[ChrisRackauckas]

🚧 UNREVIEWED — awaiting review by @ChrisRackauckas. Filed by an AI agent running as @ChrisRackauckas; Chris has not seen this.
Harness: Devin CLI 3000.11.3 (diagnosis and fix), Claude Code 2.1.285 (PR) · Model: fusion-claude-opus-5-5-high-sidekick-swe-2-medium
Conversation: local Claude session 3c311569-dda6-4687-bc9f-65febb212c33 (fleet master)

This fixes a failure on the default branch's CI. The diagnosis and the evidence below come from the investigating agent's report; "confirmed by running" and "read in logs" are kept apart there.

Part of a three-PR stack implementing Chris's direction (2026-10-14) to replace the post-solve SCC reversal with an SCC form of SteadyStateProblem stored in the compressed layout: SciMLBase (new type) → ModelingToolkit (constructs it) → SteadyStateDiffEq (solves it). Supersedes #175 and #177; fixes #171. CI on the downstream PRs cannot pass until the upstream ones are released.

SteadyStateDiffEq.jl

Branch scc-steadystate (on master)

• a3b9bd1 Solve SCCSteadyStateProblem; stop routing SteadyStateProblem through a lowering

• Files: src/solve.jl:

  • __solve_scc_blocks is shared by both SCC types and skips stateless blocks.
  • solve(::SCCSteadyStateProblem, …; save_idxs): DynamicSS/SICNM solve block by
    block; SSRootfind, a bare solve, or a nonlinear alg go through SCCNonlinearProblem(prob).
    save_idxs takes integers or states (resolved via variable_index).
  • Deletes __solve_scc_lowering, the solve(::SteadyStateProblem, …) interception and
    the SSRootfind lowering branch.

  Also src/SteadyStateDiffEq.jl (export), src/algorithms.jl (docstrings),
  docs/src/index.md (section "SCC-split steady states"), test/qa/qa.jl (reexport
  list), Project.toml (2.18.0 → 2.19.0, SciMLBase 3.59, MTK 11.46) and test/scc.jl.

• test/scc.jl contains the ported cases from
  Restore original state order for SCC steady-state solutions #175 and
  Map SCC-lowered steady-state solutions back onto the problem's state (stacked on #173) #177. A shared layout_cases() provides
  chain, LV, mix and the observed-XY system. Two testsets loop over it:

  • "SteadyStateProblem solutions are in the problem's layout": length of u/resid =
    length(prob.u0), u[variable_index], sol[states], observed, resid ≈ 0,
    save_idxs = [2], plus Map SCC-lowered steady-state solutions back onto the problem's state (stacked on #173) #177's eltype case.
  • "SCCSteadyStateProblem from ModelingToolkit": sol.prob === prob, compressed layout,
    sol.u[variable_index(prob, s)] == sol[s], integer and symbolic save_idxs, original.

  Block-built SCCSteadyStateProblems are tested for every algorithm, plus save_idxs and
  short-tspan failures. LV runs only the nonlinear algorithms (its equilibrium is a center).

• Before (new test/scc.jl against all-master env-before, via scratch/run_scc_before.jl; ssde_scc_before_r2.log):

  SteadyStateDiffEq scc.jl (before environment)            |  388  32  58  478
    SCCSteadyStateProblem from blocks                      |   28      28 (errors)
    SteadyStateProblem solutions are in the problem's layout |  76  32  13  121
    SCCSteadyStateProblem from ModelingToolkit             |       14 (errors)
  

  • The failures are sol.prob isa …, length(sol.u) == length(prob.u0) and
    length(sol.resid) == length(prob.u0), i.e. issue solve(::SteadyStateProblem) returns the lowered problem's solution: sol.u reordered/empty, sol.prob.f.sys time-independent #171.
  • The errors are UndefVarError: SCCSteadyStateProblem not defined in SciMLBase.
  • On master, DynamicSS through the lowering also diverged on one case (dt_epsilon,
    u[2] = 1.4e47).

