fix(eve): forward each dev request on a fresh worker connection by roboeve[bot] · Pull Request #5004 · vercel/eve

roboeve · 2026-10-17T23:05:27Z

Stacked on the repro in #5003. eve dev now forwards each request to the dev worker on its own connection, so a busy parent can no longer send one on a pooled socket the worker already closed.

Why

env-runner's BaseEnvRunner.fetch calls httpxy's proxyFetch without proxy options, so every forward goes through httpxy's shared keep-alive http.Agent, which has no idle timeout. The worker's HTTP server closes idle sockets after its keep-alive timeout, and the parent only notices when its event loop reads the FIN. Under CPU load the parent can dispatch a forward before that, the forward throws, and the dev server answers 503 (socket hang up / read ECONNRESET). The request never reaches the agent.

Approach

The vendor step now patches that one call to pass { agent: false }, the same way it already patches env-runner's worker shutdown. Each forward then opens a fresh loopback connection and closes it after the response, so there is no pool to go stale. The plugin throws if env-runner's source changes, so an upgrade cannot silently drop the patch. The extra loopback handshake per request is negligible in dev, and eve start has no forwarding hop, so it is unaffected.

Also considered:

Sending connection: close on each forward (the first push here): the worker's response can override it. The MCP SDK's SSE responses set connection: keep-alive, so Node's client returned the socket to the pool while the worker's parser rejected the next request with an empty 400. The mcp-agent-channel scenario caught this.
Retrying on a reused-socket reset: the request body is streamed, so a retry would need buffering.
Raising the worker's keep-alive timeout: narrows the window but keeps the race.

Validation

Repro head (it.fails removed): forwards a request after the parent stalls past the worker keep-alive timeout gets 503 {"error":"socket hang up"}.
This head, after pnpm --filter eve build: drained-nitro-dev-server.scenario.test.ts and mcp-agent-channel.scenario.test.ts → 17 passed.
Full scenario suite in a local sandbox without Docker: 498 passed, 9 failed. All 9 fail inside eve build or packed-package loading (production-build-isolation, app-runtime-dependencies, package-consumption, react-server-module-map, ai/package), and all 9 passed in CI on the previous push.
pnpm run typecheck, oxlint, and oxfmt --check pass.
Isolated check against a live dev worker: 4 sequential MCP POSTs through proxyFetch with connection: close → 200 400 200 400; with { agent: false } → 200 200 200 200, each on a new connection.

Fixes #5001 Co-authored-by: ruiconti <1834568+ruiconti@users.noreply.github.com>

vercel · 2026-10-17T23:05:34Z

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Project	Deployment	Actions	Updated
eve-docs	Ready	Preview, v0	Oct 17, 2026 11:39pm UTC
eve-pkg	Ready	Preview, v0	Oct 17, 2026 11:39pm UTC

github-actions · 2026-10-17T23:08:11Z

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

Key takeaways

No notable deltas vs triage/5001-repro (ab8cb30).

Delta vs `triage/5001-repro (ab8cb30)`

Area	Metric	Baseline	Current	Delta
Package	Packed tarball	7.66 MB	7.66 MB	-174 B ✅
Package	Unpacked publish size	26.97 MB	26.98 MB	+11 B ⚠️
Package	Installed footprint	75.11 MB	75.11 MB	+11 B ⚠️
Package	Published files	4171	4171	0
Package	Installed files	8182	8182	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	20.77 MB	20.77 MB	-120 B ✅
Runtime	Public routes	18	18	0

Changed function payloads vs triage/5001-repro (ab8cb30) (2)

Function	Status	Baseline	Current	Delta	Route changes
`functions/__server.func`	changed	10.38 MB	10.38 MB	-60 B ✅	none
`functions/.well-known/workflow/v1/flow.func`	changed	10.38 MB	10.38 MB	-60 B ✅	none

`eve init` install

Metric	Baseline	Current	Delta
Installed footprint	113.50 MB	113.50 MB	+11 B ⚠️
Installed packages	97	97	0
dependencies	4	4	0
devDependencies	2	2	0
Dependency package bytes	45.25 MB	45.25 MB	+11 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: 2026-10-17T23:39:19.801Z
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@0.80.1
Package directory: packages/eve
Tarball: 7.66 MB (eve-0.80.1.tgz)
Unpacked payload: 26.98 MB across 4171 published files
Installed footprint: 75.11 MB across 8182 installed files
Installed root package: 26.97 MB
Installed dependencies: 48.14 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: 26.97 MB (35.9%)
@rolldown/binding-linux-x64-gnu: 19.81 MB (26.4%)
ai: 8.24 MB (11.0%)
zod: 6.15 MB (8.2%)
undici: 3.58 MB (4.8%)

