diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index b45820b..889e4ea 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -67,7 +67,7 @@ Tables can also:
 - opt into **`ScanResult.ProjectRoot` attribution** — the Python table sets the matched root on every result because its artifacts sit at arbitrary depth and output grouping needs explicit attribution (nested project roots win over parents);
 - declare a **`PruneRoot`** hook that skips the ecosystem's own SDK checkout.
 
-**Pruned trees** are skipped whole — no artifact matching, no project context, no descent — so nothing inside is ever a deletion target: installed-package trees (pnpm store version dirs, macOS `.app` bundles) engine-wide, and an ecosystem's SDK checkout via `PruneRoot` — [decision](decisions/prune-non-project-trees.md).
+**Pruned trees** are skipped whole — no artifact matching, no project context, no descent — so nothing inside is ever a deletion target: installed-package trees (`node_modules` outside a project, `site-packages`, package stores, app bundles) engine-wide, and an ecosystem's SDK checkout via `PruneRoot` — [decision](decisions/prune-non-project-trees.md).
 
 **Deduplication**: a directory matching rules of several active ecosystems is reported once, attributed to the first table in order (node → rust → ruby → python → go → flutter → android), and no scanner descends into another's matched artifact (`__pycache__` inside `node_modules` is not reported separately). Attribution can therefore differ between a full scan and an `--eco` subset scan.
 
diff --git a/docs/SPEC.md b/docs/SPEC.md
index 0069535..73b31bb 100644
--- a/docs/SPEC.md
+++ b/docs/SPEC.md
@@ -328,7 +328,7 @@ Sizes use **decimal SI units** (1 KB = 1000 B). `--min-size` uses the same conve
 
 **pnpm-installed `node_modules`**: pnpm clones (macOS) or hard-links (Linux) pnpm store files into `node_modules` by default, so deleting it alone frees little; running `pnpm store prune` afterwards removes packages no project references. The result carries this note when `node_modules/.modules.yaml` (written by pnpm on install, JSON since pnpm 12; `pnpm-lock.yaml` alone does not prove pnpm populated the folder) names a `storeDir` that exists on the same volume. On macOS its `size` leaves out blocks shared with the store or other projects. A store on another volume forces pnpm to copy, so no note is given. `packageImportMethod: copy` is not recorded in `.modules.yaml`, so such installs still get the note.
 
-**Installed-package trees are excluded**: `node_modules`/`dist` inside a pnpm store or a macOS `.app` bundle are package content, not project output — see [decision](decisions/prune-non-project-trees.md).
+**Installed-package trees are excluded**: nothing inside a `node_modules` that is not a project's own, Python `site-packages`/`dist-packages`, the Go module cache, a pnpm store or a macOS `.app` bundle is reported — see [decision](decisions/prune-non-project-trees.md).
 
 **Monorepo support**: artifacts in sub-packages (apps/, packages/) are grouped under the git root project. Sub-packages are displayed with headers showing their path and total size.
 
diff --git a/docs/decisions/prune-non-project-trees.md b/docs/decisions/prune-non-project-trees.md
index 4673c65..ff99fca 100644
--- a/docs/decisions/prune-non-project-trees.md
+++ b/docs/decisions/prune-non-project-trees.md
@@ -4,9 +4,9 @@
 
 The walk engine skips these trees whole — no artifact matching, no project context, no descent — so nothing inside is ever a deletion target:
 
-- **Installed-package trees**, engine-wide regardless of which ecosystems are active: their marker files belong to shipped packages, not projects. Detected by invariant layout, never install path.
-  - pnpm store version dir (`v<N>` with `files/` and `index/` or `index.db`)
-  - macOS app bundle (`*.app` with `Contents/`)
+- **Installed-package trees**, whichever ecosystems are active. Detected by layout or by a name only package managers create, never by install path:
+  - by name, when the directory is not itself a matched artifact: `node_modules`, `site-packages`, `dist-packages`
+  - by entries: pnpm store version dir (`v<N>` with `files/` and `index/` or `index.db`), macOS app bundle (`*.app` with `Contents/`), Go module cache (`mod/` with `cache/download/`)
 - **An ecosystem's own SDK checkout**, via the table's `PruneRoot` hook. The SDK root is detected by its invariant bootstrap layout, never a hardcoded install path (Flutter: `bin/flutter` + `bin/internal/engine.version`).
 
