatch
@workflow/nestPatch
@workflow/rollupPatch
@workflow/sveltekitPatch
@workflow/vitePatch
@workflow/nuxtPatch
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ProjectDeploymentActionsUpdated (UTC)
example-nextjs-workflow-turbopackReadyReadyPreviewJul 31, 2026 1:13am
example-nextjs-workflow-webpackReadyReadyPreviewJul 31, 2026 1:13am
example-workflowReadyReadyPreviewJul 31, 2026 1:13am
workbench-astro-workflowReadyReadyPreviewJul 31, 2026 1:13am
workbench-express-workflowReadyReadyPreviewJul 31, 2026 1:13am
workbench-fastify-workflowReadyReadyPreviewJul 31, 2026 1:13am
workbench-hono-workflowReadyReadyPreviewJul 31, 2026 1:13am
workbench-nestjs-workflowReadyReadyPreviewJul 31, 2026 1:13am
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workbench-tanstack-start-workflowReadyReadyPreviewJul 31, 2026 1:13am
workbench-vite-workflowReadyReadyPreviewJul 31, 2026 1:13am
workflow-docsBuildingBuildingPreview, v0Jul 31, 2026 1:13am
workflow-swc-playgroundReadyReadyPreviewJul 31, 2026 1:13am
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🧪 E2E Test Results
✅ All tests passed
E2E Test Summary
Summary
PassedFailedSkippedTotal
✅ ▲ Vercel Production145502391694
✅ 💻 Local Development312604163542
✅ 📦 Local Production339604543850
✅ 🐘 Local Postgres339604543850
✅ 🪟 Windows30800308
✅ 📋 Other178803682156
✅ vercel-multi-region270027
Total134960193115427
Details by Category
✅ ▲ Vercel Production
AppPassedFailedSkipped
✅ astro-node126028
✅ example-node126028
✅ express-node126028
✅ fastify-node126028
✅ hono-node126028
✅ nextjs-turbopack-node15103
✅ nextjs-webpack-node15103
✅ nitro-node126028
✅ nuxt-node126028
✅ sveltekit-node14509
✅ vite-node126028
✅ 💻 Local Development
AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026
✅ 📦 Local Production
AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-canary-node135019
✅ nextjs-webpack-canary-quickjs135019
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026
✅ 🐘 Local Postgres
AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-canary-node135019
✅ nextjs-webpack-canary-quickjs135019
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) by TooTallNate · Pull Request #3053 · vercel/workflow · GitHub
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Stacked PR — based on #3050 (quickjs-vm-snapshots) ← #3049 ← #3048. Review only the top commit here until the bases merge.
Summary
PR 4 of the QuickJS VM roadmap: threshold-based VM-memory snapshotting — the middle ground that motivated reviving this effort (see #1298 / #1300 discussion). Instead of snapshotting at every suspension (the original branch's model, which cost ~25% on e2e wall clock), snapshots are taken only once WORKFLOW_SNAPSHOT_THRESHOLD events have been processed since the last one:
  • Short-lived runs never snapshot — they keep PR 1/2's pure replay behavior with zero snapshot overhead.
  • Long/forever runs stop scaling their resume cost with event-log length — a resumption restores the VM heap and replays only the delta events since the snapshot's cursor.
How it works
  • WORKFLOW_SNAPSHOT_THRESHOLD env var (default 0 = disabled) or per-run executionContext.snapshotThreshold, stamped at start() for run affinity like WORKFLOW_VM.
  • Save (suspension exit, threshold met): capture live VM memory (session.snapshot()) → compress (zstd/gzip via the shared serialization pipeline; QuickJS heaps compress ~4×, measured 16.5 MB → 3.9 MB) → encrypt with the run's key when configured → world.snapshots.save with the events cursor at the VM's feed frontier.
  • Restore (subsequent invocation): world.snapshots.load → decrypt → decompress → QuickJS.restore over the cached WASM module, re-register host callbacks, fetch events from the snapshot's cursor and feed only the delta. Runs seamlessly through PR 2's inline continuation loop.
  • Delete on run completion/failure.
