diff --git a/crates/ffi/include/agent_desktop.h b/crates/ffi/include/agent_desktop.h
index f720d26..be5ad83 100644
--- a/crates/ffi/include/agent_desktop.h
+++ b/crates/ffi/include/agent_desktop.h
@@ -196,6 +196,12 @@ typedef struct AdPoint {
   double y;
 } AdPoint;
 
+/*
+ * Caller-allocated drag parameters. Zero-initialize the whole struct before
+ * setting fields: `duration_ms`/`drop_delay_ms` treat 0 as the adapter-default
+ * sentinel, so stack garbage in an unset field would become a real delay.
+ * Validate layout with `AD_DRAG_PARAMS_SIZE` / `ad_drag_params_size()`.
+ */
 typedef struct AdDragParams {
   struct AdPoint from;
   struct AdPoint to;
@@ -205,6 +211,10 @@ typedef struct AdDragParams {
   uint64_t drop_delay_ms;
 } AdDragParams;
 
+#define AD_DRAG_PARAMS_SIZE (sizeof(AdDragParams))
+
+uintptr_t ad_drag_params_size(void);
+
 /**
  * Action dispatched by `ad_execute_action`.
  *
diff --git a/crates/ffi/src/types/drag_params.rs b/crates/ffi/src/types/drag_params.rs
index 43ebafb..4b63880 100644
--- a/crates/ffi/src/types/drag_params.rs
+++ b/crates/ffi/src/types/drag_params.rs
@@ -1,6 +1,11 @@
 use crate::types::point::AdPoint;
 use agent_desktop_core::action::{DragParams as CoreDragParams, Point as CorePoint};
 
+/// Caller-allocated drag parameters. Callers must zero-initialize the whole
+/// struct before setting fields so unset numeric fields read as the `0`
+/// adapter-default sentinel rather than stack garbage. Verify layout against
+/// `AD_DRAG_PARAMS_SIZE` / `ad_drag_params_size()` when binding from a language
+/// whose struct layout may diverge.
 #[repr(C)]
 pub struct AdDragParams {
     pub from: AdPoint,
@@ -9,6 +14,15 @@ pub struct AdDragParams {
     pub drop_delay_ms: u64,
 }
 
+pub const AD_DRAG_PARAMS_SIZE: usize = 48;
+
+const _: () = assert!(std::mem::size_of::<AdDragParams>() == AD_DRAG_PARAMS_SIZE);
+
+#[unsafe(no_mangle)]
+pub extern "C" fn ad_drag_params_size() -> usize {
+    std::mem::size_of::<AdDragParams>()
+}
+
 impl AdDragParams {
     /// Converts the C drag params into the core type. `duration_ms` and
     /// `drop_delay_ms` use `0` as the "adapter default" sentinel because the