diff --git a/TODO.org b/TODO.org
index b92a4f8e..32de7f60 100644
--- a/TODO.org
+++ b/TODO.org
@@ -630,12 +630,6 @@ generic binding — saying =$= marks a
 type variable and naming the bare spelling. A =defn= parameter was already
 refused, as a type in a name slot.
 
-** NEXT A container parameter the function grows is warned at
-Decided 2026-09-25: Odin's behaviour stays — a Vec or Map passed by value is a
-copy of its header, so growth inside the callee does not reach the caller. A
-parameter the function grows (push, put, reserve, anything that can reallocate)
-gets a warning at the parameter suggesting (Ptr ...).
-
 ** CANCELLED not= as a spelling of !=
 CLOSED: [2026-09-25]
 One spelling for one operation; != stays, and not= is refused with a suggestion
diff --git a/lib/check.ml b/lib/check.ml
index aee64ee2..c077e403 100644
--- a/lib/check.ml
+++ b/lib/check.ml
@@ -4150,6 +4150,39 @@ let tracked_call loc env name (tr : Shim.track) ret (args : Tast.expr list) =
 let truthy_failed : (Ast.expr * ctx * Loc.diag) list ref = ref []
 let truthy_depth = ref 0
 
+
+(* A Vec or a Map parameter is a copy of the caller's header — Odin's rule —
+   so growing it reallocates a block only this function's copy points at, and
+   the caller's container never sees the elements. The function being checked
+   and its container parameters, by slot, and the warnings found so far, one
+   per parameter, printed by [build_program]. A stack because a generic's copy
+   is checked from inside the body that called it. *)
+let grow_params : (ctx * (int * Ast.field) list) list ref = ref []
+let grow_warnings : Loc.diag list ref = ref []
+
+let note_grown ctx op loc (target : Tast.expr) =
+  match target.Tast.e, target.Tast.ty, !grow_params with
+  | Tast.Local s, ((Types.Vec _ | Types.Map _) as t), (c, ps) :: _ when c == ctx ->
+    (match List.assoc_opt s ps with
+     | Some (p : Ast.field)
+       when not
+              (List.exists
+                 (fun (d : Loc.diag) -> d.Loc.dloc = p.Ast.floc)
+                 !grow_warnings) ->
+       let ts = Types.to_string t in
+       grow_warnings :=
+         Loc.diag ~kind:"check/grown-parameter" p.Ast.floc
+           (Printf.sprintf
+              "%s is a %s passed by value, a copy of the caller's header, so \
+               the %s at %s grows this function's copy and the caller's \
+               container never sees it. Take it as (Ptr %s) and write (%s \
+               (deref %s) ...), and each caller passes (addr c) for its \
+               container c"
+              p.Ast.fname ts op (Loc.to_string loc) ts op p.Ast.fname)
+         :: !grow_warnings
+     | _ -> ())
+  | _ -> ()
+
 let rec check ctx ?want (e : Ast.expr) : Tast.expr =
   let place = ctx.place_ok in
   ctx.place_ok <- false;
@@ -9197,6 +9230,7 @@ and named_call ?(qualified = false) ctx ~want loc name args =
               [ target; check ctx ~want:Types.Dyn x; here loc ])
        else begin
        let elem = vec_elem loc "push" target.Tast.ty in
+       note_grown ctx "push" loc target;
        let x = check ctx ~want:elem x in
        (* The element is bound before the loop so that a [retry] re-attempts
           the allocation and not the expression that produced the value. *)
@@ -9228,6 +9262,7 @@ and named_call ?(qualified = false) ctx ~want loc name args =
        let target = check_target ctx target in
        refuse_const_change ctx loc target;
        let n = check ctx ~want:index_ty n in
+       note_grown ctx "reserve" loc target;
        let n64 =
          mk loc (Types.Int Types.I64) (Tast.Prim (Tast.Cast (Types.Int Types.I64), [ n ]))
        in
@@ -9524,6 +9559,7 @@ and named_call ?(qualified = false) ctx ~want loc name args =
                 check ctx ~want:Types.Dyn v; here loc ])
        else begin
        let kt, vt = map_kv loc "put" target.Tast.ty in
+       note_grown ctx "put" loc target;
        let k = check ctx ~want:kt k in
        let v = check ctx ~want:vt v in
        (* Deferred: the arguments are checked — so a move here is still a move
@@ -12866,6 +12902,15 @@ let rec check_fn env (fn : Ast.fn) : Tast.fn =
        end;
        ignore (bind ctx p.Ast.fname ty ~assignable:false))
     fn.Ast.params params;
+  let grow_saved = !grow_params in
+  grow_params :=
+    ( ctx,
+      List.filter_map
+        (fun (p : Ast.field) ->
+           Option.map (fun b -> (b.slot, p)) (List.assoc_opt p.Ast.fname ctx.scope))
+        fn.Ast.params )
+    :: grow_saved;
+  Fun.protect ~finally:(fun () -> grow_params := grow_saved) @@ fun () ->
   let body =
     match fn.Ast.fbody with
     | [] ->
@@ -14158,6 +14203,7 @@ let build_program ~keep_going ?tolerate (decls : Ast.decl list) :
      time it runs every signature is sound, so a body that fails to check
      cannot make the next body fail — which is what makes a declaration a
      resync point that needs no resynchronising. *)
+  grow_warnings := [];
   let decls = collect env decls in
   if !print_warnings then
     List.iter
@@ -14211,6 +14257,12 @@ let build_program ~keep_going ?tolerate (decls : Ast.decl list) :
          | _ -> None)
       decls
   in
+  if !print_warnings then
+    List.iter
+      (fun (d : Loc.diag) ->
+         prerr_endline
+           (Loc.entry ~mark:'~' ~label:"warning: " d.Loc.dloc d.Loc.dmsg))
+      (List.rev !grow_warnings);
   Loc.finish s;
   (* The handler clauses lifted out along the way. They are ordinary functions
      from here down; nothing in the backend knows they were written inside
diff --git a/test/programs/grow-param.flan b/test/programs/grow-param.flan
new file mode 100644
index 00000000..a3f12362
--- /dev/null
+++ b/test/programs/grow-param.flan
@@ -0,0 +1,23 @@
+;;;; A container parameter is a copy of the caller's header. Growing it grows
+;;;; the copy, so the caller's container does not see the push; the function
+;;;; is warned at, at the parameter, and the fix it names is the (Ptr ...)
