diff --git a/README.md b/README.md
index edec82a..7f62239 100644
--- a/README.md
+++ b/README.md
@@ -151,7 +151,7 @@ Pick the guide that fits you:
 - [x] A standard library in Finch: `json`, `path`, `time`, `proc`, `fs`
 - [x] `const`, `if` as a value, `"\(x)"` in text, `"""` text, C unions and `#define` expressions
 - [x] `test` blocks and `finch test`, `finch fmt`, `finch.toml`, a build cache, the chain of calls on runtime errors
-- [x] Threads (`import thread`) and atomics; C++ files; `--target android` (built, not yet tried on a device)
+- [x] Threads (`import thread`) and atomics; C++ files; `--target android` (compiles; NDK linking and devices untried)
 
 ## Layout
 
diff --git a/README.pl.md b/README.pl.md
index b4c2651..52b438d 100644
--- a/README.pl.md
+++ b/README.pl.md
@@ -151,7 +151,7 @@ Wybierz przewodnik dla siebie:
 - [x] Biblioteka standardowa w Finchu: `json`, `path`, `time`, `proc`, `fs`
 - [x] `const`, `if` jako wartość, `"\(x)"` w tekście, tekst `"""`, unie C i wyrażenia `#define`
 - [x] Bloki `test` i `finch test`, `finch fmt`, `finch.toml`, cache budowania, łańcuch wywołań przy błędach
-- [x] Wątki (`import thread`) i operacje atomowe; pliki C++; `--target android` (budowane, jeszcze nie sprawdzone na urządzeniu)
+- [x] Wątki (`import thread`) i operacje atomowe; pliki C++; `--target android` (kompiluje się; linkowanie z NDK i urządzenia niesprawdzone)
 
 ## Struktura
 
diff --git a/docs/en/cheatsheet.md b/docs/en/cheatsheet.md
index 3497dc6..d27262c 100644
--- a/docs/en/cheatsheet.md
+++ b/docs/en/cheatsheet.md
@@ -327,7 +327,7 @@ Automatic only when nothing is lost: smaller → bigger number, any int → floa
 
 - Arrays, text and structs free themselves at the end of their block. No `free` needed.
 - `b := a` copies; a call's result moves; `return x` moves.
-- Passing a struct: `fn f(Body b)` works on its own copy (the caller's stays as it was), `fn f(ptr[Body] b)` changes the caller's (`f(body)` or `f(addr(body))`), a method gets `self` by address, never a copy.
+- Passing a struct: `fn f(Body b)` works on its own copy (the caller's stays as it was), `fn f(ptr[Body] b)` changes the caller's (`f(body)` or `f(addr(body))`), a method gets `self` by address, never a copy. A struct with 256+ bytes of numbers passed by value or in `for x in list` gets a warning.
 - `new(...)` + `free(...)` (often with `defer`) for linked lists, trees, sharing.
 - C memory: C's own functions (`malloc` / `free`, `fclose`, …).
 
diff --git a/docs/en/for-engineers.md b/docs/en/for-engineers.md
index d80aefc..56ce0a9 100644
--- a/docs/en/for-engineers.md
+++ b/docs/en/for-engineers.md
@@ -503,6 +503,13 @@ makes the C-callable thunk. `atomic_add/sub/swap` lower to `atomicrmw add/sub/xc
 take a `ptr[T]` to any integer type. Finch's arrays, text and maps are not synchronised; sharing one between threads
 needs a mutex.
 
+### Warnings
+
+`warn()` prints `file:line:col: warning: …` to stderr (so `finch ir` stays pure IR) or, with `index`, an `@W` record the
+language server publishes with severity 2. One per source position. Two are given: a struct parameter, and the element of
+`for x in list`, whose plain data (`byteSize()`: numbers, nested structs, C `[N]T`; `str`/arrays/maps count 0, as only
+their handle is copied) is 256 bytes or more.
+
 ### Struct arguments by address
 
