diff --git a/docs/src/guide/development.md b/docs/src/guide/development.md
index bea1de3b..e860ee2f 100644
--- a/docs/src/guide/development.md
+++ b/docs/src/guide/development.md
@@ -112,6 +112,10 @@ automatically when the asset is requested.
 This is very convenient when running integration tests, or when a developer
 does not want to start the Vite development server (at the expense of a slower feedback loop).
 
+When tests run in parallel processes, builds and manifest reads are synchronized
+across workers. The first worker builds stale assets while the others wait and
+reuse its result.
+
 ::: tip Enabled locally
 By [default][json config], <kbd>[autoBuild]</kbd> is enabled in the `test` and `development` environments.
 :::
@@ -178,10 +182,14 @@ When running tests locally, you can test the production build by not starting th
 When running tests in the CI, it's more reliable if assets are available
 __before__ tests start to run, as it:
 
-- Prevents timeouts in Capybara during <kbd>[autoBuild]</kbd>
-- Prevents race conditions when running tests in parallel (each thread could start a build)
+- Prevents timeouts in Capybara while waiting for <kbd>[autoBuild]</kbd>
+- Verifies the production asset build independently from the test suite
+
+Parallel test workers are synchronized when using <kbd>[autoBuild]</kbd>, so
+precompilation is not required for correctness. It remains recommended in CI
+for predictable test startup and build failures.
 
-To achieve that, it's recommended to run `bin/rake assets:precompile`—which should [also run `vite build`][deployment]—in a previous CI step.
+To achieve that, run `bin/rake assets:precompile`—which should [also run `vite build`][deployment]—in a previous CI step.
 
 You can verify your setup is working by disabling <kbd>[autoBuild]</kbd>. A convenient way to do that is to add `VITE_RUBY_AUTO_BUILD="false"` to the build environment variables.
 
diff --git a/test/builder_test.rb b/test/builder_test.rb
index b189502b..5dd10cdb 100644
--- a/test/builder_test.rb
+++ b/test/builder_test.rb
@@ -81,6 +81,12 @@ def test_last_build_path
     assert_equal builder.send(:last_build_path, ssr: true).basename.to_s, "last-ssr-build-#{ViteRuby.config.mode}.json"
   end
 
+  def test_build_is_synchronized_between_processes
+    skip "Process.fork is not supported" unless Process.respond_to?(:fork)
+
+    assert_build_is_synchronized_between_processes
+  end
+
   def test_watched_files_digest
     previous_digest = ViteRuby.digest
     refresh_config
@@ -158,6 +164,39 @@ def test_build_cache
 
 private
 
+  def assert_build_is_synchronized_between_processes
+    build_count_path = ViteRuby.config.build_cache_dir.join("build-count")
+    build_count_path.dirname.mkpath
+    reader, writer = IO.pipe
+    result = ["stdout", "", MockProcessStatus.new(success: true)]
+
+    ViteRuby::IO.stub(:capture, ->(*) {
+      build_count_path.open("a") { |file| file.puts Process.pid }
+      sleep 0.2
+      result
+    }) do
+      pids = Array.new(2) do
+        Process.fork do
+          writer.close
+          reader.read(1)
+          exit! builder.build ? 0 : 1
+        end
+      end
+
+      reader.close
+      2.times { writer.write("1") }
+      writer.close
+
+      assert pids.map { |pid| Process.wait2(pid).last }.all?(&:success?)
+    end
+
+    assert_equal 1, build_count_path.readlines.size
+  ensure
+    reader&.close unless reader&.closed?
+    writer&.close unless writer&.closed?
+    build_count_path&.delete if build_count_path&.exist?
+  end
+
   def watched_file
     Pathname.new(path_to_test_app).join(WATCHED_FILE)
   end
diff --git a/test/manifest_test.rb b/test/manifest_test.rb
index 516196b6..050cab62 100644
--- a/test/manifest_test.rb
+++ b/test/manifest_test.rb
@@ -117,6 +117,16 @@ def test_lookup_with_type_exception!
     assert_match "Vite Ruby can't find entrypoints/#{asset_file}.js in the manifests", error.message
   end
 