  • Fallback is always full replay: missing snapshot, load error, corrupt bytes, or restore failure logs a warning and boots fresh against the full event log — the log remains the source of truth; snapshots are strictly an optimization.
Determinism model (restore + partial replay)
The threshold model's new mechanism vs. the original branch: a resumption may restore a snapshot older than the log head (suspensions since the snapshot weren't persisted) and must deterministically re-derive everything in between:
  • The PRNG seed mixes in the restored snapshot's eventsCursor: the heap already consumed pre-snapshot draws, so re-seeding from the base would replay the first-N draws and collide with recorded correlationIds. The cursor is identical for every resume from the same snapshot (concurrent resumes still collide ids for the world's dedup) and advances only when a newer snapshot is taken.
  • Feeding already-consumed events is harmless by construction (consumed resolvers are gone; hook deliveries are deduped by eventId in the VM heap, which travels with the snapshot), so imprecise cursors only cost redundant scanning.
  • Covered by dedicated unit tests, including restore-from-older-snapshot with multi-suspension partial replay and identical post-restore correlationIds across concurrent resumes.
Validation
  • 135/135 e2e on nextjs-turbopack with WORKFLOW_SNAPSHOT_THRESHOLD=1 (maximum churn: snapshot on every qualifying suspension), wall clock within ~10% of the node baseline
  • Verified via debug diagnostics: restored: true resumptions, save/restore/delete lifecycle, and threshold gating (threshold=100 short run ⇒ zero snapshots, pure replay)
  • Full core unit suite green (1,573 tests); new tests for the config knobs and the snapshot/restore/partial-replay determinism
  • CI: new quickjs-snapshot matrix leg (nextjs-turbopack, threshold=1) across local dev/prod/postgres e2e jobs
Notes / follow-ups
  • Version skew: snapshot bytes are tied to the quickjs-wasi build that produced them. Per the deployment contract (runs continue on the version they started on — free on Vercel), this is a non-issue in production; environments without skew protection are covered by the restore-failure fallback to full replay.
  • An ID-divergence window exists when concurrent invocations resume from different snapshot generations; the world's per-(run, correlation) uniqueness rejects duplicates and the log-consistent invocation drives progress, with full-replay convergence as the backstop. Noted in code comments.
  • Docs: WORKFLOW_SNAPSHOT_THRESHOLD section added to v5 Runtime Tuning.
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🧪 E2E Test Results
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✅ 💻 Local Development312604163542
✅ 📦 Local Production339604543850
✅ 🐘 Local Postgres339604543850
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✅ nextjs-webpack-stable-quickjs15400
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✅ sveltekit-stable-node14707
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✅ vite-stable-quickjs128026
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) by TooTallNate · Pull Request #3053 · vercel/workflow · GitHub
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) - #3053
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD)#3053
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Stacked PR — based on #3050 (quickjs-vm-snapshots) ← #3049 ← #3048. Review only the top commit here until the bases merge.
Summary
PR 4 of the QuickJS VM roadmap: threshold-based VM-memory snapshotting — the middle ground that motivated reviving this effort (see #1298 / #1300 discussion). Instead of snapshotting at every suspension (the original branch's model, which cost ~25% on e2e wall clock), snapshots are taken only once WORKFLOW_SNAPSHOT_THRESHOLD events have been processed since the last one:
  • Short-lived runs never snapshot — they keep PR 1/2's pure replay behavior with zero snapshot overhead.
  • Long/forever runs stop scaling their resume cost with event-log length — a resumption restores the VM heap and replays only the delta events since the snapshot's cursor.
How it works
  • WORKFLOW_SNAPSHOT_THRESHOLD env var (default 0 = disabled) or per-run executionContext.snapshotThreshold, stamped at start() for run affinity like WORKFLOW_VM.
  • Save (suspension exit, threshold met): capture live VM memory (session.snapshot()) → compress (zstd/gzip via the shared serialization pipeline; QuickJS heaps compress ~4×, measured 16.5 MB → 3.9 MB) → encrypt with the run's key when configured → world.snapshots.save with the events cursor at the VM's feed frontier.
  • Restore (subsequent invocation): world.snapshots.load → decrypt → decompress → QuickJS.restore over the cached WASM module, re-register host callbacks, fetch events from the snapshot's cursor and feed only the delta. Runs seamlessly through PR 2's inline continuation loop.
