From df02627134cabd99d92d521b91125e0cdaf90f59 Mon Sep 17 00:00:00 2001
From: mixelpixx <chris_stockslager@comcast.net>
Date: Sat, 30 May 2026 09:47:25 -0400
Subject: [PATCH] fix(windows): derive PYTHONPATH from the detected KiCAD
 python (#84)
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findPythonExecutable already auto-detects KiCAD's bundled python across versions
(10.0/9.0/…) and both Program Files and per-user %LOCALAPPDATA% installs, but the
spawn still defaulted PYTHONPATH to a hardcoded "C:/Program Files/KiCad/9.0/..."
path. So a KiCad 10 or per-user user whose python.exe was found correctly still
got the wrong PYTHONPATH unless they set it by hand — the exact manual fixup
people keep posting in #84.

Add deriveKiCadSitePackages(pythonExe): for a detected KiCAD python at
<root>/<version>/bin/python.exe it returns the matching site-packages
(<...>/bin/Lib/site-packages, or the older lib/python3/dist-packages layout),
verified to exist. Use it as the PYTHONPATH default. Precedence is unchanged at
the top: explicit PYTHONPATH env override still wins; the legacy 9.0 string
remains only as a last-resort fallback, so this can't regress below current
behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 src/server.ts | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/src/server.ts b/src/server.ts
index f6f5264..fd92766 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -71,6 +71,29 @@ function getWindowsKiCadPythonCandidates(): string[] {
   return [...new Set(candidates)];
 }
 
+/**
+ * Derive the KiCAD bundled-Python site-packages path for a detected python.exe,
+ * so PYTHONPATH follows the *same* install we picked (any version, Program Files
+ * or per-user %LOCALAPPDATA%) instead of a hardcoded KiCad 9.0 path.
+ *
+ * KiCAD on Windows installs python at `<root>/<version>/bin/python.exe`, with
+ * pcbnew under `<...>/bin/Lib/site-packages` (older/alt layouts use
+ * `<version>/lib/python3/dist-packages`). Returns the first existing candidate,
+ * or undefined if pythonExe isn't a KiCAD bundled python.
+ */
+function deriveKiCadSitePackages(pythonExe: string): string | undefined {
+  if (process.platform !== "win32") return undefined;
+  const lower = pythonExe.toLowerCase();
+  if (!lower.endsWith("python.exe") || !lower.includes("kicad")) return undefined;
+  const binDir = dirname(pythonExe); // <root>/<version>/bin
+  const versionDir = dirname(binDir); // <root>/<version>
+  const candidates = [
+    join(binDir, "Lib", "site-packages"),
+    join(versionDir, "lib", "python3", "dist-packages"),
+  ];
+  return candidates.find((p) => existsSync(p));
+}
+
 /**
  * Find the Python executable to use.
  * Prioritizes project venvs, then explicit overrides, then KiCAD-bundled Python
@@ -475,12 +498,21 @@ export class KiCADMcpServer {
       if (!isValid) {
         throw new Error("Prerequisites validation failed. See logs above for details.");
       }
+      // PYTHONPATH precedence: explicit env override → site-packages derived
+      // from the detected KiCAD python (any version / install location) →
+      // legacy 9.0 fallback as a last resort.
+      const derivedSitePackages = deriveKiCadSitePackages(pythonExe);
+      if (derivedSitePackages && !process.env.PYTHONPATH) {
+        logger.info(`Using KiCAD site-packages: ${derivedSitePackages}`);
+      }
       this.pythonProcess = spawn(pythonExe, [this.kicadScriptPath], {
         stdio: ["pipe", "pipe", "pipe"],
         env: {
           ...process.env,
           PYTHONPATH:
-            process.env.PYTHONPATH || "C:/Program Files/KiCad/9.0/lib/python3/dist-packages",
+            process.env.PYTHONPATH ||
+            derivedSitePackages ||
+            "C:/Program Files/KiCad/9.0/lib/python3/dist-packages",
         },
       });