diff --git a/crates/xlsx/src/fmla.rs b/crates/xlsx/src/fmla.rs
index b4298c95d..018cbf2a9 100644
--- a/crates/xlsx/src/fmla.rs
+++ b/crates/xlsx/src/fmla.rs
@@ -21,7 +21,56 @@ pub fn text_to_file(text: &str) -> String {
     }
 }
 
+const PARAM_PREFIX: &str = "_xlpm.";
+
+/// Adds Excel's `_xlpm.` prefix to LET/LAMBDA parameter names, at the declaration and at every use in scope.
+fn prefix_params(e: Expr, scope: &[String]) -> Expr {
+    let sub = |x: Box<Expr>, sc: &[String]| Box::new(prefix_params(*x, sc));
+    match e {
+        Expr::Name(n) if scope.iter().any(|s| s.eq_ignore_ascii_case(&n)) => Expr::Name(format!("{PARAM_PREFIX}{n}")),
+        Expr::Unary(op, x) => Expr::Unary(op, sub(x, scope)),
+        Expr::Paren(x) => Expr::Paren(sub(x, scope)),
+        Expr::Binary(op, a, b) => Expr::Binary(op, sub(a, scope), sub(b, scope)),
+        Expr::Invoke(c, args) => Expr::Invoke(sub(c, scope), args.into_iter().map(|a| prefix_params(a, scope)).collect()),
+        Expr::Call(name, args) if name == "LET" || name == "LAMBDA" => {
+            let last = args.len().saturating_sub(1);
+            let is_let = name == "LET";
+            let mut sc = scope.to_vec();
+            let mut pending: Option<String> = None;
+            let mut out = Vec::with_capacity(args.len());
+            for (i, a) in args.into_iter().enumerate() {
+                let declares = i < last && (!is_let || i % 2 == 0);
+                match a {
+                    Expr::Name(n) if declares => {
+                        out.push(Expr::Name(format!("{PARAM_PREFIX}{n}")));
+                        if is_let {
+                            pending = Some(n);
+                        } else {
+                            sc.push(n);
+                        }
+                    }
+                    other => {
+                        out.push(prefix_params(other, &sc));
+                        // A LET name is in scope only after its value expression.
+                        if let Some(n) = pending.take() {
+                            sc.push(n);
+                        }
+                    }
+                }
+            }
+            Expr::Call(name, out)
+        }
+        // A LET-bound LAMBDA called by name, e.g. `f(3)`.
+        Expr::Call(name, args) if scope.iter().any(|s| s.eq_ignore_ascii_case(&name)) => {
+            Expr::Call(format!("{PARAM_PREFIX}{name}"), args.into_iter().map(|a| prefix_params(a, scope)).collect())
+        }
+        Expr::Call(name, args) => Expr::Call(name, args.into_iter().map(|a| prefix_params(a, scope)).collect()),
+        other => other,
+    }
+}
+
 fn expr_to_file(e: Expr) -> String {
+    let e = prefix_params(e, &[]);
     let e = e.map(&mut |x| match x {
         Expr::Call(name, args) => match crate::tables::file_function_name(&name) {
             Some(n) => Expr::Call(n, args),
@@ -34,11 +83,23 @@ fn expr_to_file(e: Expr) -> String {
     gridcraft_formula::print(&e)
 }
 
+/// `_xlpm.x` → `x` (any case, as the parser upper-cases function names).
+fn strip_param_prefix(n: String) -> String {
+    match (n.get(..PARAM_PREFIX.len()), n.get(PARAM_PREFIX.len()..)) {
+        (Some(p), Some(rest)) if p.eq_ignore_ascii_case(PARAM_PREFIX) => rest.to_string(),
+        _ => n,
+    }
+}
+
 /// Formula text read from a file → model text (strips prefixes when it parses).
 pub fn from_file(text: &str) -> String {
     let body = text.strip_prefix('=').unwrap_or(text);
     match gridcraft_formula::parse(body) {
-        Ok(e) => gridcraft_formula::print(&e),
+        Ok(e) => gridcraft_formula::print(&e.map(&mut |x| match x {
+            Expr::Name(n) => Expr::Name(strip_param_prefix(n)),
+            Expr::Call(n, args) => Expr::Call(strip_param_prefix(n), args),
+            other => other,
+        })),
         Err(_) => body.to_string(),
     }
 }
@@ -61,4 +122,18 @@ mod tests {
         assert_eq!(from_file("_xlfn.XLOOKUP(1,A:A,B:B)"), "XLOOKUP(1,A:A,B:B)");
         assert_eq!(text_to_file("=IFS(A1>0,1)"), "_xlfn.IFS(A1>0,1)");
     }
+
+    #[test]
+    fn let_lambda_params() {
+        assert_eq!(to_file(&Formula::new("LET(x,1,x+1)")), "_xlfn.LET(_xlpm.x,1,_xlpm.x+1)");
+        assert_eq!(to_file(&Formula::new("LET(x,A1*2,y,x+1,x*y)")), "_xlfn.LET(_xlpm.x,A1*2,_xlpm.y,_xlpm.x+1,_xlpm.x*_xlpm.y)");
+        assert_eq!(to_file(&Formula::new("LAMBDA(a,b,a^2+b)(3,4)")), "_xlfn.LAMBDA(_xlpm.a,_xlpm.b,_xlpm.a^2+_xlpm.b)(3,4)");
+        // A name outside the scope stays a plain defined name.
+        assert_eq!(to_file(&Formula::new("LET(x,1,x)+x")), "_xlfn.LET(_xlpm.x,1,_xlpm.x)+x");
+        assert_eq!(from_file("_xlfn.LET(_xlpm.x,1,_xlpm.x+1)"), "LET(x,1,x+1)");
+        assert_eq!(from_file("_xlfn.LAMBDA(_xlpm.a,_xlpm.a*2)(3)"), "LAMBDA(a,a*2)(3)");
+        // A LET-bound LAMBDA called by name.
+        assert_eq!(to_file(&Formula::new("LET(f,LAMBDA(x,x*2),f(3))")), "_xlfn.LET(_xlpm.f,_xlfn.LAMBDA(_xlpm.x,_xlpm.x*2),_xlpm.F(3))");
+        assert_eq!(from_file("_xlfn.LET(_xlpm.f,_xlfn.LAMBDA(_xlpm.x,_xlpm.x*2),_xlpm.f(3))"), "LET(f,LAMBDA(x,x*2),F(3))");
+    }
 }