perf(eve-channel): open session event stream alongside tail lookup and respect backpressure by alangenfeld · Pull Request #4547 · vercel/eve

alangenfeld · 2026-10-08T22:05:12Z

Summary & Motivation

The stream route used to wait for the tail-index lookup before opening the event stream, which added a workflow round trip before the first event. Both now start at once, and the tail still has to resolve before any bytes are sent. If the lookup fails, the event stream that was opened early is cancelled.

parseNdjsonStream now reads its source on each pull instead of reading the whole source inside start(). Before this, a fast upstream and a slow client made eve hold the entire history in memory. A parse error now also cancels the source.

Test Plan

Added tests. They check that the parser reads only as fast as the consumer pulls, and that a bounded tail-indexed read, a client abort and a cancelled response body each cancel the durable source. Ran typecheck plus the affected unit and stream integration tests locally. CI covers the rest.

…d respect backpressure Signed-off-by: Alex Langenfeld <alex.langenfeld@vercel.com>

vercel · 2026-10-08T22:05:21Z

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Project	Deployment	Actions	Updated
eve-docs	Ready	Preview, v0	Oct 8, 2026 10:06pm UTC
eve-pkg	Ready	Preview, v0	Oct 8, 2026 10:06pm UTC

github-actions · 2026-10-08T22:11:34Z

Bundle + Package Summary: `apps/fixtures/weather-agent`

Key takeaways

No notable deltas vs main (30c88ee).

Delta vs `main (30c88ee)`

Area	Metric	Baseline	Current	Delta
Package	Packed tarball	7.52 MB	7.52 MB	+88 B ⚠️
Package	Unpacked publish size	26.48 MB	26.48 MB	+178 B ⚠️
Package	Installed footprint	74.28 MB	74.28 MB	+178 B ⚠️
Package	Published files	4191	4191	0
Package	Installed files	8196	8196	0
Package	Installed package instances	34	34	0
Package	Distinct installed package names	32	32	0
Package	Installed dependency edges	51	51	0
Package	Installed optional peer edges	9	9	0
Runtime	Unique function payloads	2	2	0
Runtime	Total function bytes	20.30 MB	20.30 MB	+336 B ⚠️
Runtime	Public routes	18	18	0

Changed function payloads vs main (30c88ee) (2)

Function	Status	Baseline	Current	Delta	Route changes
`functions/__server.func`	changed	10.15 MB	10.15 MB	+168 B ⚠️	none
`functions/.well-known/workflow/v1/flow.func`	changed	10.15 MB	10.15 MB	+168 B ⚠️	none

`eve init` install

Metric	Baseline	Current	Delta
Installed footprint	112.57 MB	112.57 MB	+178 B ⚠️
Installed packages	103	103	0
dependencies	4	4	0
devDependencies	2	2	0
Dependency package bytes	44.26 MB	44.26 MB	+178 B ⚠️
devDependency package bytes	5.11 MB	5.11 MB	0 B ➖

Build Metadata

Preset: vercel
Nitro: nitro@3.0.260903-beta
Output directory: apps/fixtures/weather-agent/.vercel/output
Build metadata timestamp: 2026-10-08T22:10:47.479Z
Route aliases: 18 public, 1 internal (19 total aliases)
Vercel routes in config: 21
Severity legend: 🔴 dominant/large, 🟠 notable, 🟡 watch, ⚪ small

Package Drill-Down

Package Details

Package: eve@0.75.0
Package directory: packages/eve
Tarball: 7.52 MB (eve-0.75.0.tgz)
Unpacked payload: 26.48 MB across 4191 published files
Installed footprint: 74.28 MB across 8196 installed files
Installed root package: 26.18 MB
Installed dependencies: 48.10 MB
Installed package instances: 34
Distinct installed package names: 32
Installed dependency edges: 51
Installed optional peer edges: 9
Runtime dependencies: 2
Peer dependencies: 7 (6 optional)

Installed footprint is measured from an isolated temporary npm install of the packed tarball.
Graph metrics read only package.json files in package directories directly beneath a node_modules boundary, including nested boundaries. Each directory is one package instance; distinct names come from those manifests. Dependency edges count each unique name in dependencies or optionalDependencies per instance; optional peer edges count peerDependencies marked optional.