  • Delete on run completion/failure.
  • Fallback is always full replay: missing snapshot, load error, corrupt bytes, or restore failure logs a warning and boots fresh against the full event log — the log remains the source of truth; snapshots are strictly an optimization.
Determinism model (restore + partial replay)
The threshold model's new mechanism vs. the original branch: a resumption may restore a snapshot older than the log head (suspensions since the snapshot weren't persisted) and must deterministically re-derive everything in between:
  • The PRNG seed mixes in the restored snapshot's eventsCursor: the heap already consumed pre-snapshot draws, so re-seeding from the base would replay the first-N draws and collide with recorded correlationIds. The cursor is identical for every resume from the same snapshot (concurrent resumes still collide ids for the world's dedup) and advances only when a newer snapshot is taken.
  • Feeding already-consumed events is harmless by construction (consumed resolvers are gone; hook deliveries are deduped by eventId in the VM heap, which travels with the snapshot), so imprecise cursors only cost redundant scanning.
  • Covered by dedicated unit tests, including restore-from-older-snapshot with multi-suspension partial replay and identical post-restore correlationIds across concurrent resumes.
Validation
  • 135/135 e2e on nextjs-turbopack with WORKFLOW_SNAPSHOT_THRESHOLD=1 (maximum churn: snapshot on every qualifying suspension), wall clock within ~10% of the node baseline
  • Verified via debug diagnostics: restored: true resumptions, save/restore/delete lifecycle, and threshold gating (threshold=100 short run ⇒ zero snapshots, pure replay)
  • Full core unit suite green (1,573 tests); new tests for the config knobs and the snapshot/restore/partial-replay determinism
  • CI: new quickjs-snapshot matrix leg (nextjs-turbopack, threshold=1) across local dev/prod/postgres e2e jobs
Notes / follow-ups
  • Version skew: snapshot bytes are tied to the quickjs-wasi build that produced them. Per the deployment contract (runs continue on the version they started on — free on Vercel), this is a non-issue in production; environments without skew protection are covered by the restore-failure fallback to full replay.
  • An ID-divergence window exists when concurrent invocations resume from different snapshot generations; the world's per-(run, correlation) uniqueness rejects duplicates and the log-consistent invocation drives progress, with full-replay convergence as the backstop. Noted in code comments.
  • Docs: WORKFLOW_SNAPSHOT_THRESHOLD section added to v5 Runtime Tuning.
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Latest commit: f1b1e10
The changes in this PR will be included in the next version bump.
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) by TooTallNate · Pull Request #3053 · vercel/workflow · GitHub
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) - #3053
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD)#3053
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Stacked PR — based on #3050 (quickjs-vm-snapshots) ← #3049 ← #3048. Review only the top commit here until the bases merge.
Summary
PR 4 of the QuickJS VM roadmap: threshold-based VM-memory snapshotting — the middle ground that motivated reviving this effort (see #1298 / #1300 discussion). Instead of snapshotting at every suspension (the original branch's model, which cost ~25% on e2e wall clock), snapshots are taken only once WORKFLOW_SNAPSHOT_THRESHOLD events have been processed since the last one:
  • Short-lived runs never snapshot — they keep PR 1/2's pure replay behavior with zero snapshot overhead.
  • Long/forever runs stop scaling their resume cost with event-log length — a resumption restores the VM heap and replays only the delta events since the snapshot's cursor.
How it works
  • WORKFLOW_SNAPSHOT_THRESHOLD env var (default 0 = disabled) or per-run executionContext.snapshotThreshold, stamped at start() for run affinity like WORKFLOW_VM.
  • Save (suspension exit, threshold met): capture live VM memory (session.snapshot()) → compress (zstd/gzip via the shared serialization pipeline; QuickJS heaps compress ~4×, measured 16.5 MB → 3.9 MB) → encrypt with the run's key when configured → world.snapshots.save with the events cursor at the VM's feed frontier.
  • Restore (subsequent invocation): world.snapshots.load → decrypt → decompress → QuickJS.restore over the cached WASM module, re-register host callbacks, fetch events from the snapshot's cursor and feed only the delta. Runs seamlessly through PR 2's inline continuation loop.
