diff --git a/scripts/build-python-env-win.ps1 b/scripts/build-python-env-win.ps1
index ae0d332c..a1f853c0 100644
--- a/scripts/build-python-env-win.ps1
+++ b/scripts/build-python-env-win.ps1
@@ -67,10 +67,16 @@ if ($LASTEXITCODE -ne 0) {
     if ($LASTEXITCODE -ne 0) { throw "ensurepip failed" }
 }
 
-Write-Host "Installing backend dependencies..."
+# Install from the fully-pinned, hash-locked file so the shipped python-env is
+# byte-for-byte reproducible (pillar 3). requirements.txt is the human-edited
+# source; regenerate the lock after editing it with:
+#   uv pip compile backend/requirements.txt --python-version 3.13 `
+#       --generate-hashes --output-file backend/requirements.lock
+# --require-hashes is implied because every entry carries a hash.
+Write-Host "Installing backend dependencies (from requirements.lock)..."
 & $PythonBin -m pip install --upgrade pip
 if ($LASTEXITCODE -ne 0) { throw "pip upgrade failed" }
-& $PythonBin -m pip install -r (Join-Path $ProjectRoot 'backend\requirements.txt')
+& $PythonBin -m pip install -r (Join-Path $ProjectRoot 'backend\requirements.lock')
 if ($LASTEXITCODE -ne 0) { throw "pip install requirements failed" }
 
 Write-Host "Installing debugger module..."
@@ -100,6 +106,7 @@ $ToStrip = @(
     (Join-Path $PythonEnvDir 'lib\python3.13\tkinter'),           # Tk GUI toolkit — same
     (Join-Path $PythonEnvDir 'lib\python3.13\ensurepip'),         # Pip bootstrap — backend never installs at runtime
     (Join-Path $PythonEnvDir 'lib\python3.13\turtledemo'),        # Educational drawing examples
+    (Join-Path $PythonEnvDir 'lib\python3.13\turtle.py'),         # Tk-based turtle graphics; imports stripped tkinter, backend never uses it
     (Join-Path $PythonEnvDir 'lib\python3.13\pydoc_data'),        # pydoc topics/keywords; only `help()` reads them
     (Join-Path $PythonEnvDir 'lib\python3.13\_pyrepl'),           # Python 3.13 interactive REPL, never started in packaged app
     (Join-Path $PythonEnvDir 'share')                             # Man pages / desktop integration
diff --git a/scripts/build-python-env.sh b/scripts/build-python-env.sh
index 71c5526f..577e5d33 100755
--- a/scripts/build-python-env.sh
+++ b/scripts/build-python-env.sh
@@ -79,10 +79,15 @@ if ! "$PYTHON_BIN" -m pip --version &>/dev/null; then
     "$PYTHON_BIN" -m ensurepip --upgrade
 fi
 
-# Install backend dependencies
-echo "Installing backend dependencies..."
+# Install backend dependencies from the fully-pinned, hash-locked file so the
+# shipped python-env is byte-for-byte reproducible (pillar 3). requirements.txt
+# is the human-edited source; regenerate the lock after editing it with:
+#   uv pip compile backend/requirements.txt --python-version 3.13 \
+#       --generate-hashes --output-file backend/requirements.lock
+# --require-hashes is implied because every entry carries a hash.
+echo "Installing backend dependencies (from requirements.lock)..."
 "$PYTHON_BIN" -m pip install --upgrade pip
-"$PYTHON_BIN" -m pip install -r "$PROJECT_ROOT/backend/requirements.txt"
+"$PYTHON_BIN" -m pip install -r "$PROJECT_ROOT/backend/requirements.lock"
 
 # Install the debugger module
 echo "Installing debugger module..."
@@ -128,6 +133,9 @@ rm -rf "$PYTHON_ENV_DIR/lib/python3.13/tkinter"
 rm -rf "$PYTHON_ENV_DIR/lib/python3.13/ensurepip"
 # Educational drawing examples that ship with stdlib — never imported.
 rm -rf "$PYTHON_ENV_DIR/lib/python3.13/turtledemo"
+# turtle itself: a Tk-based graphics module. It imports tkinter (stripped
+# above), so it's already non-functional here, and the backend never uses it.
+rm -rf "$PYTHON_ENV_DIR/lib/python3.13/turtle.py"
 # Man pages / desktop-integration files — embedded Python doesn't read these.
 rm -rf "$PYTHON_ENV_DIR/share"
 # pip itself + launcher shims. Verified the packaged backend never invokes