feat(streams): trace first WebSocket writes by alangenfeld · Pull Request #4074 · vercel/workflow

alangenfeld · 2026-09-10T18:07:53Z

Summary & Motivation

Tags the first write of a session, and the first write on each reconnect, with phase timings on the existing workflow.stream.write span — token/config resolution, connect, wait-for-open, send, and ack round trip — so cold WebSocket write latency can be attributed to a client-side phase before changing first-chunk transport behavior. Later writes keep the two attributes they had, since the per-phase clocks only mean anything while a connection is being established.

Test Plan

Unit tests added; existing world-vercel suite and typecheck pass.

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

changeset-bot · 2026-09-10T18:07:58Z

🦋 Changeset detected

Latest commit: 585005f

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

This PR includes changesets to release 17 packages

Name	Type
@workflow/world-vercel	Patch
@workflow/cli	Patch
@workflow/core	Patch
@workflow/web	Patch
workflow	Patch
@workflow/world-testing	Patch
@workflow/builders	Patch
@workflow/next	Patch
@workflow/nitro	Patch
@workflow/vitest	Patch
@workflow/web-shared	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 · 2026-09-10T18:08:10Z

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

Project	Deployment	Actions	Updated
example-nextjs-workflow-turbopack	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
example-nextjs-workflow-webpack	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
example-workflow	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
workbench-astro-workflow	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
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workbench-nuxt-workflow	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
workbench-python-workflow	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
workbench-sveltekit-workflow	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
workbench-tanstack-start-workflow	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
workbench-vite-workflow	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
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workflow-tarballs	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC
workflow-web	Ready	Preview, v0	Sep 10, 2026 6:12pm UTC

github-actions · 2026-09-10T18:08:14Z

🧪 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.

cancelRun via CLI - cancelling a running workflow (nextjs-turbopack)
deploymentId: 'latest' is a no-op in non-Vercel worlds (fastify)
deploymentId: 'latest' is a no-op in non-Vercel worlds (nuxt)
errorSubclassRoundTripWorkflow - first-class Error subclasses survive every serialization boundary (nextjs-turbopack)

🛠 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 18:13:35Z · abandoned wrun_01M268C5X1A149WYA0V6C4YF7D

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 · 2026-09-10T18:08:22Z

📊 Workflow Benchmarks

commit 585005f · Thu, 10 Sep 2026 18:25:57 GMT · run logs

Backend: vercel · app: nextjs-turbopack

Metric	Scenario	Best (ms)	P75 (ms)	P90 (ms)	P99 (ms)	Samples
TTFS	step	1163 (+74%) 🔻	1247 🔴 (+20%) 🔻	1268 🔴 (+18%) 🔻	1289 🔴 (-8.3%)	30
TTFS	stream	1170 (+465%) 🔻	1210 🔴 (+17%) 🔻	1234 🔴 (+17%) 🔻	1297 🔴 (+7.4%)	30
TTFS	hook + stream	571 (-53%) 💚	1522 🔴 (+16%) 🔻	1569 🔴 (+17%) 🔻	6024 🔴 (+258%) 🔻	30
Fan-out TTFS	Promise.all(100 steps)	653 (+9.0%)	1105 (+46%) 🔻	1912 (+101%) 🔻	1930 (+41%) 🔻	10
Fan-out TTLS	Promise.all(100 steps)	1605 (-3.1%)	8009 (+24%) 🔻	8297 (-3.1%)	8416 (-6.7%)	10
STSO	1020 steps (inline)	77 (-8.3%)	120 (-2.4%)	137 (-2.1%)	204 (+5.2%)	1019
WO	1020 steps	120776 (-2.7%)	120776 (-2.7%)	120776 (-2.7%)	120776 (-2.7%)	1
CRTT	first chunk (pooled)	50 (-19%) 💚	84 (-12%)	139 (-0.7%)	159 (-14%)	28

Streams

Scenario	CRTT 1st	p75	p90	p99	CDV max	iters
paced control (100/s, 60B)	67.5 (-17%)	131 (+7%)	197 (+25%)	467 (+43%)	160 (+21%)	10
size sweep (100/s, 160B-12KB)	73.5 (-10%)	122 (-44%)	163 (-59%)	304 (-70%)	134 (+6%)	10
replay gateway-gpt-5.4-nano-2000t (1x)	70 (-26%)	119 (-16%)	151 (-40%)	325 (-44%)	212 (-51%)	3
replay eve-gpt-5.6-sol-2000t (1x)	150 (+91%)	115 (-15%)	150 (-24%)	259 (-65%)	236 (-53%)	2
replay eve-gpt-5.6-sol-2000t (2x)	66 (-31%)	150 (-28%)	199 (-27%)	542 (+6%)	252 (-21%)	3

