Simulate Modelica

Simulate Modelica Model

This skill simulates Modelica models (.mo files) with WSMKernelX: it flattens the model, generates C++ code, compiles it, and runs the simulation. It produces a .mat results file.

Before you run anything

This skill drives WSMKernelX through the shared launcher ../scripts/wsm_run.py. Read [the shared-conventions appendix at the end of this file](#appendix-shared-conventions-for-the-modelica-skills) first — launcher resolution, the Windows-vs-Unix shell/Python rules, the temp-dir and cleanup conventions, the JSON-array output gotcha, and the MSL 4.x dialect notes that every step below assumes.

The launcher works in _wsm_simulate_temp/ next to the .mo file and leaves simulate.out.json and the .mat there. Tell the user: "Working in temporary directory _wsm_simulate_temp/. This will be deleted after simulation."

Workflow

1. Identify the model file and name

Identify the .mo file and extract the model name — see [Appendix → Picking the model name](#picking-the-model-name). For a directory-form (multi-file) library, point --model at the library folder (not one class file) and pass the full dotted --name — see [Appendix → Directory-form (multi-file) libraries](#directory-form-multi-file-libraries).

2. Run the simulation

python3 "/wsm_run.py" --mode simulate \
  --model "" --name ModelName --timeout 180

Terser loop: add --quiet to print only a one-line outcome (status + integration time + result file) instead of the full kernel log, and --report "Vout,x.T" to print a compact min/max/mean/pp/final table of those variables straight from the result .mat — so the common "simulate then inspect a few values" step is a single call. (--report runs mat_summary.py, which you can also call standalone on any .mat.)

The launcher auto-detects MSL usage and loads the right version; force it with --msl yes|no or --msl-version 4.1.0. If the model uses an installed non-MSL library (e.g. Hydraulic), add --load-library — see [Appendix → Using non-MSL libraries](#using-non-msl-libraries-hydraulic-and-other-installed-libraries). Timeout: allow up to 180 seconds — compilation and simulation of complex models can take time.

Compiling needs a per-OS C++ toolchain (Visual Studio Build Tools / Xcode CLT / gcc) — the launcher finds it; if it can't, see [Appendix → When the install or compiler isn't found](#when-the-install-or-compiler-isnt-found).

3. Parse the output

WSMKernelX writes structured results to simulate.out.json in the temp directory — a JSON array; take the first element. Check status.flatten, status.build, and status.result in that order; on success simulation.resultFile holds the path to the .mat (also printed as a SimulationResult record). Field reference: [Appendix → Reading the JSON output](#reading-the-json-output).

4. Report results

Summarize clearly:

• Pass: State the simulation completed successfully. Report key stats from the log (integration time, number of events, number of function evaluations).
• Build Fail: The C++ compilation or linking failed. Show the build errors.
• Flatten Fail: The model has structural errors. Show the flatten errors.
• Fatal error: exception ...(_) / no out.json: a normal error (assertion, parameter/init, type, lookup) that escaped catching — not a crash. Read the launcher's === actual kernel diagnostic === block, which prints the recovered message; don't assume a compiler bug.

A clean "Pass" does not mean the result is correct. A model can compile and simulate with zero errors while being physically wrong (mis-biased circuit with ~0 output, a state pinned at a saturation limit, a node that silently went NaN). After a successful run, sanity-check the trajectories:

python3 "/check_sanity.py" "/.mat"

It flags NaN/Inf, variables that never move, and signals that swung then flatlined (possible saturation / mis-bias). Treat the flags as prompts to inspect operating points, not as failures. (It self-provisions DyMat/numpy.)

When you simulate a system to settle, get the operating point — a domain-neutral steady-state report that works for thermal, hydraulic, mechanical, electrical, ... models. It checks each state's derivative to report whether the run reached equilibrium (and lists states still drifting), then prints the settled values:

python3 "/op_report.py" "/.mat" --vars tank.T pump.dp

Domain add-ons fire automatically when applicable — e.g. for circuits it also classifies BJT regions (ACTIVE / SATURATED / CUTOFF from Vbe/Vbc), catching mis-bias that no error would report. (Self-provisions DyMat/numpy.)

4b. Parameter studies WITHOUT recompiling (--override / --sweep)

To try different parameter values, do not rebuild or hand-write wrapper models. The launcher builds once and re-runs the compiled executable per value by editing the .sim init file's value= attribute:

# one value set
python3 "/wsm_run.py" --mode simulate --model M.mo --name Pkg.M \
  --override "kfb=0.04,I0=45e-6"
# sweep one parameter (one .mat per value), holding others via --override
python3 "/wsm_run.py" --mode simulate --model M.mo --name Pkg.M \
  --sweep "I0=12e-6,32.5e-6,90e-6"

Each run writes run_.mat in the temp dir; plot/compare them directly. Caveat (this is enforced): only parameters with initType="exact" in the .sim can be overridden this way. Structural parameters (used in array sizes, conditional components, etc.) are constant-folded into the compiled code — the launcher detects these from the .sim and warns that they need a real rebuild (set them in the model or build model X = Pkg.M(param=value)).

