diff --git a/lib/erlang/runtime.sx b/lib/erlang/runtime.sx
index 4dc1a709..b1f258f2 100644
--- a/lib/erlang/runtime.sx
+++ b/lib/erlang/runtime.sx
@@ -814,3 +814,58 @@
                 (er-mk-atom mod)
                 (er-mk-atom name)))))
         (er-apply-fun (get mod-env name) vs)))))
+
+;; ── gen_server (OTP-lite) ───────────────────────────────────────
+;; A minimal gen_server behaviour — `start_link/2`, `call/2`, `cast/2`,
+;; `stop/1`, plus the receive loop dispatching `Mod:handle_call/3`,
+;; `Mod:handle_cast/2`, `Mod:handle_info/2`. Loaded into the user
+;; module registry on demand via `(er-load-gen-server!)`.
+(define
+  er-gen-server-source
+  "-module(gen_server).
+   start_link(Mod, Args) ->
+     spawn(fun () ->
+       case Mod:init(Args) of
+         {ok, State} -> gen_server:loop(Mod, State);
+         {stop, Reason} -> exit(Reason)
+       end
+     end).
+   call(Pid, Req) ->
+     Ref = make_ref(),
+     Pid ! {'$gen_call', {self(), Ref}, Req},
+     receive {Ref, Reply} -> Reply end.
+   cast(Pid, Msg) ->
+     Pid ! {'$gen_cast', Msg},
+     ok.
+   stop(Pid) ->
+     gen_server:call(Pid, '$gen_stop').
+   loop(Mod, State) ->
+     receive
+       {'$gen_call', {From, Ref}, '$gen_stop'} ->
+         From ! {Ref, ok};
+       {'$gen_call', {From, Ref}, Req} ->
+         case Mod:handle_call(Req, From, State) of
+           {reply, Reply, NewState} ->
+             From ! {Ref, Reply},
+             gen_server:loop(Mod, NewState);
+           {noreply, NewState} ->
+             gen_server:loop(Mod, NewState);
+           {stop, Reason, Reply, NewState} ->
+             From ! {Ref, Reply},
+             exit(Reason)
+         end;
+       {'$gen_cast', Msg} ->
+         case Mod:handle_cast(Msg, State) of
+           {noreply, NewState} -> gen_server:loop(Mod, NewState);
+           {stop, Reason, NewState} -> exit(Reason)
+         end;
+       Other ->
+         case Mod:handle_info(Other, State) of
+           {noreply, NewState} -> gen_server:loop(Mod, NewState);
+           {stop, Reason, NewState} -> exit(Reason)
+         end
+     end.")
+
+(define
+  er-load-gen-server!
+  (fn () (erlang-load-module er-gen-server-source)))
diff --git a/lib/erlang/scoreboard.json b/lib/erlang/scoreboard.json
index 9960afbe..b60173dc 100644
--- a/lib/erlang/scoreboard.json
+++ b/lib/erlang/scoreboard.json
@@ -1,11 +1,11 @@
 {
   "language": "erlang",
-  "total_pass": 415,
-  "total": 415,
+  "total_pass": 425,
+  "total": 425,
   "suites": [
     {"name":"tokenize","pass":62,"total":62,"status":"ok"},
     {"name":"parse","pass":52,"total":52,"status":"ok"},
-    {"name":"eval","pass":231,"total":231,"status":"ok"},
+    {"name":"eval","pass":241,"total":241,"status":"ok"},
     {"name":"runtime","pass":39,"total":39,"status":"ok"},
     {"name":"ring","pass":4,"total":4,"status":"ok"},
     {"name":"ping-pong","pass":4,"total":4,"status":"ok"},
diff --git a/lib/erlang/scoreboard.md b/lib/erlang/scoreboard.md
index 6f9e96f3..ee7e06fd 100644
--- a/lib/erlang/scoreboard.md
+++ b/lib/erlang/scoreboard.md
@@ -1,12 +1,12 @@
 # Erlang-on-SX Scoreboard
 
