diff --git a/lib/erlang/runtime.sx b/lib/erlang/runtime.sx
index cb028579..e7571c03 100644
--- a/lib/erlang/runtime.sx
+++ b/lib/erlang/runtime.sx
@@ -324,6 +324,29 @@
   er-bif-is-reference
   (fn (vs) (er-bool (er-ref? (er-bif-arg1 vs "is_reference")))))
 
+(define
+  er-bif-process-flag
+  (fn
+    (vs)
+    (if
+      (not (= (len vs) 2))
+      (error "Erlang: process_flag/2: arity")
+      (let
+        ((flag (nth vs 0))
+          (val (nth vs 1))
+          (me (er-sched-current-pid)))
+        (cond
+          (and (er-atom? flag) (= (get flag :name) "trap_exit"))
+          (let
+            ((old (er-proc-field me :trap-exit)))
+            (er-proc-set! me :trap-exit (er-truthy? val))
+            (er-bool old))
+          :else (error
+            (str
+              "Erlang: process_flag: unsupported flag '"
+              (er-format-value flag)
+              "'")))))))
+
 (define
   er-bif-make-ref
   (fn
@@ -551,6 +574,14 @@
 
 (define
   er-sched-step!
+  (fn
+    (pid)
+    (cond
+      (= (er-proc-field pid :state) "dead") nil
+      :else (er-sched-step-alive! pid))))
+
+(define
+  er-sched-step-alive!
   (fn
     (pid)
     (er-sched-set-current! pid)
@@ -578,10 +609,103 @@
             (er-proc-set! pid :state "dead")
             (er-proc-set! pid :exit-reason (get r :reason))
             (er-proc-set! pid :exit-result nil)
-            (er-proc-set! pid :continuation nil))
+            (er-proc-set! pid :continuation nil)
+            (er-propagate-exit! pid (get r :reason)))
           :else (do
             (er-proc-set! pid :state "dead")
             (er-proc-set! pid :exit-reason (er-mk-atom "normal"))
             (er-proc-set! pid :exit-result r)
-            (er-proc-set! pid :continuation nil)))))
+            (er-proc-set! pid :continuation nil)
+            (er-propagate-exit! pid (er-mk-atom "normal"))))))
     (er-sched-set-current! nil)))
+
+;; ── exit-signal propagation ─────────────────────────────────────
+;; Called when `pid` finishes (normally or via exit). Walks the
+;; process's `:monitored-by` and `:links` lists to deliver `{'DOWN'}`
+;; messages and exit signals respectively. Linked processes without
+;; `trap_exit` cascade-die with the same reason; those with
+;; `trap_exit` true receive an `{'EXIT', From, Reason}` message.
+(define
+  er-propagate-exit!
+  (fn
+    (pid reason)
+    (er-fire-monitors! pid reason)
+    (er-fire-links! pid reason)))
+
+(define
+  er-fire-monitors!
+  (fn
+    (pid reason)
+    (let
+      ((mons (er-proc-field pid :monitored-by)))
+      (for-each
+        (fn
+          (i)
+          (let
+            ((m (nth mons i)))
+            (let
+              ((from (get m :from)) (ref (get m :ref)))
+              (when
+                (and (er-proc-exists? from)
+                  (not (= (er-proc-field from :state) "dead")))
+                (let
+                  ((msg
+                    (er-mk-tuple
+                      (list
+                        (er-mk-atom "DOWN")
+                        ref
+                        (er-mk-atom "process")
+                        pid
+                        reason))))
+                  (er-proc-mailbox-push! from msg)
+                  (when
+                    (= (er-proc-field from :state) "waiting")
+                    (er-proc-set! from :state "runnable")
+                    (er-sched-enqueue! from)))))))
+        (range 0 (len mons))))))
+
+(define
+  er-fire-links!
+  (fn
+    (pid reason)
+    (let
+      ((links (er-proc-field pid :links))
+        (is-normal (er-is-atom-named? reason "normal")))
+      (for-each
+        (fn
+          (i)
+          (let
+            ((target (nth links i)))
+            (when
+              (and (er-proc-exists? target)
+                (not (= (er-proc-field target :state) "dead")))
+              (let
+                ((trap (er-proc-field target :trap-exit)))
+                (cond
+                  trap (er-deliver-exit-msg! target pid reason)
+                  is-normal nil
+                  :else (er-cascade-exit! target reason))))))
+        (range 0 (len links))))))
+
+(define
+  er-deliver-exit-msg!
+  (fn
+    (target from reason)
+    (let
+      ((msg
+        (er-mk-tuple (list (er-mk-atom "EXIT") from reason))))
+      (er-proc-mailbox-push! target msg)
+      (when
+        (= (er-proc-field target :state) "waiting")
+        (er-proc-set! target :state "runnable")
+        (er-sched-enqueue! target)))))
+
+(define
+  er-cascade-exit!
+  (fn
+    (target reason)
+    (er-proc-set! target :state "dead")
+    (er-proc-set! target :exit-reason reason)
+    (er-proc-set! target :exit-result nil)
+    (er-proc-set! target :continuation nil)
+    (er-propagate-exit! target reason)))
diff --git a/lib/erlang/scoreboard.json b/lib/erlang/scoreboard.json
index 6cdffc67..2426a635 100644
--- a/lib/erlang/scoreboard.json
+++ b/lib/erlang/scoreboard.json
@@ -1,11 +1,11 @@
 {
   "language": "erlang",
-  "total_pass": 375,
-  "total": 375,
+  "total_pass": 386,
+  "total": 386,
   "suites": [
     {"name":"tokenize","pass":62,"total":62,"status":"ok"},
     {"name":"parse","pass":52,"total":52,"status":"ok"},
-    {"name":"eval","pass":191,"total":191,"status":"ok"},
+    {"name":"eval","pass":202,"total":202,"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 47c31770..d3146fa0 100644
--- a/lib/erlang/scoreboard.md
+++ b/lib/erlang/scoreboard.md
@@ -1,12 +1,12 @@
 # Erlang-on-SX Scoreboard
 
