feat(streams): add writer session seam by alangenfeld · Pull Request #3832 · vercel/workflow

alangenfeld · 2026-08-26T22:13:57Z

Summary & Motivation

Gives one in-memory stream writer a stable identity and its own sequence space, so a transport can preserve chunk ordering across a mid-stream HTTP/WebSocket transition. Streamer.streams.createWriteSession is optional — Worlds that don't implement it keep using write/writeMulti/close unchanged.

Abort disposes the session rather than closing it, since a producer failure is transport cleanup, not stream completion.

Test Plan

Tests added, plus the full @workflow/world-vercel suite and package builds/typecheck pass. Root build/typecheck is blocked locally by a missing Rust toolchain for the unrelated @workflow/swc-plugin.

changeset-bot · 2026-08-26T22:14:03Z

🦋 Changeset detected

Latest commit: 3611997

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

This PR includes changesets to release 20 packages

Name	Type
@workflow/core	Patch
@workflow/world	Minor
@workflow/builders	Patch
@workflow/cli	Patch
@workflow/next	Patch
@workflow/nitro	Patch
@workflow/vitest	Patch
@workflow/web-shared	Patch
@workflow/web	Patch
workflow	Patch
@workflow/world-testing	Patch
@workflow/world-local	Patch
@workflow/world-postgres	Patch
@workflow/world-vercel	Patch
@workflow/astro	Patch
@workflow/nest	Patch
@workflow/rollup	Patch
@workflow/sveltekit	Patch
@workflow/vite	Patch
@workflow/nuxt	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

vercel · 2026-08-26T22:14:04Z

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated
example-nextjs-workflow-turbopack	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
example-nextjs-workflow-webpack	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
example-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-astro-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-express-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-fastify-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-hono-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-nestjs-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-nitro-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-nuxt-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-python-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-sveltekit-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-tanstack-start-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workbench-vite-workflow	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workflow-docs	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workflow-swc-playground	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workflow-tarballs	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC
workflow-web	Ready	Preview, v0	Sep 9, 2026 4:52pm UTC

github-actions · 2026-08-26T22:14:16Z

🧪 E2E Test Results

✅ All tests passed

⚠️ Flaky E2E Tests (passed on retry)

These tests failed at least once and passed on a retry. A recurring entry here is a real race worth investigating.

addTenWorkflow (fastify)
addTenWorkflow (nuxt)
cancelRun via CLI - cancelling a running workflow (hono)
cancelRun via CLI - cancelling a running workflow (nextjs-turbopack)
cancelRun via CLI - cancelling a running workflow (sveltekit)
promiseAllWorkflow (sveltekit)
promiseAllWorkflow (tanstack-start)

🛠 Infra Events (absorbed by the harness)

Platform anomalies the e2e harness detected and worked around (e.g. a run the queue never picked up, replaced by a fresh run). Clustered timestamps indicate a backend blip; a steady drip indicates a platform issue worth escalating.

cold-start-warmup · suite warmup (tanstack-start) · at 16:54:42Z · abandoned wrun_01M23HEVF34EHZ1DQ706P0MEZR
run-pickup-stall · hookWithSleepWorkflow - hook payloads delivered correctly with concurrent sleep (nextjs-webpack) · at 16:59:04Z · abandoned wrun_01M23HQ7H3C6M5FKBJ0ST1F437

E2E Test Summary

Summary

	Passed	Skipped	Total
✅ ▲ Vercel Production	3662	685	4347
✅ 💻 Local Development	3922	586	4508
✅ 📦 Local Production	3922	586	4508
✅ 🐘 Local Postgres	3922	586	4508
✅ 🪟 Windows	320	2	322
✅ 🌐 Cross-language Conformance	68	74	142
✅ vercel-http-transport	823	143	966
✅ vercel-multi-region	27	0	27
✅ vercel-ws-transport	557	87	644
Total	17223	2749	19972

