diff --git a/crates/but-core/src/worktree/checkout/function.rs b/crates/but-core/src/worktree/checkout/function.rs
index c63f263d2c9..5689127f585 100644
--- a/crates/but-core/src/worktree/checkout/function.rs
+++ b/crates/but-core/src/worktree/checkout/function.rs
@@ -47,6 +47,15 @@ pub fn safe_checkout_from_head(
         bail!("Refusing to check out conflicted commit {new_head_id}");
     }
 
+    // This probe briefly creates and removes index.lock, and only detects a lock held now.
+    // It does not serialize checkout: libgit2 acquires the lock when persisting results.
+    let index_lock = gix::lock::File::acquire_to_update_resource(
+        repo.index_path(),
+        gix::lock::acquire::Fail::Immediately,
+        None,
+    )?;
+    drop(index_lock);
+
     let git2_repo = git2::Repository::open(repo.git_dir())?;
     let head_tree_id = repo.head_tree_id_or_empty()?;
     let head_tree = git2_repo.find_tree(head_tree_id.to_git2())?;
diff --git a/crates/but-core/tests/core/worktree/checkout_index_lock.rs b/crates/but-core/tests/core/worktree/checkout_index_lock.rs
new file mode 100644
index 00000000000..6151ca9db8d
--- /dev/null
+++ b/crates/but-core/tests/core/worktree/checkout_index_lock.rs
@@ -0,0 +1,154 @@
+use std::io::{Read as _, Seek as _, Write as _};
+
+use but_core::worktree::safe_checkout_from_head;
+use but_testsupport::{CommandExt, git, git_status, open_repo, writable_scenario};
+use gix::object::tree::EntryKind;
+
+use crate::worktree::utils::build_commit;
+
+#[test]
+fn external_index_lock_prevents_all_checkout_changes() -> anyhow::Result<()> {
+    let (repo, _tmp) = writable_scenario("unborn-empty");
+    let mut tree = repo.empty_tree().edit()?;
+    tree.upsert(
+        "existing.txt",
+        EntryKind::Blob,
+        repo.write_blob(b"original\n")?,
+    )?;
+    let initial = repo.new_commit("initial", tree.write()?.detach(), None::<gix::ObjectId>)?;
+    safe_checkout_from_head(initial.id, &repo, Default::default())?;
+
+    let next = build_commit(
+        &repo,
+        |tree| {
+            tree.upsert(
+                "existing.txt",
+                EntryKind::Blob,
+                repo.write_blob(b"changed\n")?,
+            )?;
+            tree.upsert("added.txt", EntryKind::Blob, repo.write_blob(b"added\n")?)?;
+            Ok(())
+        },
+        "change existing and add file",
+    )?;
+
+    let existing_path = repo.workdir_path("existing.txt").expect("non-bare repo");
+    let added_path = repo.workdir_path("added.txt").expect("non-bare repo");
+    let index_path = repo.index_path();
+    let index_before = std::fs::read(&index_path)?;
+    let head_name_before = repo.head_name()?.expect("symbolic HEAD").to_owned();
+    let head_id_before = repo.head_id()?.detach();
+    let head_ref_id_before = repo.find_reference(&head_name_before)?.id().detach();
+
+    let mut external_lock = gix::lock::File::acquire_to_update_resource(
+        &index_path,
+        gix::lock::acquire::Fail::Immediately,
+        None,
+    )?;
+    external_lock.write_all(b"external owner")?;
+    let lock_path = external_lock.lock_path().to_owned();
+
+    let err = safe_checkout_from_head(next.id, &repo, Default::default())
+        .expect_err("the external index lock must prevent checkout");
+
+    assert_eq!(
+        std::fs::read_to_string(&existing_path)?,
+        "original\n",
+        "checkout must not change existing files before reporting contention"
+    );
+    assert!(
+        !added_path.exists(),
+        "checkout must not add files before reporting contention"
+    );
+    assert_eq!(
+        std::fs::read(&index_path)?,
