diff --git a/pyproject.toml b/pyproject.toml
index 0e28833..3782ed3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -41,6 +41,11 @@ include = ["pywr_utils*"]
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 addopts = "--cov=pywr_utils --cov-report=term-missing --cov-report=xml"
+markers = [
+    "unit: a single function/class in isolation, no DB/network/subprocess",
+    "integration: real internal components wired together via the code's own Python API, no mocks",
+    "e2e: drives the installed CLI entrypoint end to end",
+]
 
 [tool.coverage.report]
 # Ratchet, not an aspirational target: set at today's measured coverage (99.23%, after a full unit/integration/e2e pass on model_runner.py) minus a small buffer for environment variance, so coverage can't silently regress further without failing CI. Raise this number as coverage genuinely improves.
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..926e6e1
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,20 @@
+import sys
+
+import pytest
+
+BLOCKED_MODULES = ("pandas", "pywr")
+
+
+@pytest.fixture
+def block_pandas_and_pywr(monkeypatch):
+    """Simulate a fresh install with neither pandas nor pywr present.
+
+    Setting `sys.modules[name] = None` is the standard idiom for this: Python's import system raises ImportError for any name mapped to None in sys.modules, without needing to intercept `__import__` or actually uninstall the package.
+    """
+    for name in BLOCKED_MODULES:
+        monkeypatch.setitem(sys.modules, name, None)
+    for name in ("pywr_utils.cli", "pywr_utils.model_runner"):
+        monkeypatch.delitem(sys.modules, name, raising=False)
+    yield
+    for name in ("pywr_utils.cli", "pywr_utils.model_runner"):
+        monkeypatch.delitem(sys.modules, name, raising=False)
diff --git a/tests/e2e/test_cli_commands.py b/tests/e2e/test_cli_commands.py
index b276596..7356262 100644
--- a/tests/e2e/test_cli_commands.py
+++ b/tests/e2e/test_cli_commands.py
@@ -15,6 +15,8 @@
 from pywr_utils import model_runner
 from pywr_utils.cli import main
 
+pytestmark = pytest.mark.e2e
+
 
 @pytest.fixture
 def runner():
diff --git a/tests/test_cli_lazy_import.py b/tests/e2e/test_cli_lazy_import.py
similarity index 58%
rename from tests/test_cli_lazy_import.py
rename to tests/e2e/test_cli_lazy_import.py
index 8090335..6f67b20 100644
--- a/tests/test_cli_lazy_import.py
+++ b/tests/e2e/test_cli_lazy_import.py
@@ -1,34 +1,13 @@
-"""Regression tests for the model_runner lazy-import fix.
+"""E2E tests for the model_runner lazy-import fix, driven through the Click entrypoint.
 
-create-synthetic-model must not require pandas or pywr to be installed, since it only depends on model_creation (stdlib json/typing). Before the fix, cli.py imported model_runner (which needs both) at module load time, so every CLI invocation failed on a fresh install before pywr's own native build was ever reached.
+create-synthetic-model must not require pandas or pywr to be installed, since it only depends on model_creation (stdlib json/typing). Before the fix, cli.py imported model_runner (which needs both) at module load time, so every CLI invocation failed on a fresh install before pywr's own native build was ever reached. See tests/unit/test_cli_lazy_import.py for the isolated import-only check.
 """
 import importlib
-import sys
 
 import pytest
 from click.testing import CliRunner
 
-BLOCKED_MODULES = ("pandas", "pywr")
-
-
-@pytest.fixture
-def block_pandas_and_pywr(monkeypatch):
-    """Simulate a fresh install with neither pandas nor pywr present.
-
-    Setting `sys.modules[name] = None` is the standard idiom for this: Python's import system raises ImportError for any name mapped to None in sys.modules, without needing to intercept `__import__` or actually uninstall the package.
-    """
-    for name in BLOCKED_MODULES:
-        monkeypatch.setitem(sys.modules, name, None)
-    for name in ("pywr_utils.cli", "pywr_utils.model_runner"):
-        monkeypatch.delitem(sys.modules, name, raising=False)
-    yield
-    for name in ("pywr_utils.cli", "pywr_utils.model_runner"):
-        monkeypatch.delitem(sys.modules, name, raising=False)
-
-
-def test_cli_module_imports_without_pandas_or_pywr(block_pandas_and_pywr):
-    module = importlib.import_module("pywr_utils.cli")
-    assert hasattr(module, "create_synthetic_model")
+pytestmark = pytest.mark.e2e
 