Details by Category

✅ ▲ Vercel Production

App	Passed	Skipped
✅ astro-node	133	28
✅ astro-quickjs	133	28
✅ example-node	133	28
✅ example-quickjs	133	28
✅ express-node	133	28
✅ express-quickjs	133	28
✅ fastify-node	133	28
✅ fastify-quickjs	133	28
✅ hono-node	133	28
✅ hono-quickjs	133	28
✅ nest-node	133	28
✅ nest-quickjs	133	28
✅ nextjs-turbopack-node	158	3
✅ nextjs-turbopack-quickjs	158	3
✅ nextjs-webpack-node	158	3
✅ nextjs-webpack-quickjs	158	3
✅ nitro-node	133	28
✅ nitro-quickjs	133	28
✅ nuxt-node	133	28
✅ nuxt-quickjs	133	28
✅ python-node	66	95
✅ sveltekit-node	152	9
✅ sveltekit-quickjs	152	9
✅ tanstack-start-node	133	28
✅ tanstack-start-quickjs	133	28
✅ vite-node	133	28
✅ vite-quickjs	133	28

✅ 💻 Local Development

App	Passed	Skipped
✅ astro-stable-node	134	27
✅ astro-stable-quickjs	134	27
✅ express-stable-node	134	27
✅ express-stable-quickjs	134	27
✅ fastify-stable-node	134	27
✅ fastify-stable-quickjs	134	27
✅ hono-stable-node	134	27
✅ hono-stable-quickjs	134	27
✅ nest-stable-node	134	27
✅ nest-stable-quickjs	134	27
✅ nextjs-turbopack-canary-node	141	20
✅ nextjs-turbopack-canary-quickjs	141	20
✅ nextjs-turbopack-stable-node	160	1
✅ nextjs-turbopack-stable-quickjs	160	1
✅ nextjs-webpack-canary-node	141	20
✅ nextjs-webpack-canary-quickjs	141	20
✅ nextjs-webpack-stable-node	160	1
✅ nextjs-webpack-stable-quickjs	160	1
✅ nitro-stable-node	134	27
✅ nitro-stable-quickjs	134	27
✅ nuxt-stable-node	134	27
✅ nuxt-stable-quickjs	134	27
✅ sveltekit-stable-node	153	8
✅ sveltekit-stable-quickjs	153	8
✅ tanstack-start-node	134	27
✅ tanstack-start-quickjs	134	27
✅ vite-stable-node	134	27
✅ vite-stable-quickjs	134	27

✅ 📦 Local Production

App	Passed	Skipped
✅ astro-stable-node	134	27
✅ astro-stable-quickjs	134	27
✅ express-stable-node	134	27
✅ express-stable-quickjs	134	27
✅ fastify-stable-node	134	27
✅ fastify-stable-quickjs	134	27
✅ hono-stable-node	134	27
✅ hono-stable-quickjs	134	27
✅ nest-stable-node	134	27
✅ nest-stable-quickjs	134	27
✅ nextjs-turbopack-canary-node	141	20
✅ nextjs-turbopack-canary-quickjs	141	20
✅ nextjs-turbopack-stable-node	160	1
✅ nextjs-turbopack-stable-quickjs	160	1
✅ nextjs-webpack-canary-node	141	20
✅ nextjs-webpack-canary-quickjs	141	20
✅ nextjs-webpack-stable-node	160	1
✅ nextjs-webpack-stable-quickjs	160	1
✅ nitro-stable-node	134	27
✅ nitro-stable-quickjs	134	27
✅ nuxt-stable-node	134	27
✅ nuxt-stable-quickjs	134	27
✅ sveltekit-stable-node	153	8
✅ sveltekit-stable-quickjs	153	8
✅ tanstack-start-node	134	27
✅ tanstack-start-quickjs	134	27
✅ vite-stable-node	134	27
✅ vite-stable-quickjs	134	27