Publish payload breakdown

Published file size
🟠 dist/src/compiled/shadcn-registry/index.js       [#####...................] 3.85 MB 14.3%
🟠 dist/src/compiled/@photon-ai/chat-adapter-ime... [###.....................] 2.29 MB 8.5%
🟠 dist/src/compiled/@ai-sdk/code-mode/index.js     [#.......................] 1.03 MB 3.8%
🟡 dist/src/compiled/@vercel/blob/index.js          [#.......................] 604.9 kB 2.2%
🟡 dist/src/compiled/_chunks/workflow/undici-D-I... [#.......................] 521.9 kB 1.9%
🔴 Other published files                            [########################] 18.68 MB 69.3%

Installed footprint breakdown

Installed package size
🔴 eve                             [########################] 26.97 MB 35.9%
🔴 @rolldown/binding-linux-x64-gnu [##################......] 19.81 MB 26.4%
🔴 ai                              [#######.................] 8.24 MB 11.0%
🔴 zod                             [#####...................] 6.15 MB 8.2%
🟠 undici                          [###.....................] 3.58 MB 4.8%
🟠 nitro                           [##......................] 1.89 MB 2.5%
🔴 Other installed packages        [########................] 8.46 MB 11.3%

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`	`^4.42.0`	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: 113.50 MB across 10099 installed files
Installed packages: 97 total (91 transitive-only)
dependencies: 4 direct packages totaling 45.25 MB
devDependencies: 2 direct packages totaling 5.11 MB
Other transitive package files: 63.14 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 (24.6%)
eve: 26.97 MB (23.8%)
@rolldown/binding-linux-x64-gnu: 19.81 MB (17.5%)
zod: 9.76 MB (8.6%)
ai: 8.24 MB (7.3%)

Installed footprint breakdown

Installed package size
🔴 @typescript/typescript-linux-x64 [########################] 27.95 MB 24.6%
🔴 eve                              [#######################.] 26.97 MB 23.8%
🔴 @rolldown/binding-linux-x64-gnu  [#################.......] 19.81 MB 17.5%
🔴 zod                              [########................] 9.76 MB 8.6%
🔴 ai                               [#######.................] 8.24 MB 7.3%
🟠 undici                           [###.....................] 3.58 MB 3.1%
🔴 Other installed packages         [###############.........] 17.19 MB 15.1%

dependencies (4)

Package	Range	Installed size	Share
`@vercel/connect`	`2.5.2`	276.3 kB	0.2%
`ai`	`^7.0.137`	8.24 MB	7.3%
`eve`	`file:eve-0.80.1.tgz`	26.97 MB	23.8%
`zod`	`4.5.4`	9.76 MB	8.6%

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     [########################] 10.38 MB 50.0%
🔴 functions/__server.func                         [########################] 10.38 MB 50.0%

Top Function Payloads

🟠 functions/.well-known/workflow/v1/flow.func • 1 public route • 10.38 MB

Metric	Value
Public routes	`/.well-known/workflow/v1/flow`
Runtime	`nodejs24.x`
Handler	`index.mjs`
Payload	10.38 MB
Function files	10.38 MB across 122 files
Traced dependencies	0 B
Signal	🟠 Bundled file `_chunks/vercel.web.mjs` is 2.38 MB (22.9%)

🟠 🔎 Dependency Analysis

📦 Bundled files:

Bundled file size
🟠 _chunks/vercel.web.mjs                         [############............] 2.38 MB 22.9%
🟠 _libs/undici.mjs                               [#####...................] 1.01 MB 9.7%
🟡 _chunks/sandbox3.mjs                           [####....................] 815.4 kB 7.9%
🟡 _chunks/compiled-artifacts-instrumentation.mjs [####....................] 734.8 kB 7.1%
🟡 _chunks/token-util-Bla5Z0G9.mjs                [####....................] 704.7 kB 6.8%
🔴 Other bundled files                            [########################] 4.74 MB 45.7%

🧾 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 • 10.38 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	10.38 MB
Function files	10.38 MB across 122 files
Traced dependencies	0 B
Signal	🟠 Bundled file `_chunks/vercel.web.mjs` is 2.38 MB (22.9%)

🟠 🔎 Dependency Analysis

📦 Bundled files:

Bundled file size
🟠 _chunks/vercel.web.mjs                         [############............] 2.38 MB 22.9%
🟠 _libs/undici.mjs                               [#####...................] 1.01 MB 9.7%
🟡 _chunks/sandbox3.mjs                           [####....................] 815.4 kB 7.9%
🟡 _chunks/compiled-artifacts-instrumentation.mjs [####....................] 734.8 kB 7.1%
🟡 _chunks/token-util-Bla5Z0G9.mjs                [####....................] 704.7 kB 6.8%
🔴 Other bundled files                            [########################] 4.74 MB 45.7%

🧾 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: 3.76 s -> 3.63 s (-130.1 ms) vs triage/5001-repro (ab8cb30).
Timing is informational: shared GitHub runners are too variable for a hard timing budget.