  • Delete on run completion/failure.
  • Fallback is always full replay: missing snapshot, load error, corrupt bytes, or restore failure logs a warning and boots fresh against the full event log — the log remains the source of truth; snapshots are strictly an optimization.
Determinism model (restore + partial replay)
The threshold model's new mechanism vs. the original branch: a resumption may restore a snapshot older than the log head (suspensions since the snapshot weren't persisted) and must deterministically re-derive everything in between:
  • The PRNG seed mixes in the restored snapshot's eventsCursor: the heap already consumed pre-snapshot draws, so re-seeding from the base would replay the first-N draws and collide with recorded correlationIds. The cursor is identical for every resume from the same snapshot (concurrent resumes still collide ids for the world's dedup) and advances only when a newer snapshot is taken.
  • Feeding already-consumed events is harmless by construction (consumed resolvers are gone; hook deliveries are deduped by eventId in the VM heap, which travels with the snapshot), so imprecise cursors only cost redundant scanning.
  • Covered by dedicated unit tests, including restore-from-older-snapshot with multi-suspension partial replay and identical post-restore correlationIds across concurrent resumes.
Validation
  • 135/135 e2e on nextjs-turbopack with WORKFLOW_SNAPSHOT_THRESHOLD=1 (maximum churn: snapshot on every qualifying suspension), wall clock within ~10% of the node baseline
  • Verified via debug diagnostics: restored: true resumptions, save/restore/delete lifecycle, and threshold gating (threshold=100 short run ⇒ zero snapshots, pure replay)
  • Full core unit suite green (1,573 tests); new tests for the config knobs and the snapshot/restore/partial-replay determinism
  • CI: new quickjs-snapshot matrix leg (nextjs-turbopack, threshold=1) across local dev/prod/postgres e2e jobs
Notes / follow-ups
  • Version skew: snapshot bytes are tied to the quickjs-wasi build that produced them. Per the deployment contract (runs continue on the version they started on — free on Vercel), this is a non-issue in production; environments without skew protection are covered by the restore-failure fallback to full replay.
  • An ID-divergence window exists when concurrent invocations resume from different snapshot generations; the world's per-(run, correlation) uniqueness rejects duplicates and the log-consistent invocation drives progress, with full-replay convergence as the backstop. Noted in code comments.
  • Docs: WORKFLOW_SNAPSHOT_THRESHOLD section added to v5 Runtime Tuning.
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Latest commit: f1b1e10
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 16 packages
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✅ 🐘 Local Postgres339604543850
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) by TooTallNate · Pull Request #3053 · vercel/workflow · GitHub
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) - #3053
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD)#3053
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Stacked PR — based on #3050 (quickjs-vm-snapshots) ← #3049 ← #3048. Review only the top commit here until the bases merge.
Summary
PR 4 of the QuickJS VM roadmap: threshold-based VM-memory snapshotting — the middle ground that motivated reviving this effort (see #1298 / #1300 discussion). Instead of snapshotting at every suspension (the original branch's model, which cost ~25% on e2e wall clock), snapshots are taken only once WORKFLOW_SNAPSHOT_THRESHOLD events have been processed since the last one:
  • Short-lived runs never snapshot — they keep PR 1/2's pure replay behavior with zero snapshot overhead.
  • Long/forever runs stop scaling their resume cost with event-log length — a resumption restores the VM heap and replays only the delta events since the snapshot's cursor.
How it works
  • WORKFLOW_SNAPSHOT_THRESHOLD env var (default 0 = disabled) or per-run executionContext.snapshotThreshold, stamped at start() for run affinity like WORKFLOW_VM.
  • Save (suspension exit, threshold met): capture live VM memory (session.snapshot()) → compress (zstd/gzip via the shared serialization pipeline; QuickJS heaps compress ~4×, measured 16.5 MB → 3.9 MB) → encrypt with the run's key when configured → world.snapshots.save with the events cursor at the VM's feed frontier.