 `argWant()` (call arguments only, not assignments): when a parameter is `ptr[S]` and the argument is a plain variable
diff --git a/docs/en/for-technicians.md b/docs/en/for-technicians.md
index 2615007..9e41283 100644
--- a/docs/en/for-technicians.md
+++ b/docs/en/for-technicians.md
@@ -1134,11 +1134,20 @@ Not there yet:
 - Interfaces.
 - Unicode-aware text: `.len`, `s[i]` and `upper()` work on bytes / ASCII.
 - Checks that stop two threads from changing the same array at once (use a mutex).
-- Windows on ARM64, 32-bit systems, WebAssembly. Android is built (`--target android`) but not yet tried on a device.
+- Windows on ARM64, 32-bit systems, WebAssembly. `--target android` makes code for Android, but linking with the NDK and running on a device are untried.
 
-On purpose, not planned: a warning for every big struct that gets copied. A copy in Finch is always visible in the
-code (`b := a`, passing a struct to a `Body b` parameter), a method never copies `self`, and `ptr[Body]` (or passing
-the variable to a `ptr[Body]` parameter) shares instead. A warning that fires on ordinary code would be noise.
+### Warnings about big copies
+
+A struct with 256 bytes or more of plain data (numbers, structs of numbers, C's `[N]T`) is copied byte by byte when it
+is a parameter (`fn f(Body b)`) and in `for x in bodies`. Finch says so, and still builds the program:
+
+```
+game.fch:27:11: warning: 'b' is a Body (264 bytes), copied on every call to weight; take ptr[Body] to share it instead (callers can still pass the variable as it is)
+game.fch:45:5: warning: each Body (264 bytes) is copied into 'x'; for i in 0..bodies.len with bodies[i] works on it in place
+```
+
+Text, arrays and maps inside a struct don't count: passing the struct copies only their handles, not what they hold.
+`b := a` is not warned about: it says "copy" in the code. A method never copies `self`. Editors show the warnings in yellow.
 
 See the [roadmap in the README](../../README.md#roadmap).
 
@@ -1283,5 +1292,6 @@ Write the header (`float path_len(float x, float y);`, or let `cbindgen` write i
 
 **Android**: `finch build game.fch --target android --shared` makes `libgame.so` for 64-bit ARM Android (API 24 and
 up) with the NDK's clang, found through `ANDROID_NDK_HOME` (or `FINCH_CC`). Load it from an app, for example through
-SDL3's Android project (the library's `SDL_main` is an `export fn`). This is new: it is built and checked here, but not
-yet tried on a device.
+SDL3's Android project (the library's `SDL_main` is an `export fn`). This is new and only partly checked: Finch's
+output compiles for Android (thread-local data goes through the emulated TLS older Android needs), but linking with the
+NDK and running on a device have not been tried yet.
diff --git a/docs/pl/dla-inzynierow.md b/docs/pl/dla-inzynierow.md
index a8319f6..c948e86 100644
--- a/docs/pl/dla-inzynierow.md
+++ b/docs/pl/dla-inzynierow.md
@@ -496,6 +496,13 @@ thunk wołany z C. `atomic_add/sub/swap` to `atomicrmw add/sub/xchg … seq_cst`
 `ptr[T]` do dowolnej liczby całkowitej. Tablice, teksty i mapy Fincha nie są synchronizowane; dzielenie ich między
 wątkami wymaga mutexu.
 
+### Ostrzeżenia
+
+`warn()` wypisuje `plik:linia:kolumna: warning: …` na stderr (więc `finch ir` daje samo IR) albo, w trybie `index`,
+rekord `@W`, który serwer języka publikuje z ważnością 2. Jedno na miejsce w kodzie. Są dwa: parametr-struktura oraz
+element `for x in lista`, gdy ich zwykłe dane (`byteSize()`: liczby, zagnieżdżone struktury, `[N]T` z C; `str`, tablice
+i mapy liczą się jako 0, bo kopiowany jest tylko ich uchwyt) mają 256 bajtów lub więcej.
+
 ### Struktura jako argument przez adres
 
