diff --git a/crates/engine/src/store.rs b/crates/engine/src/store.rs
index a56b29d9d..fd2543751 100644
--- a/crates/engine/src/store.rs
+++ b/crates/engine/src/store.rs
@@ -191,6 +191,9 @@ fn lock_disabled(_: &Path) -> Option<std::fs::File> {
 thread_local! {
     /// Tests: [`Session::put_back_plugin`] fails on this thread, as on a full volume.
     pub(crate) static PUT_BACK_FAILS: std::cell::Cell<bool> = const { std::cell::Cell::new(false) };
+    /// Tests: removing [`GRANTS_FILE`] fails on this thread while the file still reads, as when
+    /// another program holds it open.
+    pub(crate) static GRANTS_DELETE_FAILS: std::cell::Cell<bool> = const { std::cell::Cell::new(false) };
     /// Tests: [`Session::put_back_plugin`] writes the old module back on this thread and then
     /// reports a failure, as when the last flush of a copy fails.
     pub(crate) static PUT_BACK_FAILS_AFTER_WRITE: std::cell::Cell<bool> = const { std::cell::Cell::new(false) };
@@ -205,7 +208,15 @@ thread_local! {
 /// Remove `dir/file` → whether it is gone; a file that isn't there is what was wanted.
 fn take(dir: &Path, file: &str) -> bool {
     let path = dir.join(file);
-    match std::fs::remove_file(&path) {
+    #[cfg(test)]
+    let removed = if file == GRANTS_FILE && GRANTS_DELETE_FAILS.get() {
+        Err(std::io::Error::from(std::io::ErrorKind::PermissionDenied))
+    } else {
+        std::fs::remove_file(&path)
+    };
+    #[cfg(not(test))]
+    let removed = std::fs::remove_file(&path);
+    match removed {
         Err(e) if e.kind() != std::io::ErrorKind::NotFound => {
             log::warn!("can't remove {}: {e}", path.display());
             false
diff --git a/crates/engine/src/tests/plugins.rs b/crates/engine/src/tests/plugins.rs
index d21754e36..36dbc5ba8 100644
--- a/crates/engine/src/tests/plugins.rs
+++ b/crates/engine/src/tests/plugins.rs
@@ -2123,6 +2123,35 @@ fn a_grants_file_that_cannot_be_deleted_keeps_the_stored_module() {
     run(&mut s, "plugin.remove", json!({"id": id}));
 }
 
+/// When the last grant can't be removed because `grants.json` can't be deleted, a replacing module
+/// that asks for no permissions doesn't stay in the store: the store gets back the module the grant
+/// still covers, and the grant is left as it was.
+#[test]
+fn a_grants_file_whose_deletion_fails_keeps_the_stored_module() {
+    let folder = crate::tests_links::Folder::new("plugin-store-undeletable-grants");
+    let id = "test.engine.permissions.undeletablegrants";
+    let empty = json!({"objects": []});
+    let approved = asking(id, r#"["doc.read"]"#, &empty);
+    let approved_data = vectorcraft_format::base64_encode(&approved);
+    let plain = asking(id, "null", &empty);
+    let grants = folder.0.join("grants.json");
+    let mut s = session();
+    s.attach_plugin_store(folder.0.clone());
+    run(&mut s, "plugin.install", json!({"dataBase64": approved_data, "grant": ["doc.read"]}));
+    let file = s.stored_plugin_file(id).unwrap();
+    let recorded = std::fs::read(&grants).unwrap();
+
+    crate::store::GRANTS_DELETE_FAILS.set(true);
+    run(&mut s, "plugin.install", json!({"dataBase64": vectorcraft_format::base64_encode(&plain), "replace": true}));
+    crate::store::GRANTS_DELETE_FAILS.set(false);
+    assert_eq!(std::fs::read(&file).unwrap(), approved, "the store gets the module the grant covers back");
+    assert_eq!(std::fs::read(&grants).unwrap(), recorded, "the grant stays as it was");
+    assert_eq!(run(&mut s, "plugin.inspect", json!({"dataBase64": approved_data}))["needsApproval"], false, "the grant still covers the module");
+
+    run(&mut s, "plugin.remove", json!({"id": id}));
+    assert!(!grants.exists(), "a removal that can delete the file forgets the grant");
+}
+
 /// A plug-in folder that leaves a module listing no permissions under an id forgets the process's
 /// grant for it, unless a later file in the folder leaves a granted module there. A grant kept in
 /// the plug-in store stays, with the stored module it covers.