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Option A: an AI app

The app brings the AI model. openPASO brings the solvers. You only connect the two once.

Run this once, from the openPASO folder:

claude mcp add openpaso -s user \
  -e PYTHONPATH="$PWD/src" \
  -e VIRTUAL_ENV="$PWD/.venv" \
  -e PYVISTA_OFF_SCREEN=true \
  -- "$PWD/.venv/bin/python" -m server

$PWD means "the folder I am in right now", so it fills in the full path for you. Three details matter, and each one fails silently if you get it wrong:

  • -e must come before the --. Everything after -- is the command itself.
  • The paths must be full paths. Your app starts openPASO from its own folder, so a relative path like src points at nothing.
  • -s user makes openPASO available in every folder. Without it, it only exists in this one.

Check that it worked:

claude mcp list          # openpaso should be listed, and ✔ Connected

If Claude Code was already running, restart it. If you see ⏸ Pending approval, you used -s project or -s local: remove it with claude mcp remove openpaso and run the command above again.

First get your full path. In the openPASO folder run pwd and copy what it prints.

Then open Settings → Developer → Edit Config and add this block. /path/to/openPASO appears three times. Replace all three.

{
  "mcpServers": {
    "openpaso": {
      "command": "/path/to/openPASO/.venv/bin/python",
      "args": ["-m", "server"],
      "env": {
        "PYTHONPATH": "/path/to/openPASO/src",
        "VIRTUAL_ENV": "/path/to/openPASO/.venv",
        "PYVISTA_OFF_SCREEN": "true"
      }
    }
  }
}
  • If the file is empty, paste the whole block.
  • If it already has an "mcpServers" section, add only the "openpaso": { ... } part inside it, with a comma after the entry before it. A second outer { ... } makes the file invalid, and the app ignores it without a word.
  • Before saving, search the block for /path/to/. If anything is left, you missed one.

Then quit Claude Desktop completely and start it again. Closing the window is not enough.

Check: open the tools menu in the message box; openpaso should be listed. If not: you did not fully restart; the file is not valid JSON (paste it into a JSON checker); or the command path does not exist (ls /path/to/openPASO/.venv/bin/python).

Two things change, and both fail silently if you skip them: the Python program is at .venv\Scripts\python.exe, and every backslash must be written twice in JSON.

{
  "mcpServers": {
    "openpaso": {
      "command": "C:\\Users\\you\\openPASO\\.venv\\Scripts\\python.exe",
      "args": ["-m", "server"],
      "env": {
        "PYTHONPATH": "C:\\Users\\you\\openPASO\\src",
        "VIRTUAL_ENV": "C:\\Users\\you\\openPASO\\.venv",
        "PYVISTA_OFF_SCREEN": "true"
      }
    }
  }
}

Get your own path with cd (Command Prompt) or pwd (PowerShell) in the openPASO folder, then double every backslash when you paste it.

Use the command /path/to/openPASO/.venv/bin/python, the arguments -m server, and all three settings from the Claude Desktop env block. Then quit the app completely and start it again.

Solver settings must also go into the app

Your AI app does not start openPASO from your terminal, so it never sees your export lines or your ~/.bashrc. Every solver variable you set, such as FEBIO_BINARY, FENICS_PYTHON, FOURC_BINARY, SPARTA_BINARY or DUNE_PYTHON, must also go into the env block as another "NAME": "value" line. Otherwise the solver works in your terminal and openPASO still reports it as missing.

Ask for a simulation

Inside the app, simply write:

Solve the Poisson equation on a unit square with a known exact solution, and check that the error falls at the expected rate.

See what a run looks like, and Troubleshooting if the app does not see openPASO.