 When adding an ecosystem or rule, scan that tool's real SDK/toolchain checkout and verify what gets flagged. A scanner's own false positive is fixed in that scanner's PR.
@@ -19,14 +19,15 @@ go test ./internal/scanner/ -run 'TestWalkExcludesInstalledPackageTrees|TestFlut
 
 ## Rationale
 
+- Each package in an installed-package tree, including the runtime an app ships inside its bundle, carries its own marker (`package.json`, `setup.py`, `go.mod`). Without pruning it roots a project, and its shipped `dist/`, `__pycache__` or `vendor/` matches as `safe` output — deleting the `dist/` of the npm bundled in a GitHub Actions runner breaks the runner. A project's own `node_modules` matches as an artifact before the name check, so it is still reported.
+- pnpm v11's `links/` holds unpacked packages; deleting it corrupts the store every project hard-links from. The store itself is reported by the Global Caches scanner.
 - A directory name like `build`/`dist` is not always regenerable output. Inside an SDK checkout it can be committed *source*: the Flutter SDK's `engine/src/build` / `engine/src/flutter/build` are GN build-system source trees tracked in git.
 - Gitignore-aware protection does not catch this: committed-clean files are not `protected`, so they classify as `safe` and become deletion targets.
-- pnpm v11's `links/` unpacks packages whose `dist/` would match; deleting it corrupts the store every project hard-links from. The store itself is reported by the Global Caches scanner.
-- Apps ship their runtime inside the bundle (e.g. Electron `node_modules` in VS Code update copies under `~/Library/Caches`).
 - The Android SDK ships no `build.gradle`, so it never establishes a project context and needs no `PruneRoot`.
 
 ## Rejected alternatives
 
 - **Detect by install path** — SDKs and stores live wherever the user put them; a hardcoded path misses relocated checkouts.
 - **Rely on gitignore-aware protection** — misses committed-clean source (above).
+- **Prune every directory a deps rule names** — would also prune `vendor/` and `venv/` outside projects, which are not package-manager trees.
 - **Defer a scanner's false positive to a separate change** — the scanner ships able to delete real source in the meantime.
diff --git a/internal/scanner/walk.go b/internal/scanner/walk.go
index 29e9769..7ac60e1 100644
--- a/internal/scanner/walk.go
+++ b/internal/scanner/walk.go
@@ -186,6 +186,11 @@ func runWalk(ctx context.Context, root string, tables []walkEcosystem) ([]model.
 				ReportProgress(ctx, len(results))
 				return false // matched artifact: do not descend
 			}
+			// After artifact matching, so a project's own node_modules is
+			// still reported.
+			if installedPackageDirNames[name] {
+				return false
+			}
 			// The root is always entered: the hidden-dir rule applies below it.
 			return dir == root || !strings.HasPrefix(name, ".") || hiddenDescend[name]
 		},
@@ -197,7 +202,7 @@ func runWalk(ctx context.Context, root string, tables []walkEcosystem) ([]model.
 
 			// Prune before establishing any context or descending, so nothing
 			// inside is ever a deletion target.
-			if isInstalledPackageTree(filepath.Base(dir), names) {
+			if isInstalledPackageTree(dir, names) {
 				return false
 			}
 			for i := range tables {
@@ -263,19 +268,34 @@ func runWalk(ctx context.Context, root string, tables []walkEcosystem) ([]model.
 	return results, nil
 }
 
+// installedPackageDirNames hold only installed packages when the walk reaches
+// them without matching an artifact rule (a global lib/node_modules, a tool's
+// bundled dependencies, a Python environment's site-packages). Enter prunes
+// them by name, so they are never read.
+var installedPackageDirNames = map[string]bool{
+	"node_modules":  true,
+	"site-packages": true,
+	"dist-packages": true,
+}
+
 // isInstalledPackageTree reports whether a directory roots a tree of installed
-// packages, whose marker files belong to shipped packages, not projects.
-// Checked for every ecosystem, so results don't depend on the --eco subset.
+// packages identified by its entries:
 //
 //   - pnpm store version dir: v<N> with files/ and index/ (v10) or index.db
-//     (v11). v11's links/ unpacks packages whose dist/ would match; deleting
-//     it corrupts the store every pnpm project hard-links from.
-//   - macOS app bundle: *.app with Contents/ (e.g. Electron node_modules).
-func isInstalledPackageTree(name string, names map[string]bool) bool {
-	if strings.HasSuffix(name, ".app") && names["Contents"] {
+//     (v11). Deleting v11's links/ corrupts the store.
+//   - macOS app bundle: *.app with Contents/.
+//   - Go module cache: mod/ with cache/download/.
+func isInstalledPackageTree(dir string, names map[string]bool) bool {
+	name := filepath.Base(dir)
+	switch {
+	case strings.HasSuffix(name, ".app") && names["Contents"]:
+		return true
+	case name == "mod" && names["cache"]:
+		return isDir(filepath.Join(dir, "cache", "download"))
+	case isPnpmStoreVersionDir(name) && names["files"] && (names["index"] || names["index.db"]):
 		return true
 	}
-	return isPnpmStoreVersionDir(name) && names["files"] && (names["index"] || names["index.db"])
+	return false
 }
 
