diff --git a/docs/website/src/impl-spec/appendix/manager-socket.rst b/docs/website/src/impl-spec/appendix/manager-socket.rst
index d2874ff..662455e 100644
--- a/docs/website/src/impl-spec/appendix/manager-socket.rst
+++ b/docs/website/src/impl-spec/appendix/manager-socket.rst
@@ -142,6 +142,13 @@ and process spawn continue asynchronously and report through ``event``
 notifications. The requesting connection is subscribed to the run's events
 automatically.
 
+Only decoding is synchronous, because the request has to be read before an id
+can be attached to it: calldata that fails to decode, or a payload shape that
+names no ``run``, is answered with ``error``. Everything decided from the
+decoded request is asynchronous and arrives as a terminal event. A non-empty
+``host_hello_data[1]``, which is manager-owned, and a request that needs
+modules while they are stopped, both report ``failed_to_start`` this way.
+
 ``attach``
 ----------
 
diff --git a/implementation/src/manager/socket.rs b/implementation/src/manager/socket.rs
index fe96254..aab06bf 100644
--- a/implementation/src/manager/socket.rs
+++ b/implementation/src/manager/socket.rs
@@ -491,35 +491,13 @@ where
                 return Ok(());
             }
         };
-        if req
-            .host_hello_data
-            .get(1)
-            .is_some_and(|data| !data.is_empty())
-        {
-            self.writer
-                .error(
-                    request_id,
-                    Errors::MalformedFrame,
-                    "host_hello_data for manager-owned host index 1 is not allowed",
-                )
-                .await?;
-            return Ok(());
-        }
-
+        // `host_hello_data[1]` and a modules-stopped request are rejected by
+        // supervision (`run_genvm_process`), not here: the protocol returns the
+        // genvm_id immediately and reports those failures as a terminal event.
+        // Only the module locks are taken on this path, to pin the handlers
+        // before the run spawns.
         let modules_lock = if req.needs_modules() {
-            match super::modules::Ctx::get_module_locks(self.ctx.gep(|x| &x.mod_ctx)).await {
-                Some(lock) => Some(lock),
-                None => {
-                    self.writer
-                        .error(
-                            request_id,
-                            Errors::Internal,
-                            "modules are required but not running",
-                        )
-                        .await?;
-                    return Ok(());
-                }
-            }
+            super::modules::Ctx::get_module_locks(self.ctx.gep(|x| &x.mod_ctx)).await
         } else {
             None
         };
diff --git a/tests/system/manager-socket/test.py b/tests/system/manager-socket/test.py
index d6a73ee..e462884 100644
--- a/tests/system/manager-socket/test.py
+++ b/tests/system/manager-socket/test.py
@@ -402,6 +402,46 @@ async def _oversized_closes_connection(self):
 					aiohttp.WSCloseCode.MESSAGE_TOO_BIG,
 				), close_code
 
+	async def _run_returns_id_before_semantic_rejection(self):
+		"""
+		Both semantic rejections the socket used to answer synchronously must now
+		come back as a terminal event, after the genvm_id has been returned.
+		"""
+		async with await self._manager('async-validation') as manager:
+			host_path = manager.work_dir / 'unused-host.sock'
+
+			# `host_hello_data[1]` is manager-owned, so a non-empty slot is a
+			# semantic rejection rather than a malformed frame.
+			async with ManagerWsClient(manager.uri) as client:
+				await _read_hello(client)
+				req = _run_request(self.case.shared.root_dir, host_path)
+				req['run']['host_hello_data'] = [b'', b'injected']
+				await client.send(Methods.RUN, 10, req)
+				method, request_id, payload = await client.read_frame()
+				assert (method, request_id) == (Methods.RUN, 10)
+				genvm_id = payload['genvm_id']
+				variant, event = await _wait_terminal(client, timeout=5)
+				assert variant == 'failed_to_start', variant
+				assert event['genvm_id'] == genvm_id
+				assert 'host index 1' in event['error'], event
+
+			# A request needing modules with the modules stopped is the other
+			# class: it used to answer with an error and no genvm_id.
+			async with ManagerWsClient(manager.uri) as client:
+				await _read_hello(client)
+				req = _run_request(self.case.shared.root_dir, host_path)
+				req['run']['no_modules'] = False
+				req['run']['is_sync'] = False
+				req['run']['permissions'] = 'n'
+				await client.send(Methods.RUN, 20, req)
+				method, request_id, payload = await client.read_frame()
+				assert (method, request_id) == (Methods.RUN, 20)
+				genvm_id = payload['genvm_id']
+				variant, event = await _wait_terminal(client, timeout=5)
+				assert variant == 'failed_to_start', variant
+				assert event['genvm_id'] == genvm_id
+				assert 'modules are required' in event['error'], event
+
 	async def _startup_failure_events_and_permits(self):
 		async with await self._manager('startup-failures') as manager:
 			async with ManagerWsClient(manager.uri) as client:
@@ -958,6 +998,13 @@ def service(
 			None,
 			genvm.ManagerService,
 		),
+		(
+			'async-validation',
+			'_run_returns_id_before_semantic_rejection',
+			{},
+			None,
+			genvm.ManagerService,
+		),
 		('happy-path', '_happy_path_artifact_ack_attach', {}, None, genvm.ManagerService),
 		(
 			'reconnect-mid-run',