+  def test_lookup_does_not_initialize_build_dependencies_when_auto_build_is_disabled
+    refute ViteRuby.instance.instance_variable_defined?(:@builder)
+    refute ViteRuby.instance.instance_variable_defined?(:@build_lock)
+
+    assert_equal prefixed("app.517bf154.css"), path_for("app", type: :stylesheet)
+
+    refute ViteRuby.instance.instance_variable_defined?(:@builder)
+    refute ViteRuby.instance.instance_variable_defined?(:@build_lock)
+  end
+
   def test_lookup_success!
     vendor_chunk = {
       "file" => prefixed("vendor.0f7c0ec3.js"),
@@ -263,6 +273,12 @@ def test_skip_proxy_has_no_effect_without_dev_server
     assert_equal prefixed("logo.f42fb7ea.png"), path_for("images/logo.png")
   end
 
+  def test_manifest_read_waits_for_build_in_another_process
+    skip "Process.fork is not supported" unless Process.respond_to?(:fork)
+
+    assert_manifest_read_waits_for_build_in_another_process
+  end
+
   def test_lookup_nil
     assert_nil lookup("foo.js")
   end
@@ -302,6 +318,40 @@ def test_lookup_success
 
 private
 
+  def assert_manifest_read_waits_for_build_in_another_process
+    refresh_config(auto_build: true)
+    path = ViteRuby.config.manifest_paths.first
+    contents = path.read
+    reader, writer = IO.pipe
+
+    pid = Process.fork do
+      reader.close
+      ViteRuby.instance.build_lock.synchronize(File::LOCK_EX) do
+        path.write("")
+        writer.write("1")
+        writer.close
+        sleep 0.2
+        path.write(contents)
+      end
+      exit! 0
+    end
+
+    writer.close
+    reader.read(1)
+
+    assert ViteRuby.instance.manifest.refresh
+    assert_predicate Process.wait2(pid).last, :success?
+  ensure
+    reader&.close unless reader&.closed?
+    writer&.close unless writer&.closed?
+    path&.write(contents) if path && contents
+    begin
+      Process.wait(pid) if pid
+    rescue Errno::ECHILD
+      nil
+    end
+  end
+
   def assert_raises_manifest_missing_entry_error(auto_build: false, &block)
     error = nil
 
diff --git a/vite_ruby/lib/vite_ruby.rb b/vite_ruby/lib/vite_ruby.rb
index 71a59ae8..02f72136 100644
--- a/vite_ruby/lib/vite_ruby.rb
+++ b/vite_ruby/lib/vite_ruby.rb
@@ -113,6 +113,11 @@ def run(argv, **options)
     (@runner ||= ViteRuby::Runner.new(self)).run(argv, **options)
   end
 