✅ 🐘 Local Postgres

App	Passed	Skipped
✅ astro-stable-node	134	27
✅ astro-stable-quickjs	134	27
✅ express-stable-node	134	27
✅ express-stable-quickjs	134	27
✅ fastify-stable-node	134	27
✅ fastify-stable-quickjs	134	27
✅ hono-stable-node	134	27
✅ hono-stable-quickjs	134	27
✅ nest-stable-node	134	27
✅ nest-stable-quickjs	134	27
✅ nextjs-turbopack-canary-node	141	20
✅ nextjs-turbopack-canary-quickjs	141	20
✅ nextjs-turbopack-stable-node	160	1
✅ nextjs-turbopack-stable-quickjs	160	1
✅ nextjs-webpack-canary-node	141	20
✅ nextjs-webpack-canary-quickjs	141	20
✅ nextjs-webpack-stable-node	160	1
✅ nextjs-webpack-stable-quickjs	160	1
✅ nitro-stable-node	134	27
✅ nitro-stable-quickjs	134	27
✅ nuxt-stable-node	134	27
✅ nuxt-stable-quickjs	134	27
✅ sveltekit-stable-node	153	8
✅ sveltekit-stable-quickjs	153	8
✅ tanstack-start-node	134	27
✅ tanstack-start-quickjs	134	27
✅ vite-stable-node	134	27
✅ vite-stable-quickjs	134	27

✅ 🪟 Windows

App	Passed	Failed	Skipped
✅ nextjs-turbopack-node	160	0	1
✅ nextjs-turbopack-quickjs	160	0	1

✅ 🌐 Cross-language Conformance

App	Passed	Failed	Skipped
✅ python	68	0	74

✅ vercel-http-transport

App	Passed	Skipped
✅ example	133	28
✅ express	133	28
✅ hono	133	28
✅ nextjs-turbopack	158	3
✅ nitro	133	28
✅ vite	133	28

✅ vercel-multi-region

App	Passed	Failed	Skipped
✅ nextjs-turbopack	27	0	0

✅ vercel-ws-transport

App	Passed	Skipped
✅ example	133	28
✅ express	133	28
✅ nextjs-turbopack	158	3
✅ vite	133	28

📋 View full workflow run

github-actions · 2026-08-26T22:17:10Z

Sim World

Simulated world deterministic testing for races. Traces

🟠 world-sim scenario book — 1 fail of 41 total

fence=per-spec

scenario	outcome	events	virt	replay
✅ `smoke-no-steps`	completed	3	0ms	ok
✅ `smoke-one-step`	completed	6	0ms	ok
✅ `hook-at-step-started`	completed	12	0ms	ok
✅ `hook-at-step-completed`	completed	12	0ms	ok
✅ `hook-at-hook-created`	completed	12	0ms	ok
✅ `deadline-hook-wins`	completed	7	1.0h	ok
✅ `deadline-expires`	completed	7	1.0h	ok
✅ `long-sleep`	completed	11	30.0d	ok
✅ `hook-never-arrives`	stalled	3	0ms	skipped
✅ `step-retries-twice`	completed	10	2.0s	ok
✅ `parallel-steps`	completed	9	0ms	ok
✅ `hook-on-execution-state`	completed	12	0ms	ok
✅ `peek-hook-before-branch`	completed	12	0ms	ok
✅ `peek-hook-after-branch`	completed	12	0ms	ok
✅ `peek-hook-at-registration`	completed	12	0ms	ok
✅ `race-hook-before-probe`	completed	12	0ms	ok
✅ `race-hook-after-probe`	completed	12	0ms	ok
✅ `race-duplicate-delivery`	completed	13	0ms	ok
✅ `attr-hook-before-step`	completed	11	0ms	ok
✅ `attr-hook-after-step`	completed	11	0ms	ok
✅ `attr-from-step-body`	completed	13	0ms	ok
✅ `fork-hook-after-timeout`	completed	14	1.0m	ok
✅ `fork-hook-before-timeout`	completed	14	1.0m	ok
✅ `count-hook-after-timeout`	completed	17	1.0m	ok
✅ `count-hook-before-timeout`	completed	20	1.0m	ok
✅ `stale-read-step-count-fork`	completed	20	1.0m	ok
✅ `stale-read-equal-step-counts`	completed	14	1.0m	ok
✅ `step-vs-step-fork`	completed	12	0ms	ok
✅ `step-vs-step-fork-fenced`	completed	12	0ms	ok
✅ `fence-catches-benign-direction`	completed	12	5ms	ok
✅ `in-flight-before-decision`	completed	17	1.0m	ok
❌ `in-flight-before-decision-counted`	completed	17	1.0m	ok
✅ `in-flight-after-decision`	completed	19	2.0m	ok
✅ `stale-read-step-count-fork-fenced`	completed	20	1.0m	ok
✅ `fork-hook-wins`	completed	13	1.0m	ok
✅ `fork-timeout-wins`	completed	13	1.0m	ok
✅ `unclaimed-payload-under-fork`	completed	17	1.0m	ok
✅ `claimed-payload-under-fork`	completed	17	1.0m	ok
✅ `writers-independent-step-bodies`	completed	12	0ms	ok
✅ `writers-scripted-tempo`	completed	12	0ms	ok
✅ `cancel-mid-step`	cancelled	7	0ms	skipped