+        index_before,
+        "checkout must leave the raw index unchanged"
+    );
+    assert_eq!(
+        repo.head_name()?.as_ref(),
+        Some(&head_name_before),
+        "checkout must leave HEAD attached to the same reference"
+    );
+    assert_eq!(
+        repo.head_id()?.detach(),
+        head_id_before,
+        "checkout must leave HEAD at the same commit"
+    );
+    assert_eq!(
+        repo.find_reference(&head_name_before)?.id().detach(),
+        head_ref_id_before,
+        "checkout must leave the HEAD reference unchanged"
+    );
+    assert!(
+        lock_path.exists(),
+        "checkout must not remove an external index lock"
+    );
+    external_lock.rewind()?;
+    let mut lock_contents = String::new();
+    external_lock.read_to_string(&mut lock_contents)?;
+    assert_eq!(
+        lock_contents, "external owner",
+        "checkout must not overwrite an external index lock"
+    );
+    match err.downcast_ref::<gix::lock::acquire::Error>() {
+        Some(gix::lock::acquire::Error::PermanentlyLocked { resource_path, .. }) => {
+            assert_eq!(
+                resource_path, &index_path,
+                "contention must be reported for this worktree's index path"
+            );
+        }
+        other => panic!("expected gix index-lock contention, got {other:?}: {err:#}"),
+    }
+
+    drop(external_lock);
+    safe_checkout_from_head(next.id, &repo, Default::default())?;
+    assert_eq!(git_status(&repo)?, "", "successful checkout must be clean");
+    git(&repo).args(["diff", "--exit-code", "HEAD", "--"]).run();
+    Ok(())
+}
+
+#[test]
+fn linked_worktree_preflight_uses_its_own_index_path() -> anyhow::Result<()> {
+    let (repo, _tmp) = writable_scenario("single-unsigned");
+    let linked_root = but_testsupport::gix_testtools::tempfile::TempDir::new()?;
+    let linked_path = linked_root.path().join("linked");
+    git(&repo)
+        .args(["worktree", "add", "--detach"])
+        .arg(&linked_path)
+        .run();
+    let linked_repo = open_repo(&linked_path)?;
+    assert_ne!(
+        linked_repo.index_path(),
+        repo.index_path(),
+        "a linked worktree must have its own index"
+    );
+
+    let linked_index_path = linked_repo.index_path();
+    let external_lock = gix::lock::File::acquire_to_update_resource(
+        &linked_index_path,
+        gix::lock::acquire::Fail::Immediately,
+        None,
+    )?;
+    let err = safe_checkout_from_head(
+        linked_repo.head_id()?.detach(),
+        &linked_repo,
+        Default::default(),
+    )
+    .expect_err("the linked worktree's index lock must prevent checkout");
+
+    match err.downcast_ref::<gix::lock::acquire::Error>() {
+        Some(gix::lock::acquire::Error::PermanentlyLocked { resource_path, .. }) => {
+            assert_eq!(
+                resource_path, &linked_index_path,
+                "contention must name the linked worktree's index"
+            );
+        }
+        other => panic!("expected gix index-lock contention, got {other:?}: {err:#}"),
+    }
+    assert!(
+        external_lock.lock_path().exists(),
+        "checkout must preserve the linked worktree's external lock"
+    );
+    Ok(())
+}
diff --git a/crates/but-core/tests/core/worktree/mod.rs b/crates/but-core/tests/core/worktree/mod.rs
index 8475e3007c4..59401b672d4 100644
--- a/crates/but-core/tests/core/worktree/mod.rs
+++ b/crates/but-core/tests/core/worktree/mod.rs
@@ -1,4 +1,5 @@
 mod checkout;
+mod checkout_index_lock;
 
 mod utils {
     /// Using the `repo` `HEAD` commit, build a new commit based on its tree with `edit` and `message`, and return the new commit.