diff --git a/rockcraft/extensions/extension.py b/rockcraft/extensions/extension.py
index 707e453fc..e0972c8d9 100644
--- a/rockcraft/extensions/extension.py
+++ b/rockcraft/extensions/extension.py
@@ -106,7 +106,10 @@ def validate(self, extension_name: str) -> None:
             # There is nothing to validate, the extension will set the preferred base.
             return
 
-        base: str = self.yaml_data["base"]
+        # Use get_project_base() so that build-base takes precedence over base (e.g. when
+        # base is "bare", build-base carries the effective Ubuntu series used for dispatch).
+        # Fall back to the raw base field if get_project_base() cannot resolve it.
+        base: str = get_project_base(self.yaml_data) or self.yaml_data["base"]
 
         if self.is_experimental(base) and not os.getenv(
             "ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS"
diff --git a/tests/unit/extensions/test_expressjs.py b/tests/unit/extensions/test_expressjs.py
index fd8e747e6..dfcf4f9e5 100644
--- a/tests/unit/extensions/test_expressjs.py
+++ b/tests/unit/extensions/test_expressjs.py
@@ -398,6 +398,26 @@ def test_expressjs_invalid_package_json_scripts_error(
     )
 
 
+@pytest.mark.usefixtures("expressjs_extension", "package_json_file")
+def test_expressjs_extension_bare_base_with_ubuntu2604_build_base(
+    tmp_path, monkeypatch, expressjs_input_yaml
+):
+    """base:bare + build-base:ubuntu@26.04 should dispatch to V2 without error."""
+    monkeypatch.setenv("ROCKCRAFT_ENABLE_EXPERIMENTAL_EXTENSIONS", "1")
+    expressjs_input_yaml["base"] = "bare"
+    expressjs_input_yaml["build-base"] = "ubuntu@26.04"
+    applied = extensions.apply_extensions(tmp_path, expressjs_input_yaml)
+    assert isinstance(
+        extensions.ExpressJSFrameworkFactory(
+            project_root=tmp_path,
+            yaml_data={"name": "x", "base": "bare", "build-base": "ubuntu@26.04"},
+        ),
+        extensions.ExpressJSFrameworkV2,
+    )
+    assert applied["base"] == "bare"
+    assert applied["build-base"] == "ubuntu@26.04"
+
+
 def test_expressjs_factory_dispatch(tmp_path):
     factory = extensions.ExpressJSFrameworkFactory
 
diff --git a/tests/unit/extensions/test_extensions.py b/tests/unit/extensions/test_extensions.py
index ea7801265..cba058319 100644
--- a/tests/unit/extensions/test_extensions.py
+++ b/tests/unit/extensions/test_extensions.py
@@ -57,13 +57,15 @@ def test_experimental_no_env(tmp_path, input_yaml):
     assert str(exc.value) == expected_message
 
 
-@pytest.mark.parametrize("base", ["ubuntu:20.04", "bare"])
+@pytest.mark.parametrize("base", ["ubuntu:20.04", "ubuntu@20.04", "bare"])
 def test_wrong_base(tmp_path, input_yaml, base):
     input_yaml["extensions"] = [FakeExtension.NAME]
     input_yaml["base"] = base
     with pytest.raises(errors.ExtensionError) as exc:
         extensions.apply_extensions(tmp_path, input_yaml)
 
+    # get_project_base() normalizes legacy format so we also do the same here to match the expected message.
+    base = base.replace(":", "@")
     expected_message = (
         f"Extension '{FakeExtension.NAME}' does not support base: '{base}'"
     )