 func isPnpmStoreVersionDir(name string) bool {
diff --git a/internal/scanner/walk_prune_test.go b/internal/scanner/walk_prune_test.go
index bb9a7f9..2db924d 100644
--- a/internal/scanner/walk_prune_test.go
+++ b/internal/scanner/walk_prune_test.go
@@ -39,6 +39,36 @@ func TestWalkExcludesInstalledPackageTrees(t *testing.T) {
 			},
 			want: []artifact{{"app/node_modules", model.EcoNode, model.CatDeps, model.SafetySafe, ""}},
 		},
+		{
+			name: "node_modules outside a project",
+			ecos: node,
+			tree: []string{
+				"sdk-cache/darwin-arm64/node_modules/hono/package.json",
+				"sdk-cache/darwin-arm64/node_modules/hono/dist/index.js",
+				"runner/node20/lib/node_modules/npm/package.json",
+				"runner/node20/lib/node_modules/npm/node_modules/abbrev/",
+				"app/package.json", "app/tools/node_modules/x/package.json", "app/tools/node_modules/x/dist/",
+			},
+		},
+		{
+			name: "python site-packages",
+			ecos: []model.Ecosystem{model.EcoPython},
+			tree: []string{
+				"tool/.venv/lib/python3.12/site-packages/numpy/f2py/setup.py",
+				"tool/.venv/lib/python3.12/site-packages/numpy/f2py/__pycache__/x.pyc",
+				"usr/lib/python3/dist-packages/pkg/setup.py",
+				"usr/lib/python3/dist-packages/pkg/__pycache__/x.pyc",
+			},
+		},
+		{
+			name: "go module cache",
+			ecos: []model.Ecosystem{model.EcoGo},
+			tree: []string{
+				"go/pkg/mod/cache/download/x/@v/list",
+				"go/pkg/mod/github.com/briandowns/spinner@v1.23.2/go.mod",
+				"go/pkg/mod/github.com/briandowns/spinner@v1.23.2/vendor/",
+			},
+		},
 		{
 			name: "app bundle without node in the scan",
 			ecos: []model.Ecosystem{model.EcoPython},
diff --git a/internal/scanner/walk_test.go b/internal/scanner/walk_test.go
index 717f8bf..ba1c5ca 100644
--- a/internal/scanner/walk_test.go
+++ b/internal/scanner/walk_test.go
@@ -74,8 +74,8 @@ func TestWalkScan_DedupCoverage(t *testing.T) {
 
 // TestWalkScan_DedupNodeModulesShieldsPython pins the second double-count
 // fix: a matched artifact is skipped entirely, so no other active ecosystem
-// reports anything inside it. With python alone, node_modules is not an
-// artifact and its contents are scanned as before.
+// reports anything inside it. With python alone, node_modules is pruned as an
+// installed-package tree.
 func TestWalkScan_DedupNodeModulesShieldsPython(t *testing.T) {
 	root := t.TempDir()
 
@@ -108,16 +108,13 @@ func TestWalkScan_DedupNodeModulesShieldsPython(t *testing.T) {
 		t.Errorf("expected node_modules attributed to node, got %s (%s)", results[0].Path, results[0].Ecosystem)
 	}
 
-	// python alone: descends into node_modules and finds the __pycache__.
+	// python alone: node_modules is pruned, so nothing inside is reported.
 	results, err = WalkScan(context.Background(), root, model.EcoPython)
 	if err != nil {
 		t.Fatalf("WalkScan error: %v", err)
 	}
-	if len(results) != 1 {
-		t.Fatalf("expected exactly 1 result for python alone, got %d", len(results))
-	}
-	if results[0].Path != pyc || results[0].Ecosystem != model.EcoPython {
-		t.Errorf("expected __pycache__ attributed to python, got %s (%s)", results[0].Path, results[0].Ecosystem)
+	if len(results) != 0 {
+		t.Fatalf("expected no result for python alone, got %d (%s)", len(results), results[0].Path)
 	}
 }