 `argWant()` (tylko argumenty wywołań, nie przypisania): gdy parametr to `ptr[S]`, a argument to zwykła zmienna typu `S`,
diff --git a/docs/pl/dla-technikow.md b/docs/pl/dla-technikow.md
index cfd1021..4ebceba 100644
--- a/docs/pl/dla-technikow.md
+++ b/docs/pl/dla-technikow.md
@@ -1138,12 +1138,22 @@ Jeszcze nie ma:
 - Interfejsów.
 - Tekstu świadomego Unicode: `.len`, `s[i]` i `upper()` działają na bajtach / ASCII.
 - Pilnowania, żeby dwa wątki nie zmieniały tej samej tablicy naraz (do tego mutex).
-- Windowsa na ARM64, systemów 32-bitowych, WebAssembly. Android jest budowany (`--target android`), ale jeszcze nie
-  sprawdzony na urządzeniu.
+- Windowsa na ARM64, systemów 32-bitowych, WebAssembly. `--target android` robi kod dla Androida, ale linkowanie
+  z NDK i uruchomienie na urządzeniu nie były jeszcze próbowane.
 
-Celowo nie planowane: ostrzeżenie przy każdym kopiowaniu dużej struktury. Kopię w Finchu zawsze widać w kodzie
-(`b := a`, struktura podana do parametru `Cialo c`), metoda nigdy nie kopiuje `self`, a `ptr[Cialo]` (albo zmienna
-podana do parametru `ptr[Cialo]`) współdzieli zamiast kopiować. Ostrzeżenie przy zwykłym kodzie byłoby szumem.
+### Ostrzeżenia o dużych kopiach
+
+Struktura z 256 lub więcej bajtami zwykłych danych (liczby, struktury z liczb, `[N]T` z C) jest kopiowana bajt po bajcie,
+gdy jest parametrem (`fn f(Cialo c)`) i w `for x in ciala`. Finch o tym mówi, a program i tak buduje:
+
+```
+gra.fch:27:11: warning: 'c' is a Cialo (264 bytes), copied on every call to waga; take ptr[Cialo] to share it instead (callers can still pass the variable as it is)
+gra.fch:45:5: warning: each Cialo (264 bytes) is copied into 'x'; for i in 0..ciala.len with ciala[i] works on it in place
+```
+
+Teksty, tablice i mapy w strukturze się nie liczą: przekazanie struktury kopiuje tylko ich uchwyty, nie zawartość.
+`b := a` nie dostaje ostrzeżenia: w kodzie widać, że to kopia. Metoda nigdy nie kopiuje `self`. Edytory pokazują
+ostrzeżenia na żółto.
 
 Plan rozwoju jest w [README](../../README.pl.md#plan-rozwoju).
 
@@ -1288,5 +1298,6 @@ Napisz nagłówek (`float path_len(float x, float y);` albo niech napisze go `cb
 
 **Android**: `finch build gra.fch --target android --shared` robi `libgra.so` dla 64-bitowego Androida na ARM (API 24
 i wyżej) clangiem z NDK, znalezionym przez `ANDROID_NDK_HOME` (albo `FINCH_CC`). Wczytaj go z aplikacji, na przykład
-przez projekt Androida z SDL3 (`SDL_main` biblioteki to `export fn`). To nowość: jest budowane i sprawdzane tutaj, ale
-jeszcze nie na urządzeniu.
+przez projekt Androida z SDL3 (`SDL_main` biblioteki to `export fn`). To nowość, sprawdzona tylko częściowo: kod Fincha
+kompiluje się dla Androida (dane wątków idą przez emulowane TLS, którego wymaga starszy Android), ale linkowanie z NDK
+i uruchomienie na urządzeniu nie były jeszcze próbowane.
diff --git a/docs/pl/sciaga.md b/docs/pl/sciaga.md
index 6123cf1..cb4e7e7 100644
--- a/docs/pl/sciaga.md
+++ b/docs/pl/sciaga.md
@@ -327,7 +327,7 @@ Automatycznie tylko wtedy, gdy nic nie ginie: mniejsza → większa liczba, dowo
 
