Fix pnpm workspace detection

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Fix pnpm workspace detection. ([#1528](https://github.com/heroku/heroku-buildpack-nodejs/pull/1528))
 
 ## [v323] - 2026-01-09
 

lib/dependencies.sh

@@ -347,7 +347,7 @@ pnpm_prune_devdependencies() {
     echo "Skipping because PNPM_SKIP_PRUNING is '$PNPM_SKIP_PRUNING'"
     build_data::set_raw "skipped_prune" "true"
     return 0
-  elif [ -f "$build_dir/pnpm-workspace.yaml" ] || [ -f "$build_dir/pnpm-workspace.yml" ]; then
+  elif [[ "$(pnpm_workspace_configured "$build_dir")" == "true" ]]; then
     echo "Skipping because pruning is not supported for pnpm workspaces (https://pnpm.io/cli/prune)"
     build_data::set_raw "skipped_prune" "true"
     return 0
@@ -385,3 +385,25 @@ pnpm_prune_devdependencies() {
 
   build_data::set_raw "skipped_prune" "false"
 }
+
+pnpm_workspace_configured() {
+  local build_dir=${1:-}
+  local workspace_file="$build_dir/pnpm-workspace.yaml"
+  local yq
+  local result
+
+  yq="$BP_DIR/lib/vendor/yq-$(get_os)"
+
+  if [[ -f "$workspace_file" ]]; then
+    # prior to pnpm 10.5.0, the `packages` key was mandatory, but now, you can store
+    # other pnpm-related config settings in `pnpm-workspace.yaml`.
+    result=$($yq r "$workspace_file" 'packages' 2>&1)
+
+    if [[ -n "$result" && "$result" != "null" ]]; then
+      echo "true"
+      return
+    fi
+  fi
+
+  echo "false"
+}

test/fixtures/pnpm-workspace-file-exists-but-not-configured-as-workspace/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "pnpm-workspace-file-exists-but-not-configured-as-workspace",
+  "version": "1.0.0",
+  "packageManager": "[email protected]+sha512.11106a5916c7406fe4b8cb8e3067974b8728f47308a4f5ac5e850304afa6f57e2847d7950dfe78877d8d36bfb401d381c4215db3a4c3547ffa63c14333a6fa51",
+  "engines": {
+    "node": "20.x"
+  },
+  "devDependencies": {
+    "dotenv": "^17.2.3"
+  }
+}

test/fixtures/pnpm-workspace-file-exists-but-not-configured-as-workspace/pnpm-workspace.yaml

@@ -0,0 +1,2 @@
+# just a random config setting that shouldn't affect anything, the important thing is there's no `packages` field
+peersSuffixMaxLength: 1001

test/run

@@ -1992,6 +1992,13 @@ testPnpmWorkspaceBinaries() {
   assertCapturedSuccess
 }
 
+testPnpmWorkspaceFileExistsButNotConfiguredAsWorkspace() {
+  compile "pnpm-workspace-file-exists-but-not-configured-as-workspace"
+  assertCaptured "+ dotenv"
+  assertCaptured "- dotenv"
+  assertCapturedSuccess
+}
+
 testWarningNode_22_5_0() {
   compile "node-22.5.0-warning"
   assertCaptured "Issues with Node.js v22.5.0"