Full trace: world-sim.txt

github-actions · 2026-08-27T21:15:56Z

📊 Workflow Benchmarks

commit 3611997 · Wed, 09 Sep 2026 17:14:26 GMT · run logs

Backend: vercel · app: nextjs-turbopack

Metric	Scenario	Best (ms)	P75 (ms)	P90 (ms)	P99 (ms)	Samples
TTFS	step	279 (+43%) 🔻	1350 🔴 (+44%) 🔻	1397 🔴 (+11%)	1688 🔴 (+15%)	30
TTFS	stream	969 (+506%) 🔻	1328 🔴 (+44%) 🔻	1391 🔴 (+47%) 🔻	1607 🔴 (+62%) 🔻	30
TTFS	hook + stream	1131 (+139%) 🔻	1550 🔴 (+27%) 🔻	1591 🔴 (+23%) 🔻	1693 🔴 (-66%) 💚	30
Fan-out TTFS	Promise.all(100 steps)	625 (-2.0%)	2087 (+29%) 🔻	2582 (+58%) 🔻	6255 (+279%) 🔻	10
Fan-out TTLS	Promise.all(100 steps)	2141 (+24%) 🔻	4000 (+26%) 🔻	4659 (+26%) 🔻	12540 (+80%) 🔻	10
STSO	1020 steps (inline)	110 (+43%) 🔻	162 (+36%) 🔻	185 (+33%) 🔻	291 (+27%) 🔻	1019
WO	1020 steps	156719 (+29%) 🔻	156719 (+29%) 🔻	156719 (+29%) 🔻	156719 (+29%) 🔻	1
CRTT	first chunk (pooled)	64 (+25%) 🔻	111 (+23%) 🔻	136 (+6.3%)	301 (+28%) 🔻	28

Streams

Scenario	CRTT 1st	p75	p90	p99	CDV max	iters
paced control (100/s, 60B)	81 (+5%)	215 (+54%)	329 (+87%)	486 (+37%)	210 (+42%)	10
size sweep (100/s, 160B-12KB)	94.5 (+24%)	184 (+29%)	277 (-18%)	450 (-34%)	143 (-10%)	10
replay gateway-gpt-5.4-nano-2000t (1x)	81 (-10%)	847 (+547%)	3219 (+1518%)	4722 (+805%)	272 (-31%)	3
replay eve-gpt-5.6-sol-2000t (1x)	96 (+22%)	147 (+19%)	179 (-5%)	270 (-57%)	211 (-91%)	2
replay eve-gpt-5.6-sol-2000t (2x)	99 (+41%)	302 (-6%)	485 (+7%)	702 (+13%)	383 (+45%)	3

📈 STSO distribution vs main (inline / queue-hop histograms)

1020 steps (inline)

Cumulative STSO time: main 121054ms → this run 156516ms (Δ +35462ms, +29%)