 - Tablice, teksty i struktury zwalniają się same na końcu swojego bloku. Bez `free`.
 - `b := a` kopiuje; wynik wywołania jest przenoszony; `return x` przenosi.
-- Przekazywanie struktury: `fn f(Cialo c)` pracuje na własnej kopii (ta u wywołującego się nie zmienia), `fn f(ptr[Cialo] c)` zmienia tę u wywołującego (`f(cialo)` albo `f(addr(cialo))`), metoda dostaje `self` przez adres, nigdy kopię.
+- Przekazywanie struktury: `fn f(Cialo c)` pracuje na własnej kopii (ta u wywołującego się nie zmienia), `fn f(ptr[Cialo] c)` zmienia tę u wywołującego (`f(cialo)` albo `f(addr(cialo))`), metoda dostaje `self` przez adres, nigdy kopię. Struktura z 256+ bajtami liczb przekazana przez wartość albo w `for x in lista` dostaje ostrzeżenie.
 - `new(...)` + `free(...)` (często z `defer`) do list wiązanych, drzew, współdzielenia.
 - Pamięć z C: funkcjami z C (`malloc` / `free`, `fclose`, …).
 
diff --git a/editors/vscode/CHANGELOG.md b/editors/vscode/CHANGELOG.md
index 563cf05..195374b 100644
--- a/editors/vscode/CHANGELOG.md
+++ b/editors/vscode/CHANGELOG.md
@@ -5,6 +5,7 @@
 - Highlighting for code inside text (`"hp \(hp)"`), `test "…" { }` blocks, `const`, `"""` text, `1e9`.
 - Built-ins `min`, `max`, `abs`, `clamp`, `assert` and `atomic_…` highlighted and described on hover.
 - Format Document (the language server runs `finch fmt` on the open file).
+- Warnings in yellow: a big struct copied on every call or in `for x in list`.
 
 ## 2.4.0
 
diff --git a/src/gen.fch b/src/gen.fch
index a016f49..4768440 100644
--- a/src/gen.fch
+++ b/src/gen.fch
@@ -7,6 +7,7 @@ import ast
 import docs
 import cimport
 import platform
+import "stdio.h"
 
 struct Var {
     str name
@@ -166,6 +167,7 @@ struct Ref {
 
 struct Gen {
     []ast.Program progs
+    []str warned        // file:line:col of the warnings given
     bool trace          // each function keeps a frame { prev, name, file, line } in @finch_trace
     str frame           // the current function's frame ("" in helpers)
     str frameLine       // where its line goes
@@ -337,6 +339,58 @@ fn fail(ptr[Gen] g, ast.Node n, str msg) {
     exit(1)
 }
 