  • Restore (subsequent invocation): world.snapshots.load → decrypt → decompress → QuickJS.restore over the cached WASM module, re-register host callbacks, fetch events from the snapshot's cursor and feed only the delta. Runs seamlessly through PR 2's inline continuation loop.
  • Delete on run completion/failure.
  • Fallback is always full replay: missing snapshot, load error, corrupt bytes, or restore failure logs a warning and boots fresh against the full event log — the log remains the source of truth; snapshots are strictly an optimization.
Determinism model (restore + partial replay)
The threshold model's new mechanism vs. the original branch: a resumption may restore a snapshot older than the log head (suspensions since the snapshot weren't persisted) and must deterministically re-derive everything in between:
  • The PRNG seed mixes in the restored snapshot's eventsCursor: the heap already consumed pre-snapshot draws, so re-seeding from the base would replay the first-N draws and collide with recorded correlationIds. The cursor is identical for every resume from the same snapshot (concurrent resumes still collide ids for the world's dedup) and advances only when a newer snapshot is taken.
  • Feeding already-consumed events is harmless by construction (consumed resolvers are gone; hook deliveries are deduped by eventId in the VM heap, which travels with the snapshot), so imprecise cursors only cost redundant scanning.
  • Covered by dedicated unit tests, including restore-from-older-snapshot with multi-suspension partial replay and identical post-restore correlationIds across concurrent resumes.
Validation
  • 135/135 e2e on nextjs-turbopack with WORKFLOW_SNAPSHOT_THRESHOLD=1 (maximum churn: snapshot on every qualifying suspension), wall clock within ~10% of the node baseline
  • Verified via debug diagnostics: restored: true resumptions, save/restore/delete lifecycle, and threshold gating (threshold=100 short run ⇒ zero snapshots, pure replay)
  • Full core unit suite green (1,573 tests); new tests for the config knobs and the snapshot/restore/partial-replay determinism
  • CI: new quickjs-snapshot matrix leg (nextjs-turbopack, threshold=1) across local dev/prod/postgres e2e jobs
Notes / follow-ups
  • Version skew: snapshot bytes are tied to the quickjs-wasi build that produced them. Per the deployment contract (runs continue on the version they started on — free on Vercel), this is a non-issue in production; environments without skew protection are covered by the restore-failure fallback to full replay.
  • An ID-divergence window exists when concurrent invocations resume from different snapshot generations; the world's per-(run, correlation) uniqueness rejects duplicates and the log-consistent invocation drives progress, with full-replay convergence as the backstop. Noted in code comments.
  • Docs: WORKFLOW_SNAPSHOT_THRESHOLD section added to v5 Runtime Tuning.
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🦋 Changeset detected
Latest commit: f1b1e10
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 16 packages
NameType
@workflow/coreMinor
workflowMinor
@workflow/buildersPatch
@workflow/cliPatch
@workflow/nextPatch
@workflow/nitroPatch
@workflow/vitestPatch
@workflow/web-sharedPatch
@workflow/webPatch
@workflow/world-testingPatch
@workflow/astroPatch
@workflow/nestPatch
@workflow/rollupPatch
@workflow/sveltekitPatch
@workflow/vitePatch
@workflow/nuxtPatch
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ProjectDeploymentActionsUpdated (UTC)
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🧪 E2E Test Results
✅ All tests passed
E2E Test Summary
Summary
PassedFailedSkippedTotal
✅ ▲ Vercel Production145502391694
✅ 💻 Local Development312604163542
✅ 📦 Local Production339604543850
✅ 🐘 Local Postgres339604543850
✅ 🪟 Windows30800308
✅ 📋 Other178803682156
✅ vercel-multi-region270027
Total134960193115427
Details by Category
✅ ▲ Vercel Production
AppPassedFailedSkipped
✅ astro-node126028
✅ example-node126028
✅ express-node126028
✅ fastify-node126028
✅ hono-node126028
✅ nextjs-turbopack-node15103
✅ nextjs-webpack-node15103
✅ nitro-node126028
✅ nuxt-node126028
✅ sveltekit-node14509
✅ vite-node126028
✅ 💻 Local Development
AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026
✅ 📦 Local Production
AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-canary-node135019
✅ nextjs-webpack-canary-quickjs135019
