diff --git a/flake.nix b/flake.nix
index b5fa93e58..0207134cd 100644
--- a/flake.nix
+++ b/flake.nix
@@ -220,6 +220,11 @@
             '';
           repl-completion = nixpkgsFor.${system}.native.callPackage ./tests/repl-completion.nix { };
 
+          lazyTrees = nixpkgsFor.${system}.native.nixComponents2.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/paths.cc b/src/libexpr/paths.cc
index 40c0a23b6..65b8212e1 100644
--- a/src/libexpr/paths.cc
+++ b/src/libexpr/paths.cc
@@ -77,25 +77,31 @@ StorePath EvalState::mountInput(
 
     allowPath(storePath); // FIXME: should just whitelist the entire virtual store
 
+    std::optional<Hash> _narHash;
+
+    auto getNarHash = [&]() {
+        if (!_narHash) {
+            if (store->isValidPath(storePath))
+                _narHash = store->queryPathInfo(storePath)->narHash;
+            else
+                // FIXME: use fetchToStore to make it cache this
+                _narHash = accessor->hashPath(CanonPath::root);
+        }
+        return _narHash;
+    };
+
     storeFS->mount(CanonPath(store->printStorePath(storePath)), accessor);
 
-    if (requireLockable && (!settings.lazyTrees || !input.isLocked()) && !input.getNarHash()) {
-        // FIXME: use fetchToStore to make it cache this
-        auto narHash = accessor->hashPath(CanonPath::root);
-        input.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
-    }
+    if (requireLockable && (!settings.lazyTrees || !input.isLocked()) && !input.getNarHash())
+        input.attrs.insert_or_assign("narHash", getNarHash()->to_string(HashFormat::SRI, true));
 
-    // FIXME: what to do with the NAR hash in lazy mode?
-    if (!settings.lazyTrees && originalInput.getNarHash()) {
-        auto expected = originalInput.computeStorePath(*store);
-        if (storePath != expected)
-            throw Error(
-                (unsigned int) 102,
-                "NAR hash mismatch in input '%s', expected '%s' but got '%s'",
-                originalInput.to_string(),
-                store->printStorePath(storePath),
-                store->printStorePath(expected));
-    }
+    if (originalInput.getNarHash() && *getNarHash() != *originalInput.getNarHash())
+        throw Error(
+            (unsigned int) 102,
+            "NAR hash mismatch in input '%s', expected '%s' but got '%s'",
+            originalInput.to_string(),
+            getNarHash()->to_string(HashFormat::SRI, true),
+            originalInput.getNarHash()->to_string(HashFormat::SRI, true));
 
     return storePath;
 }
diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index 7730e0db4..2825b72ab 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -882,7 +882,8 @@ struct GitInputScheme : InputScheme
 
     bool isLocked(const Input & input) const override
     {
-        return (bool) input.getRev();
+        auto rev = input.getRev();
+        return rev && rev != nullRev;
     }
 };
 
diff --git a/tests/functional/common/init.sh b/tests/functional/common/init.sh
index 6e9bffec5..7f28a09d7 100755
--- a/tests/functional/common/init.sh
+++ b/tests/functional/common/init.sh
@@ -54,6 +54,7 @@ flake-registry = $TEST_ROOT/registry.json
 show-trace = true
 include nix.conf.extra
 trusted-users = $(whoami)
+${_NIX_TEST_EXTRA_CONFIG:-}
 EOF
 
 cat > "$NIX_CONF_DIR"/nix.conf.extra <<EOF
diff --git a/tests/functional/flakes/flakes.sh b/tests/functional/flakes/flakes.sh
index 0a52ba08c..e335fe6f3 100755
--- a/tests/functional/flakes/flakes.sh
+++ b/tests/functional/flakes/flakes.sh
@@ -69,7 +69,9 @@ nix flake metadata "$flake1Dir" | grepQuiet 'URL:.*flake1.*'
 # Test 'nix flake metadata --json'.
 json=$(nix flake metadata flake1 --json | jq .)
 [[ $(echo "$json" | jq -r .description) = 'Bla bla' ]]
-[[ -d $(echo "$json" | jq -r .path) ]]
+if [[ $(nix config show lazy-trees) = false ]]; then
+    [[ -d $(echo "$json" | jq -r .path) ]]
+fi
 [[ $(echo "$json" | jq -r .lastModified) = $(git -C "$flake1Dir" log -n1 --format=%ct) ]]
 hash1=$(echo "$json" | jq -r .revision)
 [[ -n $(echo "$json" | jq -r .fingerprint) ]]
@@ -161,7 +163,11 @@ expect 1 nix build -o "$TEST_ROOT/result" "$flake2Dir#bar" --no-update-lock-file
 nix build -o "$TEST_ROOT/result" "$flake2Dir#bar" --commit-lock-file
 [[ -e "$flake2Dir/flake.lock" ]]
 [[ -z $(git -C "$flake2Dir" diff main || echo failed) ]]
-[[ $(jq --indent 0 . < "$flake2Dir/flake.lock") =~ ^'{"nodes":{"flake1":{"locked":{"lastModified":'.*',"narHash":"sha256-'.*'","ref":"refs/heads/master","rev":"'.*'","revCount":2,"type":"git","url":"file:///'.*'"},"original":{"id":"flake1","type":"indirect"}},"root":{"inputs":{"flake1":"flake1"}}},"root":"root","version":7}'$ ]]
+if [[ $(nix config show lazy-trees) = false ]]; then
+    [[ $(jq --indent 0 . < "$flake2Dir/flake.lock") =~ ^'{"nodes":{"flake1":{"locked":{"lastModified":'.*',"narHash":"sha256-'.*'","ref":"refs/heads/master","rev":"'.*'","revCount":2,"type":"git","url":"file:///'.*'"},"original":{"id":"flake1","type":"indirect"}},"root":{"inputs":{"flake1":"flake1"}}},"root":"root","version":7}'$ ]]
+else
+    [[ $(jq --indent 0 . < "$flake2Dir/flake.lock") =~ ^'{"nodes":{"flake1":{"locked":{"lastModified":'.*',"ref":"refs/heads/master","rev":"'.*'","revCount":2,"type":"git","url":"file:///'.*'"},"original":{"id":"flake1","type":"indirect"}},"root":{"inputs":{"flake1":"flake1"}}},"root":"root","version":7}'$ ]]
+fi
 
 # Rerunning the build should not change the lockfile.
 nix build -o "$TEST_ROOT/result" "$flake2Dir#bar"
diff --git a/tests/functional/package.nix b/tests/functional/package.nix
index 43f2f25a2..799026ebe 100644
--- a/tests/functional/package.nix
+++ b/tests/functional/package.nix
@@ -26,6 +26,9 @@
 
   # For running the functional tests against a different pre-built Nix.
   test-daemon ? null,
+
+  # Whether to run tests with lazy trees enabled.
+  lazyTrees ? false,
 }:
 
 let
@@ -95,6 +98,8 @@ mkMesonDerivation (
       mkdir $out
     '';
 
+    _NIX_TEST_EXTRA_CONFIG = lib.optionalString lazyTrees "lazy-trees = true";
+
     meta = {
       platforms = lib.platforms.unix;
     };