diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2a2346a25..24760a5f2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - `ModeMonitor` and `ModeSolverMonitor` now use the default `td.ModeSpec()` with `num_modes=1` when `mode_spec` is not provided.
 - Update sidewall angle validator to clarify angle should be specified in radians.
+- Reduced the complex tolerance in the mode solver below which permittivity is considered lossless, in order to correctly compute very low-loss modes.
 
 ### Fixed
 - NumPy 2.1 compatibility issue where `numpy.float64` values passed to xarray interpolation would raise TypeError.
diff --git a/tidy3d/plugins/mode/solver.py b/tidy3d/plugins/mode/solver.py
index 1b2d35c28..70ad0817e 100644
--- a/tidy3d/plugins/mode/solver.py
+++ b/tidy3d/plugins/mode/solver.py
@@ -15,7 +15,7 @@
 from .transforms import angled_transform, radial_transform
 
 # Consider vec to be complex if norm(vec.imag)/norm(vec) > TOL_COMPLEX
-TOL_COMPLEX = fp_eps
+TOL_COMPLEX = 1e-10
 # Tolerance for eigs
 TOL_EIGS = fp_eps
 # Tolerance for deciding on the matrix to be diagonal or tensorial