diff --git a/src/agents/pi-embedded-runner/system-prompt.test.ts b/src/agents/pi-embedded-runner/system-prompt.test.ts
index 355b2c67ae9..8e20a95bda7 100644
--- a/src/agents/pi-embedded-runner/system-prompt.test.ts
+++ b/src/agents/pi-embedded-runner/system-prompt.test.ts
@@ -2,6 +2,11 @@ import type { AgentSession } from "@mariozechner/pi-coding-agent";
 import { describe, expect, it, vi } from "vitest";
 import { applySystemPromptOverrideToSession, createSystemPromptOverride } from "./system-prompt.js";
 
+type MutableSession = AgentSession & {
+  _baseSystemPrompt?: string;
+  _rebuildSystemPrompt?: (toolNames: string[]) => string;
+};
+
 function createMockSession() {
   const setSystemPrompt = vi.fn();
   const session = {
@@ -10,42 +15,41 @@ function createMockSession() {
   return { session, setSystemPrompt };
 }
 
+function applyAndGetMutableSession(
+  prompt: Parameters<typeof applySystemPromptOverrideToSession>[1],
+) {
+  const { session, setSystemPrompt } = createMockSession();
+  applySystemPromptOverrideToSession(session, prompt);
+  return {
+    mutable: session as MutableSession,
+    setSystemPrompt,
+  };
+}
+
 describe("applySystemPromptOverrideToSession", () => {
   it("applies a string override to the session system prompt", () => {
-    const { session, setSystemPrompt } = createMockSession();
     const prompt = "You are a helpful assistant with custom context.";
-
-    applySystemPromptOverrideToSession(session, prompt);
+    const { mutable, setSystemPrompt } = applyAndGetMutableSession(prompt);
 
     expect(setSystemPrompt).toHaveBeenCalledWith(prompt);
-    const mutable = session as unknown as { _baseSystemPrompt?: string };
     expect(mutable._baseSystemPrompt).toBe(prompt);
   });
 
   it("trims whitespace from string overrides", () => {
-    const { session, setSystemPrompt } = createMockSession();
-
-    applySystemPromptOverrideToSession(session, "  padded prompt  ");
+    const { setSystemPrompt } = applyAndGetMutableSession("  padded prompt  ");
 
     expect(setSystemPrompt).toHaveBeenCalledWith("padded prompt");
   });
 
   it("applies a function override to the session system prompt", () => {
-    const { session, setSystemPrompt } = createMockSession();
     const override = createSystemPromptOverride("function-based prompt");
-
-    applySystemPromptOverrideToSession(session, override);
+    const { setSystemPrompt } = applyAndGetMutableSession(override);
 
     expect(setSystemPrompt).toHaveBeenCalledWith("function-based prompt");
   });
 
   it("sets _rebuildSystemPrompt that returns the override", () => {
-    const { session } = createMockSession();
-    applySystemPromptOverrideToSession(session, "rebuild test");
-
-    const mutable = session as unknown as {
-      _rebuildSystemPrompt?: (toolNames: string[]) => string;
-    };
+    const { mutable } = applyAndGetMutableSession("rebuild test");
     expect(mutable._rebuildSystemPrompt?.(["tool1"])).toBe("rebuild test");
   });
 });