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

1020 steps (inline)

Cumulative STSO time: main 123953ms → this run 120635ms (Δ -3318ms, -3%)

 50-100 ms  ┃                         main   4  this   2    -2
100-150 ms  ███████████████████████┃  main 966  this 969    +3
150-200 ms  ┃                         main  39  this  37    -2
200-250 ms  ┃                         main   6  this   6    +0
250-300 ms  ┃                         main   2  this   3    +1
300-350 ms  ┃                         main   1  this   1    +0
350-400 ms  ┃                         main   1  this   0    -1
500-550 ms  ┃                         main   0  this   1    +1

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

variant  RTT 1ms→5s+             avg         p50         p90         p99     n
control  ······██▁····   111.2 (+6%)    99 (-2%)  197 (+25%)  467 (+43%)  3000
sweep    ······█▇▁····  104.4 (-31%)   96 (-16%)  163 (-59%)  304 (-70%)  3000
gw 1x    ·····▁█▆▁····  102.5 (-17%)   96 (-10%)  151 (-40%)  325 (-44%)  5295
eve 1x   ·····▁█▇▁····  101.9 (-18%)    95 (-4%)  150 (-24%)  259 (-65%)  5186
eve 2x   ·····▁▅█▂▁···  129.3 (-20%)  115 (-15%)  199 (-27%)   542 (+6%)  7779

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

control  ▆▄▁▅██▃▂▄▂  94–133ms
sweep    █▄▇▂▁▂▁▁▁▁  95–129ms
gw 1x    █▂▁▂▁▃▂▄▁▃  97–120ms
eve 1x   ▅▂▂▂▁▄█▄▃▂  93–119ms
eve 2x   █▃▃▃▁▂▄▅▃▂  100–188ms

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

sweep  ▆▅█▅▄▄▁  102–107ms

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

control  ▃▁▁█▄▆▂▁▁▂  30–39ms
sweep    ▄██▄▃▇▂▃▃▁  32–49ms
gw 1x    █▃▂▁▃▄▅▆▂▅  26–34ms
eve 1x   ▆▅▅▃▂▃█▃▁▄  19–23ms
eve 2x   █▂▂▂▁▂▃▂▁▃  19–29ms

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 · 2026-09-10T18:11:52Z

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 · 2026-09-10T18:12:44Z

Framework	Flow route	Step reg.	Framework output
hono	201.6 KiB (±0)	42.3 KiB (±0)	1.79 MiB (+858 B)
nextjs-turbopack	207.8 KiB (±0)	439 B (±0)	775.6 KiB (+561 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.

585005f · run

github-actions · 2026-09-10T18:29:49Z

No backport to stable for 7740388 (AI decision).

This is additive observability work: it introduces new phase-timing attributes on the existing workflow.stream.write span and the plumbing (timing structs, per-connection state, refactored request/write paths) to compute them. Nothing here fixes a correctness, crash, or leak defect on stable — the PR explicitly frames it as instrumentation gathered ahead of a future transport change — so it counts as a feature enhancement and belongs on main only.

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

7740388d7fe1a744f04974c67fd583d2526658f4

feat(streams): trace first WebSocket writes

585005f

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

vercel Bot deployed to Preview – workbench-python-workflow September 10, 2026 18:08

View deployment

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alangenfeld marked this pull request as ready for review September 10, 2026 18:15

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alangenfeld added this pull request to stack #4077 September 10, 2026 18:25

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alangenfeld merged commit 7740388 into main Sep 10, 2026
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alangenfeld deleted the alangenfeld/ws-stream-first-write-telemetry branch September 10, 2026 18:29

This was referenced Sep 10, 2026

Version Packages (beta) #4071

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Version Packages (beta) Jokacar10/jkworkflow#548

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Version Packages ben-vargas/workflow#104

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Version Packages (beta) erickirt/workflow#523

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VaguelySerious mentioned this pull request Oct 2, 2026

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

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feat(streams): trace first WebSocket writes - #4074

feat(streams): trace first WebSocket writes#4074
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Summary & Motivation

Test Plan

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🧪 E2E Test Results

⚠️ Flaky E2E Tests (passed on retry)

🛠 Infra Events (absorbed by the harness)

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📊 Workflow Benchmarks

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