Heavy installed dependencies

eve: 26.18 MB (35.2%)
@rolldown/binding-linux-x64-gnu: 19.62 MB (26.4%)
ai: 8.14 MB (11.0%)
zod: 6.14 MB (8.3%)
undici: 3.56 MB (4.8%)

Publish payload breakdown

Published file size
🟠 dist/src/compiled/shadcn-registry/index.js       [#####...................] 3.85 MB 14.5%
🟠 dist/src/compiled/@photon-ai/chat-adapter-ime... [###.....................] 2.29 MB 8.6%
🟠 dist/src/compiled/@ai-sdk/code-mode/index.js     [#.......................] 1.03 MB 3.9%
🟡 dist/src/compiled/@vercel/blob/index.js          [#.......................] 604.9 kB 2.3%
🟡 dist/src/compiled/_chunks/workflow/undici-D-I... [#.......................] 521.9 kB 2.0%
🔴 Other published files                            [########################] 18.19 MB 68.7%

Installed footprint breakdown

Installed package size
🔴 eve                             [########################] 26.18 MB 35.2%
🔴 @rolldown/binding-linux-x64-gnu [##################......] 19.62 MB 26.4%
🔴 ai                              [#######.................] 8.14 MB 11.0%
🔴 zod                             [######..................] 6.14 MB 8.3%
🟠 undici                          [###.....................] 3.56 MB 4.8%
🟠 nitro                           [##......................] 1.89 MB 2.5%
🔴 Other installed packages        [########................] 8.75 MB 11.8%

Runtime dependencies (2)

Package	Range	Notes
`nitro`	`3.0.260903-beta`
`undici`	`8.10.2`

Peer dependencies (7)

Package	Range	Notes
`@opentelemetry/api`	`^1.0.0`	optional peer
`ai`	`catalog:`
`braintrust`	`^3.0.0`	optional peer
`chat`	`^4.41.0`	optional peer
`dd-trace`	`^6.13.0`	optional peer
`just-bash`	`^3.1.0`	optional peer
`microsandbox`	`^0.5.0`	optional peer

eve init install drill-down

`eve init` install details

Command: eve init my-agent
Package manager: npm
Installed footprint: 112.57 MB across 10091 installed files
Installed packages: 103 total (97 transitive-only)
dependencies: 4 direct packages totaling 44.26 MB
devDependencies: 2 direct packages totaling 5.11 MB
Other transitive package files: 63.21 MB

Installed footprint is measured from an isolated temporary eve init my-agent using the current packed eve tarball.

Heavy installed dependencies

@typescript/typescript-linux-x64: 27.95 MB (24.8%)
eve: 26.18 MB (23.3%)
@rolldown/binding-linux-x64-gnu: 19.62 MB (17.4%)
zod: 9.76 MB (8.7%)
ai: 8.14 MB (7.2%)

Installed footprint breakdown

Installed package size
🔴 @typescript/typescript-linux-x64 [########################] 27.95 MB 24.8%
🔴 eve                              [######################..] 26.18 MB 23.3%
🔴 @rolldown/binding-linux-x64-gnu  [#################.......] 19.62 MB 17.4%
🔴 zod                              [########................] 9.76 MB 8.7%
🔴 ai                               [#######.................] 8.14 MB 7.2%
🟠 undici                           [###.....................] 3.56 MB 3.2%
🔴 Other installed packages         [###############.........] 17.37 MB 15.4%

dependencies (4)

Package	Range	Installed size	Share
`@vercel/connect`	`2.2.0`	188.7 kB	0.2%
`ai`	`^7.0.128`	8.14 MB	7.2%
`eve`	`file:eve-0.75.0.tgz`	26.18 MB	23.3%
`zod`	`4.5.4`	9.76 MB	8.7%

devDependencies (2)

Package	Range	Installed size	Share
`@types/node`	`24.x`	2.61 MB	2.3%
`typescript`	`7.0.2`	2.50 MB	2.2%

Function Drill-Down

Payload Size Graph

Unique function payload size and share of total
🔴 functions/.well-known/workflow/v1/flow.func     [########################] 10.15 MB 50.0%
🔴 functions/__server.func                         [########################] 10.15 MB 50.0%

