diff --git a/pi-extension/index.js b/pi-extension/index.js
index 5f176270..9bf56d17 100644
--- a/pi-extension/index.js
+++ b/pi-extension/index.js
@@ -184,6 +184,7 @@ export default function ponytailExtension(pi) {
 
   pi.on("before_agent_start", async (event) => {
     if (!currentMode || currentMode === "off") return;
-    return { systemPrompt: `${event.systemPrompt}\n\n${getPonytailInstructions(currentMode)}` };
+    return { systemPrompt: `${event?.systemPrompt ?? ""}\n\n${getPonytailInstructions(currentMode)}` };
   });
+
 }
diff --git a/pi-extension/test/extension.test.js b/pi-extension/test/extension.test.js
index b5f3918d..680c0416 100644
--- a/pi-extension/test/extension.test.js
+++ b/pi-extension/test/extension.test.js
@@ -165,3 +165,39 @@ test("status bar stays silent when ui lacks a theme", async () => withTempConfig
 
   assert.deepEqual(calls, []);
 }));
+
+// Regression: before_agent_start must not throw when event is null/undefined
+// or when event is a truthy object without a systemPrompt key (issue #439).
+test("before_agent_start: null event does not throw", async () => withTempConfig(async () => {
+  const { commands, events } = createPiHarness();
+  const ctx = createCommandContext();
+  await events.get("session_start")({ reason: "startup" }, ctx);
+  await commands.get("ponytail").handler("full", ctx);
+
+  const result = await events.get("before_agent_start")(null, ctx);
+  assert.ok(result.systemPrompt.includes("PONYTAIL MODE ACTIVE"), "instructions present even with null event");
+  assert.ok(!result.systemPrompt.includes("undefined"), "no literal 'undefined' in output");
+}));
+
+test("before_agent_start: undefined event does not throw", async () => withTempConfig(async () => {
+  const { commands, events } = createPiHarness();
+  const ctx = createCommandContext();
+  await events.get("session_start")({ reason: "startup" }, ctx);
+  await commands.get("ponytail").handler("full", ctx);
+
+  const result = await events.get("before_agent_start")(undefined, ctx);
+  assert.ok(result.systemPrompt.includes("PONYTAIL MODE ACTIVE"), "instructions present even with undefined event");
+  assert.ok(!result.systemPrompt.includes("undefined"), "no literal 'undefined' in output");
+}));
+
+test("before_agent_start: truthy event without systemPrompt does not produce 'undefined' prefix", async () => withTempConfig(async () => {
+  const { commands, events } = createPiHarness();
+  const ctx = createCommandContext();
+  await events.get("session_start")({ reason: "startup" }, ctx);
+  await commands.get("ponytail").handler("full", ctx);
+
+  const result = await events.get("before_agent_start")({}, ctx);
+  assert.ok(result.systemPrompt.includes("PONYTAIL MODE ACTIVE"), "instructions present");
+  assert.ok(!result.systemPrompt.startsWith("undefined"), "no literal 'undefined' prefix");
+}));
+