• After (ssde_scc_r2.log, env-stack):

  SSRootfind forwards SCCNonlinearProblem                  |  204  204
  DynamicSS sequentially solves an SCCNonlinearProblem     |   33   33
  SICNM sequentially solves an SCCNonlinearProblem         |   33   33
  SCCSteadyStateProblem from blocks                        |  228  228
  SSRootfind on a ModelingToolkit SCCSteadyStateProblem    |    3    3
  DynamicSS on a ModelingToolkit SCCSteadyStateProblem     |    4    4
  SICNM on a ModelingToolkit SCCSteadyStateProblem         |    4    4
  SteadyStateProblem solutions are in the problem's layout |  123  123
  SCCSteadyStateProblem from ModelingToolkit               |  134  134
  SSRootfind on a ModelingToolkit SCC decomposition        |   30   30
  DynamicSS on a ModelingToolkit SCC decomposition         |   12   12
  

  ssde_core_r2.log and ssde_sicnm.log: all green. Runic --check and typos: clean.

• IntegrationTest evidence (SciMLBase test/downstream/comprehensive_indexing.jl):

  • Before (comprehensive_indexing_before.log): State indexing / Observed error with
    BoundsError: attempt to access 1-element Vector{Float64} at index [[1, 2]] (solve(::SteadyStateProblem) returns the lowered problem's solution: sol.u reordered/empty, sol.prob.f.sys time-independent #171).
  • After (comprehensive_indexing_after_r2.log): fully green.
  • The remaining before-run failures in OptimizationSolution indexing
    (comprehensive_indexing_before_rest.log) also occur on master with the current deps,
    and pass on env-stack.

• Not verified: GROUP=QA (JET/Aqua/ExplicitImports), the full SSDE suite outside
  scc.jl/core.jl/sicnm.jl, and the docs build.

Open design questions for Chris

1. Plain SteadyStateProblem + nonlinear alg. With NewtonRaphson() the solution's
   sol.prob is the converted NonlinearProblem, as before the lowering existed (only
   SSRootfind/DynamicSS/SICNM give sol.prob isa SteadyStateProblem). The tests
   encode this. Should SSDE rewrap it? I left it alone because it changes the dispatch
   surface that SciMLSensitivity uses.
2. Release order for removing the field. Should lowered_problem go now as SciMLBase
   4.0 (scc-steadystate-remove-lowered), or stay deprecated-but-unused in 3.x? The 4.0
   path needs SciMLBase = "3.59, 4" compat in MTK/SSDE and in every package that caps
   SciMLBase below 4.
3. SemVer of the MTKBase accessor removal. The SCCNonlinearProblem(::SteadyStateProblem)
   method was a public-ish extension in MTKBase 1.77. I released its removal as a minor
   (1.78.0), on the grounds that it was only meaningful with the lowering. A strict reading
   makes it a major.
4. Construction cost. SCCSteadyStateProblem(sys, op) builds a full
   SteadyStateProblem (including its initialization problem) only to process op, then
   compiles the residual system. build_initializeprob = false breaks hierarchical
   initialization_eqs. Is a lighter op-processing path worth adding?
5. Monolithic DynamicSS with Tsit5. On the plain mix system, DynamicSS(Tsit5()) with
   the default tspan = Inf runs out of maxiters before the termination callback fires,
   even though the ODE is globally stable. A hand-written SteadyStateProblem behaves the
   same, so this is not caused by this change. The tests use tspan = 100. Worth an SSDE
   issue?
6. No dedicated SCC solution type. SteadyStateSolution (= NonlinearSolution) on an
   SCCSteadyStateProblem carries the compressed layout through sol.prob.f.sys, and no
   dedicated SCC solution type was added. Is that the intended end state?

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🤖 Posted by an AI agent — harness: Devin CLI 3000.11.3 (fix), Claude Code 2.1.285 (PR) · model: fusion-claude-opus-5-5-high-sidekick-swe-2-medium / claude-opus-5-5
Conversation: local Claude session 3c311569-dda6-4687-bc9f-65febb212c33 (fleet master)

🤖 Generated with Claude Code

🤖 Automated comment from an AI agent running as @ChrisRackauckas — not written or reviewed by Chris.

