diff --git a/CHANGELOG.md b/CHANGELOG.md
index e05a297..b8a5d8b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -51,8 +51,19 @@ All notable changes to EigenScript are documented here.
   preemption between the acquire check passing and the claim. Surfaced by the
   `eddy` consumer (concurrency-control DST): a critical section that spans a
   yield needs an explicit lock, since only a non-yielding region is implicitly
-  atomic. Still open under #488: a debug-mode *assertion* that a delimited
-  region issued no scheduler yield (needs VM support).
+  atomic.
+- **`must_not_yield of fn` — assert a critical region is atomic (#488).** The
+  other half of #488: a region relies on cooperative atomicity (mutual
+  exclusion for free) *only* while it issues no yield, but nothing marked such
+  a region, so a `task_yield` — or a builtin that yields internally — introduced
+  later broke atomicity **silently** (a rare, seed-dependent anomaly).
+  `must_not_yield of fn` runs `fn of null` as an atomic section: any suspending
+  task builtin inside (`task_yield`, a blocking `task_recv`/`task_join`,
+  `task_sleep`) raises `value` instead of suspending, so the mistake fails
+  loudly. Non-suspending ops (`task_try_recv`, `task_send`, joining a finished
+  task) are allowed. The region depth is balanced even when the body raises,
+  and it nests. Surfaced by the `eddy` consumer's snapshot-isolation commit
+  loop (atomic only because it contains no yield).
 
 ### Changed
 - **`deadlock` is now catchable (#509).** A cooperative-task deadlock (all
diff --git a/docs/BUILTINS.md b/docs/BUILTINS.md
index 0ecf204..1359bfe 100644
--- a/docs/BUILTINS.md
+++ b/docs/BUILTINS.md
@@ -571,6 +571,7 @@ Requires full build. Transformer model inference and training.
 | `task_spawn` | `task_spawn of fn` or `task_spawn of [fn, arg1, ...]` | Create a cooperative task (#408) running `fn` on the single OS thread — deterministic by construction, unlike `spawn`'s OS thread. Args are deep-COPIED (share-nothing, like channel sends), not shared by reference. Returns a numeric task id. (Increment 1a: the task is recorded and reported by `task_alive`; the copying-stack scheduler that runs and interleaves tasks — `task_yield`/`task_join` — lands in a later increment.) |
 | `task_alive` | `task_alive of id` | Returns 1 while the task is runnable or suspended, 0 once it has finished (or for an unknown id). |
 | `task_yield` | `task_yield of null` | Cooperatively hand control to the next ready task; this task resumes round-robin. A no-op when no task has been spawned. Forbidden inside an `arena_mark`…`arena_reset` scope or a nested evaluation (raises `value`). |
+| `must_not_yield` | `must_not_yield of fn` | Run `fn of null` as an **atomic** critical section, asserting it issues no scheduler yield (#488). Any *suspending* task builtin inside — `task_yield`, a blocking `task_recv`/`task_join`, `task_sleep` — raises `value` instead of suspending, so a yield introduced into a region that relies on cooperative atomicity fails loudly rather than corrupting under a rare interleaving. Non-suspending ops (`task_try_recv`, `task_send`, joining an already-finished task) are allowed. Returns `fn`'s result (or propagates its error); the region depth is balanced even if the body raises. Nestable. |
 | `task_join` | `task_join of id` | Block until task `id` finishes, then return its deep-copied result — or re-raise its uncaught error (as the same `{kind, message, line}` it died with). Joining an already-finished task returns immediately; an unknown id (or self) returns null. All tasks blocked with none runnable is a `deadlock` error, not a hang — catchable by a `try`/`catch` around the join on the main task (`e.kind == "deadlock"`); terminal only if unhandled. |
 | `task_send` | `task_send of [id, value]` | Append a deep-copied message to task `id`'s unbounded FIFO mailbox, waking it if it waits in `task_recv`. Returns 1 if delivered, 0 if `id` is finished/unknown (a silent drop — send-to-dead is never an error). Never blocks. |
 | `task_recv` | `task_recv of null` | Return the next message from this task's mailbox, or block cooperatively until one arrives. Forbidden inside an `arena_mark`…`arena_reset` scope or a nested evaluation (raises `value`). |
diff --git a/src/builtins.c b/src/builtins.c
index 7530845..9848c34 100644
--- a/src/builtins.c
+++ b/src/builtins.c
@@ -4731,6 +4731,27 @@ Value* builtin_task_spawn(Value *arg) {
     return make_num((double)id);
 }
 
