diff --git a/crates/workspace-server/src/repositories.rs b/crates/workspace-server/src/repositories.rs
index aa273fe1..f6ce9824 100644
--- a/crates/workspace-server/src/repositories.rs
+++ b/crates/workspace-server/src/repositories.rs
@@ -319,10 +319,24 @@ impl RepositoryRegistryReader {
         result_commit: &str,
     ) -> Result<(), RepositoryLookupError> {
         let repository = self.merge_repository(id)?;
-        if !selector.starts_with("refs/heads/")
-            || selector.starts_with('-')
-            || selector.as_bytes().contains(&0)
-        {
+        let target_ref = if selector.starts_with("refs/heads/") {
+            selector.to_owned()
+        } else if selector.starts_with("refs/") {
+            return Err(RepositoryLookupError::InvalidSelector {
+                id: id.into(),
+                selector: selector.into(),
+            });
+        } else {
+            format!("refs/heads/{selector}")
+        };
+        let valid_ref = Command::new("git")
+            .args(["check-ref-format", target_ref.as_str()])
+            .status()
+            .map_err(|_| RepositoryLookupError::ProviderFailure {
+                id: id.into(),
+                operation: "validate target branch ref".into(),
+            })?;
+        if !valid_ref.success() || selector.starts_with('-') || selector.as_bytes().contains(&0) {
             return Err(RepositoryLookupError::InvalidSelector {
                 id: id.into(),
                 selector: selector.into(),
@@ -332,7 +346,12 @@ impl RepositoryRegistryReader {
         let status = Command::new("git")
             .arg("-C")
             .arg(&repository.path)
-            .args(["update-ref", selector, result_commit, expected_target])
+            .args([
+                "update-ref",
+                target_ref.as_str(),
+                result_commit,
+                expected_target,
+            ])
             .status()
             .map_err(|_| RepositoryLookupError::ProviderFailure {
                 id: id.into(),
@@ -724,7 +743,7 @@ mod tests {
         );
         reader.ensure_ancestor("main", &base, &source).unwrap();
         reader
-            .update_merge_target("main", "refs/heads/main", &base, &source)
+            .update_merge_target("main", "main", &base, &source)
             .unwrap();
         assert_eq!(
             reader
diff --git a/crates/workspace-server/src/server.rs b/crates/workspace-server/src/server.rs
index 0eb18daf..664a8866 100644
--- a/crates/workspace-server/src/server.rs
+++ b/crates/workspace-server/src/server.rs
@@ -4028,6 +4028,35 @@ async fn scoped_complete_merge_request(
             )
             .map_err(repository_merge_evidence_error)?;
     }
+    let verified_target = repositories.observe_merge_target(&mr.repository_id, Some(selector));
+    let verified_result = matches!(
+        verified_target.as_ref(),
+        Ok(target) if target.commit == input.result_commit
+    );
+    if !verified_result {
+        if !target_was_already_updated {
+            if let Err(rollback_error) = repositories.update_merge_target(
+                &mr.repository_id,
+                selector,
+                &input.result_commit,
+                &input.target_commit,
+            ) {
+                return Err(Error::InvalidInput(format!(
+                    "post-update target verification failed and guarded rollback also failed: verification={verified_target:?}; rollback={rollback_error:?}"
+                ))
+                .into());
+            }
+        }
+        return Err(Error::InvalidInput(format!(
+            "post-update target verification failed: expected result {}, observed {:?}",
+            input.result_commit,
+            verified_target
+                .as_ref()
+                .map(|target| target.commit.as_str())
+                .map_err(|error| error)
+        ))
+        .into());
+    }
     let completion = merge_request::CompleteMergeRequest {
         operation_id: input.operation_id,
         ticket_id,