-**Total: 415 / 415 tests passing**
+**Total: 425 / 425 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | tokenize | 62 | 62 |
 | ✅ | parse | 52 | 52 |
-| ✅ | eval | 231 | 231 |
+| ✅ | eval | 241 | 241 |
 | ✅ | runtime | 39 | 39 |
 | ✅ | ring | 4 | 4 |
 | ✅ | ping-pong | 4 | 4 |
diff --git a/lib/erlang/tests/eval.sx b/lib/erlang/tests/eval.sx
index 530f8faf..93563c28 100644
--- a/lib/erlang/tests/eval.sx
+++ b/lib/erlang/tests/eval.sx
@@ -699,6 +699,75 @@
     (ev "Me = self(), spawn(fun () -> m9:work(Me) end), receive done -> ok end"))
   (er-mk-atom "ok"))
 
+;; ── gen_server (OTP-lite) ──────────────────────────────────────
+(do
+  (er-load-gen-server!)
+  (erlang-load-module
+    "-module(ctr).
+     init(N) -> {ok, N}.
+     handle_call(get, _F, S) -> {reply, S, S}.
+     handle_call({set, V}, _F, _S) -> {reply, ok, V}.
+     handle_call({add, K}, _F, S) -> {reply, S + K, S + K}.
+     handle_cast(inc, S) -> {noreply, S + 1}.
+     handle_cast(dec, S) -> {noreply, S - 1}.
+     handle_cast({add, K}, S) -> {noreply, S + K}.
+     handle_info(_M, S) -> {noreply, S}.")
+  nil)
+
+(er-eval-test "gen_server start + call get"
+  (ev "P = gen_server:start_link(ctr, 10), gen_server:call(P, get)")
+  10)
+
+(er-eval-test "gen_server cast then call"
+  (ev "P = gen_server:start_link(ctr, 0), gen_server:cast(P, inc), gen_server:cast(P, inc), gen_server:cast(P, inc), gen_server:call(P, get)")
+  3)
+
+(er-eval-test "gen_server call returns reply"
+  (ev "P = gen_server:start_link(ctr, 5), gen_server:call(P, {add, 7})")
+  12)
+
+(er-eval-test "gen_server state mutation"
+  (ev "P = gen_server:start_link(ctr, 5), gen_server:call(P, {set, 99}), gen_server:call(P, get)")
+  99)
+
+(er-eval-test "gen_server stop returns ok"
+  (nm (ev "P = gen_server:start_link(ctr, 0), gen_server:stop(P)"))
+  "ok")
+
+(er-eval-test "gen_server cast returns ok immediately"
+  (nm (ev "P = gen_server:start_link(ctr, 0), gen_server:cast(P, inc)"))
+  "ok")
+
+(er-eval-test "gen_server multi-state mutations"
+  (ev "P = gen_server:start_link(ctr, 0), gen_server:cast(P, {add, 100}), gen_server:cast(P, dec), gen_server:cast(P, dec), gen_server:call(P, get)")
+  98)
+
+;; Stack server — exercises a different state shape.
+(do
+  (erlang-load-module
+    "-module(stk).
+     init(_) -> {ok, []}.
+     handle_call(pop, _F, []) -> {reply, empty, []};
+     handle_call(pop, _F, [H | T]) -> {reply, {ok, H}, T};
+     handle_call(peek, _F, []) -> {reply, empty, []};
+     handle_call(peek, _F, [H | T]) -> {reply, {ok, H}, [H | T]};
+     handle_call(size, _F, S) -> {reply, length(S), S}.
+     handle_cast({push, V}, S) -> {noreply, [V | S]}.
+     handle_info(_M, S) -> {noreply, S}.")
+  nil)
+
+(er-eval-test "stack push/pop"
+  (ev "P = gen_server:start_link(stk, ignored), gen_server:cast(P, {push, 1}), gen_server:cast(P, {push, 2}), gen_server:cast(P, {push, 3}), gen_server:call(P, size)")
+  3)
+
+(er-eval-test "stack lifo"
+  (ev "P = gen_server:start_link(stk, ignored), gen_server:cast(P, {push, 1}), gen_server:cast(P, {push, 2}), gen_server:cast(P, {push, 3}), {ok, V} = gen_server:call(P, pop), V")
+  3)
+
+(er-eval-test "stack empty pop"
+  (nm (ev "P = gen_server:start_link(stk, ignored), gen_server:call(P, pop)"))
+  "empty")
+
 (define
   er-eval-test-summary
   (str "eval " er-eval-test-pass "/" er-eval-test-count))
diff --git a/lib/erlang/transpile.sx b/lib/erlang/transpile.sx
index 00afed25..0f4189ae 100644
--- a/lib/erlang/transpile.sx
+++ b/lib/erlang/transpile.sx
@@ -486,10 +486,10 @@
             (er-apply-fun (get env name) vs)
             :else (er-apply-bif name vs)))
         (= (get fun-node :type) "remote")
-        (er-apply-remote-bif
-          (get (get fun-node :mod) :value)
-          (get (get fun-node :fun) :value)
-          (er-eval-args args env))
+        (let
+          ((mod-name (er-resolve-call-name (get fun-node :mod) env "module"))
+            (fn-name (er-resolve-call-name (get fun-node :fun) env "function")))
+          (er-apply-remote-bif mod-name fn-name (er-eval-args args env)))
         :else
         (let
           ((fv (er-eval-expr fun-node env)))
@@ -509,6 +509,24 @@
         (range 0 (len args)))
       out)))
 
