diff --git a/scripts/build-web-worker.sh b/scripts/build-web-worker.sh
index 7a278ed7d..2c96d5448 100644
--- a/scripts/build-web-worker.sh
+++ b/scripts/build-web-worker.sh
@@ -2,7 +2,11 @@
 set -eu
 
 cargo build --locked --release --target wasm32-unknown-unknown --package ocs_web_worker
-worker_out="${TRUNK_STAGING_DIR:?}/worker_pkg"
+# On Windows, Trunk passes an extended-length path (\\?\C:\...) that MSYS sh
+# tools cannot handle; strip the prefix so local web builds work there too.
+# CI (Linux) paths never carry the prefix and pass through unchanged.
+staging=$(printf '%s\n' "${TRUNK_STAGING_DIR:?}" | sed 's|^\\\\?\\||')
+worker_out="${staging:?}/worker_pkg"
 mkdir -p "$worker_out"
 wasm-bindgen \
   --target web \