5. Clean up

Remove _wsm_simulate_temp/ entirely — commands per OS: [Appendix → Temporary directories](#temporary-directories).

Edge cases

• Packages / name mismatches: parse the actual model/package declaration, not the filename — see [Appendix → Picking the model name](#picking-the-model-name).
• Unknown library: X on a multi-file library: you pointed --model at a single class file; point it at the library folder instead — see [Appendix → Directory-form (multi-file) libraries](#directory-form-multi-file-libraries).
• Element not found ... in Modelica...: the MSL path is wrong — look up the right one with the search-modelica-docs skill, don't grep the install tree. See [Appendix → MSL 4.x dialect](#msl-4x-dialect).
• WSMKernelX or compiler not found: see [Appendix → When the install or compiler isn't found](#when-the-install-or-compiler-isnt-found).
• Simulation times out or stalls at initialization: First distinguish build from solve — check whether the .exe was produced (build done) and whether any result rows were written. If it builds but the solver makes no progress:

1. Re-run with a short StopTime (override the model's experiment annotation, or use a small wrapper model) to confirm it integrates at all before committing to a long run.
2. If it stalls at/near t=0, suspect a nonlinear algebraic loop with no dynamic states — common in high-gain feedback (active circuits, control loops) where every variable is algebraic. Run the diagnose-modelica skill, then the check_singularity.py / check_tearing.py scripts (documented in [../scripts/README.md](../scripts/README.md)) — they reveal the offending algebraic systems.
3. The usual physical fix for active analog circuits is to add the parasitic dynamic elements the idealized model omitted (junction/winding capacitances, lead inductances). They turn the stiff algebraic loop into an integrable ODE and bandwidth-limit feedback, which also defines the operating point.
4. As a numerical lever, loosen tolerance or raise --timeout, but prefer fixing the model structure — a model that needs a huge timeout for a short horizon is usually telling you something.

---

Appendix: shared conventions for the Modelica skills

> Shared by every Modelica skill that drives WSMKernelX through the > bundled launcher; inlined here at release time. For the CLI/option > reference, environment variables (WSM_HOME, WSM_VSDEVCMD), > install discovery, and the analysis scripts, see > [../scripts/README.md](../scripts/README.md).

Locating the launcher

` (used throughout the skills) is the shared scripts/` folder. Some installs symlink the skill directories without it, so resolve it in this order and use the first that exists:

1. $WSM_SKILLS_SCRIPTS (bash) or $env:WSM_SKILLS_SCRIPTS (PowerShell), if set.
2. ../scripts relative to the skill directory — in a normal install

../scripts/wsm_run.py already exists, so use that path directly; do not run a shell probe to "resolve" it.

1. The repo checkout you installed from, e.g. .../agentskills/scripts.
2. Last resort, search the home directory:

• PowerShell: Get-ChildItem $HOME -Recurse -Filter wsm_run.py -ErrorAction SilentlyContinue | Select-Object -First 1
• bash/zsh: find ~ -name wsm_run.py -path '*scripts*' 2>/dev/null | head -1

If only #4 finds it, the install is missing the scripts/ link — tell the user to run install.sh (or install.ps1) from the repo, which links scripts/ too.

Shell and Python

On Windows, use PowerShell. The Git-Bash/cygwin layer may be broken (even ls/find can be absent, giving a misleading "exit 127 / command not found"). Run wsm_run.py with python (not python3); those calls are single-line and shell-agnostic. For cleanup use Remove-Item -Recurse -Force, not rm -rf. On macOS/Linux any POSIX shell is fine and python3 is the usual name.

Let the launcher own .mos/.bat and paths

Do not hand-write .mos scripts, .bat files, or hardcode install/compiler paths. The bundled scripts/wsm_run.py handles every OS difference — it finds the System Modeler install and kernel binary (macOS / Windows / Linux), finds and loads the right MSL files, generates the .mos, and runs the kernel with a working compiler environment per platform (system clang/gcc on macOS/Linux; the Visual Studio dev environment via VsDevCmd.bat on Windows). See [../scripts/README.md](../scripts/README.md) for WSM_HOME, the Windows compiler prerequisites, and the full option table.

When the install or compiler isn't found

The launcher searches each OS's standard install locations. If it prints ERROR: Could not locate a Wolfram System Modeler installation, the install is in a non-standard place — ask the user for it and re-run with --wsm-home "" (or have them set WSM_HOME).

