diff --git a/src/commands.rs b/src/commands.rs
index 37cabc2..7d7a219 100644
--- a/src/commands.rs
+++ b/src/commands.rs
@@ -733,6 +733,28 @@ pub(crate) fn run_push(preflight: &env::PreflightContext) -> ExitCode {
 
     for index in state.completed_pushes..state.push_branches.len() {
         let branch = &state.push_branches[index];
+
+        let remote_ref = format!("refs/remotes/origin/{branch}");
+        let local_ref = format!("refs/heads/{branch}");
+        match gitops::is_ancestor(&remote_ref, &local_ref) {
+            Ok(true) => {}
+            Ok(false) => {
+                if let Err(save_error) = sync_state::save_push(&state) {
+                    eprintln!("error: {save_error}");
+                    return ExitCode::from(1);
+                }
+                eprintln!(
+                    "error: remote branch `origin/{branch}` has commits not in local `{branch}`; \
+                     pull or rebase to integrate remote changes before pushing"
+                );
+                return ExitCode::from(1);
+            }
+            Err(message) => {
+                eprintln!("error: {message}");
+                return ExitCode::from(1);
+            }
+        }
+
         println!(
             "Pushing branch {}/{}: {}",
             index + 1,
diff --git a/tests/harness/mod.rs b/tests/harness/mod.rs
index 618259a..d7d5467 100644
--- a/tests/harness/mod.rs
+++ b/tests/harness/mod.rs
@@ -228,6 +228,16 @@ if [[ "${1:-}" == "merge-base" && "${2:-}" == "--is-ancestor" ]]; then
   descendant_branch="${descendant#refs/heads/}"
   descendant_branch="${descendant_branch#refs/remotes/origin/}"
 
+  if [[ -n "${STCK_TEST_NOT_ANCESTOR_PAIRS:-}" ]]; then
+    IFS=',' read -ra pairs <<< "${STCK_TEST_NOT_ANCESTOR_PAIRS}"
+    for pair in "${pairs[@]}"; do
+      IFS=':' read -r pa pd <<< "${pair}"
+      if [[ "${ancestor_branch}" == "${pa}" && "${descendant_branch}" == "${pd}" ]]; then
+        exit 1
+      fi
+    done
+  fi
+
   if [[ -n "${STCK_TEST_ANCESTOR_PAIRS:-}" ]]; then
     IFS=',' read -ra pairs <<< "${STCK_TEST_ANCESTOR_PAIRS}"
     for pair in "${pairs[@]}"; do
@@ -238,6 +248,10 @@ if [[ "${1:-}" == "merge-base" && "${2:-}" == "--is-ancestor" ]]; then
     done
   fi
 
+  if [[ "${ancestor_branch}" == "${descendant_branch}" ]]; then
+    exit 0
+  fi
+
   case "${ancestor_branch}:${descendant_branch}" in
     main:feature-base|main:feature-branch|main:feature-child) exit 0 ;;
     feature-base:feature-branch|feature-base:feature-child) exit 0 ;;
diff --git a/tests/push.rs b/tests/push.rs
index c736df5..504dceb 100644
--- a/tests/push.rs
+++ b/tests/push.rs
@@ -364,6 +364,34 @@ fn sync_then_push_after_squash_merge_produces_correct_retargets() {
         ));
 }
 
+#[test]
+fn push_aborts_when_remote_has_commits_not_in_local_branch() {
+    let (temp, mut cmd) = stck_cmd_with_stubbed_tools();
+    let log_path = log_path(&temp, "stck-push-diverged.log");
+    cmd.env("STCK_TEST_LOG", log_path.as_os_str());
+    cmd.env("STCK_TEST_NEEDS_PUSH_BRANCHES", "feature-branch");
+    // Simulate remote/feature-branch NOT being an ancestor of local feature-branch,
+    // meaning the remote has commits that would be lost on force push.
+    cmd.env(
+        "STCK_TEST_NOT_ANCESTOR_PAIRS",
+        "feature-branch:feature-branch",
+    );
+    cmd.arg("push");
+
+    cmd.assert()
+        .code(1)
+        .stderr(predicate::str::contains("feature-branch"));
+
+    // The force push should NOT have been executed.
+    if log_path.exists() {
+        let log = fs::read_to_string(&log_path).expect("push log should be readable");
+        assert!(
+            !log.contains("push --force-with-lease origin feature-branch"),
+            "push should not force-push a branch whose remote has diverged"
+        );
+    }
+}
+
 #[test]
 fn push_blocked_while_sync_state_exists() {
     let (temp, mut sync) = stck_cmd_with_stubbed_tools();