diff --git a/parser/llguidance.h b/parser/llguidance.h
index b26420e..9ea56e8 100644
--- a/parser/llguidance.h
+++ b/parser/llguidance.h
@@ -9,8 +9,6 @@
 
 typedef struct LlgConstraint LlgConstraint;
 
-typedef struct LlgConstraintStep LlgConstraintStep;
-
 typedef struct LlgTokenizer LlgTokenizer;
 
 typedef struct LlgParserLimits {
@@ -110,6 +108,21 @@ typedef struct LlgCommitResult {
   bool is_stop;
 } LlgCommitResult;
 
+typedef struct LlgConstraintStep {
+  /**
+   * The constraint to compute mask for.
+   */
+  struct LlgConstraint *constraint;
+  /**
+   * Pointer to memory where the mask should be written.
+   */
+  uint32_t *mask_dest;
+  /**
+   * The length of the mask_dest array in bytes (not elements).
+   */
+  size_t mask_byte_len;
+} LlgConstraintStep;
+
 /**
  * Function which llg calls when an operation is done.
  */
diff --git a/parser/src/ffi.rs b/parser/src/ffi.rs
index 5d65ad9..68617c6 100644
--- a/parser/src/ffi.rs
+++ b/parser/src/ffi.rs
@@ -213,6 +213,7 @@ pub struct LlgConstraintInit {
 }
 
 #[derive(Clone)]
+#[repr(C)]
 pub struct LlgConstraintStep {
     /// The constraint to compute mask for.
     pub constraint: *mut LlgConstraint,