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026
✅ 🐘 Local Postgres
AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-canary-node135019
✅ nextjs-webpack-canary-quickjs135019
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026
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✅ nextjs-turbopack-quickjs15400
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✅ e2e-local-dev-nest-stable-quickjs128026
✅ e2e-local-dev-tanstack-start-node128026
✅ e2e-local-dev-tanstack-start-quickjs128026
✅ e2e-local-postgres-nest-stable-node128026
✅ e2e-local-postgres-nest-stable-quickjs128026
✅ e2e-local-postgres-tanstack-start-node128026
✅ e2e-local-postgres-tanstack-start-quickjs128026
✅ e2e-local-prod-nest-stable-node128026
✅ e2e-local-prod-nest-stable-quickjs128026
✅ e2e-local-prod-tanstack-start-node128026
✅ e2e-local-prod-tanstack-start-quickjs128026
✅ e2e-vercel-prod-nest-node126028
✅ e2e-vercel-prod-tanstack-start-node126028
✅ vercel-multi-region
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✅ nextjs-turbopack2700
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) by TooTallNate · Pull Request #3053 · vercel/workflow · GitHub
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) - #3053
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quickjs-vm-snapshotsfrom 
quickjs-vm-threshold-snapshots
Jul 31, 2026
Merged
QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD)#3053
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Stacked PR — based on #3050 (quickjs-vm-snapshots) ← #3049 ← #3048. Review only the top commit here until the bases merge.
Summary
PR 4 of the QuickJS VM roadmap: threshold-based VM-memory snapshotting — the middle ground that motivated reviving this effort (see #1298 / #1300 discussion). Instead of snapshotting at every suspension (the original branch's model, which cost ~25% on e2e wall clock), snapshots are taken only once WORKFLOW_SNAPSHOT_THRESHOLD events have been processed since the last one:
  • Short-lived runs never snapshot — they keep PR 1/2's pure replay behavior with zero snapshot overhead.
  • Long/forever runs stop scaling their resume cost with event-log length — a resumption restores the VM heap and replays only the delta events since the snapshot's cursor.
How it works
  • WORKFLOW_SNAPSHOT_THRESHOLD env var (default 0 = disabled) or per-run executionContext.snapshotThreshold, stamped at start() for run affinity like WORKFLOW_VM.
  • Save (suspension exit, threshold met): capture live VM memory (session.snapshot()) → compress (zstd/gzip via the shared serialization pipeline; QuickJS heaps compress ~4×, measured 16.5 MB → 3.9 MB) → encrypt with the run's key when configured → world.snapshots.save with the events cursor at the VM's feed frontier.
  • Restore (subsequent invocation): world.snapshots.load → decrypt → decompress → QuickJS.restore over the cached WASM module, re-register host callbacks, fetch events from the snapshot's cursor and feed only the delta. Runs seamlessly through PR 2's inline continuation loop.
  • Delete on run completion/failure.
  • Fallback is always full replay: missing snapshot, load error, corrupt bytes, or restore failure logs a warning and boots fresh against the full event log — the log remains the source of truth; snapshots are strictly an optimization.
Determinism model (restore + partial replay)
The threshold model's new mechanism vs. the original branch: a resumption may restore a snapshot older than the log head (suspensions since the snapshot weren't persisted) and must deterministically re-derive everything in between:
  • The PRNG seed mixes in the restored snapshot's eventsCursor: the heap already consumed pre-snapshot draws, so re-seeding from the base would replay the first-N draws and collide with recorded correlationIds. The cursor is identical for every resume from the same snapshot (concurrent resumes still collide ids for the world's dedup) and advances only when a newer snapshot is taken.
  • Feeding already-consumed events is harmless by construction (consumed resolvers are gone; hook deliveries are deduped by eventId in the VM heap, which travels with the snapshot), so imprecise cursors only cost redundant scanning.
  • Covered by dedicated unit tests, including restore-from-older-snapshot with multi-suspension partial replay and identical post-restore correlationIds across concurrent resumes.
