{
  "$type": "site.standard.document",
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  "path": "/viewtopic.php?p=1278592#p1278592",
  "publishedAt": "2026-03-17T19:50:50.000Z",
  "site": "https://www.tt-forums.net",
  "tags": [
    "https://github.com/samisalreadytaken/sqdbg-vs",
    "https://github.com/PaSaSaP/OpenTTD-sqdb ... penttd.exe",
    "https://github.com/PaSaSaP/OpenTTD-sqdbg-debugger",
    "openttd-sqdbg.gif",
    "MustBe"
  ],
  "textContent": "For quite a long time, debugging AI / GameScripts in OpenTTD has mostly meant printing values and reading logs:\n\nCode:\n\n\n    AILog.Info(\"value = \" + value);\n\nor similar approaches.\n\nI experimented with integrating the Squirrel debugger (sqdbg) into OpenTTD, and it turns out it is possible to attach a real debugger to running AI / GameScript instances.\n\nCurrent state\n\nThe prototype currently supports:\n\n  * attaching an external debugger\n  * breakpoints\n  * stepping\n  * inspecting variables\n  * works with both AI and GameScripts\n\nTested with VS Code using the sqdbg extension: https://github.com/samisalreadytaken/sqdbg-vs\n\nHow it works\n\nEach Squirrel VM exposes a debug port.\n\nPorts are assigned sequentially starting from:\n\nCode:\n\n\n    2222\n\nSo typically:\n\nCode:\n\n\n    GS -> 2222AI0 -> 2223AI1 -> 2224AI2 -> 2225...\n\nPorts are printed to the console, so OpenTTD needs to be started with:\n\nCode:\n\n\n    -d\n\nThen you can attach the debugger to the port corresponding to the AI / GS instance.\n\nExample workflow\n\n  * start OpenTTD with -d\n  * load a game with AI or GameScript\n  * check console output for assigned port\n  * attach debugger (e.g. from VS Code)\n  * set breakpoint\n  * execution stops at that point\n\nFunction breakpoints\n\nFunction name matching is currently limited.\n\nThere is no support for:\n\nCode:\n\n\n    Class.MethodClass::Method\n\nOnly the function name itself is used:\n\nCode:\n\n\n    Method\n\nThis means that if multiple classes define functions with the same name, function breakpoints may trigger in multiple places.\n\nIn such cases, regular line breakpoints are more reliable.\n\nDownloads\nWindows x64 binary: https://github.com/PaSaSaP/OpenTTD-sqdb ... penttd.exe\n\nSource code: https://github.com/PaSaSaP/OpenTTD-sqdbg-debugger\n\nVS Code setup (sqdbg)\n\nExample launch.json configuration:\n\nCode:\n\n\n    { \"version\": \"0.2.0\", \"configurations\": [ { \"type\": \"squirrel\", \"request\": \"attach\", \"name\": \"Attach to OpenTTD\", \"port\": 2222 } ]}\n\nHow to use\n\n\n  * Install the sqdbg VS Code extension: https://github.com/samisalreadytaken/sqdbg-vs\n  * Create or edit .vscode/launch.json in your project.\n  * Set the port to match the one printed by OpenTTD (requires running with -d).\n  * Start OpenTTD, load a game with AI / GameScript.\n  * In VS Code:\n    * select \"Attach to OpenTTD\"\n    * press Start Debugging (F5)\n  * Set breakpoints in .nut files and run the script.\n\nExecution will pause when a breakpoint is hit.\n\nDemo\n\n\n\nopenttd-sqdbg.gif\n\nNotes\n\n  * based on OpenTTD 15.2\n  * currently implemented as a patched build\n  * each VM uses a separate port (no multiplexing)\n\n\n\nStatistics: Posted by MustBe — 17 Mar 2026 19:50\n\n* * *",
  "title": "OpenTTD Development • Squirrel debugger support for OpenTTD (AI / GameScript) – prototype",
  "updatedAt": "2026-03-17T19:50:50.000Z"
}