Top Function Payloads

🟠 functions/.well-known/workflow/v1/flow.func • 1 public route • 10.15 MB

Metric	Value
Public routes	`/.well-known/workflow/v1/flow`
Runtime	`nodejs24.x`
Handler	`index.mjs`
Payload	10.15 MB
Function files	10.15 MB across 127 files
Traced dependencies	0 B
Signal	🟠 Bundled file `_chunks/vercel.web.mjs` is 2.20 MB (21.7%)

🟠 🔎 Dependency Analysis

📦 Bundled files:

Bundled file size
🟠 _chunks/vercel.web.mjs                         [###########.............] 2.20 MB 21.7%
🟠 _libs/undici.mjs                               [#####...................] 1.01 MB 9.9%
🟡 _chunks/sandbox3.mjs                           [####....................] 815.4 kB 8.0%
🟡 _chunks/compiled-artifacts-instrumentation.mjs [####....................] 762.7 kB 7.5%
🟡 _chunks/token-util-Bla5Z0G9.mjs                [####....................] 704.7 kB 6.9%
🔴 Other bundled files                            [########################] 4.66 MB 45.9%

🧾 Vercel Config

{
  "handler": "index.mjs",
  "launcherType": "Nodejs",
  "shouldAddHelpers": false,
  "supportsResponseStreaming": true,
  "runtime": "nodejs24.x",
  "maxDuration": "max",
  "experimentalTriggers": [
    {
      "type": "queue/v2beta",
      "topic": "__eve776561746865722d6167656e74_wkf_workflow_*",
      "consumer": "default",
      "retryAfterSeconds": 5,
      "initialDelaySeconds": 0
    }
  ],
  "environment": {
    "WORKFLOW_PRECONDITION_GUARD": "1"
  }
}

🟠 functions/__server.func • 17 public routes, 1 internal alias • 10.15 MB

Metric	Value
Public routes	`/` `/.well-known/workflow/v1/webhook/[token]` `/eve/v1/callback/[token]` `/eve/v1/connections/[name]/callback/[attemptId]/[token]` `/eve/v1/connections/[name]/callback/[token]` `/eve/v1/health` `/eve/v1/info` `/eve/v1/session` `/eve/v1/session/[parentSessionId]/subagents/[callId]/[childSessionId]/stream` `/eve/v1/session/[sessionId]` `/eve/v1/session/[sessionId]/cancel` `/eve/v1/session/[sessionId]/clear` `/eve/v1/session/[sessionId]/compact` `/eve/v1/session/[sessionId]/reset` `/eve/v1/session/[sessionId]/stream` `/eve/v1/session/[sessionId]/stubs` `/eve/v1/task-input/[token]`
Internal aliases	`/__server`
Runtime	`nodejs24.x`
Handler	`index.mjs`
Payload	10.15 MB
Function files	10.15 MB across 127 files
Traced dependencies	0 B
Signal	🟠 Bundled file `_chunks/vercel.web.mjs` is 2.20 MB (21.7%)

🟠 🔎 Dependency Analysis

📦 Bundled files:

Bundled file size
🟠 _chunks/vercel.web.mjs                         [###########.............] 2.20 MB 21.7%
🟠 _libs/undici.mjs                               [#####...................] 1.01 MB 9.9%
🟡 _chunks/sandbox3.mjs                           [####....................] 815.4 kB 8.0%
🟡 _chunks/compiled-artifacts-instrumentation.mjs [####....................] 762.7 kB 7.5%
🟡 _chunks/token-util-Bla5Z0G9.mjs                [####....................] 704.7 kB 6.9%
🔴 Other bundled files                            [########################] 4.66 MB 45.9%

🧾 Vercel Config

{
  "handler": "index.mjs",
  "launcherType": "Nodejs",
  "shouldAddHelpers": false,
  "supportsResponseStreaming": true,
  "runtime": "nodejs24.x"
}

Build Timing: `e2e/fixtures/agent-tools-sandbox`

This is an informational timing measurement inside eve build, from preflight through publication. Output-size measurement and profile writing are excluded.

