fix[invdes]: handle pixel size validation for simulations without sources

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - NumPy 2.1 compatibility issue where `numpy.float64` values passed to xarray interpolation would raise TypeError.
 - Support for quasi-uniform grid specifications via `QuasiUniformGrid` that subclasses from `GridSpec1d`. The grids are almost uniform, but can adjust locally to the edge of structure bounding boxes, and snapping points.
 - New field `min_steps_per_sim_size` in `AutoGrid` that sets minimal number of grid steps per longest edge length of simulation domain.
+- Validation error in inverse design plugin when simulation has no sources by adding a source existence check before validating pixel size.
 
 ## [2.8.0rc1] - 2024-12-17
 

tests/test_plugins/test_invdes.py

@@ -625,3 +625,23 @@ def test_validate_invdes_metric():
     invdes = invdes.updated_copy(simulation=simulation.updated_copy(monitors=[monitor]))
     with pytest.raises(ValueError, match="must return a real"):
         invdes.updated_copy(metric=metric)
+
+
+def test_pixel_size_warn_validator_no_sources(log_capture):
+    """Test that pixel size validator handles simulations without sources."""
+
+    sourceless_sim = simulation.updated_copy(sources=[])
+
+    invdes = make_invdes()
+    invdes = invdes.updated_copy(simulation=sourceless_sim)
+
+    with AssertLogLevel(log_capture, "WARNING", contains_str="Cannot validate pixel size"):
+        region_coarse = invdes.design_region.updated_copy(pixel_size=1.0)
+        invdes = invdes.updated_copy(design_region=region_coarse)
+
+    invdes_multi = make_invdes_multi()
+    sourceless_sims = [sourceless_sim for _ in invdes_multi.simulations]
+    invdes_multi = invdes_multi.updated_copy(simulations=sourceless_sims)
+
+    with AssertLogLevel(log_capture, "WARNING", contains_str="Cannot validate pixel size"):
+        invdes_multi = invdes_multi.updated_copy(design_region=region_coarse)

tidy3d/plugins/invdes/validators.py

@@ -33,6 +33,13 @@ def check_pixel_size(sim_field_name: str):
 
     def check_pixel_size_sim(sim: td.Simulation, pixel_size: float, index: int = None) -> None:
         """Check a pixel size compared to the simulation min wvl in material."""
+        if not sim.sources:
+            td.log.warning(
+                "Cannot validate pixel size in design region: simulation has no sources. "
+                "Please ensure the pixel size is appropriate for your target wavelength."
+            )
+            return
+
         if pixel_size > PIXEL_SIZE_WARNING_THRESHOLD * sim.wvl_mat_min:
             sim_string = f"simulations[{index}]" if index else "the simulation"