Risk assessment (independent review, Codex gpt-6.1-sol, head e941945)

• Verdict: CHANGES
• Rating: Codex gpt-6.1-sol rated it medium (M1) at head e941945
• Risk: medium (M1: adds methods on SciMLBase.solve and introduces a public saved-problem dispatch/layout contract). Also matches L1/L2/L5/L7 for documentation, tests, versioning and internal refactoring, ML1 for the verified parameter-override fix, ML4 for raised compat floors, ML7 for the additive export, and ML8 for shared saved-problem views; none exceeds M1.
• Blast radius: Users of SCCSteadyStateProblem, saved solutions and their reconstructed problems; solve-time overrides also affect existing SCCNonlinearProblem solves with SSRootfind/DynamicSS/SICNM. The existing SteadyStateProblem lowering routes remain present at this head, despite the title/body describing their removal. No new dependency, copyleft addition, public-name removal or signature removal appears in this diff; the minor version bump fits the additive API. The reexported SCCSteadyStateProblem has a docstring and rendered docs entry in its owning SciMLBase dependency PR, plus a manual section here.
• Evidence: Read the linked issue, full git diff origin/HEAD...HEAD, upstream type/remake implementation, and all failing-check logs. git diff --check origin/HEAD...HEAD passed; the checkout remained clean. Ran Julia 1.12.4 with four threads, the requested depot path and timeout 3600, using scratch source copies and SciMLBase prerequisite PR head 867dba9a21cbf2e280be419d8b5bc74aa1be70e0. julia --project="$S/env" "$S/head-run.jl" passed all 422 assertions copied unchanged from test/scc.jl's six block-focused testsets (96/24/128/32/102/40). The standalone probes in probe.jl and probe-small.jl confirmed both findings below; selection-array mutation and symbolic parameter-remake probes passed. python3 "$S/run-final-checks.py" ran the same reduced parameter-override test against base and head, then ordinary head dependency resolution: base 0 passed / 2 failed (u=[1], reported p=[1]); head 2 passed (u=[3], reported p=[3]); registry resolution failed because available SciMLBase versions end at 3.58.0 while this head requires 3.59. Here $S is this report's sibling scratch directory. G1 pass: analytic equilibria and independent linear-system references; the reduced override regression also uses the analytic root u=p. G2 not applicable: no GPU kernel, AD-rule, Reactant or MPI implementation is changed; Dual-value preservation is covered by the selected tests. G3 pass: tests exercise parameter propagation and remake isolation, and the caller-selection mutation probe preserved the saved layout.
• Merge: needs changes (fix both saved-selection errors and add regressions; release prerequisites and rerun failing CI before merging).

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🤖 Posted by an AI agent — harness: Codex CLI 0.162.1 · model: gpt-6.1-sol
Conversation: local headless review run (no public session URL); transcript kept by the fleet master.

[ChrisRackauckas]

🤖 Automated comment from an AI agent running as @ChrisRackauckas — not written or reviewed by Chris.

New commit 86e034a addresses the three blocking findings from the third independent review (gpt-6.1-sol):

• Saved problems now forward set_parameter! and finalize_parameters_hook! to the wrapped SCC problem, so parameter updates reach the blocks. Before the fix the new testset had 12 failures out of 24 (the re-solve gave y = 2, not 6). After the fix: 24/24.
• save_idxs works on linear blocks that have no initial guesses: state_values stays nothing and there is no MethodError.
• remake(saved.prob; u0) promotes the element type instead of converting back to the old one. Tested with BigFloat and ForwardDiff dual numbers.

The fix was written by Devin (SWE-2 Fusion, Opus 5.5). A fourth review is queued.

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🤖 Posted by an AI agent — harness: Claude Code 2.1.285 · model: claude-opus-5-5
Conversation: local session 3c311569-dda6-4687-bc9f-65febb212c33

[ChrisRackauckas]

🤖 Automated comment from an AI agent running as @ChrisRackauckas — not written or reviewed by Chris.

New commit f278d45 addresses the two blocking findings from the fourth independent review (gpt-6.1-sol):

• Solve-time u0 and p overrides are now applied before the blocks are solved, and the solution is built on the overridden problem. solve(prob, alg; p = [3.0]) now gives the same answer and the same reported parameters as solve(remake(prob; p = [3.0]), alg).
• An initial guess passed to a saved problem is now mapped from the saved layout to the full layout, so it no longer goes to the wrong state or throws a BoundsError.

The new testset "solve-time u0 and p overrides" had 20 passes, 50 failures and 2 errors without the fix, and passes all 102 checks with it. test/core.jl and test/sicnm.jl pass. QA has one error, which also occurs without this change in the same local environment. The fix was written by Devin (SWE-2 Fusion, Opus 5.5). A fifth review is queued.

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🤖 Posted by an AI agent — harness: Claude Code 2.1.285 · model: claude-opus-5-5
Conversation: local session 3c311569-dda6-4687-bc9f-65febb212c33