 
 def test_create_synthetic_model_command_without_pandas_or_pywr(tmp_path, block_pandas_and_pywr):
diff --git a/tests/integration/test_incremental_sweep_failures.py b/tests/integration/test_incremental_sweep_failures.py
index 2aa59c8..9cf9dc0 100644
--- a/tests/integration/test_incremental_sweep_failures.py
+++ b/tests/integration/test_incremental_sweep_failures.py
@@ -10,6 +10,8 @@
 from pywr_utils import model_runner
 from pywr_utils.model_runner import PywrFileRunner, run_incremental_sizes
 
+pytestmark = pytest.mark.integration
+
 SWEEP_ARGS = {"max_zones": 2, "zone_increment": 2, "transfer_increment": 2, "output_csv": "timings.csv"}
 
 
diff --git a/tests/integration/test_model_runner_api.py b/tests/integration/test_model_runner_api.py
index 14e3f59..d05f059 100644
--- a/tests/integration/test_model_runner_api.py
+++ b/tests/integration/test_model_runner_api.py
@@ -12,6 +12,8 @@
 from pywr_utils.model_creation import SyntheticModelCreator
 from pywr_utils.model_runner import PywrFileRunner, run_file, run_incremental_sizes
 
+pytestmark = pytest.mark.integration
+
 DEFAULT_SOLVER = "glpk"
 ALTERNATIVE_SOLVER = "glpk-edge"
 
diff --git a/tests/unit/test_cli_module_import_without_deps.py b/tests/unit/test_cli_module_import_without_deps.py
new file mode 100644
index 0000000..9f96255
--- /dev/null
+++ b/tests/unit/test_cli_module_import_without_deps.py
@@ -0,0 +1,14 @@
+"""UNIT test for the model_runner lazy-import fix.
+
+create-synthetic-model must not require pandas or pywr to be installed, since it only depends on model_creation (stdlib json/typing). Before the fix, cli.py imported model_runner (which needs both) at module load time, so every CLI invocation failed on a fresh install before pywr's own native build was ever reached. This checks only the import itself, in isolation from the Click entrypoint — see tests/e2e/test_cli_lazy_import.py for the command-level regression coverage.
+"""
+import importlib
+
+import pytest
+
+pytestmark = pytest.mark.unit
+
+
+def test_cli_module_imports_without_pandas_or_pywr(block_pandas_and_pywr):
+    module = importlib.import_module("pywr_utils.cli")
+    assert hasattr(module, "create_synthetic_model")
diff --git a/tests/unit/test_model_creation.py b/tests/unit/test_model_creation.py
index 559330c..a209f50 100644
--- a/tests/unit/test_model_creation.py
+++ b/tests/unit/test_model_creation.py
@@ -8,6 +8,8 @@
 
 from pywr_utils.model_creation import SyntheticModelCreator, create_synthetic_model
 
+pytestmark = pytest.mark.unit
+
 # One input produces three nodes: an input, a link and a demand. Each transfer adds one further link node.
 NODES_PER_INPUT = 3
 
diff --git a/tests/unit/test_terminal_graph.py b/tests/unit/test_terminal_graph.py
index 20e324f..006f8e9 100644
--- a/tests/unit/test_terminal_graph.py
+++ b/tests/unit/test_terminal_graph.py
@@ -7,6 +7,8 @@
 
 from pywr_utils.model_runner import _display_terminal_graph, _draw_line
 
+pytestmark = pytest.mark.unit
+
 
 def timing_frame(zones, transfers, setup_times):
     return pd.DataFrame({"zones": zones, "transfers": transfers, "setup_time": setup_times})