Master CI red: 'Characteristic Ideal Diodes' test failing with Unstable retcode

[ChrisRackauckas-Claude]

Summary

Tests workflow on main has been failing on every run since 2026-04-27 due to a single test failure in test/test_antlr_parser.jl:400. The bump for OrdinaryDiffEq v7 / SciMLBase v3 ecosystem (PR #120) is already merged and Project.toml correctly admits both — this failure is unrelated to that effort.

Failing test

Characteristic Ideal Diodes: Test Failed at /home/runner/work/BaseModelica.jl/BaseModelica.jl/test/test_antlr_parser.jl:400
  Expression: sol.retcode == ReturnCode.Success
   Evaluated: SciMLBase.ReturnCode.Unstable == SciMLBase.ReturnCode.Success

Stack trace points to test/test_antlr_parser.jl:9, 388, 400. ODE solver fails with:

At t= 0.0, dt was forced below floating point epsilon 5.0e-324, and step error estimate = NaN.
Aborting. There is either an error in your model specification or the true solution is unstable…

Reproduction

• Repo: SciML/BaseModelica.jl @ main (809bcdd "Bump version from 1.7.0 to 1.8.0")
• julia --project -e 'using Pkg; Pkg.test()'
• Test summary: BaseModelica | 237 passed, 1 failed, 0 errored, 0 broken. The single failure is in ANTLR Parser Tests → Characteristic Ideal Diodes.

When did it start

Master Tests started failing 2026-04-27, coincident with the OrdinaryDiffEq v7 / SciMLBase v3 ecosystem bump (PR #120, merged 2026-04-27 22:21 UTC). Behavior change suggests the new solver in OrdinaryDiffEq v7 (or a new default in SciMLBase v3) is initializing this DAE differently and the Modelica Characteristic Ideal Diodes model needs an updated guess/initial condition or a different solver/algorithm hint.

Suggested investigation

1. Bisect: revert PR Bump compat for OrdinaryDiffEq v7 / SciMLBase v3 ecosystem #120 locally and confirm test passes — would confirm v7 stack as cause.
2. Inspect the model definition in test_antlr_parser.jl line ~388–400 — try other solvers (Rodas5, FBDF) and see if any succeed.
3. Compare initial guesses with what was used pre-v7.

This is not silent — visible on every push and weekly schedule. Filed proactively per the master-failure investigation rule.