+;; Resolve a remote call's module/function reference into a string.
+;; Atom AST nodes use their `:value` directly. For any other shape
+;; (typically a var or another expression), evaluate it and require
+;; the result to be an atom.
+(define
+  er-resolve-call-name
+  (fn
+    (node env kind)
+    (cond
+      (= (get node :type) "atom") (get node :value)
+      :else (let
+        ((v (er-eval-expr node env)))
+        (if
+          (er-atom? v)
+          (get v :name)
+          (error
+            (str "Erlang: call " kind " must be an atom, got " (er-format-value v))))))))
+
 ;; ── fun values ───────────────────────────────────────────────────
 (define
   er-mk-fun
diff --git a/plans/erlang-on-sx.md b/plans/erlang-on-sx.md
index 152c18ec..ace6a470 100644
--- a/plans/erlang-on-sx.md
+++ b/plans/erlang-on-sx.md
@@ -85,7 +85,7 @@ Core mapping:
 
 ### Phase 5 — modules + OTP-lite
 - [x] `-module(M).` loading, `M:F(...)` calls across modules — **10 new eval tests**; multi-arity, sibling calls, cross-module dispatch via `er-modules` registry
-- [ ] `gen_server` behaviour (the big OTP win)
+- [x] `gen_server` behaviour (the big OTP win) — **10 new eval tests**; counter + LIFO stack callback modules driven via `gen_server:start_link/call/cast/stop`
 - [ ] `supervisor` (simple one-for-one)
 - [ ] Registered processes: `register/2`, `whereis/1`
 
@@ -99,6 +99,7 @@ Core mapping:
 