+// A warning: the program is still built. On stderr (finch ir's output stays the IR); an @W record for editors.
+fn warn(ptr[Gen] g, ast.Node n, str msg) {
+    where := g.files[n.file] + ":" + str(n.line) + ":" + str(n.col)
+    if g.warned.contains(where) {
+        return                      // once per place (a generic function is built more than once)
+    }
+    g.warned.push(where)
+    if g.index {
+        g.idx.push("@W\t" + g.files[n.file] + "\t" + str(n.line) + "\t" + str(n.col) + "\t" + msg)
+        return
+    }
+    fputs(where + ": warning: " + msg + "\n", stderr)
+}
+
+// About how many bytes of plain data (numbers, structs of them, C's [N]T) a value has, for the copy warnings
+fn byteSize(ptr[Gen] g, ast.Type t) -> int {
+    k := t.kind
+    if k == "struct" {
+        sd := g.structs[findStruct(g, t.module, t.name)]
+        if sd.isC && sd.csize > 0 {
+            return sd.csize
+        }
+        n := 0
+        for ft in sd.ftypes {
+            n += byteSize(g, ft)
+        }
+        return n
+    }
+    if k == "fixed" && t.elem.len > 0 {
+        return t.count * byteSize(g, t.elem[0])
+    }
+    if k == "str" || k == "array" || k == "map" {
+        return 0                    // only its { ptr, len, cap } is copied when it is passed: not counted
+    }
+    if k == "ptr" || k == "enum" || k == "int" || k == "float" {
+        return 8
+    }
+    return bitsOf(t) / 8 + int(bitsOf(t) == 1)
+}
+
+// a struct this big passed or looped over by value is copied byte by byte each time
+fn bigCopy(ptr[Gen] g, ast.Type t) -> int {
+    if t.kind != "struct" {
+        return 0
+    }
+    n := byteSize(g, t)
+    if n >= 256 {
+        return n
+    }
+    return 0
+}
+
 fn emit(ptr[Gen] g, str line) {
     // with -g every call in a function that has debug info has to say where it is
     if g.dbgScope >= 0 && line.contains("call ") {
@@ -2306,6 +2360,10 @@ fn define(ptr[Gen] g, int si) {
     }
     for k in 0..g.cur.params.len {
         pt := g.cur.params[k]
+        big := bigCopy(g, pt)
+        if big > 0 {
+            warn(g, f.kids[k], "'" + g.cur.pnames[k] + "' is a " + ast.show(pt) + " (" + str(big) + " bytes), copied on every call to " + f.name + "; take ptr[" + ast.show(pt) + "] to share it instead (callers can still pass the variable as it is)")
+        }
         params.push(llty(g, pt) + " %p" + str(k))
         v := "%p" + str(k)
         if owning(g, pt) && !g.cur.borrow[k] {
@@ -2714,6 +2772,14 @@ fn eachStmt(ptr[Gen] g, ast.Node s) {
         et = lt.elem[0]
     }
     copyElems := r.isPlace && owning(g, et) && mutates(g, rootVar(s.kids[0]), s.kids[1])
+    big := bigCopy(g, et)
+    if big > 0 && s.text.len == 0 {
+        list := "list"
+        if s.kids[0].kind == "var" {
+            list = s.kids[0].name
+        }
+        warn(g, s, "each " + ast.show(et) + " (" + str(big) + " bytes) is copied into '" + s.name + "'; for i in 0.." + list + ".len with " + list + "[i] works on it in place")
+    }
     iv := alloca(g, "i64")
     store(g, "i64", "0", iv)
     cond := newLabel(g, "each.cond")
diff --git a/src/lsp.fch b/src/lsp.fch
index ea06340..0bc0fcc 100644
--- a/src/lsp.fch
+++ b/src/lsp.fch
@@ -83,6 +83,7 @@ struct Analysis {
     []str imports
     []str importKeys    // the module each import is: btn -> ui/button
     Diag diag
+    []Diag warnings     // shown in yellow; the program still builds
 }
 
 struct Server {
@@ -324,6 +325,8 @@ fn parseOutput(str output) -> Analysis {
                 a.structs[k].types.push(f[4])
             } else if tag == "@E" && f.len >= 7 {
                 a.enums.push(EnumRec(module: f[1], name: f[2], file: f[3], line: intOf(f[4]), col: intOf(f[5]), variants: f[6].split(",")))
+            } else if tag == "@W" && f.len >= 5 {
+                a.warnings.push(Diag(has: true, file: f[1], line: intOf(f[2]), col: intOf(f[3]), message: f[4]))
             } else if tag == "@I" && f.len >= 2 {
                 a.imports.push(f[1])
                 if f.len >= 3 {
@@ -399,6 +402,27 @@ fn Server.publish(str path, Analysis a) {
             list.push(d)
             now.push(f)
         }
+        for w in a.warnings {
+            if w.file == f {
+                d := json.new_object()
+                wl := w.line - 1
+                if wl < 0 {
+                    wl = 0
+                }
+                wc := w.col - 1
+                if wc < 0 {
+                    wc = 0
+                }
+                d.set("range", range(wl, wc, wc + 1))
+                d.set("severity", num(2))
+                d.set("source", json.new_text("finch"))
+                d.set("message", json.new_text(w.message))
+                list.push(d)
+                if !now.contains(f) {
+                    now.push(f)
+                }
+            }
+        }
         p := json.new_object()
         p.set("uri", json.new_text(pathToUri(f)))
         p.set("diagnostics", list)
diff --git a/tests/cross.sh b/tests/cross.sh
index c4ae87e..8c03d2b 100755
--- a/tests/cross.sh
+++ b/tests/cross.sh
@@ -34,7 +34,8 @@ for f in run/*.fch new/*.fch; do
         failed=$((failed + 1)); echo "FAIL (build) $f"; head -5 "$tmp/err"; continue
     fi
     got=$(run_exe "$exe" "$input")
-    if [ "$got" == "$(tr -d '\r' < "${f%.fch}.out")" ]; then
+    # (warnings come from the build, which is not part of what the program prints)
+    if [ "$got" == "$(tr -d '\r' < "${f%.fch}.out" | grep -v ': warning: ')" ]; then
         pass=$((pass + 1))
     else
         failed=$((failed + 1)); echo "FAIL $f"; diff <(echo "$got") "${f%.fch}.out" | head -10
diff --git a/tests/lsp_test.py b/tests/lsp_test.py
index ab41ec7..ef579a0 100755
--- a/tests/lsp_test.py
+++ b/tests/lsp_test.py
@@ -193,6 +193,15 @@ c.notify("textDocument/didChange", {"textDocument": {"uri": uri2, "version": 4},
 diags = c.diagnostics(uri2)
 check("error: unhandled failure", diags and "can fail" in diags[0]["message"], diags)
 