Detailed phase timings vs `triage/5001-repro (ab8cb30)`

Phase	Baseline	Current	Delta
`extension.check`	18.0 ms	17.8 ms	-0.2 ms
`project.resolve`	7.0 ms	0.4 ms	-6.6 ms
`workspace.create`	1.0 ms	0.6 ms	-0.4 ms
`host.prepare`	749.2 ms	716.4 ms	-32.8 ms
`vercel.service-prefix.resolve`	1.9 ms	1.6 ms	-0.3 ms
`nitro.create`	691.2 ms	682.5 ms	-8.7 ms
`sandbox.prewarm`	306.4 ms	248.2 ms	-58.2 ms
`nitro.cache.prepare`	0.3 ms	0.3 ms	0.0 ms
`nitro.prepare`	0.7 ms	0.7 ms	0.0 ms
`nitro.public-assets`	0.8 ms	0.8 ms	0.0 ms
`nitro.prerender`	0.6 ms	0.6 ms	0.0 ms
`nitro.bundle`	1.84 s	1.82 s	-17.4 ms
`nitro.cache.write`	0.4 ms	0.4 ms	0.0 ms
`vercel.workflow-function.materialize`	59.3 ms	56.4 ms	-2.9 ms
`agent-summary.emit`	0.6 ms	0.7 ms	+0.1 ms
`connect-manifest.emit`	0.3 ms	0.3 ms	0.0 ms
`nitro.close`	0.1 ms	0.1 ms	0.0 ms
`output.publish`	3.6 ms	3.6 ms	0.0 ms
`workspace.remove`	2.9 ms	2.9 ms	0.0 ms

roboeve · 2026-10-17T23:22:44Z

CI's test-scenario failure is a real regression from this PR, not a flake. mcp-agent-channel.scenario.test.ts fails here and passes on the repro base (#5003). I reproduced both locally on 55b902d.

What breaks: the legacy MCP agent_start call gets HTTP 400 with an empty body. A clientError hook in the worker shows Node rejecting the forwarded request with HPE_CLOSED_CONNECTION: Data after Connection: close at bytesParsed: 1. The parent sent it on a socket where the previous request already had connection: close.

Why: the legacy initialize reply is SSE, and the MCP SDK sets an explicit Connection: keep-alive response header. Node's client decides reuse from the response, so httpxy's keep-alive agent pools the socket. Node's server decides from the request and closes after responding, then rejects the next request on that socket. So connection: close on the request does not reliably keep the pool empty. Any worker response that sets its own Connection header defeats it.

Options:

Forward with agent: false or a short idle timeout. env-runner's BaseEnvRunner.fetch calls httpxy proxyFetch(address, input, init) with no proxy options, so this needs an upstream env-runner option or our own forward through private _address.
Strip hop-by-hop Connection from worker responses. That would have to live in the Nitro dev entry and only patches this symptom.

I haven't pushed anything since this changes the approach. Leaving it as a draft. The E2E workflow-retention rerun I triggered is unrelated to this.

Sending connection: close broke MCP SSE: the worker answers with connection: keep-alive, so the socket went back to the pool and the next request got an empty 400. Patch env-runner's proxyFetch call to use agent: false instead, so each forward opens its own loopback connection. Refs #5001 Co-authored-by: ruiconti <1834568+ruiconti@users.noreply.github.com>

fix(eve): forward each dev request on a fresh worker connection

55b902d

Fixes #5001 Co-authored-by: ruiconti <1834568+ruiconti@users.noreply.github.com>

roboeve Bot added this pull request to stack #5005 October 17, 2026 23:05

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roboeve Bot requested a review from ruiconti October 17, 2026 23:36

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roboeve Bot marked this pull request as ready for review October 17, 2026 23:44

roboeve Bot requested a review from a team as a code owner October 17, 2026 23:44

roboeve Bot mentioned this pull request Oct 17, 2026

eve dev intermittently answers 503 {"error":"read ECONNRESET"} to channel requests under CPU load #5001

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fix(eve): forward each dev request on a fresh worker connection - #5004

fix(eve): forward each dev request on a fresh worker connection#5004
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Package Details

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Payload Size Graph

Top Function Payloads

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

Delta vs triage/5001-repro (ab8cb30)

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

Delta vs `triage/5001-repro (ab8cb30)`

`eve init` install

`eve init` install details

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