 _Newest first._
 
+- **2026-04-25 gen_server (OTP-lite) green** — `er-gen-server-source` in `lib/erlang/runtime.sx` is the canonical Erlang text of the behaviour; `er-load-gen-server!` registers it in the user-module table. Implements `start_link/2`, `call/2` (sync via `make_ref` + selective `receive {Ref, Reply}`), `cast/2` (async fire-and-forget returning `ok`), `stop/1`, and the receive loop dispatching `{'$gen_call', {From, Ref}, Req}` → `Mod:handle_call/3`, `{'$gen_cast', Msg}` → `Mod:handle_cast/2`, anything else → `Mod:handle_info/2`. handle_call reply tuples supported: `{reply, R, S}`, `{noreply, S}`, `{stop, R, Reply, S}`. handle_cast/info: `{noreply, S}`, `{stop, R, S}`. `Mod:F` and `M:F` where `M` is a runtime variable now work via new `er-resolve-call-name` (was bug: passed unevaluated AST node `:value` to remote dispatch). 10 new eval tests: counter callback module (start/call/cast/stop, repeated state mutations), LIFO stack callback module (`{push, V}` cast, pop returns `{ok, V}` or `empty`, size). Total suite 425/425.
 - **2026-04-25 modules + cross-module calls green** — `er-modules` global registry (`{module-name -> mod-env}`) in `lib/erlang/runtime.sx`. `erlang-load-module SRC` parses a module declaration, groups functions by name (concatenating clauses across arities so multi-arity falls out of `er-apply-fun-clauses`'s arity filter), creates fun-values capturing the same `mod-env` so siblings see each other recursively, registers under `:name`. `er-apply-remote-bif` checks user modules first, then built-ins (`lists`, `io`, `erlang`). `er-eval-call` for atom-typed call targets now consults the current env first — local calls inside a module body resolve sibling functions via `mod-env`. Undefined cross-module call raises `error({undef, Mod, Fun})`. 10 new eval tests: load returns module name, zero-/n-ary cross-module call, recursive fact/6 = 720, sibling-call `c:a/1` ↦ `c:b/1`, multi-arity dispatch (`/1`, `/2`, `/3`), pattern + guard clauses, cross-module call from within another module, undefined fn raises `undef`, module fn used in spawn. Total suite 415/415.
 - **2026-04-25 try/catch/of/after green — Phase 4 complete** — Three new exception markers in runtime: `er-mk-throw-marker`, `er-mk-error-marker` alongside the existing `er-mk-exit-marker`; `er-thrown?`, `er-errored?` predicates. `throw/1` and `error/1` BIFs raise their respective markers. Scheduler step's guard now also catches throw/error: an uncaught throw becomes `exit({nocatch, X})`, an uncaught error becomes `exit(X)`. `er-eval-try` uses two-layer guard: outer captures any exception so the `after` body runs (then re-raises); inner catches throw/error/exit and dispatches to `catch` clauses by class name + pattern + guard. No matching catch clause re-raises with the same class via `er-mk-class-marker`. `of` clauses run on success; no-match raises `error({try_clause, V})`. 19 new eval tests: plain success, all three classes caught, default-class behaviour (throw), of-clause matching incl. fallthrough + guard, after on success/error/value-preservation, nested try, class re-raise wrapping, multi-clause catch dispatch. Total suite 405/405. **Phase 4 complete — Phase 5 (modules + OTP-lite) is next.** Gotcha: SX's `dynamic-wind` doesn't interact with `guard` — exceptions inside dynamic-wind body propagate past the surrounding guard untouched, so the `after`-runs-on-exception semantics had to be wired with two manual nested guards instead.
 - **2026-04-25 exit-signal propagation + trap_exit green** — `process_flag(trap_exit, Bool)` BIF returns the prior value. After every scheduler step that ends with a process dead, `er-propagate-exit!` walks `:monitored-by` (delivers `{'DOWN', Ref, process, From, Reason}` to each monitor + re-enqueues if waiting) and `:links` (with `trap_exit=true` -> deliver `{'EXIT', From, Reason}` and re-enqueue; `trap_exit=false` + abnormal reason -> recursive `er-cascade-exit!`; normal reason without trap_exit -> no signal). `er-sched-step!` short-circuits if the popped pid is already dead (could be cascade-killed mid-drain). 11 new eval tests: process_flag default + persistence, monitor DOWN on normal/abnormal/ref-bound, two monitors both fire, trap_exit catches abnormal/normal, cascade reason recorded on linked proc, normal-link no cascade (proc returns via `after` clause), monitor without trap_exit doesn't kill the monitor. Total suite 386/386. `kill`-as-special-reason and `exit/2` (signal to another) deferred.