diff --git a/fc-agent/src/agent.rs b/fc-agent/src/agent.rs
index c9391e1b..9696ab6c 100644
--- a/fc-agent/src/agent.rs
+++ b/fc-agent/src/agent.rs
@@ -162,11 +162,14 @@ pub async fn run() -> Result<()> {
     // Store prefix globally so exec server and health checks can use it
     container::set_podman_cmd_prefix(cmd_prefix.clone());
 
-    // Reset podman state to match storage.conf before the first real podman operation.
-    // By this point, storage setup is complete (btrfs loopback mounted, storage.conf
-    // written with correct driver). Reset ensures db.sql matches storage.conf even if
-    // concurrent health monitor `podman inspect` created stale state during setup.
-    container::reset_podman_state();
+    // Reset root podman state to match storage.conf. The health monitor may have
+    // run `podman inspect` via the exec server during setup, creating a stale
+    // db.sql with the wrong graph driver. Only needed for root podman — user mode
+    // already resets in create_vm_user(), and a root reset would destroy the
+    // user's storage directory.
+    if cmd_prefix.is_empty() {
+        container::reset_podman_state();
+    }
 
     // Prepare image based on delivery mode
     let image_ref = match (plan.image_mode.as_deref(), &plan.image_device) {
diff --git a/fc-agent/src/container.rs b/fc-agent/src/container.rs
index 89adee5e..3596a708 100644
--- a/fc-agent/src/container.rs
+++ b/fc-agent/src/container.rs
@@ -356,17 +356,15 @@ pub fn setup_btrfs_storage_if_available() {
     );
 }
 
-/// Reset podman state to match the current storage.conf.
+/// Reset root podman state to match the current storage.conf.
 ///
-/// Must be called after storage.conf is written (by btrfs/overlay setup) and
-/// immediately before the first real podman operation (pull/load/run).
+/// Fixes "database graph driver does not match" errors caused by the health
+/// monitor running `podman inspect` via exec before storage setup completes,
+/// creating db.sql with an empty or wrong driver.
 ///
-/// This fixes "database graph driver does not match" errors caused by:
-/// 1. Stale db.sql from rootfs build (apt post-install creates it with driver="")
-/// 2. Concurrent health monitor `podman inspect` recreating db.sql during setup
-///
-/// `podman system reset --force` atomically drops and recreates all podman state
-/// to match the current storage.conf, eliminating any driver mismatch.
+/// Only call for root podman (empty cmd_prefix). User-mode podman already
+/// resets in create_vm_user(). A root reset would destroy the user's btrfs
+/// storage subdirectory at /var/lib/containers/storage/user-{uid}.
 pub fn reset_podman_state() {
     match std::process::Command::new("podman")
         .args(["system", "reset", "--force"])
diff --git a/tests/common/mod.rs b/tests/common/mod.rs
index 915f2fa2..4d4b78c7 100644
--- a/tests/common/mod.rs
+++ b/tests/common/mod.rs
@@ -1042,9 +1042,10 @@ pub async fn ensure_nested_image() -> anyhow::Result<()> {
     ensure_nested_container("localhost/nested-test", "Containerfile.nested").await
 }
 
-/// Build a container image for nested testing.
+/// Build a container image for nested testing, serialized via file lock.
 ///
-/// Always runs podman build - relies on podman's layer caching for speed.
+/// Uses an exclusive file lock to prevent concurrent builds from racing on
+/// overlay unmount. Redundant builds are fast due to podman's layer caching.
 /// If the container extends localhost/nested-test, call ensure_nested_image() first.
 ///
 /// # Arguments
@@ -1054,6 +1055,19 @@ pub async fn ensure_nested_container(image_name: &str, containerfile: &str) -> a
     let fcvm_path = find_fcvm_binary()?;
     let fcvm_dir = fcvm_path.parent().unwrap();
 
+    // Serialize concurrent builds with a file lock. Multiple nextest processes
+    // may call this simultaneously; without locking, concurrent `podman build`
+    // races on overlay unmount and corrupts the build cache (x64-specific).
+    let lock_name = image_name.replace('/', "-");
+    let lock_path = format!("/tmp/fcvm-build-{}.lock", lock_name);
+    let lock_file = std::fs::OpenOptions::new()
+        .create(true)
+        .write(true)
+        .truncate(false)
+        .open(&lock_path)
+        .context("creating build lock file")?;
+    lock_file.lock_exclusive().context("acquiring build lock")?;
+
     // Copy binaries to build context (needed for nested-test base)
     if image_name == "localhost/nested-test" {
         let profile = fcvm::setup::get_kernel_profile("nested")?
@@ -1080,6 +1094,7 @@ pub async fn ensure_nested_container(image_name: &str, containerfile: &str) -> a
         .context("running podman build")?;
 
     if !output.status.success() {
+        drop(lock_file);
         let stderr = String::from_utf8_lossy(&output.stderr);
         anyhow::bail!("Failed to build {}: {}", image_name, stderr);
     }
@@ -1130,6 +1145,7 @@ pub async fn ensure_nested_container(image_name: &str, containerfile: &str) -> a
         println!("✓ {} built", image_name);
     }
 
+    drop(lock_file);
     Ok(())
 }