+/* #488: a `must_not_yield` region asserts atomicity. A critical section under
+ * cooperative scheduling is atomic *only* while it issues no yield — the
+ * implicit mutual exclusion a non-yielding region gets for free. Nothing marked
+ * such a region, so a yield introduced later (directly, or via a builtin that
+ * yields internally) broke atomicity silently. While this depth is nonzero,
+ * any task builtin that would actually SUSPEND raises instead, so the mistake
+ * fails loudly. Only nonzero for the running task — a yield cannot happen
+ * inside, so no task switch occurs while it is set, and a plain counter is
+ * safe. */
+static int g_no_yield_depth = 0;
+
+static int no_yield_forbidden(const char *what) {
+    if (g_no_yield_depth > 0) {
+        rt_error(EK_VALUE, 0,
+                 "%s inside a must_not_yield region — the critical section "
+                 "would suspend, breaking its atomicity", what);
+        return 1;
+    }
+    return 0;
+}
+
 /* task_yield of null — cooperatively hand control to the next ready task.
  * Returns null (the value of the yield expression); the scheduler saves and
  * re-enqueues this task at the tail. Forbidden inside an active arena scope
@@ -4749,6 +4770,7 @@ Value* builtin_task_yield(Value *arg) {
         return make_null();
     }
     if (!g_task_sched) return make_null();   /* no scheduler: yield is a no-op */
+    if (no_yield_forbidden("task_yield")) return make_null();   /* #488 */
     task_request_yield();
     return make_null();   /* placeholder: execution resumes right after this */
 }
@@ -4785,6 +4807,7 @@ Value* builtin_task_join(Value *arg) {
                  "(arena_mark…arena_reset)");
         return make_null();
     }
+    if (no_yield_forbidden("blocking task_join")) return make_null();   /* #488 */
     if (!task_request_join(target)) return make_null();
     return make_null();   /* placeholder: the scheduler fills it with the result on resume */
 }
@@ -4821,6 +4844,7 @@ Value* builtin_task_recv(Value *arg) {
         return make_null();
     }
     if (!g_task_sched) return make_null();   /* no tasks: nothing can arrive */
+    if (no_yield_forbidden("blocking task_recv")) return make_null();   /* #488 */
     task_request_recv();
     return make_null();   /* placeholder: the scheduler delivers the message on resume */
 }
@@ -4871,10 +4895,32 @@ Value* builtin_task_sleep(Value *arg) {
         rt_error(EK_TYPE, 0, "task_sleep requires a number of ticks");
         return make_null();
     }
+    if (no_yield_forbidden("task_sleep")) return make_null();   /* #488 */
     task_request_sleep(arg->data.num);
     return make_null();   /* placeholder: execution resumes when the clock wakes it */
 }
 
+/* must_not_yield of fn — run `fn of null` as an ATOMIC critical section and
+ * assert it issues no scheduler yield (#488). Any suspending task builtin
+ * inside — task_yield, a blocking task_recv/task_join, task_sleep — raises
+ * instead of suspending, so a yield introduced into a region that relies on
+ * cooperative atomicity (e.g. eddy's snapshot-isolation commit loop) fails
+ * loudly rather than corrupting silently under a rare interleaving. Returns
+ * fn's result, or propagates its error; the region depth is balanced even when
+ * the body raises (call_eigs_fn returns control either way). Nestable. */
+Value* builtin_must_not_yield(Value *arg) {
+    if (!arg || (arg->type != VAL_FN && arg->type != VAL_BUILTIN)) {
+        rt_error(EK_TYPE, 0, "must_not_yield requires a function");
+        return make_null();
+    }
+    g_no_yield_depth++;
+    Value *nul = make_null();
+    Value *r = call_eigs_fn(arg, nul);
+    val_decref(nul);
+    g_no_yield_depth--;
+    return r ? r : make_null();
+}
+
 /* task_now of null → the current virtual-clock value (a number, 0 before any
  * task_sleep and 0 when no scheduler is active). Deterministic; reads a logical
  * counter, so it records no tape nondet. */
@@ -5948,6 +5994,7 @@ void register_builtins(Env *env) {
     env_set_local_owned(env, "task_spawn", make_builtin(builtin_task_spawn));
     env_set_local_owned(env, "task_alive", make_builtin(builtin_task_alive));
     env_set_local_owned(env, "task_yield", make_builtin(builtin_task_yield));
+    env_set_local_owned(env, "must_not_yield", make_builtin(builtin_must_not_yield));
     env_set_local_owned(env, "task_join", make_builtin(builtin_task_join));
     env_set_local_owned(env, "task_send", make_builtin(builtin_task_send));
     env_set_local_owned(env, "task_recv", make_builtin(builtin_task_recv));
diff --git a/tests/test_tasks.eigs b/tests/test_tasks.eigs
index 8873f42..16722a2 100644
--- a/tests/test_tasks.eigs
+++ b/tests/test_tasks.eigs
@@ -384,4 +384,50 @@ catch e:
     assert_eq of [e.message, "all tasks are blocked — deadlock", "deadlock carries its message"]
 assert_eq of [dl_caught, 1, "the deadlock handler ran and code after the try continues"]
 
+# --- #488: must_not_yield asserts a critical region issues no scheduler yield ---
+# A yield inside the region raises a catchable `value` error.
+define _mny_yielder() as:
+    task_yield of null
+    return 0
+define _mny_probe_yield() as:
+    caught is 0
+    try:
+        must_not_yield of _mny_yielder
+    catch e:
+        caught is 1
+        assert_eq of [e.kind, "value", "must_not_yield violation is a catchable value error"]
+    # depth is balanced even though the body raised: a yield now must work
+    task_yield of null
+    return caught
+_mny_w0 is task_spawn of _mny_probe_yield
+assert_eq of [task_join of _mny_w0, 1, "yield inside must_not_yield is caught, region depth restored"]
+
+# A region that issues no yield returns its body's result, no raise.
+_mny_acc is 0
+define _mny_clean() as:
+    _mny_acc is _mny_acc + 5
+    _mny_acc is _mny_acc + 5
+    return _mny_acc
+assert_eq of [must_not_yield of _mny_clean, 10, "non-yielding region returns its body result"]
+
+# Non-blocking ops (task_try_recv) inside a region do NOT trip the assertion.
+define _mny_nonblock() as:
+    m is task_try_recv of null
+    return 1
+assert_eq of [must_not_yield of _mny_nonblock, 1, "non-suspending ops are allowed in a region"]
+
+# A blocking task_recv inside a region raises (would suspend).
+define _mny_blockrecv() as:
+    m is task_recv of null
+    return 0
+define _mny_probe_recv() as:
+    caught is 0
+    try:
+        must_not_yield of _mny_blockrecv
+    catch e:
+        caught is 1
+    return caught
+_mny_w1 is task_spawn of _mny_probe_recv
+assert_eq of [task_join of _mny_w1, 1, "blocking task_recv inside must_not_yield raises"]
+
 test_summary of null