diff --git a/ci/gha/tests/default.nix b/ci/gha/tests/default.nix
index c179174e6e2..4a30a96a475 100644
--- a/ci/gha/tests/default.nix
+++ b/ci/gha/tests/default.nix
@@ -82,6 +82,11 @@ rec {
     '';
     repl-completion = pkgs.callPackage ../../../tests/repl-completion.nix { inherit (packages') nix; };
 
+    lazyTrees = nixComponents.nix-functional-tests.override {
+      pname = "nix-lazy-trees-tests";
+      lazyTrees = true;
+    };
+
     /**
       Checks for our packaging expressions.
       This shouldn't build anything significant; just check that things
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index e1df04f2624..1a4b01805ad 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -4729,8 +4729,9 @@ static void prim_hashString(EvalState & state, const PosIdx pos, Value ** args,
         state.error<EvalError>("unknown hash algorithm '%1%'", algo).atPos(pos).debugThrow();
 
     NixStringContext context; // discarded
-    auto s =
-        state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.hashString");
+    auto s = state.devirtualize(
+        state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.hashString"),
+        context);
 
     v.mkString(hashString(*ha, s).to_string(HashFormat::Base16, false), state.mem);
 }
diff --git a/src/nix/app.cc b/src/nix/app.cc
index 07c7c55cfdb..4a8ca815c0f 100644
--- a/src/nix/app.cc
+++ b/src/nix/app.cc
@@ -90,7 +90,7 @@ UnresolvedApp InstallableValue::toApp(EvalState & state)
                         },
                         [&](const NixStringContextElem::Opaque & o) -> DerivedPath {
                             return DerivedPath::Opaque{
-                                .path = o.path,
+                                .path = state.devirtualize(o.path),
                             };
                         },
                         [&](const NixStringContextElem::Path & p) -> DerivedPath {
@@ -102,7 +102,7 @@ UnresolvedApp InstallableValue::toApp(EvalState & state)
 
         return UnresolvedApp{App{
             .context = std::move(context2),
-            .program = program,
+            .program = state.devirtualize(program, context),
         }};
     }
 
diff --git a/tests/functional/fetchGit.sh b/tests/functional/fetchGit.sh
index 1d8a8e75224..d43108e6d2a 100755
--- a/tests/functional/fetchGit.sh
+++ b/tests/functional/fetchGit.sh
@@ -363,3 +363,17 @@ rm "$TEST_ROOT"/flake/flake.lock
 nix eval "path:$TEST_ROOT/flake"#isModern
 nix eval --nix-219-compat "path:$TEST_ROOT/flake"#isModern
 [[ $(jq -r .nodes.eol.locked.narHash < "$TEST_ROOT"/flake/flake.lock) = "$newHash" ]]
+
+
+# Test that builtins.hashString devirtualizes lazy paths (https://github.com/DeterminateSystems/determinate/issues/160).
+hashStringRepo="$TEST_ROOT/hashString"
+createGitRepo "$hashStringRepo"
+echo hello > "$hashStringRepo"/hello
+git -C "$hashStringRepo" add hello
+git -C "$hashStringRepo" commit -m 'Initial'
+hashStringRev=$(git -C "$hashStringRepo" rev-parse HEAD)
+
+hash1=$(nix eval --lazy-trees --raw --expr "builtins.hashString \"sha256\" (toString ((builtins.fetchGit { url = file://$hashStringRepo; rev = \"$hashStringRev\"; })))")
+hash2=$(nix eval --lazy-trees --raw --expr "builtins.hashString \"sha256\" (toString ((builtins.fetchGit { url = file://$hashStringRepo; rev = \"$hashStringRev\"; })))")
+
+[[ "$hash1" = "$hash2" ]]