+  # Internal: Synchronizes Vite builds and manifest reads.
+  def build_lock
+    @build_lock ||= ViteRuby::BuildLock.new(self)
+  end
+
   # Public: Keeps track of watched files and triggers builds as needed.
   def builder
     @builder ||= ViteRuby::Builder.new(self)
diff --git a/vite_ruby/lib/vite_ruby/build_lock.rb b/vite_ruby/lib/vite_ruby/build_lock.rb
new file mode 100644
index 00000000..ef96a4f9
--- /dev/null
+++ b/vite_ruby/lib/vite_ruby/build_lock.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+# Internal: Synchronizes Vite builds and manifest reads across threads and processes.
+class ViteRuby::BuildLock
+  def initialize(vite_ruby)
+    @vite_ruby = vite_ruby
+    @mutex = Mutex.new
+  end
+
+  def synchronize(mode)
+    @mutex.synchronize do
+      lock_path.dirname.mkpath
+      lock_path.open(File::RDWR | File::CREAT, 0o644) do |lock|
+        lock.flock(mode)
+        yield
+      end
+    end
+  end
+
+private
+
+  extend Forwardable
+
+  def_delegator :@vite_ruby, :config
+
+  # The lock is kept outside the build cache so clobbering the cache cannot
+  # replace the locked file while another process is waiting on it.
+  def lock_path
+    Pathname.new("#{config.build_cache_dir}.lock")
+  end
+end
diff --git a/vite_ruby/lib/vite_ruby/builder.rb b/vite_ruby/lib/vite_ruby/builder.rb
index bfdefe71..1a0483c5 100644
--- a/vite_ruby/lib/vite_ruby/builder.rb
+++ b/vite_ruby/lib/vite_ruby/builder.rb
@@ -12,19 +12,21 @@ def initialize(vite_ruby)
   # Public: Checks if the watched files have changed since the last compilation,
   # and triggers a Vite build if any files have changed.
   def build(*args)
-    last_build = last_build_metadata(ssr: args.include?("--ssr"))
-
-    if args.delete("--force") || last_build.stale? || config.manifest_paths.empty?
-      stdout, stderr, status = build_with_vite(*args)
-      log_build_result(stdout, stderr, status)
-      record_build_metadata(last_build, errors: stderr, success: status.success?)
-      status.success?
-    elsif last_build.success
-      logger.debug "Skipping vite build. Watched files have not changed since the last build at #{last_build.timestamp}"
-      true
-    else
-      logger.error "Skipping vite build. Watched files have not changed since the build failed at #{last_build.timestamp} ❌"
-      false
+    build_lock.synchronize(File::LOCK_EX) do
+      last_build = last_build_metadata(ssr: args.include?("--ssr"))
+
+      if args.delete("--force") || last_build.stale? || config.manifest_paths.empty?
+        stdout, stderr, status = build_with_vite(*args)
+        log_build_result(stdout, stderr, status)
+        record_build_metadata(last_build, errors: stderr, success: status.success?)
+        status.success?
+      elsif last_build.success
+        logger.debug "Skipping vite build. Watched files have not changed since the last build at #{last_build.timestamp}"
+        true
+      else
+        logger.error "Skipping vite build. Watched files have not changed since the build failed at #{last_build.timestamp} ❌"
+        false
+      end
     end
   end
 
@@ -37,7 +39,7 @@ def last_build_metadata(ssr: false)
 
   extend Forwardable
 
-  def_delegators :@vite_ruby, :config, :logger, :run
+  def_delegators :@vite_ruby, :build_lock, :config, :logger, :run
 
   # Internal: Writes a digest of the watched files to disk for future checks.
   def record_build_metadata(build, **attrs)
diff --git a/vite_ruby/lib/vite_ruby/manifest.rb b/vite_ruby/lib/vite_ruby/manifest.rb
index 1a151323..b22324a6 100644
--- a/vite_ruby/lib/vite_ruby/manifest.rb
+++ b/vite_ruby/lib/vite_ruby/manifest.rb
@@ -12,7 +12,6 @@
 class ViteRuby::Manifest
   def initialize(vite_ruby)
     @vite_ruby = vite_ruby
-    @build_mutex = Mutex.new if config.auto_build
   end
 
   # Public: Returns the path for the specified Vite entrypoint file.
@@ -104,7 +103,9 @@ def lookup!(name, **options)
   #   manifest.lookup('calendar.js')
   #   => { "file" => "/vite/assets/calendar-1016838bab065ae1e122.js", "imports" => [] }
   def lookup(name, **options)
-    @build_mutex.synchronize { builder.build || (return nil) } if should_build?
+    if should_build?
+      return unless builder.build
+    end
 
     find_manifest_entry resolve_entry_name(name, **options)
   end
@@ -116,7 +117,7 @@ def lookup(name, **options)
 
   extend Forwardable
 
-  def_delegators :@vite_ruby, :config, :builder, :dev_server_running?
+  def_delegators :@vite_ruby, :build_lock, :config, :builder, :dev_server_running?
 
   # NOTE: Auto compilation is convenient when running tests, when the developer
   # won't focus on the frontend, or when running the Vite server is not desired.
@@ -145,6 +146,12 @@ def manifest
 
   # Internal: Loads and merges the manifest files, resolving the asset paths.
   def load_manifest
+    return read_manifest unless config.auto_build
+
+    build_lock.synchronize(File::LOCK_SH) { read_manifest }
+  end
+
+  def read_manifest
     config.manifest_paths
       .map { |path| JSON.parse(path.read) }
       .inject({}, &:merge)