Hosted yarn classic pins give no berry-migration warning, so a yarn 2+ install silently drops them (vendored warns about the same trap)

[mikolalysenko]

[agent] Found by the scheduled Yarn classic (1.x) bug-hunt routine (ledger #304).

Summary

When yarn 2+ (berry) installs over a classic (v1) yarn.lock, it migrates the lock and re-resolves every entry from the registry. Socket-patch knows about this trap. Vendored mode emits yarn_classic_berry_migration_risk (crates/socket-patch-core/src/vendor/mod.rs:177) unless package.json pins packageManager: yarn@1…. Hosted mode pins the same lock, and berry drops that pin the same way, but hosted never runs the probe. scan --mode hosted and get <uuid> --mode hosted report success, redirected: 1 and no warning. This holds even when package.json already declares "packageManager": "yarn@4.x", where the next non-immutable install is certain to discard the pin.

Impact

A developer runs scan --mode hosted on a v1 lock that is mid-migration to berry (or unpinned) and commits the result. The next yarn install under berry quietly rewrites the lock to left-pad@npm:1.3.0 and installs the upstream, unpatched bytes, with nothing printed by either tool. vex correctly fails closed afterwards (the pin is gone, so manifest_not_found / exit 2), so nothing is falsely attested. The patch is lost silently, though, which is exactly the outcome the vendored warning exists to prevent. Under --immutable (berry's CI default), the same install fails YN0028 instead. That's loud, but there's still no hint that socket-patch's pin is the cause.

Repro

# mock patch API on 127.0.0.1:8787 serving a free left-pad@1.3.0 patch (run-18 mock from the #304 ledger)
export SOCKET_PATCH_SERVER_URL=http://127.0.0.1:8787
API="--api-url http://127.0.0.1:8787 --org o --api-token x"
mkdir p && cd p
echo '{"name":"p","version":"1.0.0","private":true,"dependencies":{"left-pad":"1.3.0"}}' > package.json
yarn@1.22.22 install                      # v1 lock
# mid-migration: the project now declares berry
echo '{"name":"p","version":"1.0.0","private":true,"packageManager":"yarn@4.18.1","dependencies":{"left-pad":"1.3.0"}}' > package.json
socket-patch scan --mode hosted --json --yes $API
#   status "success", redirect.redirected 1, redirect.warnings [] , top-level warnings []
grep resolved yarn.lock                   # http://127.0.0.1:8787/artifacts/…/left-pad-1.3.0.tgz#81960ff…
rm -rf node_modules
printf 'nodeLinker: node-modules\nenableGlobalCache: false\n' > .yarnrc.yml
YARN_ENABLE_IMMUTABLE_INSTALLS=0 yarn@4.18.1 install   # exit 0, migrates the lock
grep -A3 '"left-pad@' yarn.lock           # resolution: "left-pad@npm:1.3.0"  (pin gone)
head -1 node_modules/left-pad/index.js    # upstream bytes, no patch marker

# control: identical flow with --mode vendored
#   warnings: [yarn_classic_berry_migration_risk]  ("…installing with yarn 2+ (berry) migrates the lockfile and silently drops them…")

Expected vs actual

• Expected: hosted pins in a classic lock are subject to the same migration loss the vendored probe describes ("installing with yarn 2+ (berry) migrates the lockfile and silently drops them — packages install unpatched from the registry"), so hosted should emit the same advisory (yarn_classic_berry_migration_risk, or a redirect_* twin). It should be suppressed by a packageManager: yarn@1… pin, and fire when there's no pin or when a non-1 yarn is declared. CLI_CONTRACT's hosted section says a dep counts as redirected only when its pin "actually landed in a project file". Here it lands, but the project's own declared package manager discards it on the next install with no signal.
• Actual: hosted exits 0 success with no warning, while vendored on the same project warns.

OS × version

Linux, main 9c43dfc, each cell run at least once; the 1.22.22 scan cells twice. Berry is yarn 4.18.1 (@yarnpkg/cli-dist), nodeLinker: node-modules.

lock written by	command	packageManager	socket-patch result	berry install	pin kept / installed patched
yarn 1.7.0	scan --mode hosted	none	success, redirected 1, no warning	exit 0, migrated	no / no
yarn 1.7.0	scan --mode hosted	yarn@4.18.1	success, redirected 1, no warning	exit 0, migrated	no / no
yarn 1.7.0	get <uuid> --mode hosted	none / yarn@4.18.1	success, redirected 1, no warning	exit 0, migrated	no / no
yarn 1.10.1	scan / get --mode hosted	none / yarn@4.18.1	success, redirected 1, no warning	exit 0, migrated	no / no
yarn 1.22.22	scan / get --mode hosted	none / yarn@4.18.1 (×2)	success, redirected 1, no warning	exit 0, migrated	no / no
yarn 1.22.22	scan --mode vendored (control)	none / yarn@4.18.1 (×2)	success + yarn_classic_berry_migration_risk	exit 0, migrated	no / no
yarn 1.22.22	scan --mode hosted, packageManager: yarn@1.22.22	pinned	success, no warning (correct)	n/a (corepack would refuse berry)	—
yarn 1.22.22	hosted, then berry install --immutable	none	success, no warning	YN0028, lockfile would be modified	—

macOS / Windows weren't probed. The behaviour is in the shared engine, not OS-specific code. No bisect: hosted mode has never called the probe.

Suspect code

• crates/socket-patch-core/src/vendor/mod.rs:177 yarn_classic_berry_migration_risk only looks for .socket/vendor/ wiring (lock.contains(".socket/vendor/")), so it can't see a hosted pin.
• crates/socket-patch-cli/src/commands/vendor.rs:689 note_classic_migration_risk is called only from the vendor paths (vendor.rs:949, vendor.rs:1600, scan/vendor_flow.rs:371). The hosted flow and rewrite_yarn_classic (crates/socket-patch-core/src/patch/redirect/mod.rs:3217) have no equivalent.

[mikolalysenko]

[agent] Claiming this issue (single-issue cluster; root cause: the hosted yarn classic rewriter never runs the berry-migration check that vendored mode runs, and the hosted engine doesn't read package.json beside a classic yarn.lock). Branch: agent/fix-yarn-classic-hosted-berry-risk. Claim-ID: 2026-10-06T05:21:16Z-a150ce

---

Generated by Claude Code

[mikolalysenko]

[agent] Draft PR: #917

---

Generated by Claude Code