Implements native dependency patching per RFC #862: a first-class way to apply small, local modifications to an installed dependency and have them re-applied automatically on every install, with no external tooling or postinstall scripts.

Patches are declared in a new patchedDependencies field of the root package.json, stored as plain unified diffs under patches/, and recorded with a content hash in package-lock.json. Because the patch is applied during the install itself, it works for transitive dependencies, across every install-strategy, and is not disabled by --ignore-scripts.

The npm patch command

A new command with five subcommands (and a bare npm patch <pkg> shorthand for add):

• npm patch add <pkg>[@<version>] — extracts a clean copy of the resolved registry tarball into a temp directory outside node_modules and prints the path to edit. Ambiguous when multiple versions are installed; the error lists the exact selectors to retry with.
• npm patch commit <edit-dir> — diffs the edited directory against a fresh copy of the original tarball, writes <patches-dir>/<name>@<version>.patch, adds the patchedDependencies entry, and reifies to apply the patch and record its integrity in the lockfile. package.json is excluded from the diff — Arborist resolves the pre-patch manifest, so a patched manifest would change resolution-affecting fields on disk without being honored (silent partial application); commit warns when an edit only touches it.
• npm patch update <pkg>[@<old-version>] [--to <new-version>] — rebases an existing patch onto a new version. It reads the target from --to or the lockfile, 3-way-merges the existing patch onto the new tarball in a throwaway git repo, and rewrites package.json + package-lock.json without touching node_modules (so it works from a failed-install state). On conflict it leaves an edit dir with <<<<<<< markers, finalized by npm patch commit. Exact selectors are renamed; range/name-only selectors gain a new exact entry and keep the old one while it still wins another installed node.
• npm patch ls — lists registered patches and how many installed nodes each matches (flagging overlapping range selectors that conflict on a node).
• npm patch rm <pkg>[@<version>] — removes the matching entries, deletes the patch file when no other entry references it, and reifies to revert the files.

Install-time apply pipeline

Patch resolution and application live in Arborist so every install path honors them:

• resolvePatchedDependencies resolves the root patchedDependencies map against the ideal tree, attaching node.patched = { path, integrity } to each matched node. Selector precedence is exact > range-subset > name-only, with ambiguous overlapping ranges surfaced as a hard error.
• reify applies the diff after extraction and records the patched integrity in the lockfile. diff.js forces re-extraction when a node's patch integrity changes, and re-extracts to revert when a previously-patched node loses its selector (patchRemoved).
• install-strategy=linked is supported via a content-addressed side-store: the store key is suffixed with the patch identity (+patch) so a patched and unpatched copy of the same version coexist without collision. A failed patch under linked strategy is always a hard error (the side-store cannot represent unpatched contents at a patched key without later installs silently trusting it).

Lockfile

Patches require lockfileVersion: 4 so that older npm clients abort rather than silently installing unpatched code. When any node is patched, npm writes version 4 and warns if this upgrades a lower pinned lockfile-version (the safety gate cannot be honored otherwise). npm ci revalidates each patch's existence and integrity against the lockfile before installing.

Failure modes

By default any patch problem is a hard error that aborts the install: a patch that fails to apply, a registered patch that matches no installed package, a missing patch file, or a patch whose hash does not match the lockfile. Two CLI-only relax flags cover one-off cases — --allow-unused-patches and --ignore-patch-failures — and are rejected in npm ci and when set anywhere other than the command line.

Non-registry dependencies

Patches need a stable registry tarball as their baseline, so a dependency reached through a non-registry consumer edge (file:, git:, http(s):) is rejected with EPATCHNONREGISTRY, both by npm patch add and at install time. The check is edge-based (the consuming spec's type), not node-based, so it does not falsely reject edgeless nodes such as linked-store entries or extraneous installs, which are still registry deps. npm: registry aliases are correctly classified as registry deps and are supported by the install engine; the npm patch add <alias> ergonomics will land in a fast-follow.

Publish / pack

patchedDependencies is stripped from the published registry manifest (libnpmpublish) so the field never leaks to the packument. Stripping it from the tarball's own package.json and excluding the patches/ directory from the tarball is a coordinated follow-up in pacote + npm-packlist (those packages own the packed file list and the manifest written into the tarball, neither editable from the CLI) — see Follow-up work.

Other surfaces

• npm ls annotates patched dependencies in its output.
• New config: patches-dir, edit-dir, ignore-existing, keep-edit-dir, plus the two relax flags.
• New npm-patch man page and nav entry.

Tests

Unit and integration coverage for every subcommand (including update's clean rebase, conflict→commit, and selector-rename/range-fork paths), the apply pipeline, selector matching, linked-strategy apply/removal, lockfile validation, publish stripping, and the relax flags. Arborist and CLI suites pass at 100% coverage.

Follow-up work

A few additive pieces are deliberately deferred — nothing in this PR depends on them.

• Tarball-side strip for publish/pack — stripping patchedDependencies from the tarball's own package.json and excluding the patches/ directory from the published tarball. This can't be done in the CLI: the tarball's file list and manifest come from pacote (packs the raw on-disk files) and npm-packlist, so it needs coordinated changes there. Raised in the RFC review; the registry-manifest strip in this PR already prevents the field from being honored or appearing in the packument.

• npm patch add <alias> ergonomics for npm: registry aliases — the install engine already treats npm: aliases as registry dependencies and applies a hand-written <alias>@<version> selector correctly today. What remains is the add/commit convenience: resolving the alias to its real name@version tarball as the baseline and keying the written selector on the alias name. Currently npm patch add <alias> resolves the alias name as a real package and fails.

• Binary files — patches are unified text diffs, so binary files (images, wasm, native addons) cannot be patched. This is a limitation of the whole feature (shared with patch-package), not a regression; a binary-aware path could be added later.

References

Implements npm/rfcs#862