Build mode: deployable Vercel build with sandbox template prewarm included.

Build pipeline: 3.13 s -> 3.04 s (-91.5 ms) vs main (30c88ee).
Timing is informational: shared GitHub runners are too variable for a hard timing budget.

Detailed phase timings vs `main (30c88ee)`

Phase	Baseline	Current	Delta
`extension.check`	10.3 ms	0.4 ms	-9.9 ms
`project.resolve`	0.4 ms	0.3 ms	-0.1 ms
`workspace.create`	0.6 ms	0.6 ms	0.0 ms
`host.prepare`	718.6 ms	630.2 ms	-88.4 ms
`vercel.service-prefix.resolve`	1.3 ms	1.3 ms	0.0 ms
`nitro.create`	485.3 ms	506.6 ms	+21.3 ms
`sandbox.prewarm`	279.9 ms	289.7 ms	+9.8 ms
`nitro.cache.prepare`	0.3 ms	0.3 ms	0.0 ms
`nitro.prepare`	0.9 ms	0.6 ms	-0.3 ms
`nitro.public-assets`	0.7 ms	0.7 ms	0.0 ms
`nitro.prerender`	0.4 ms	0.4 ms	0.0 ms
`nitro.bundle`	1.43 s	1.47 s	+35.5 ms
`nitro.cache.write`	0.3 ms	0.2 ms	-0.1 ms
`vercel.workflow-function.materialize`	141.1 ms	80.1 ms	-61.0 ms
`agent-summary.emit`	0.7 ms	0.8 ms	+0.1 ms
`connect-manifest.emit`	0.3 ms	0.3 ms	0.0 ms
`nitro.close`	0.1 ms	0.2 ms	+0.1 ms
`output.publish`	4.0 ms	4.4 ms	+0.4 ms
`workspace.remove`	2.8 ms	2.8 ms	0.0 ms

karthikscale3

Reviewed the concurrent stream/tail lookup, HTTP error handling, NDJSON backpressure, and cancellation paths. No blocking findings.

Validation: all 14 focused parser and stream-response tests passed locally. A comparison against the base revision confirmed that an unconsumed source now stops after two pulls instead of draining all 100 chunks, and a parse error cancels its source. CI checks are green for this revision. The broader channel test could not run locally because the existing dependency build lacks the generated js-yaml module; CI covers that suite.

perf(eve-channel): open session event stream alongside tail lookup an…

d448da9

…d respect backpressure Signed-off-by: Alex Langenfeld <alex.langenfeld@vercel.com>

vercel Bot deployed to Preview – eve-pkg October 8, 2026 22:06

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vercel Bot deployed to Preview – eve-docs October 8, 2026 22:06

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alangenfeld marked this pull request as ready for review October 8, 2026 22:29

alangenfeld requested a review from a team as a code owner October 8, 2026 22:29

karthikscale3 approved these changes Oct 8, 2026

View reviewed changes

prha approved these changes Oct 8, 2026

View reviewed changes

AndrewBarba merged commit 13059ba into main Oct 8, 2026
157 of 158 checks passed

AndrewBarba deleted the alangenfeld/eve-session-stream-route-latency branch October 8, 2026 23:15

roboeve Bot mentioned this pull request Oct 11, 2026

NDJSON stream decoder leaks source on parse failure and ignores reader backpressure #4649

Closed

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perf(eve-channel): open session event stream alongside tail lookup and respect backpressure - #4547

perf(eve-channel): open session event stream alongside tail lookup and respect backpressure#4547
AndrewBarba merged 1 commit into
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Package Details

`eve init` install details

Payload Size Graph

Top Function Payloads

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Summary & Motivation

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Bundle + Package Summary: apps/fixtures/weather-agent

Delta vs main (30c88ee)

eve init install

Package Details

eve init install details

Payload Size Graph

Top Function Payloads

Build Timing: e2e/fixtures/agent-tools-sandbox

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alangenfeld commented Oct 8, 2026 •

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Bundle + Package Summary: `apps/fixtures/weather-agent`

Delta vs `main (30c88ee)`

`eve init` install

`eve init` install details

Build Timing: `e2e/fixtures/agent-tools-sandbox`