diff --git a/src/msg/core/executor.py b/src/msg/core/executor.py
index 6c417e68..7023104a 100644
--- a/src/msg/core/executor.py
+++ b/src/msg/core/executor.py
@@ -55,7 +55,8 @@ async def execute(self,request, *, entry='network'):
                     capacity_writes={'identity.register','identity.temporary','identity.custodial_create','content.topic_create',
                         'content.post_create','content.post_edit','content.file_put','content.attach',
                         'content.template_put','discussion.reply','discussion.quote','discussion.repost',
-                        'transfer.part_put','git.create','git.push','git.receive','hosting.deploy','keystore.put'}
+                        'transfer.part_put','git.create','git.push','git.receive','hosting.deploy','keystore.put',
+                        'achievement.start'}
                     if spec.name in capacity_writes:
                         require(session.setting('runtime_config',{}).get('accept_writes',True),'writes_paused')
                     for rid,generation in request.expected_generations:
diff --git a/src/msg/plugins/achievements.py b/src/msg/plugins/achievements.py
index 04a59d38..d567a92a 100644
--- a/src/msg/plugins/achievements.py
+++ b/src/msg/plugins/achievements.py
@@ -143,6 +143,18 @@ async def list_grants(ctx, request, tx):
     async def start(ctx, request, tx):
         subject = _owner(ctx)
         await tx.subject(subject)
+        grant = tx.one(
+            'SELECT 1 FROM achievement_grants WHERE subject=? AND achievement_id=? AND spec_version=?',
+            (subject, I_AM_NOT_HUMAN.id, I_AM_NOT_HUMAN.version),
+        )
+        require(grant is None, 'achievement_already_granted')
+        ceremonies = [loads(row[0]) for row in tx.rows(
+            'SELECT body FROM achievement_ceremonies WHERE subject=?', (subject,))]
+        require(not any(item['status'] == 'active' and ctx.now < parse_time(item['expires_at'])
+                        for item in ceremonies), 'achievement_ceremony_active')
+        # A subject can retain at most one ceremony. Reclaim failed, completed, and
+        # expired attempts before allocating a replacement challenge.
+        tx.execute('DELETE FROM achievement_ceremonies WHERE subject=?', (subject,), write=True)
         state = {'id': new_id('achc'), 'subject_id': subject, 'achievement_id': I_AM_NOT_HUMAN.id,
                  'spec_version': I_AM_NOT_HUMAN.version, 'status': 'active', 'round': 1,
                  'started_at': wire(ctx.now), 'expires_at': wire(ctx.now + timedelta(seconds=CEREMONY_TTL_SECONDS)),
diff --git a/tests/test_achievements.py b/tests/test_achievements.py
index d7dd183e..e120315e 100644
--- a/tests/test_achievements.py
+++ b/tests/test_achievements.py
@@ -90,6 +90,36 @@ async def test_wrong_nonce_and_round_ttl_fail_closed(installed):
     assert expired.data['reason'] == 'round_expired'
 
 
+@pytest.mark.asyncio
+async def test_start_keeps_only_one_ceremony_per_subject(installed):
+    app, _ = installed
+    key, subject, _ = await register(app, 'achievement-bounded')
+    started = await call(app, 'achievement.start', {}, key=key, subject=subject)
+    duplicate = await call(app, 'achievement.start', {}, key=key, subject=subject)
+    assert duplicate.status == 'error' and duplicate.error.code == 'achievement_ceremony_active'
+    async with app.metadata.transaction(write=False) as tx:
+        assert tx.one('SELECT COUNT(*) FROM achievement_ceremonies WHERE subject=?', (subject,))[0] == 1
+
+    app.executor.clock = lambda: NOW + timedelta(seconds=301)
+    replacement = await call(app, 'achievement.start', {}, key=key, subject=subject)
+    assert replacement.status == 'ok', wire(replacement)
+    assert replacement.data['challenge_id'] != started.data['challenge_id']
+    async with app.metadata.transaction(write=False) as tx:
+        assert tx.one('SELECT COUNT(*) FROM achievement_ceremonies WHERE subject=?', (subject,))[0] == 1
+
+
+@pytest.mark.asyncio
+async def test_start_obeys_capacity_pause(installed):
+    app, _ = installed
+    key, subject, _ = await register(app, 'achievement-paused')
+    async with app.metadata.transaction(write=True) as tx:
+        tx.set_setting('runtime_config', {'accept_writes': False, 'cleanup_enabled': True})
+    blocked = await call(app, 'achievement.start', {}, key=key, subject=subject)
+    assert blocked.status == 'error' and blocked.error.code == 'writes_paused'
+    async with app.metadata.transaction(write=False) as tx:
+        assert tx.one('SELECT COUNT(*) FROM achievement_ceremonies WHERE subject=?', (subject,))[0] == 0
+
+
 @pytest.mark.asyncio
 async def test_machine_payload_and_final_digest_are_required(installed):
     app, _ = installed