From 84c0f345612fb1672be05af732c80f7893aa9276 Mon Sep 17 00:00:00 2001
From: Pluviobyte <Pluviobyte@users.noreply.github.com>
Date: Fri, 29 May 2026 06:30:48 +0000
Subject: [PATCH] fix(prompt-files): avoid crash when <system> tag is unclosed

parsePromptFile only checked for an opening `<system>` tag before running
`prompt.split("</system>")[1].trim()`. A `.prompt` file that contains
`<system>` but omits the closing `</system>` makes that split return a
single-element array, so `[1]` is undefined and `.trim()` throws
`TypeError: Cannot read properties of undefined (reading 'trim')`,
failing config load over a simple authoring typo.

Require both tags before parsing the system block; an unclosed tag now
falls through and the whole body is treated as the prompt. Adds tests for
the closed, unclosed, and no-system cases.

Co-authored-by: Cursor <cursoragent@cursor.com>
---
 core/promptFiles/parsePromptFile.ts        |  4 ++-
 core/promptFiles/parsePromptFile.vitest.ts | 32 ++++++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 core/promptFiles/parsePromptFile.vitest.ts

diff --git a/core/promptFiles/parsePromptFile.ts b/core/promptFiles/parsePromptFile.ts
index 4b6cf6917b8..06eb0458c76 100644
--- a/core/promptFiles/parsePromptFile.ts
+++ b/core/promptFiles/parsePromptFile.ts
@@ -15,7 +15,9 @@ export function parsePromptFile(path: string, content: string) {
   const version = preamble.version ?? 2;
 
   let systemMessage: string | undefined = undefined;
-  if (prompt.includes("<system>")) {
+  // Require both tags: a `<system>` with no matching `</system>` would make
+  // `split("</system>")[1]` undefined and crash on `.trim()`.
+  if (prompt.includes("<system>") && prompt.includes("</system>")) {
     systemMessage = prompt.split("<system>")[1].split("</system>")[0].trim();
     prompt = prompt.split("</system>")[1].trim();
   }
diff --git a/core/promptFiles/parsePromptFile.vitest.ts b/core/promptFiles/parsePromptFile.vitest.ts
new file mode 100644
index 00000000000..dd8f508d090
--- /dev/null
+++ b/core/promptFiles/parsePromptFile.vitest.ts
@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+
+import { parsePromptFile } from "./parsePromptFile.js";
+
+describe("parsePromptFile", () => {
+  it("extracts the system message when both tags are present", () => {
+    const result = parsePromptFile(
+      "greeting.prompt",
+      "<system>You are helpful</system>\nSummarize the file",
+    );
+
+    expect(result.systemMessage).toBe("You are helpful");
+    expect(result.prompt).toBe("Summarize the file");
+  });
+
+  it("does not throw when <system> has no closing tag", () => {
+    const content = "<system>You forgot to close the tag\nSummarize the file";
+
+    expect(() => parsePromptFile("broken.prompt", content)).not.toThrow();
+
+    const result = parsePromptFile("broken.prompt", content);
+    expect(result.systemMessage).toBeUndefined();
+    expect(result.prompt).toBe(content);
+  });
+
+  it("leaves the prompt untouched when there is no system block", () => {
+    const result = parsePromptFile("plain.prompt", "Just a prompt body");
+
+    expect(result.systemMessage).toBeUndefined();
+    expect(result.prompt).toBe("Just a prompt body");
+  });
+});