diff --git a/scripts/build-python-env-win.ps1 b/scripts/build-python-env-win.ps1
index 3fb059b7..fe6166ea 100644
--- a/scripts/build-python-env-win.ps1
+++ b/scripts/build-python-env-win.ps1
@@ -165,14 +165,16 @@ Write-Host "Trimming .pyi type stubs..."
 Get-ChildItem -Path $PythonEnvDir -Recurse -Filter '*.pyi' -File -ErrorAction SilentlyContinue `
     | Remove-Item -Force -ErrorAction SilentlyContinue
 
-# Pre-compile bytecode so cold backend startup skips parse+compile on
-# every imported .py. Worth ~5-10s on Windows under Defender (parsing
-# Python source is parser-bound; loading .pyc is just bytes). We cap
-# concurrency at 4 — `-j 0` (all cores) is fine on dev boxes but
-# unstable on small CI runners. Missing .pyc is non-fatal at runtime
-# (Python falls back to in-memory compile), so we warn rather than fail.
+# Pre-compile bytecode so cold backend startup skips parse+compile per import.
+# invalidation-mode unchecked-hash is load-bearing: the default timestamp mode
+# ties each .pyc to its source mtime, but the installer rewrites mtimes on extract,
+# so every .pyc looks stale and Python recompiles the whole stdlib+deps from source
+# on EVERY launch (and runtime PYTHONDONTWRITEBYTECODE means it never caches the
+# result), which is the multi-minute Windows cold-start. unchecked-hash makes the
+# .pyc valid regardless of mtime, which is the correct mode for a frozen bundle.
+# Concurrency capped at 4; missing .pyc is non-fatal (runtime in-memory fallback).
 Write-Host "Pre-compiling bytecode..."
-& $PythonBin -m compileall -q -j 4 (Join-Path $PythonEnvDir 'lib')
+& $PythonBin -m compileall -q -j 4 --invalidation-mode unchecked-hash (Join-Path $PythonEnvDir 'lib')
 if ($LASTEXITCODE -ne 0) {
     Write-Host "WARNING: some files failed to compile; runtime will fall back to in-memory compile." -ForegroundColor Yellow
 }
diff --git a/scripts/build-python-env.sh b/scripts/build-python-env.sh
index b62db1b0..c00eca93 100755
--- a/scripts/build-python-env.sh
+++ b/scripts/build-python-env.sh
@@ -224,8 +224,12 @@ find "$PYTHON_ENV_DIR" -name 'libpython*.a' -delete 2>/dev/null || true
 # version-shim packages); a non-zero exit here would rather be visible
 # than silent so we don't `|| true` the whole thing — but missing .pyc
 # is non-fatal at runtime, so a hard fail isn't warranted either.
+# invalidation-mode unchecked-hash: default timestamp mode ties each .pyc to its
+# source mtime, which installers rewrite on extract, silently invalidating every
+# .pyc so Python recompiles from source on every launch. unchecked-hash is mtime-
+# independent (correct for a frozen bundle), so the precompiled .pyc actually get used.
 echo "Pre-compiling bytecode..."
-"$PYTHON_BIN" -m compileall -q -j 4 "$PYTHON_ENV_DIR/lib" || \
+"$PYTHON_BIN" -m compileall -q -j 4 --invalidation-mode unchecked-hash "$PYTHON_ENV_DIR/lib" || \
     echo "WARNING: some files failed to compile; runtime will fall back to in-memory compile."
 
 # ----- macOS: hide bundled python from the Dock -----