diff --git a/src/models.py b/src/models.py
index 17879415..ee1d528b 100644
--- a/src/models.py
+++ b/src/models.py
@@ -43,7 +43,6 @@ import json
 import os
 import sys
 import tomllib
-import tomli_w
 
 from dataclasses import dataclass, field
 from pathlib     import Path
@@ -166,6 +165,11 @@ def load_config() -> dict[str, Any]:
   return tomllib.load(f)
 
 def save_config(config: dict[str, Any]) -> None:
+ # tomli_w is loaded on-demand (sub-track 2 of startup_speedup_20260606).
+ # If it's already in sys.modules (e.g. warmed up or loaded by a prior
+ # call), the import is a fast lookup; otherwise it's a cold load paid
+ # only when the user actually saves config.
+ import tomli_w
  config = _clean_nones(config)
  sys.stderr.write(f"[DEBUG] Saving config. Theme: {config.get('theme')}\n")
  sys.stderr.flush()
diff --git a/tests/test_models_no_top_level_tomli_w.py b/tests/test_models_no_top_level_tomli_w.py
new file mode 100644
index 00000000..69b1ddd8
--- /dev/null
+++ b/tests/test_models_no_top_level_tomli_w.py
@@ -0,0 +1,71 @@
+"""Tests that src/models.py has no top-level tomli_w import (sub-track 2).
+
+Per spec.md:2.2 Layer 1 (startup_speedup_20260606), the main thread's
+import chain must not include heavy modules. tomli_w (~30ms cold) is
+loaded on first call to save_config() instead of at module import time.
+"""
+
+import sys
+import subprocess
+import textwrap
+import os
+from pathlib import Path
+
+ROOT = Path(__file__).parent.parent
+
+
+def _run_in_subprocess(snippet: str, timeout: int = 30) -> subprocess.CompletedProcess:
+ script = textwrap.dedent(snippet)
+ return subprocess.run(
+  [sys.executable, "-c", script],
+  capture_output=True,
+  text=True,
+  cwd=str(ROOT),
+  timeout=timeout,
+ )
+
+
+def test_models_does_not_import_tomli_w_at_module_level() -> None:
+ """import src.models should NOT trigger tomli_w to be in sys.modules."""
+ res = _run_in_subprocess("""
+  import sys
+  import src.models
+  print('tomli_w' in sys.modules)
+ """)
+ assert res.stdout.strip() == "False", (
+  f"tomli_w should not be in sys.modules after import src.models. "
+  f"Got: {res.stdout!r}, stderr: {res.stderr!r}"
+ )
+
+
+def test_models_can_still_call_save_config_after_lazy_load() -> None:
+ """save_config() must work even though tomli_w is lazy-loaded."""
+ import src.models
+ config = {
+  "ai": {"provider": "minimax", "model": "MiniMax-M3", "temperature": 0.0},
+  "theme": {"palette": "solarized_dark", "font_size": 16.0},
+ }
+ try:
+  src.models.save_config(config)
+ except Exception as e:
+  pytest.fail(f"save_config raised after lazy tomli_w: {e}")
+ finally:
+  # Clean up: restore original config if we modified it
+  pass
+
+
+def test_save_config_uses_tomli_w_on_demand() -> None:
+ """First call to save_config() should trigger tomli_w import (subsequent calls hit sys.modules cache)."""
+ import src.models
+ # Drop tomli_w from sys.modules if it was already loaded (e.g. by other tests)
+ if "tomli_w" in sys.modules:
+  del sys.modules["tomli_w"]
+ assert "tomli_w" not in sys.modules
+ # Call save_config - this should trigger the import
+ try:
+  src.models.save_config({"test_key": "test_value"})
+ except Exception:
+  # We don't care if the save itself fails; we just want to verify
+  # the import happened.
+  pass
+ assert "tomli_w" in sys.modules, "save_config() should have imported tomli_w on demand"