diff --git a/shell/shell.qml b/shell/shell.qml
index 888d871f..7aecdcca 100644
--- a/shell/shell.qml
+++ b/shell/shell.qml
@@ -319,7 +319,8 @@ ShellRoot {
     if (!manifest) return null
     if (manifest.__isFirstParty) return manifest
     var copy = JSON.parse(JSON.stringify(manifest))
-    delete copy.__sourceDir
+    // Keep __sourceDir: third-party plugins find their bundled scripts through it.
+    // Only the host trust markers are withheld.
     delete copy.__isFirstParty
     delete copy.__hostCapabilities
     return copy
diff --git a/test/shell.d/plugins-test.sh b/test/shell.d/plugins-test.sh
index 5d634dc9..c10509a7 100644
--- a/test/shell.d/plugins-test.sh
+++ b/test/shell.d/plugins-test.sh
@@ -210,5 +210,18 @@ check(
   '_syncServices still drops disabled or removed services'
 )
 
+const publicManifestMatch = shellSource.match(/function publicPluginManifest\(manifest\) \{[\s\S]*?\n  \}/)
+check(!!publicManifestMatch, 'publicPluginManifest is defined')
+if (publicManifestMatch) {
+  const host = {}
+  require('vm').runInNewContext(`${publicManifestMatch[0]}\nthis.publicPluginManifest = publicPluginManifest`, host)
+  const raw = { id: 'acme.demo', __sourceDir: '/plugins/acme.demo', __isFirstParty: false, __hostCapabilities: ['authentication'] }
+  const copy = host.publicPluginManifest(raw)
+  check(copy.__sourceDir === '/plugins/acme.demo', 'third-party manifests keep their own source directory')
+  check(!('__isFirstParty' in copy) && !('__hostCapabilities' in copy), 'third-party manifests drop host trust markers')
+  copy.__sourceDir = '/elsewhere'
+  check(raw.__sourceDir === '/plugins/acme.demo', 'a plugin editing its manifest copy cannot move the registry entry')
+}
+
 assert(errors.length === 0, 'plugin manifests match shell registry contract', errors.join('\n'))
 JS