+;;;; below, which reaches the caller's own header.
+
+(defn add-copy [v (Vec i32)] () (push v 1) (free v))
+(defn add-ptr [v (Ptr (Vec i32))] () (push (deref v) 2))
+(defn put-ptr [m (Ptr (Map i32 i32))] () (put (deref m) 7 8))
+
+(defn main [] i32
+  (let [v (vec-new i32)
+        m (map-new i32 i32)]
+    (add-copy v)
+    (println (length v))                         ; 0
+    (add-ptr (addr v))
+    (add-ptr (addr v))
+    (println (length v))                         ; 2
+    (println (at v 1))                           ; 2
+    (put-ptr (addr m))
+    (println (length m))                         ; 1
+    (free v)
+    (free m))
+  0)
diff --git a/test/test_acceptance.ml b/test/test_acceptance.ml
index 23d96f21..b3869e17 100644
--- a/test/test_acceptance.ml
+++ b/test/test_acceptance.ml
@@ -622,6 +622,10 @@ let () =
       "7 8 9 \n2 4 6 8 10 12 \n4 8 12 \n21\n6 2 4 \n3 1 2 \n3 4 \n2 4 \n1\n";
     (* The fix into's refusal names for owning elements, (map clone): the
        copy's inner Vec grows on the heap and the source's is untouched. *)
+    (* A grown container parameter reaches the caller only through a Ptr. *)
+    outputs "a grown parameter" "programs/grow-param.flan" "0\n2\n2\n1\n";
+    outputs ~x86:true "a grown parameter, --x86" "programs/grow-param.flan"
+      "0\n2\n2\n1\n";
     outputs "into with (map clone)" "programs/into-owning.flan" "1\n1\n101\n99\n";
     outputs ~x86:true "into with (map clone), --x86" "programs/into-owning.flan"
       "1\n1\n101\n99\n";
diff --git a/test/test_flan.ml b/test/test_flan.ml
index 1e20a2e4..6150470e 100644
--- a/test/test_flan.ml
+++ b/test/test_flan.ml
@@ -5837,6 +5837,39 @@ let () =
           (Printf.sprintf "one pairing warning, not %d" (List.length ds))
           false)
    | exception Loc.Error _ -> check "the paired program checks" false);
+  (* A Vec or Map parameter is the caller's header copied, so growing it is
+     warned at the parameter, once, naming the (Ptr ...) that reaches the
+     caller's own. A pointer parameter and a local are not warned at. The
+     running side is programs/grow-param.flan. *)
+  let grown src =
+    match checked src with
+    | _ -> Some !Check.grow_warnings
+    | exception Loc.Error _ -> None
+  in
+  (match
+     grown "(defn f [v (Vec i32) m (Map i32 i32)] ()\n\
+            \  (push v 1) (reserve v 8) (put m 1 2))"
+   with
+   | Some [ dm; dv ] ->
+     check "a grown Vec parameter is warned at the parameter"
+       (dv.Loc.kind = "check/grown-parameter"
+        && dv.Loc.dloc.Loc.line = 1 && dv.Loc.dloc.Loc.col = 10
+        && dv.Loc.dmsg
+           = "v is a (Vec i32) passed by value, a copy of the caller's header, \
+              so the push at <test>:2:3 grows this function's copy and the \
+              caller's container never sees it. Take it as (Ptr (Vec i32)) \
+              and write (push (deref v) ...), and each caller passes (addr c) \
+              for its container c");
+     check "and a grown Map parameter names put"
+       (dm.Loc.dloc.Loc.col = 22
+        && Test_support.contains dm.Loc.dmsg "the put at <test>:2:28")
+   | Some ds ->
+     check (Printf.sprintf "two grow warnings, not %d" (List.length ds)) false
+   | None -> check "the grown-parameter program checks" false);
+  check "a pointer parameter and a local are not warned at"
+    (grown "(defn f [v (Ptr (Vec i32))] ()\n\
+            \  (push (deref v) 1) (let [w (vec-new i32)] (push w 1) (free w)))"
+     = Some []);
   check "a program that shadows nothing is warned at not at all"
     (Check.shadowed_builtins (program "(defn f [] i32 1)") = []);
   (* A prelude function's name is taken over the same way, for the calls in