[Bug?]: Cannot easily test for yarn install warnings in CI when using esm modules with pnp

[NormalGaussian]

Self-service

• I'd be willing to implement a fix

Describe the bug

I want to be able to have a CI step that checks there are no warnings in my yarn environment; however using esm and pnp creates a warning I cannot suppress or filter out. This makes it particularly complex to detect issues.

Specifically: when a project uses ESM (type: "module" or has ESM dependencies), yarn install emits:

➤ YN0000: │ ESM support for PnP uses the experimental loader API and is therefore experimental
➤ YN0000: · Done with warnings in 0s 508ms

There appears to be no way to detect or filter this:

1. logFilters cannot filter it: The warning uses message code YN0000 (MessageName.UNNAMED). The log filter implementation explicitly bypasses code 0: if(C===null||C===0) return x skips all text/pattern/code matching and returns the default level. This is tracked in [Bug?]: yarn 4 logFilters is unable to filter most messages #6221.

2. No structured output: yarn install does not support --json, so there's no machine-readable way to parse warnings from output.

3. No non-zero exit code: exitCode() only checks this.errorCount > 0. warningCount only changes the "Done with warnings" text but the process exits 0.

4. The warning itself is unavoidable: As described in [Bug?]: Implicit behaviour of pnpEnableEsmLoader: false is not documented or communicated #5747, pnpEnableEsmLoader is implicitly enabled when any dependency uses type: "module", regardless of the config defaulting to false. Setting it explicitly to false would disable ESM support, which isn't viable.

The result is that CI pipelines cannot treat warnings as failures (to catch real issues), without catching known-harmless warnings

To reproduce

package.json:

{
  "name": "example",
  "packageManager": "yarn@4.13.0",
  "type": "module",
  "dependencies": {
    "cowsay": "^1.6.0"
  }
}

.yarnrc.yml:

nodeLinker: pnp
logFilters:
  - code: YN0000
    text: "ESM support for PnP uses the experimental loader API and is therefore experimental"
    level: discard

Then run yarn install

Environment

System:
    OS: Linux 6.8 Ubuntu 24.04.4 LTS 24.04.4 LTS (Noble Numbat)
    CPU: (12) x64 AMD Ryzen 5 5600X 6-Core Processor
  Binaries:
    Node: 25.6.1 - /tmp/xfs-96160961/node
    Yarn: 4.13.0 - /tmp/xfs-96160961/yarn
    npm: 11.9.0 - /home/redacted/.nvm/versions/node/v25.6.1/bin/npm
    bun: 1.3.9 - /home/redacted/.bun/bin/bun
    Deno: 2.5.6 - /home/redacted/.deno/bin/deno

Additional context

I'd happily produce a fix for this; but its quite unclear what would be accepted.

Filtering YN0000 seems against an existing policy; adding --json to install with a {warnings:[]} that can be filtered with jq seems like a very large change; removing the warning seems possible, but may be contentious

[clemyan]

yarn install does not support --json

It does?

I'd happily produce a fix for this; but its quite unclear what would be accepted.

If that needs to change, the best way is probably create a new code for that to allow logFiler

[NormalGaussian]

It does?

you're 100% right; I don't know what mistake I made in my testing but I definitely made one! Thank you.

This gives me a functional workaround that won't break with a stiff breeze - I'm now outputting to a json file and checking for non-whitelisted warnings with jq. Its still quite janky - this is what I've done:

yarn install --json | tee >(jq -c 'select( .type == "warning" and .data != "ESM support for PnP uses the experimental loader API and is therefore experimental" and (.data | startswith("Done with warnings") | not))' > warnings.jsonl) | jq -r '.indent + .data'
test "$(wc -l < warnings.jsonl)" -gt 0 && echo "Unfiltered warnings found in 'yarn install'" && exit 1

I don't really fancy my teams chances to deal with this cleanly; but it is a solution for me so I could call it a day.

On consideration overnight, every package I write now is ESM, so this exists in all of my repo's and I assume many many users as well.

Next week I'll make a patch for the logFilter working for YN0000 and a patch for making the ESM warning have a different name, as they are the two approaches I can see working here

Either would allow filtering to work, and then the CI code can simply be:

yarn install --json | tee >(jq -c 'select( .type == "warning")' > warnings.jsonl) | jq -r '.indent + .data'
test "$(wc -l < warnings.jsonl)" -gt 0 && echo "Unfiltered warnings found in 'yarn install'" && exit 1

An --error-on-warnings patch would simplify it to just yarn install --error-on-warnings ofc.

[NormalGaussian]

If #7068 and #7069 are merged I'd consider this resolved as it would let users set a filter in .yarnrc.yml and simply do yarn install --fail-on-warnings.