  50-100 ms  ┃                         main  10  this   0   -10
 100-150 ms  ██████████████┃█████████  main 945  this 604  -341
 150-200 ms  █░░░░░░░┃                 main  43  this 348  +305
 200-250 ms  ┃                         main  14  this  47   +33
 250-300 ms  ┃                         main   6  this  10    +4
 300-350 ms  ┃                         main   0  this   5    +5
 400-450 ms  ┃                         main   1  this   2    +1
 500-550 ms  ┃                         main   0  this   1    +1
 700-750 ms  ┃                         main   0  this   1    +1
950-1000 ms  ┃                         main   0  this   1    +1

📈 CRTT drill-down vs main (RTT distributions & profiles)

variant  RTT 1ms→5s+              avg         p50            p90           p99     n
control  ······▂█▃▁···   157.2 (+41%)  138 (+34%)     329 (+87%)    486 (+37%)  3000
sweep    ······▂█▂····   146.2 (+15%)  131 (+26%)     277 (-18%)    450 (-34%)  3000
gw 1x    ·····▁▃█▂▁▁▁·  366.7 (+206%)  134 (+37%)  3219 (+1518%)  4722 (+805%)  5295
eve 1x   ·····▁▄█▁····      124 (-2%)  117 (+29%)      179 (-5%)    270 (-57%)  5186
eve 2x   ·····▁▂█▆▁···   219.9 (+23%)  183 (+29%)      485 (+7%)    702 (+13%)  7779

RTT over stream progress (avg per tenth of stream, bars scaled min→max):

control  ▇██▄▁▃█▃▅▃  130–179ms
sweep    █▂▂▄▂▁▁▃▅▃  128–183ms
gw 1x    ▁▁▁█▅▃▁▁▁▁  122–1361ms
eve 1x   ▄▄▄▅▁▅▅█▆▅  100–146ms
eve 2x   ▂▁▂▁▁▂▃█▅▃  150–406ms

RTT by chunk size (avg per log size bin, ~160B → ~12KB serialized, bars scaled min→max):

sweep  ▁▇▆█▅▂▁  145–148ms

Delivery jitter over stream progress (avg positive CDV per tenth of stream, bars scaled min→max):

control  ▄█▃▄▃▄▇▂▅▁  35–56ms
sweep    ▁▁▄█▄▆▄▄▅▇  48–64ms
gw 1x    ▁▂▂█▂▄▂▂▁▁  37–99ms
eve 1x   ▄▃▃▄▁▆▄▆█▅  22–29ms
eve 2x   █▁▁▁▃▂▆▄▃▇  25–39ms

ℹ️ Metric definitions & methodology

_{Streams: first-chunk RTT (the stream-open path, before any buffering/backpressure), CRTT percentiles, and worst delivery stall (CDV max). Cells are medians across iterations; per-run values in the artifacts. No 🔴/🟢 marks until targets attach.}

_{The collapsed STSO distribution section above buckets every step gap, split inline (same warm process — pure framework overhead) vs queue-hop (fresh process — dispatch, reinit, replay). █ = main, ┃ = this run, ░ = fill.}

_{The collapsed CRTT drill-down: per-variant RTT histograms (fixed log bins, · = empty) and mean RTT/positive-CDV profile lines over stream progress and chunk size. Histograms, avgs, and profiles merge exactly across runs; p50–p99 are percentile-of-percentiles. Per-index rows live in the artifacts.}

_{Best/P75/P90/P99 deltas compare against the most recent benchmark run on main at the time of this run. 🔻 flags a delta worse than +15%, 💚 one better than −15%.}

