diff --git a/packages/core/src/dataspace.js b/packages/core/src/dataspace.js
index 8074614..f9c29cc 100644
--- a/packages/core/src/dataspace.js
+++ b/packages/core/src/dataspace.js
@@ -544,6 +544,9 @@ Message.prototype.perform = function (ds, ac) {
 };
 
 Dataspace.send = function (body) {
+  if (!Dataspace._inScript) {
+    throw new Error("Cannot `send` during facet setup; are you missing an `on start { ... }`?");
+  }
   Dataspace._currentFacet.enqueueScriptAction(new Message(body));
 };
 
@@ -558,6 +561,9 @@ Spawn.prototype.perform = function (ds, ac) {
 };
 
 Dataspace.spawn = function (name, bootProc, initialAssertions) {
+  if (!Dataspace._inScript) {
+    throw new Error("Cannot `spawn` during facet setup; are you missing an `on start { ... }`?");
+  }
   Dataspace._currentFacet.enqueueScriptAction(new Spawn(name, bootProc, initialAssertions));
 };
 
@@ -579,6 +585,9 @@ DeferredTurn.prototype.perform = function (ds, ac) {
 };
 
 Dataspace.deferTurn = function (continuation) {
+  if (!Dataspace._inScript) {
+    throw new Error("Cannot defer turn during facet setup; are you missing an `on start { ... }`?");
+  }
   Dataspace._currentFacet.enqueueScriptAction(new DeferredTurn(Dataspace.wrap(continuation)));
 };
 
@@ -697,7 +706,17 @@ Facet.prototype.stop = function (continuation) {
   });
 };
 
+Facet.prototype.addStartScript = function (s) {
+  if (Dataspace._inScript) {
+    throw new Error("Cannot `on start` outside facet setup");
+  }
+  this.actor.scheduleScript(s);
+};
+
 Facet.prototype.addStopScript = function (s) {
+  if (Dataspace._inScript) {
+    throw new Error("Cannot `on stop` outside facet setup");
+  }
   this.stopScripts = this.stopScripts.push(s);
 };
 
diff --git a/packages/core/src/relay.js b/packages/core/src/relay.js
index 94a2e8a..240ff7f 100644
--- a/packages/core/src/relay.js
+++ b/packages/core/src/relay.js
@@ -139,7 +139,9 @@ function inNestedDataspace(bootProc) {
     const outerFacet = Dataspace.currentFacet();
     outerFacet.addDataflow(function () {});
     // ^ eww! Dummy endpoint to keep the root facet of the relay alive.
-    const innerDs = new NestedDataspace(outerFacet, bootProc);
+    const innerDs = new NestedDataspace(outerFacet, function () {
+      Dataspace.currentFacet().addStartScript(() => bootProc.call(this));
+    });
     innerDs.start();
   };
 }
diff --git a/packages/driver-streams-node/src/buffer.js b/packages/driver-streams-node/src/buffer.js
index 60447d7..f6464ab 100644
--- a/packages/driver-streams-node/src/buffer.js
+++ b/packages/driver-streams-node/src/buffer.js
@@ -25,8 +25,19 @@ export {
   PacketRequest,
 };
 
+export function onStartSpawnBufferStream() {
+  const id = genUuid('buffer-stream');
+  on start _spawnBufferStream(id);
+  return id;
+}
+
 export function spawnBufferStream() {
   const id = genUuid('buffer-stream');
+  _spawnBufferStream(id);
+  return id;
+}
+
+function _spawnBufferStream(id) {
   spawn named id {
     stop on retracted Observe(S.Duplex(id));
     assert S.Duplex(id);
diff --git a/packages/syntax-playground/src/server.js b/packages/syntax-playground/src/server.js
index 92c53d0..2a5652b 100644
--- a/packages/syntax-playground/src/server.js
+++ b/packages/syntax-playground/src/server.js
@@ -30,7 +30,7 @@ assertion type Counter(id);
 
 function counter() {
   const id = genUuid();
-  spawn named ['counter', id] {
+  on start spawn named ['counter', id] {
     const rootFacet = currentFacet();
 
     assert Counter(id);
diff --git a/packages/syntax-playground/src/socks.js b/packages/syntax-playground/src/socks.js
index 98db1ef..736c630 100644
--- a/packages/syntax-playground/src/socks.js
+++ b/packages/syntax-playground/src/socks.js
@@ -17,7 +17,7 @@ spawn named 'socks-server' {
 
       stop on retracted S.Duplex(conn);
 
-      const buf = S.spawnBufferStream();
+      const buf = S.onStartSpawnBufferStream();
       field this.bufferWanted = true;
       on start react {
         stop on (!this.bufferWanted);
@@ -210,8 +210,7 @@ spawn named 'remap-service' {
         err.errno = err.code = 'ENOTFOUND';
         err.hostname = err.host = host;
         err.port = port;
-        // TODO: should error because no 'on start':
-        send S.ConnectionRejected(id, err);
+        on start send S.ConnectionRejected(id, err);
       }
     } else {
       assert S.OutgoingConnection(id, S.TcpAddress(host, port));
diff --git a/packages/syntax/src/plugin.js b/packages/syntax/src/plugin.js
index 81212bf..9626b9a 100644
--- a/packages/syntax/src/plugin.js
+++ b/packages/syntax/src/plugin.js
@@ -439,7 +439,7 @@ export default declare((api, options) => {
       PseudoEventHandler(path, state) {
         const { node } = path;
         if (node.triggerType === "start") {
-          path.replaceWith(template(`DATASPACE._currentFacet.actor.scheduleScript(() => {
+          path.replaceWith(template(`DATASPACE._currentFacet.addStartScript(() => {
                                        BODY;
                                      });`)({
                                        DATASPACE: state.DataspaceID,