diff --git a/news/13353.bugfix.rst b/news/13353.bugfix.rst
new file mode 100644
index 00000000000..ac5edd07bd3
--- /dev/null
+++ b/news/13353.bugfix.rst
@@ -0,0 +1 @@
+Fix ``req.source_dir`` AssertionError when using the legacy resolver.
diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index 49b8fd78bef..300ae92b337 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -420,7 +420,7 @@ def run(self, options: Values, args: List[str]) -> int:
 
             reqs_to_build = [
                 r
-                for r in requirement_set.requirements.values()
+                for r in requirement_set.requirements_to_install
                 if should_build_for_install_command(r)
             ]