FXC-1617: switch tensorial mode solver to tidy3d-extras

[momchil-flex]

The tensorial mode solver in tidy3d-extras has been made more accurate. Notably, for nominally lossless, fully tensorial media, the mode solver should return very close to lossless modes.

To avoid confusion, we are removing the legacy local tensorial mode solver. Local runs are still fully supported (and more accurate!) they just require installing tidy3d-extras.

---

Note

Medium Risk
Changes affect the mode-solver execution path and error behavior for tensorial/angled/aniso cases, and rely on optional tidy3d-extras availability; diagonal/local and server paths should remain unchanged but need regression coverage.

Overview
Local mode solving now selects the best available solver at runtime: ModeSolver/ModeSimulation.run_local() will use tidy3d-extras’ mode solver when present, otherwise fall back to the base solver.

The legacy in-core fully tensorial eigenmode implementation is removed; attempting a tensorial solve (non-zero ModeSpec.angle_theta or fully anisotropic media) without tidy3d-extras now raises a clear NotImplementedError instructing users to install tidy3d[extras] or run via the server.

Tests and docs are updated accordingly: new coverage asserts the error/fallback behavior and skips tensorial-local tests when extras aren’t available, and plugin tests that directly exercised compute_modes/angled tensorial paths are removed.

^{Written by Cursor Bugbot for commit 3cb8539. This will update automatically on new commits. Configure here.}

[Copilot]

Pull request overview

This PR transitions the fully tensorial mode solver from local implementation to requiring either tidy3d-extras or server execution. The change aims to consolidate on a more accurate tensorial mode solver implementation available in tidy3d-extras while maintaining backward compatibility for diagonal (non-tensorial) mode solving.

Changes:

• Replaces the local tensorial mode solver implementation with a NotImplementedError that provides clear instructions to users
• Adds dynamic solver selection via _get_solver_func() that prefers tidy3d-extras when available
• Includes comprehensive test coverage for both with/without tidy3d-extras scenarios

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
tidy3d/components/mode/solver.py	Removed ~130 lines of tensorial solver implementation; replaced with error message directing users to tidy3d-extras or server
tidy3d/components/mode/mode_solver.py	Added _get_solver_func() to dynamically select between extras and base solver; updated solver calls to use this function
tests/test_components/test_mode.py	Added comprehensive tests for tensorial solver error handling, tidy3d-extras integration, and diagonal solver functionality
CHANGELOG.md	Documented the breaking change with clear user guidance

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with Cloud Agents, enable Autofix in the Cursor dashboard.}

[github-actions]

Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/components/mode/mode_solver.py (75.0%): Missing lines 126,131-132,134
• tidy3d/components/mode/solver.py (100%)

Summary

• Total: 18 lines
• Missing: 4 lines
• Coverage: 77%

tidy3d/components/mode/mode_solver.py

Lines 122-138

  122     ImportError
  123         If the local solver could not be imported (e.g., scipy not installed).
  124     """
  125     if not LOCAL_SOLVER_IMPORTED:
! 126         raise ImportError(IMPORT_ERROR_MSG)
  127 
  128     # Try to get tidy3d-extras solver (handles all cases including tensorial)
  129     try:
  130         _check_tidy3d_extras_available(quiet=True)
! 131         if tidy3d_extras["mod"] is not None:
! 132             from tidy3d_extras.mode import EigSolver as ExtrasEigSolver
  133 
! 134             return ExtrasEigSolver.compute_modes
  135     except (Tidy3dImportError, ImportError, AttributeError):
  136         # tidy3d-extras is optional; if unavailable or incompatible, fall back to base solver.
  137         pass

---