feat: add validation for grid spacing in AutoGrid

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - The method `Geometry.reflected` can be used to create a reflected copy of any geometry off a plane. As for other transformations, for efficiency, `reflected` `PolySlab` directly returns an updated `PolySlab` object rather than a `Transformed` object, except when the normal of the plane of reflection has a non-zero component along the slab axis, in which case `Transformed` is still returned.
 - Validation check for unit error in grid spacing.
 - Validation that when symmetry is imposed along a given axis, the boundary conditions on each side of the axis are identical.
+- Validation check for unit error in grid spacing for `AutoGrid`.
 
 ### Changed
 - Supplying autograd-traced values to geometric fields (`center`, `size`) of simulations, monitors, and sources now logs a warning and falls back to the static value instead of erroring.

tests/test_components/test_grid_spec.py

@@ -541,3 +541,41 @@ def test_uniform_grid_dl_validation(dl, expect_exception):
             grid_spec=td.GridSpec.uniform(dl=dl),
             run_time=1e-12,
         )
+
+
+@pytest.mark.parametrize(
+    ("freq0", "expect_error"),
+    [
+        (3e21, True),  # dl<1e-6 → fail
+        (1e14, False),  # dl>1e-7 → pass
+    ],
+)
+def test_autogrid_min_spacing(freq0, expect_error):
+    """
+    Test that Simulation.post_init catches too‐fine spacing produced by AutoGrid
+    (i.e. min grid spacing < 1e-7 µm).
+    """
+    L = 10.0
+    buffer = 1.0
+    source = td.PointDipole(
+        center=(-L / 2 + buffer, 0, 0),
+        source_time=td.GaussianPulse(freq0=freq0, fwidth=freq0 / 10.0),
+        polarization="Ez",
+    )
+
+    kwargs = dict(
+        size=(L, L, L),
+        sources=[source],
+        grid_spec=td.GridSpec(
+            grid_x=td.AutoGrid(min_steps_per_wvl=10),
+            grid_y=td.AutoGrid(min_steps_per_wvl=10),
+            grid_z=td.AutoGrid(min_steps_per_wvl=10),
+        ),
+        run_time=1e-11,
+    )
+
+    if expect_error:
+        with pytest.raises(SetupError):
+            _ = td.Simulation(**kwargs)
+    else:
+        _ = td.Simulation(**kwargs)

tidy3d/components/simulation.py

@@ -228,6 +228,29 @@ def boundaries_for_zero_dims(cls, val, values):
     return boundaries_for_zero_dims
 
 
+def _validate_min_grid_spacing(grid, threshold=1e-7, msg_prefix=""):
+    """
+    Ensure that the smallest grid spacing in any direction of the simulation grid
+    is not below `threshold` (µm).
+    """
+    # grab the 1D cell‐boundary arrays
+    xs, ys, zs = grid.boundaries.x, grid.boundaries.y, grid.boundaries.z
+    dx = np.diff(xs)
+    dy = np.diff(ys)
+    dz = np.diff(zs)
+    # ignore zero‐length dims
+    min_dx = dx[dx > 0].min() if np.any(dx > 0) else np.inf
+    min_dy = dy[dy > 0].min() if np.any(dy > 0) else np.inf
+    min_dz = dz[dz > 0].min() if np.any(dz > 0) else np.inf
+    overall = min(min_dx, min_dy, min_dz)
+    if (overall < threshold) and not np.isclose(overall, threshold, rtol=1e-6):
+        raise SetupError(
+            f"{msg_prefix} grid spacing is {overall:.2e} µm. "
+            "Please check your units! For more info on Tidy3D units, see: "
+            "https://docs.flexcompute.com/projects/tidy3d/en/latest/faq/docs/faq/What-are-the-units-used-in-the-simulation.html"
+        )
+
+
 class AbstractYeeGridSimulation(AbstractSimulation, ABC):
     """
     Abstract class for a simulation involving electromagnetic fields defined on a Yee grid.
@@ -3596,6 +3619,9 @@ def _post_init_validators(self) -> None:
         self._validate_custom_source_time()
         self._validate_mode_object_bends()
         self._warn_mode_object_pml()
+        _validate_min_grid_spacing(
+            self.grid, threshold=1e-7, msg_prefix=f"'{self.__class__.__name__}'"
+        )
 
     def _warn_mode_object_pml(self) -> None:
         """Warn if any mode objects have large pml."""