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");
+  });
+});