diff --git a/.gitmodules b/.gitmodules
index eb15f426a..65b8828cb 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -7,3 +7,7 @@
 [submodule "vendor/jian"]
 	path = vendor/jian
 	url = git@github.com:ZSeven-W/jian.git
+[submodule "vendor/casement"]
+	path = vendor/casement
+	url = https://github.com/ZSeven-W/casement.git
+	branch = op-file-open
diff --git a/Cargo.toml b/Cargo.toml
index 5f53e4698..5344b39d8 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,6 +9,7 @@ members = ["crates/*"]
 exclude = [
   "vendor/agent",
   "vendor/anthropic-agent-sdk",
+  "vendor/casement",
   "vendor/jian",
   "vendor/skia-safe-op",
   "node_modules",
diff --git a/crates/op-host-desktop/Cargo.toml b/crates/op-host-desktop/Cargo.toml
index fcb84a55d..65cb4a098 100644
--- a/crates/op-host-desktop/Cargo.toml
+++ b/crates/op-host-desktop/Cargo.toml
@@ -102,12 +102,12 @@ skia-safe = { version = "0.97.0", default-features = false, features = [
   "gl",
   "pdf",
 ] }
-# `casement` — ZSeven-W's winit fork (sibling repo, local path for
-# now). It adds `winit::platform::macos::drain_opened_file_urls`, the
-# macOS open-documents Apple-event hook upstream winit lacks. The
+# `casement` — ZSeven-W's winit fork (vendored as a submodule under
+# `vendor/casement`). It adds `winit::platform::macos::drain_opened_file_urls`,
+# the macOS open-documents Apple-event hook upstream winit lacks. The
 # `package` key keeps the `winit` import name, so all `use winit::…`
 # stays unchanged.
-winit = { package = "casement", path = "../../../winit", default-features = false, features = [
+winit = { package = "casement", path = "../../vendor/casement", default-features = false, features = [
   "x11",
   "wayland",
   "wayland-csd-adwaita",
diff --git a/crates/op-host-native/Cargo.toml b/crates/op-host-native/Cargo.toml
index d8282f9ef..4c422ca33 100644
--- a/crates/op-host-native/Cargo.toml
+++ b/crates/op-host-native/Cargo.toml
@@ -109,14 +109,15 @@ skia-safe = { version = "0.97.0", default-features = false, features = [
   "gl",
 ] }
 glutin = "0.32.3"
-# `casement` — ZSeven-W's winit fork (sibling repo, local path for
-# now). The `package` key keeps the `winit` import name. `glutin-winit`
+# `casement` — ZSeven-W's winit fork (vendored as a submodule under
+# `vendor/casement` so CI checks out the same source the desktop binary
+# links). The `package` key keeps the `winit` import name. `glutin-winit`
 # is intentionally NOT used: it hard-depends on the upstream `winit`
 # package, which would pull a second, incompatible winit into the
 # tree. Its only use here — `GlWindow::build_surface_attributes` — is
 # replaced by a direct `glutin::surface::SurfaceAttributesBuilder`
 # call in `context/provider.rs`.
-winit = { package = "casement", path = "../../../winit", default-features = false, features = [
+winit = { package = "casement", path = "../../vendor/casement", default-features = false, features = [
   "x11",
   "wayland",
   "wayland-csd-adwaita",
diff --git a/vendor/casement b/vendor/casement
new file mode 160000
index 000000000..5877fa83e
--- /dev/null
+++ b/vendor/casement
@@ -0,0 +1 @@
+Subproject commit 5877fa83e8f7c5ce81afec234c1cd5146262a92e