From 39bad58d40a293b2b40f4d2b1bce2f3fdea1ca26 Mon Sep 17 00:00:00 2001
From: DietrichGebert <dietrich.gebert@gmail.com>
Date: Tue, 7 Jul 2026 01:23:31 +0200
Subject: [PATCH] fix: guard pi before_agent_start against bad event and
 missing systemPrompt (#482)

The before_agent_start handler read event.systemPrompt with no guard. A
null/undefined event crashed the agent with a TypeError (#439), and a truthy
event without a systemPrompt key stringified to the literal 'undefined',
prepending garbage to every system prompt (#440). Both are fixed with one
optional-chain guard that mirrors the event?. pattern the input handler already
uses: prepend the base prompt only when present, otherwise inject the ruleset
alone. Adds a regression test covering null/undefined event, missing
systemPrompt, and the normal base-prompt path.

Closes #439
Closes #440

Co-authored-by: Aroool <90670606+Aroool@users.noreply.github.com>
---
 pi-extension/index.js               |  5 ++++-
 pi-extension/test/extension.test.js | 23 +++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/pi-extension/index.js b/pi-extension/index.js
index 5f17627..f348fb0 100644
--- a/pi-extension/index.js
+++ b/pi-extension/index.js
@@ -184,6 +184,9 @@ 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)}` };
+    // Guard a null/undefined event or a missing systemPrompt: don't crash, and
+    // don't prepend the literal string "undefined" to the prompt (#439, #440).
+    const base = event?.systemPrompt ? `${event.systemPrompt}\n\n` : "";
+    return { systemPrompt: `${base}${getPonytailInstructions(currentMode)}` };
   });
 }
diff --git a/pi-extension/test/extension.test.js b/pi-extension/test/extension.test.js
index b5f3918..2279ffb 100644
--- a/pi-extension/test/extension.test.js
+++ b/pi-extension/test/extension.test.js
@@ -77,6 +77,29 @@ test("/ponytail updates session mode and injects instructions", async () => with
   assert.ok(result.systemPrompt.includes("ultra"));
 }));
 
+test("before_agent_start guards missing event and missing systemPrompt (#439, #440)", async () => withTempConfig(async () => {
+  const { events } = createPiHarness();
+  const ctx = createCommandContext();
+  await events.get("session_start")({ reason: "startup" }, ctx); // currentMode -> default (full)
+
+  // #439: a null/undefined event must not crash, and still injects the ruleset.
+  for (const bad of [undefined, null]) {
+    const r = await events.get("before_agent_start")(bad, ctx);
+    assert.ok(r.systemPrompt.includes("PONYTAIL MODE ACTIVE"));
+    assert.ok(!r.systemPrompt.includes("undefined"), "must not contain the literal 'undefined'");
+  }
+
+  // #440: an event without a systemPrompt must not prepend the literal "undefined".
+  const empty = await events.get("before_agent_start")({}, ctx);
+  assert.ok(empty.systemPrompt.includes("PONYTAIL MODE ACTIVE"));
+  assert.ok(!empty.systemPrompt.startsWith("undefined"), "must not start with 'undefined'");
+
+  // A real base prompt is still preserved and prepended.
+  const withBase = await events.get("before_agent_start")({ systemPrompt: "BASE" }, ctx);
+  assert.ok(withBase.systemPrompt.startsWith("BASE\n\n"));
+  assert.ok(withBase.systemPrompt.includes("PONYTAIL MODE ACTIVE"));
+}));
+
 test("session_start restores latest persisted mode", async () => withTempConfig(async () => {
   const { events } = createPiHarness();
   const ctx = createCommandContext({