Building and simulating also need a C++ toolchain:

• Windows: Visual Studio Build Tools. The launcher locates VsDevCmd.bat

itself; if it reports the compiler environment is missing, pass --vsdevcmd "" (or set WSM_VSDEVCMD) and make sure Build Tools are installed.

• macOS: the Xcode command-line tools (xcode-select --install).
• Linux: gcc/g++.

Run python3 "/wsm_run.py" --mode info to see what the launcher discovered.

Temporary directories

The launcher works in a _wsm__temp/ directory next to the .mo file (_wsm_validate_temp/, _wsm_simulate_temp/, _wsm_diagnose_temp/) and leaves its outputs there. Tell the user, e.g.: "Working in temporary directory _wsm__temp/. This will be deleted afterwards." Pass --tempdir to reuse one directory across models in a session.

Clean up by removing the whole directory — use the user's shell:

rm -rf "/_wsm__temp"          # macOS / Linux
# PowerShell: Remove-Item -Recurse -Force "\_wsm__temp"

Picking the model name

• The user may provide a path to a .mo file, or you may already be working with

one in context.

• Extract the model name: the identifier after model on the first non-comment

line, e.g. model FooBar → FooBar. The filename does not always match the model name — parse the actual model/package declaration.

• For packages or nested models, use the top-level model name.
• Pick an instantiable model, not a package, for any kernel call. A package

cannot be validated or simulated ("Invalid instantiation … is a package") — use a nested model's full dotted name, e.g. Package.Model.

• Pass an absolute path to --model (relative paths break as the working

directory shifts between calls).

Directory-form (multi-file) libraries

A directory-form library stores one class per file with a package.mo at each level. You cannot validate such a class by handing the launcher only its own .mo file — the class's within Lib; clause needs the whole package loaded, and loading the single file alone fails with Internal error: ... expandLibNode: Unknown library: Lib. Instead point --model at the library folder (or its top package.mo, or any class file inside it) and pass the full dotted class name via --name:

python3 "/wsm_run.py" --mode validate \
  --model "/abs/path/InvertedPendulum" \
  --name InvertedPendulum.Controller

The launcher resolves any of those forms up to the library's root package.mo and loads the entire package (following package.order) before instantiating --name. It prints a NOTE: telling you which package.mo it loaded. Do not try to work around the unknown-library error by --load-ing individual files.

Reading the JSON output

The kernel writes .out.json into the temp dir. **It is a JSON array — take the first element**, then read:

• status.flatten: "Pass" / "Fail" (the primary result for validate).
• status.build: "Pass" / "Fail" — C++ compilation/linking (simulate).
• status.result: simulation result status (simulate).
• messages.errors / messages.warnings / messages.notifications:

arrays (empty if none).

• flat_model (validate) / simulation.resultFile (simulate): the

flattened class / path to the .mat.

See [../scripts/README.md](../scripts/README.md) (wsm_run.py section) for the full field reference.

MSL 4.x dialect

This toolchain ships MSL 4.x. When authoring models, use the 4.x names — the 3.2 names flatten with confusing "not found" errors:

• units: Modelica.Units.SI.* (not Modelica.SIunits.*)
• source frequency parameter: f (not freqHz), e.g. SineVoltage(V=.., f=..)
• declare the dependency as annotation(uses(Modelica(version="4.0.0")))

Run python3 "/wsm_run.py" --mode info to confirm the exact MSL version. wsm_run.py also warns on stderr if it spots a 3.2 name in the model.

When a flatten fails with Element not found ... in Modelica..., the MSL component path is wrong (a misremembered name, not a missing install). Resolve the correct path with the search-modelica-docs skill — e.g. sine is Modelica.Blocks.Sources.Sine (not Math.Sine), difference is Math.Feedback (not Math.Subtract), and saturation is Nonlinear.Limiter (not Nonlinear.Saturation). Do not grep or walk the System Modeler install tree to hunt for the class.

Using non-MSL libraries (Hydraulic, and other installed libraries)

MSL is automatic (--msl). For any other library a model uses — Hydraulic, or anything the user installed from the Library Store — the launcher can find it for you. Two steps, no guessing at paths:

1. See what is installed (bundled with System Modeler, user-installed archives, and

any custom folders configured in Model Center):

``bash python3 "/wsm_run.py" --mode libraries # add --json for a machine-readable array; each row has name/version/source/package ``

To get just one library's package path (e.g. for a manual --load): --mode libraries --library Hydraulic (add --library-version 2.1 to pin a version).

1. Load it into a build by name — add --load-library to a validate /

simulate / diagnose run (repeatable; pin a version with Name==Ver

…

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: WolframResearch
• Source: WolframResearch/system-modeler-ai-toolkit
• License: MIT
• Homepage: https://www.wolfram.com/system-modeler/

Install and usage instructions live in the source repository linked above.