-**Total: 375 / 375 tests passing**
+**Total: 386 / 386 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
 | ✅ | tokenize | 62 | 62 |
 | ✅ | parse | 52 | 52 |
-| ✅ | eval | 191 | 191 |
+| ✅ | eval | 202 | 202 |
 | ✅ | 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 1d531ef7..061b7794 100644
--- a/lib/erlang/tests/eval.sx
+++ b/lib/erlang/tests/eval.sx
@@ -502,6 +502,65 @@
       (= (len (er-proc-field (er-mk-pid 1) :monitored-by)) 0)))
   true)
 
+;; ── exit-signal propagation + trap_exit ────────────────────────
+(er-eval-test "process_flag default false"
+  (nm (ev "process_flag(trap_exit, true)")) "false")
+(er-eval-test "process_flag returns prev"
+  (nm (ev "process_flag(trap_exit, true), process_flag(trap_exit, false)"))
+  "true")
+
+;; Monitor fires on normal exit.
+(er-eval-test "monitor DOWN normal"
+  (nm (ev "P = spawn(fun () -> ok end), monitor(process, P), receive {'DOWN', _, process, _, R} -> R end"))
+  "normal")
+
+;; Monitor fires on abnormal exit.
+(er-eval-test "monitor DOWN abnormal"
+  (nm (ev "P = spawn(fun () -> exit(boom) end), monitor(process, P), receive {'DOWN', _, process, _, R} -> R end"))
+  "boom")
+
+;; Monitor's ref appears in DOWN message.
+(er-eval-test "monitor DOWN ref matches"
+  (nm (ev "P = spawn(fun () -> exit(bye) end), Ref = monitor(process, P), receive {'DOWN', Ref, process, _, _} -> ok_match end"))
+  "ok_match")
+
+;; Two monitors -> both fire.
+(er-eval-test "two monitors both fire"
+  (ev "P = spawn(fun () -> exit(crash) end), monitor(process, P), monitor(process, P), receive {'DOWN', _, _, _, _} -> ok end, receive {'DOWN', _, _, _, _} -> 2 end")
+  2)
+
+;; trap_exit + link + abnormal exit -> {'EXIT', From, Reason} message.
+(er-eval-test "trap_exit catches abnormal"
+  (nm (ev "process_flag(trap_exit, true), P = spawn(fun () -> exit(boom) end), link(P), receive {'EXIT', _, R} -> R end"))
+  "boom")
+
+;; trap_exit + link + normal exit -> {'EXIT', From, normal}.
+(er-eval-test "trap_exit catches normal"
+  (nm (ev "process_flag(trap_exit, true), P = spawn(fun () -> ok end), link(P), receive {'EXIT', _, R} -> R end"))
+  "normal")
+
+;; Cascade exit: A links B, B dies abnormally, A dies with same reason.
+(er-eval-test "cascade reason"
+  (do
+    (ev "A = spawn(fun () -> B = spawn(fun () -> exit(crash) end), link(B), receive forever -> ok end end), receive after 0 -> ok end")
+    (nm (er-proc-field (er-mk-pid 1) :exit-reason)))
+  "crash")
+
+;; Normal exit doesn't cascade (without trap_exit) — A's body returns
+;; "survived" via the `after` clause and A dies normally.
+(er-eval-test "normal exit no cascade"
+  (do
+    (ev "A = spawn(fun () -> B = spawn(fun () -> ok end), link(B), receive {'EXIT', _, _} -> got_exit after 50 -> survived end end), receive after 0 -> ok end")
+    (list
+      (nm (er-proc-field (er-mk-pid 1) :exit-reason))
+      (nm (er-proc-field (er-mk-pid 1) :exit-result))))
+  (list "normal" "survived"))
+
+;; Monitor without trap_exit: monitored proc abnormal doesn't kill the monitor.
+(er-eval-test "monitor doesn't cascade"
+  (nm (ev "P = spawn(fun () -> exit(boom) end), monitor(process, P), receive {'DOWN', _, _, _, _} -> alive end"))
+  "alive")
+
 (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 d475481e..fad9b55b 100644
--- a/lib/erlang/transpile.sx
+++ b/lib/erlang/transpile.sx
@@ -572,6 +572,7 @@
       (= name "unlink") (er-bif-unlink vs)
       (= name "monitor") (er-bif-monitor vs)
       (= name "demonitor") (er-bif-demonitor vs)
+      (= name "process_flag") (er-bif-process-flag vs)
       :else (error
         (str "Erlang: undefined function '" name "/" (len vs) "'")))))
 
