Simulate And Plot Modelica

Simulate and Plot

This skill simulates a Modelica model and plots the results: it runs the simulation to produce a .mat, then reads chosen variables from it with DyMat and plots them over time. No reference data is needed.

Scope check before starting: if the request goes beyond plotting — verifying limits or requirements, finding violations, parameter sweeps, Monte Carlo, fitting/calibration — and Wolfram Language is available (e.g. a Wolfram MCP tool or wolframscript), hand the task to the wolfram-language-modelica skill instead: the built-in SystemModel* functions do that analysis natively. Use this skill's Python analysis path only when Wolfram Language is not available.

Prerequisites

plot_mat.py needs DyMat (reads .mat), matplotlib, numpy and scipy. You do not install these by hand: the script self-provisions a managed venv on first use (it never touches system Python). To pre-warm it: python3 "/bootstrap_env.py".

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, and the MSL 4.x dialect notes that every step below assumes.

Temporary Directory

The launcher writes into _wsm_simulate_temp/ next to the model file (or pass --tempdir "/_wsm_simulate_temp" to reuse one dir across models in a session). Tell the user: "Working in _wsm_simulate_temp/. Will be deleted at end of session."

Workflow

1. Identify the model

The user may provide a path to a .mo file, or a fully qualified model name (for a model that lives in a loaded library).

2. Simulate

Run the launcher to produce the .mat:

python3 "/wsm_run.py" --mode simulate \
  --model "" --name  \
  --tempdir "" --timeout 180 2>&1 | grep -E "succeeded|stopped|events:|out.json"
mat_file=$(ls ""/*.mat | head -1)

MSL is auto-detected.

If the model uses a library that must be loaded first: for a library installed on the machine (e.g. Hydraulic), just add --load-library — the launcher finds and loads it (see [Appendix → Using non-MSL libraries](#using-non-msl-libraries-hydraulic-and-other-installed-libraries)). For a library that lives in the repo/on disk (e.g. HydraulicTests), pass its file with --load (repeatable). Both load before the model:

python3 "/wsm_run.py" --mode simulate \
  --model "/Model.mo" --name  \
  --load-library Hydraulic --load "/HydraulicTests/HydraulicTests.mo" \
  --tempdir "" --timeout 180

3. Plot the results

Pick the variables to plot (ask the user, or use --list to see what's in the .mat and choose a sensible few), then plot them with plot_mat.py. Favor curves that reveal emergent behavior — a dynamic response, a comparison, a limit case — over ones that just re-trace a known input signal (if you'd see the same shape by plotting the input alone, skip it):

# See what's available
python3 "/plot_mat.py" "$mat_file" --list | head -40

# Combined figure, one subplot per variable
python3 "/plot_mat.py" "$mat_file"    \
  --outdir "" --title " results"

plot_mat.py options:

• --list — print the .mat's variable names and exit (no plot)
• --separate — one PNG per variable instead of a combined figure
• --ncols N — columns in the combined grid (default 2)
• --name NAME / --title STR — output filename stem / figure title

4. Show the plot to the user

Read and display the generated .png with the Read tool. If the user wants a different set of variables or one-per-file, re-run plot_mat.py with a new variable list or --separate.

5. Switching to another model

Just re-run the launcher with the same --tempdir and a new --name/--model. It regenerates the .mos and reuses the temp dir; the build is fast for the second model since the toolchain is warm. To keep the temp dir tidy, optionally clear old build artifacts first (the .mat, .exe/binaries, .sim, logs):

find "" -maxdepth 1 \( -name "*.mat" -o -name "*.exe" -o -name "*.sim" -o -name "*.log" -o -name "*.jsonl" -o -name "*.lib" -o -name "*.libs" -o -name "*.exp" -o -name "*_units.json" -o -name "*_build.log" \) -delete
python3 "/wsm_run.py" --mode simulate --model "" --name  --tempdir ""

6. Cleanup

At end of session (or when switching projects), remove _wsm_simulate_temp/ entirely — commands per OS: [Appendix → Temporary directories](#temporary-directories).

Keep final plots in _comparison_plots/ — they are useful references.

Inspecting data inline

For one-off data inspection, use python -c "..." inline. Do not write debug_*.py files.

python -c "
import DyMat
d = DyMat.DyMatFile('path/to/file.mat')
print(d.names()[:20])
print(d.data('variable.name')[-10:])
"

For longer exploration (5+ lines of Python), use a single explore.py in the temp dir and overwrite it as needed — don't accumulate debug_a.py, debug_b.py, etc.

File-efficiency rules

1. One temp dir per session — pass the same --tempdir for every model; never _wsm_simulate_temp_plots, _wsm_diagnose_temp, etc. in parallel
2. Let the launcher manage the .mos — don't hand-write or duplicate scripts
3. Inline python via -c — don't create debug_*.py files for one-off inspection
4. Clean build artifacts between models (see step 5)
5. Final plots in _comparison_plots/ — keep. Temp simulation artifacts — delete.

Edge cases

• Variable not found in .mat: plot_mat.py warns and skips it; check the name with --list (Modelica uses dotted names, e.g. tank1.port_a.mdot).
• Too many variables: don't plot all of them — pick the few that matter for the question. --list shows what's available.
• DyMat import error: plot_mat.py auto-provisions its venv; if it still fails, pre-warm with python3 "/bootstrap_env.py" and check $WSM_SKILLS_VENV.
• Model fails to simulate: report the runtime error from the simulate step; there's nothing to plot until it runs.

---

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):

``bash python3 "/wsm_run.py" --mode validate \ --model "/M.mo" --name M --load-library Hydraulic ``

The launcher resolves the library (bundled → user-installed → Model-Center custom path, newest version wins) and loadFiles it before the model. A library usually pulls in MSL, so keep MSL on (auto, or --msl yes). Override a lookup with $WSM_LIBRARY_ (e.g. WSM_LIBRARY_HYDRAULIC=/path/to/package.moe), or fall back to an explicit --load .

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.