Validation
  • 135/135 e2e on nextjs-turbopack with WORKFLOW_SNAPSHOT_THRESHOLD=1 (maximum churn: snapshot on every qualifying suspension), wall clock within ~10% of the node baseline
  • Verified via debug diagnostics: restored: true resumptions, save/restore/delete lifecycle, and threshold gating (threshold=100 short run ⇒ zero snapshots, pure replay)
  • Full core unit suite green (1,573 tests); new tests for the config knobs and the snapshot/restore/partial-replay determinism
  • CI: new quickjs-snapshot matrix leg (nextjs-turbopack, threshold=1) across local dev/prod/postgres e2e jobs
Notes / follow-ups
  • Version skew: snapshot bytes are tied to the quickjs-wasi build that produced them. Per the deployment contract (runs continue on the version they started on — free on Vercel), this is a non-issue in production; environments without skew protection are covered by the restore-failure fallback to full replay.
  • An ID-divergence window exists when concurrent invocations resume from different snapshot generations; the world's per-(run, correlation) uniqueness rejects duplicates and the log-consistent invocation drives progress, with full-replay convergence as the backstop. Noted in code comments.
  • Docs: WORKFLOW_SNAPSHOT_THRESHOLD section added to v5 Runtime Tuning.
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🦋 Changeset detected
Latest commit: f1b1e10
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 16 packages
NameType
@workflow/coreMinor
workflowMinor
@workflow/buildersPatch
@workflow/cliPatch
@workflow/nextPatch
@workflow/nitroPatch
@workflow/vitestPatch
@workflow/web-sharedPatch
@workflow/webPatch
@workflow/world-testingPatch
@workflow/astroPatch
@workflow/nestPatch
@workflow/rollupPatch
@workflow/sveltekitPatch
@workflow/vitePatch
@workflow/nuxtPatch
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example-nextjs-workflow-webpackReadyReadyPreviewJul 31, 2026 1:13am
example-workflowReadyReadyPreviewJul 31, 2026 1:13am
workbench-astro-workflowReadyReadyPreviewJul 31, 2026 1:13am
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workbench-fastify-workflowReadyReadyPreviewJul 31, 2026 1:13am
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🧪 E2E Test Results
✅ All tests passed
E2E Test Summary
Summary
PassedFailedSkippedTotal
✅ ▲ Vercel Production145502391694
✅ 💻 Local Development312604163542
✅ 📦 Local Production339604543850
✅ 🐘 Local Postgres339604543850
✅ 🪟 Windows30800308
✅ 📋 Other178803682156
✅ vercel-multi-region270027
Total134960193115427
Details by Category
✅ ▲ Vercel Production
AppPassedFailedSkipped
✅ astro-node126028
✅ example-node126028
✅ express-node126028
✅ fastify-node126028
✅ hono-node126028
✅ nextjs-turbopack-node15103
✅ nextjs-webpack-node15103
✅ nitro-node126028
✅ nuxt-node126028
✅ sveltekit-node14509
✅ vite-node126028
✅ 💻 Local Development
AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026
✅ 📦 Local Production
AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-canary-node135019
✅ nextjs-webpack-canary-quickjs135019
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026
✅ 🐘 Local Postgres
AppPassedFailedSkipped
✅ astro-stable-node128026
✅ astro-stable-quickjs128026
✅ express-stable-node128026
✅ express-stable-quickjs128026
✅ fastify-stable-node128026
✅ fastify-stable-quickjs128026
✅ hono-stable-node128026
✅ hono-stable-quickjs128026
✅ nextjs-turbopack-canary-node135019
✅ nextjs-turbopack-canary-quickjs135019
✅ nextjs-turbopack-quickjs-snapshot15400
✅ nextjs-turbopack-stable-node15400
✅ nextjs-turbopack-stable-quickjs15400
✅ nextjs-webpack-canary-node135019
✅ nextjs-webpack-canary-quickjs135019
✅ nextjs-webpack-stable-node15400
✅ nextjs-webpack-stable-quickjs15400
✅ nitro-stable-node128026
✅ nitro-stable-quickjs128026
✅ nuxt-stable-node128026
✅ nuxt-stable-quickjs128026
✅ sveltekit-stable-node14707
✅ sveltekit-stable-quickjs14707
✅ vite-stable-node128026
✅ vite-stable-quickjs128026
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) by TooTallNate · Pull Request #3053 · vercel/workflow · GitHub
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) - #3053
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QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD)#3053
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Stacked PR — based on #3050 (quickjs-vm-snapshots) ← #3049 ← #3048. Review only the top commit here until the bases merge.