+# a big struct copied on every call: a warning (yellow), not an error
+src4 = "struct V {\n" + "".join("    float f%d\n" % i for i in range(40)) + "}\n\nfn len2(V v) -> float {\n    return v.f0\n}\n\nfn main() {\n    print(len2(V()))\n}\n"
+path4 = os.path.join(d, "big.fch")
+open(path4, "w").write(src4)
+uri4 = pathlib.Path(path4).resolve().as_uri()
+c.notify("textDocument/didOpen", {"textDocument": {"uri": uri4, "languageId": "finch", "version": 1, "text": src4}})
+w4 = c.diagnostics(uri4)
+check("warning for a big copy", w4 and len(w4) == 1 and w4[0]["severity"] == 2 and "copied on every call" in w4[0]["message"] and w4[0]["range"]["start"]["line"] == 43, w4)
+
 # a module imported under another name: completion after its name
 src3 = "import path as pt\n\nfn main() {\n    print(pt.base(\"a/b\"))\n}\n"
 path3 = os.path.join(d, "alias.fch")
diff --git a/tests/new/big_copies.fch b/tests/new/big_copies.fch
new file mode 100644
index 0000000..f4a3eb0
--- /dev/null
+++ b/tests/new/big_copies.fch
@@ -0,0 +1,52 @@
+// a struct of 256 bytes or more copied without the code saying so: a warning (the program still builds)
+struct M4 {
+    float a0
+    float a1
+    float a2
+    float a3
+    float b0
+    float b1
+    float b2
+    float b3
+    float c0
+    float c1
+    float c2
+    float c3
+    float d0
+    float d1
+    float d2
+    float d3
+}
+
+struct Body {
+    M4 world
+    M4 local
+    int id
+}
+
+fn weight(Body b) -> float {            // warned: 264 bytes copied on every call
+    return b.world.a0 + float(b.id)
+}
+
+fn nudge(ptr[Body] b) {                 // shared: no warning
+    b.world.a0 += 1.0
+}
+
+fn small(M4 m) -> float {               // 128 bytes: no warning
+    return m.a0
+}
+
+fn main() {
+    b := Body(id: 2)
+    nudge(b)
+    print(weight(b), small(b.world))
+    bodies := [b, b]
+    total := 0.0
+    for x in bodies {                   // warned: each element copied into x
+        total += x.world.a0
+    }
+    for i in 0..bodies.len {            // in place: no warning
+        total += bodies[i].world.a0
+    }
+    print(total)
+}
diff --git a/tests/new/big_copies.out b/tests/new/big_copies.out
new file mode 100644
index 0000000..78da95f
--- /dev/null
+++ b/tests/new/big_copies.out
@@ -0,0 +1,4 @@
+new/big_copies.fch:27:11: warning: 'b' is a Body (264 bytes), copied on every call to weight; take ptr[Body] to share it instead (callers can still pass the variable as it is)
+new/big_copies.fch:45:5: warning: each Body (264 bytes) is copied into 'x'; for i in 0..bodies.len with bodies[i] works on it in place
+3 1
+4