_{Metrics — TTFS: time to first step body (in-deployment start() → first step body) · Fan-out TTFS: fan-out time to first step (in-deployment start() → first of the parallel step bodies to complete) · Fan-out TTLS: fan-out time to last step (in-deployment start() → last of the parallel step bodies to complete, i.e. when the Promise.all resolves) · STSO: step-to-step overhead (gap between consecutive step bodies) · WO: workflow overhead (whole-run time outside step bodies, in-deployment anchored) · CRTT: chunk round-trip time (per-chunk write → read latency, one clock domain: deployment → stream backend → same deployment) · CDV: chunk delay variation / delivery jitter (inter-arrival gap minus inter-write gap per seq-adjacent pair; skew-free; the row is each run's MAX positive value, so one stall moves it)}

_{Scenarios — step: one trivial no-op step, no stream; no hooks, so the run stays in turbo mode (in-process fast path) · stream: one streaming step; no hooks, so the run stays in turbo mode (in-process fast path) · hook + stream: registers a hook before one step, which exits turbo mode (dispatch path) · 1020 steps: 1020 trivial sequential steps; STSO is measured between consecutive steps in the given step ranges, and WO is the whole-run overhead outside step bodies · Promise.all(100 steps): 100 trivial no-op steps started together in a single Promise.all; Fan-out TTFS is the first of them to complete and Fan-out TTLS the last, both from the in-deployment clientStart, so their gap is the spread the runtime adds across the fan-out · paced control (100/s, 60B): the control: 300 tiny (~60B) deltas metronome-paced at 100/s — zero workload structure, so it reads the transport floor and flush cadence, and disambiguates transport-wide vs workload-specific when a replay row moves · size sweep (100/s, 160B-12KB): same pacing as the control with deltas padded in rotation across seven log-spaced sizes (~160B–12KB) — rotation decouples size from stream position, so it isolates whether chunk size causes latency · replay gateway-gpt-5.4-nano-2000t (1x): raw provider SSE cadence captured at the AI gateway boundary (gpt-5.4-nano, the most popular gateway model; per-token deltas p50 208B = the modal production chunk size), replayed exactly as measured — the typical customer's workload; its CDV is the typical customer's real delivery jitter · replay eve-gpt-5.6-sol-2000t (1x): a captured eve turn (gpt-5.6-sol, the most-used demanding eve model; ~2000 output tokens = production p50 turn length) replayed exactly as measured — eve's envelope protocol re-ships the cumulative message so sizes ramp 142B→13KB; the demanding outlier tenant's reality · replay eve-gpt-5.6-sol-2000t (2x): the same eve capture at 2x — the headroom/stress row; real fast-tier models emit the same chunk sizes at proportionally higher rate, so time compression is a faithful speed model · first chunk (pooled): every run's seq-0 RTT pooled across all stream scenarios — the first chunk precedes any workload differentiation, so pooling samples one shared stream-open path with exact percentiles}

_{Replay cadences (semantic sha256) — eve-gpt-5.6-sol-2000t eaf22f5946e7c61f3c65c7006d550df180cfabd4e706254a09f22aec0cfb420d · gateway-gpt-5.4-nano-2000t 6f24ac518b6b83ff1d0e85a5fe78230db192716d66a7fc6b2fe022752001d041}

_{🔴 marks a percentile over its target (within target is left unmarked). Targets (p75/p90/p99, ms) — TTFS 200/300/600}

_{All timestamps are deployment-side; runs are triggered in-deployment, so the CI runner and api.vercel.com sit outside every measured window. TTFS = start() → first step body (includes dispatch + any cold start); Fan-out TTFS/TTLS = first/last step completion of one Promise.all from the same anchor (the gap is the runtime’s fan-out spread); STSO/WO between step bodies; CRTT inside the workflow (excludes the api.vercel.com read path).}

_{Cold starts stay in the numbers (real bursty-workload latency, inflates P75+); Best is the warm floor.}

github-actions · 2026-08-27T21:26:25Z

Framework	Flow route	Step reg.	Framework output
hono	201.6 KiB (±0)	41.7 KiB (±0)	1.78 MiB (+141 B)
nextjs-turbopack	207.8 KiB (±0)	439 B (±0)	771.7 KiB (+115 B)

About these numbers

Sizes are gzip; parentheses show the change against main.
Flow route and Step reg. gate this job, on raw bytes rather than the gzip shown, at max(2%, 50.0 KiB). Framework output is informational.