diff --git a/plans/erlang-on-sx.md b/plans/erlang-on-sx.md
index f013f50f..84251271 100644
--- a/plans/erlang-on-sx.md
+++ b/plans/erlang-on-sx.md
@@ -80,7 +80,7 @@ Core mapping:
 
 ### Phase 4 — links, monitors, exit signals
 - [x] `link/1`, `unlink/1`, `monitor/2`, `demonitor/1` — **17 new eval tests**; `make_ref/0`, `is_reference/1`, refs in `=:=`/format wired
-- [ ] Exit-signal propagation; trap_exit flag
+- [x] Exit-signal propagation; trap_exit flag — **11 new eval tests**; `process_flag/2`, monitor `{'DOWN', ...}`, `{'EXIT', From, Reason}` for trap-exit links, cascade death without trap_exit
 - [ ] `try/catch/of/end`
 
 ### Phase 5 — modules + OTP-lite
@@ -99,6 +99,7 @@ Core mapping:
 
 _Newest first._
 
+- **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.
 - **2026-04-25 link/unlink/monitor/demonitor + refs green** — Refs added to scheduler (`:next-ref`, `er-ref-new!`); `er-mk-ref`, `er-ref?`, `er-ref-equal?` in runtime. Process record gains `:monitored-by`. New BIFs in `lib/erlang/runtime.sx`: `make_ref/0`, `is_reference/1`, `link/1` (bidirectional, no-op for self, raises `noproc` for missing target), `unlink/1` (removes both sides; tolerates missing target), `monitor(process, Pid)` (returns fresh ref, adds entries to monitor's `:monitors` and target's `:monitored-by`), `demonitor(Ref)` (purges both sides). Refs participate in `er-equal?` (id compare) and render as `#Ref<N>`. 17 new eval tests covering `make_ref` distinctness, link return values, bidirectional link recording, unlink clearing both sides, monitor recording both sides, demonitor purging. Total suite 375/375. Signal propagation (the next checkbox) will hook into these data structures.
 - **2026-04-25 ring benchmark recorded — Phase 3 closed** — `lib/erlang/bench_ring.sh` runs the ring at N ∈ {10, 50, 100, 500, 1000} and times each end-to-end via wall clock. `lib/erlang/bench_ring_results.md` captures the table. Throughput plateaus at ~30-34 hops/s. 1M-process target IS NOT MET in this architecture — extrapolation = ~9h. The sub-task is ticked as complete with that fact recorded inline because the perf gap is architectural (env-copy per call, call/cc per receive, mailbox rebuild on delete-at) and out of scope for this loop's iterations. Phase 3 done; Phase 4 (links, monitors, exit signals, try/catch) is next.
 - **2026-04-25 conformance harness + scoreboard green** — `lib/erlang/conformance.sh` loads every test suite via the epoch protocol, parses pass/total per suite via the `(N M)` lists, sums to a grand total, and writes both `lib/erlang/scoreboard.json` (machine-readable) and `lib/erlang/scoreboard.md` (Markdown table with ✅/❌ markers). 9 suites × full pass = 358/358. Exits non-zero on any failure. `bash lib/erlang/conformance.sh -v` prints per-suite counts. Phase 3's only remaining checkbox is the 1M-process ring benchmark target.