Summary
PR 4 of the QuickJS VM roadmap: threshold-based VM-memory snapshotting — the middle ground that motivated reviving this effort (see #1298 / #1300 discussion). Instead of snapshotting at every suspension (the original branch's model, which cost ~25% on e2e wall clock), snapshots are taken only once WORKFLOW_SNAPSHOT_THRESHOLD events have been processed since the last one:
  • Short-lived runs never snapshot — they keep PR 1/2's pure replay behavior with zero snapshot overhead.
  • Long/forever runs stop scaling their resume cost with event-log length — a resumption restores the VM heap and replays only the delta events since the snapshot's cursor.
How it works
  • WORKFLOW_SNAPSHOT_THRESHOLD env var (default 0 = disabled) or per-run executionContext.snapshotThreshold, stamped at start() for run affinity like WORKFLOW_VM.
  • Save (suspension exit, threshold met): capture live VM memory (session.snapshot()) → compress (zstd/gzip via the shared serialization pipeline; QuickJS heaps compress ~4×, measured 16.5 MB → 3.9 MB) → encrypt with the run's key when configured → world.snapshots.save with the events cursor at the VM's feed frontier.
  • Restore (subsequent invocation): world.snapshots.load → decrypt → decompress → QuickJS.restore over the cached WASM module, re-register host callbacks, fetch events from the snapshot's cursor and feed only the delta. Runs seamlessly through PR 2's inline continuation loop.
  • Delete on run completion/failure.
  • Fallback is always full replay: missing snapshot, load error, corrupt bytes, or restore failure logs a warning and boots fresh against the full event log — the log remains the source of truth; snapshots are strictly an optimization.
Determinism model (restore + partial replay)
The threshold model's new mechanism vs. the original branch: a resumption may restore a snapshot older than the log head (suspensions since the snapshot weren't persisted) and must deterministically re-derive everything in between:
  • The PRNG seed mixes in the restored snapshot's eventsCursor: the heap already consumed pre-snapshot draws, so re-seeding from the base would replay the first-N draws and collide with recorded correlationIds. The cursor is identical for every resume from the same snapshot (concurrent resumes still collide ids for the world's dedup) and advances only when a newer snapshot is taken.
  • Feeding already-consumed events is harmless by construction (consumed resolvers are gone; hook deliveries are deduped by eventId in the VM heap, which travels with the snapshot), so imprecise cursors only cost redundant scanning.
  • Covered by dedicated unit tests, including restore-from-older-snapshot with multi-suspension partial replay and identical post-restore correlationIds across concurrent resumes.
Validation
  • 135/135 e2e on nextjs-turbopack with WORKFLOW_SNAPSHOT_THRESHOLD=1 (maximum churn: snapshot on every qualifying suspension), wall clock within ~10% of the node baseline
  • Verified via debug diagnostics: restored: true resumptions, save/restore/delete lifecycle, and threshold gating (threshold=100 short run ⇒ zero snapshots, pure replay)
  • Full core unit suite green (1,573 tests); new tests for the config knobs and the snapshot/restore/partial-replay determinism
  • CI: new quickjs-snapshot matrix leg (nextjs-turbopack, threshold=1) across local dev/prod/postgres e2e jobs
Notes / follow-ups
  • Version skew: snapshot bytes are tied to the quickjs-wasi build that produced them. Per the deployment contract (runs continue on the version they started on — free on Vercel), this is a non-issue in production; environments without skew protection are covered by the restore-failure fallback to full replay.
  • An ID-divergence window exists when concurrent invocations resume from different snapshot generations; the world's per-(run, correlation) uniqueness rejects duplicates and the log-consistent invocation drives progress, with full-replay convergence as the backstop. Noted in code comments.
  • Docs: WORKFLOW_SNAPSHOT_THRESHOLD section added to v5 Runtime Tuning.
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Latest commit: f1b1e10
The changes in this PR will be included in the next version bump.
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