3611997 · run

VaguelySerious

LGTM, with some specific issues the agent found, noted below

VaguelySerious · 2026-09-08T23:40:05Z

          await world.streams.write(runId, name, chunk);
        }
      }
+      nextChunkSeq += group.length;


AI Review: Note

nextChunkSeq is read before an await and mutated after it, so its correctness rests entirely on dispatchGroup never running concurrently. That invariant does hold, and it is stated further up in this file ("Exactly one loop runs at a time (inFlight), so groups reach the server in write order"), but it is stated about ordering, several hundred lines away, and nothing near here records that a second dispatch loop would hand two groups the same starting sequence.

Since the whole point of the sequence space is that a transport can rely on it to reconstruct ordering, a one-line comment tying this increment to the inFlight guarantee is worth having. A future change that parallelises dispatch for throughput would silently produce duplicate chunkSeq values rather than fail a test.

VaguelySerious · 2026-09-08T23:40:05Z

  StepWithoutData,
 } from './steps.js';

+export interface StreamWriteSession {


AI Review: Nit

The changeset marks @workflow/world as patch, but this adds two newly exported interfaces to the package's public surface. Adding an optional member to Streamer.streams is backward compatible for implementors, so nothing breaks, but new exported types are conventionally a minor. Worth bumping so the surface addition is visible in the changelog rather than folded into a patch line.

VaguelySerious · 2026-09-08T23:40:05Z

+    // construction as soon as the run-ready barrier permits, so transports may
+    // negotiate eagerly without allowing a write to overtake run creation.
+    const writerId = `wrtr_${defaultUlid()}` as const;
+    const writeSessionPromise: Promise<StreamWriteSession | undefined> =


AI Review: Nit

I checked the unhandled-rejection question on this eagerly-constructed promise, since it is created for every writable whether or not a World implements the seam, and it comes out fine: ensureRunReady swallows its own rejection by design ("ordering barrier only"), and the only other rejection source, worldPromise, is already awaited on the same paths that await this. Noting it because the eager .then at construction time is the kind of thing worth a comment saying it cannot strand a rejection, given a stream that is created and then never written, closed, or aborted attaches no handler until it is.

The abort path using dispose() rather than close() is the right call, and the distinction between transport cleanup and semantic completion is well drawn.

Signed-off-by: Alex Langenfeld <alex.langenfeld@vercel.com>

github-actions · 2026-09-09T17:20:58Z

No backport to stable for 4547e1a (AI decision).

This is feature work: it adds a new optional Streamer.streams.createWriteSession API surface to @workflow/world (with new StreamWriteSession / CreateStreamWriteSessionOptions exported types) plus core plumbing to use it, carrying a minor changeset. It is a forward-looking seam enabling a future transport capability, not a fix for a defect on stable.

To override, re-run the Backport to stable workflow manually via workflow_dispatch and paste this commit SHA into the ref input:

4547e1a7a95f273f3166edf45344dc827f1935fa

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alangenfeld marked this pull request as ready for review September 8, 2026 22:19

alangenfeld requested a review from a team as a code owner September 8, 2026 22:19

VaguelySerious approved these changes Sep 8, 2026

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alangenfeld added 2 commits September 9, 2026 11:48

feat(streams): add writer session seam

5383f3b

Signed-off-by: Alex Langenfeld <alex.langenfeld@vercel.com>

docs(streams): record writer session invariants

3611997

Signed-off-by: Alex Langenfeld <alex.langenfeld@vercel.com>

github-actions Bot mentioned this pull request Sep 9, 2026

Version Packages (beta) #4011

Merged

This was referenced Oct 2, 2026

[world-vercel] Document the state of WebSocket stream transports #4592

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[core] Ack'd write-channel stream writes with framed-v2 writer markers #2731

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feat(streams): add writer session seam - #3832

feat(streams): add writer session seam#3832
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