Sending too much data via vitest's provide configuration causes opaque/silent failures #9957
Description
What versions & operating system are you using?
wrangler 4.24.3, node v24.4.0, macOS
Please provide a link to a minimal reproduction
https://github.com/samscott89/cf-workers-bug-repro
Describe the Bug
Should be reproducible by comparing:
npm test-passingnpm test-failingnpm test-silently-failing
I'll attach example logs below.
The context is: I was trying to pass in fixture data for tests so I could mock out API responses. After trying various payloads I would observe these three different results: success, a loud but opaque error, and the tests just being silently omitted entirely.
To reproduce this I have a stupid simple tests that passes in arrays of varying length based on an environment variable to simulate different size payloads.
What I'm observing is:
test-passing: passtest-failing: I get an opaque error message ofAssertionError: The expression evaluated to a falsy value: assert4(webSocket !== null)test-silently-failing: seems to make the tests not even show up as tests somehow
After attaching a debugger, the webSocket !== null check is coming from here:
workers-sdk/packages/vitest-pool-workers/src/pool/index.ts
Lines 775 to 787 in 4ba9f25
I'm pretty sure what is happening is that when the payload gets large enough it leads to the websocket connection failing which triggers the assert.
I'm not sure about the silently failing one though. Possibly a race condition which leads to the test framework not seeing the error?
In terms of expected behaviour: it might be nice if the assert was a bit more descriptive? Or perhaps there could be an explicit error if the payload for "MF-Vitest-Worker-Data" is too large.
On the other hand, I'm still wrapping my head around how to be effective with Workers when not having access to things like node/the filesystem in the typical way. So the way I'm trying to use provide to pass in data might be an anti-pattern. As an aside if you have any pointers that would be appreciated :)
Please provide any relevant error logs
cf-workers-bug-repro on main via v24.4.0
❯ npm run test-passing
> cf-workers-bug-repro@0.0.0 test-passing
> ARRAY_LENGTH=100 vitest run
Using array length: 100
RUN v3.2.4 /Users/sam/work/cf-workers-bug-repro
[vpw:info] Starting isolated runtimes for vitest.config.mts...
[mf:warn] The latest compatibility date supported by the installed Cloudflare Workers Runtime is "2025-07-09",
but you've requested "2025-07-12". Falling back to "2025-07-09"...
✓ test/index.spec.ts (2 tests) 13ms
✓ Hello World worker > responds with Hello World! (unit style) 6ms
✓ Hello World worker > responds with Hello World! (integration style) 3ms
Test Files 1 passed (1)
Tests 2 passed (2)
Start at 12:49:52
Duration 620ms (transform 19ms, setup 0ms, collect 19ms, tests 13ms, environment 0ms, prepare 91ms)
[vpw:debug] Shutting down runtimes...
cf-workers-bug-repro on main via v24.4.0
❯ npm run test-failing
> cf-workers-bug-repro@0.0.0 test-failing
> ARRAY_LENGTH=64000 vitest run
Using array length: 64000
RUN v3.2.4 /Users/sam/work/cf-workers-bug-repro
[vpw:info] Starting isolated runtimes for vitest.config.mts...
[mf:warn] The latest compatibility date supported by the installed Cloudflare Workers Runtime is "2025-07-09",
but you've requested "2025-07-12". Falling back to "2025-07-09"...
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Vitest caught 1 unhandled error during the test run.
This might cause false positive tests. Resolve unhandled errors to make sure your tests are not affected.
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AssertionError: The expression evaluated to a falsy value:
assert4(webSocket !== null)
❯ runTests node_modules/@cloudflare/vitest-pool-workers/dist/pool/index.mjs:1588:3
❯ process.processTicksAndRejections node:internal/process/task_queues:105:5
❯ executeMethod node_modules/@cloudflare/vitest-pool-workers/dist/pool/index.mjs:1831:19
❯ Object.runTests node_modules/@cloudflare/vitest-pool-workers/dist/pool/index.mjs:1850:7
❯ executeTests node_modules/vitest/dist/chunks/coverage.DL5VHqXY.js:3421:4
❯ node_modules/vitest/dist/chunks/cli-api.BkDphVBG.js:9638:6
❯ Vitest.runFiles node_modules/vitest/dist/chunks/cli-api.BkDphVBG.js:9666:10
❯ Vitest.start node_modules/vitest/dist/chunks/cli-api.BkDphVBG.js:9554:18
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Test Files (1)
Tests no tests
Errors 1 error
Start at 12:49:55
Duration 1.40s (transform 0ms, setup 0ms, collect 0ms, tests 0ms, environment 0ms, prepare 0ms)
[vpw:debug] Shutting down runtimes...
cf-workers-bug-repro on main via v24.4.0
❯ npm run test-silently-failing
> cf-workers-bug-repro@0.0.0 test-silently-failing
> ARRAY_LENGTH=128000 vitest run
Using array length: 128000
RUN v3.2.4 /Users/sam/work/cf-workers-bug-repro
[vpw:info] Starting isolated runtimes for vitest.config.mts...
[mf:warn] The latest compatibility date supported by the installed Cloudflare Workers Runtime is "2025-07-09",
but you've requested "2025-07-12". Falling back to "2025-07-09"...
Test Files 0 passed (1)
Tests 0 passed (0)
Start at 12:49:58
Duration 376ms