diff --git a/codex-rs/core-plugins/src/startup_sync.rs b/codex-rs/core-plugins/src/startup_sync.rs
index 44fa95794..eb7813f35 100644
--- a/codex-rs/core-plugins/src/startup_sync.rs
+++ b/codex-rs/core-plugins/src/startup_sync.rs
@@ -34,6 +34,27 @@ const CURATED_PLUGINS_HTTP_TIMEOUT: Duration = Duration::from_secs(30);
 const CURATED_PLUGINS_BACKUP_ARCHIVE_TIMEOUT: Duration = Duration::from_secs(30);
 // Keep this comfortably above a normal sync attempt so we do not race another Codex process.
 const CURATED_PLUGINS_STALE_TEMP_DIR_MAX_AGE: Duration = Duration::from_secs(10 * 60);
+// These variables can redirect Git away from the repository selected by `-C`,
+// or inject command-scoped configuration into the sync commands.
+const REPOSITORY_LOCAL_GIT_ENVIRONMENT_VARIABLES: &[&str] = &[
+    "GIT_ALTERNATE_OBJECT_DIRECTORIES",
+    "GIT_CEILING_DIRECTORIES",
+    "GIT_COMMON_DIR",
+    "GIT_CONFIG",
+    "GIT_CONFIG_COUNT",
+    "GIT_CONFIG_PARAMETERS",
+    "GIT_DIR",
+    "GIT_DISCOVERY_ACROSS_FILESYSTEM",
+    "GIT_GRAFT_FILE",
+    "GIT_IMPLICIT_WORK_TREE",
+    "GIT_INDEX_FILE",
+    "GIT_NAMESPACE",
+    "GIT_OBJECT_DIRECTORY",
+    "GIT_PREFIX",
+    "GIT_REPLACE_REF_BASE",
+    "GIT_SHALLOW_FILE",
+    "GIT_WORK_TREE",
+];
 
 #[derive(Debug, Deserialize)]
 struct GitHubRepositorySummary {
@@ -242,17 +263,14 @@ fn fetch_curated_plugins_commit_from(
     context: &str,
 ) -> Result<(), String> {
     let fetch_refspec = format!("+{source_revision}:{CURATED_PLUGINS_FETCH_REF}");
-    let output = run_git_command_with_timeout(
-        Command::new(git_binary)
-            .env("GIT_OPTIONAL_LOCKS", "0")
-            .arg("-C")
-            .arg(repo_path)
-            .args(["fetch", "--depth", "1", "--no-tags"])
-            .arg(source)
-            .arg(fetch_refspec),
-        context,
-        CURATED_PLUGINS_GIT_TIMEOUT,
-    )?;
+    let mut command = git_command(git_binary);
+    command
+        .arg("-C")
+        .arg(repo_path)
+        .args(["fetch", "--depth", "1", "--no-tags"])
+        .arg(source)
+        .arg(fetch_refspec);
+    let output = run_git_command_with_timeout(&mut command, context, CURATED_PLUGINS_GIT_TIMEOUT)?;
     ensure_git_success(&output, context)
 }
 
@@ -277,15 +295,9 @@ fn run_git_in_repo(
     args: &[&str],
     context: &str,
 ) -> Result<(), String> {
-    let output = run_git_command_with_timeout(
-        Command::new(git_binary)
-            .env("GIT_OPTIONAL_LOCKS", "0")
-            .arg("-C")
-            .arg(repo_path)
-            .args(args),
-        context,
-        CURATED_PLUGINS_GIT_TIMEOUT,
-    )?;
+    let mut command = git_command(git_binary);
+    command.arg("-C").arg(repo_path).args(args);
+    let output = run_git_command_with_timeout(&mut command, context, CURATED_PLUGINS_GIT_TIMEOUT)?;
     ensure_git_success(&output, context)
 }
 
@@ -583,12 +595,13 @@ fn read_local_git_or_sha_file(
 }
 
 fn git_ls_remote_head_sha(git_binary: &str) -> Result<String, String> {
+    let mut command = git_command(git_binary);
+    command
+        .arg("ls-remote")
+        .arg("https://github.com/openai/plugins.git")
+        .arg("HEAD");
     let output = run_git_command_with_timeout(
-        Command::new(git_binary)
-            .env("GIT_OPTIONAL_LOCKS", "0")
-            .arg("ls-remote")
-            .arg("https://github.com/openai/plugins.git")
-            .arg("HEAD"),
+        &mut command,
         "git ls-remote curated plugins repo",
         CURATED_PLUGINS_GIT_TIMEOUT,
     )?;
@@ -610,8 +623,7 @@ fn git_ls_remote_head_sha(git_binary: &str) -> Result<String, String> {
 }
 
 fn git_head_sha(repo_path: &Path, git_binary: &str) -> Result<String, String> {
-    let output = Command::new(git_binary)
-        .env("GIT_OPTIONAL_LOCKS", "0")
+    let output = git_command(git_binary)
         .arg("-C")
         .arg(repo_path)
         .arg("rev-parse")
@@ -635,6 +647,15 @@ fn git_head_sha(repo_path: &Path, git_binary: &str) -> Result<String, String> {
     Ok(sha)
 }
 
+fn git_command(git_binary: &str) -> Command {
+    let mut command = Command::new(git_binary);
+    command.env("GIT_OPTIONAL_LOCKS", "0");
+    for name in REPOSITORY_LOCAL_GIT_ENVIRONMENT_VARIABLES {
+        command.env_remove(name);
+    }
+    command
+}
+
 fn run_git_command_with_timeout(
     command: &mut Command,
     context: &str,
diff --git a/codex-rs/core-plugins/src/startup_sync_tests.rs b/codex-rs/core-plugins/src/startup_sync_tests.rs
index f73e4ad9d..fcedf3084 100644
--- a/codex-rs/core-plugins/src/startup_sync_tests.rs
+++ b/codex-rs/core-plugins/src/startup_sync_tests.rs
@@ -1,5 +1,6 @@
 use super::*;
 use pretty_assertions::assert_eq;
+use std::ffi::OsStr;
 use std::io::Write;
 use std::path::Path;
 use std::path::PathBuf;
@@ -16,6 +17,22 @@ use zip::write::SimpleFileOptions;
 
 const TEST_CURATED_PLUGIN_SHA: &str = "0123456789abcdef0123456789abcdef01234567";
 
+#[test]
+fn git_command_sanitizes_ambient_repository_environment() {
+    let command = git_command("git");
+
+    for name in REPOSITORY_LOCAL_GIT_ENVIRONMENT_VARIABLES {
+        assert_eq!(
+            command
+                .get_envs()
+                .find(|(key, _)| *key == OsStr::new(name))
+                .map(|(_, value)| value),
+            Some(None),
+            "{name} should be removed from startup sync Git commands"
+        );
+    }
+}
+
 fn write_file(path: &Path, contents: &str) {
     std::fs::create_dir_all(path.parent().expect("file should have a parent")).unwrap();
     std::fs::write(path, contents).unwrap();