messages: AgentMessage[];

prePromptMessageCount: number;

}) => Record<string, unknown> | undefined,

onAfterTurnCheckpoint?: (messageCount: number) => void,

): () => void {

return installContextEngineLoopHook({

agent,

modelId,

...(prePromptCount !== undefined ? { getPrePromptMessageCount: () => prePromptCount } : {}),

...(getRuntimeContext ? { getRuntimeContext } : {}),

...(onAfterTurnCheckpoint ? { onAfterTurnCheckpoint } : {}),

});

}

expect(engine.afterTurn.mock.calls[1]?.[0]?.prePromptMessageCount).toBe(4);

});

it("reports the latest delivered afterTurn checkpoint", async () => {

const agent = makeGuardableAgent();

const engine = makeMockEngine();

const onAfterTurnCheckpoint = vi.fn();

installHook(agent, engine, undefined, undefined, onAfterTurnCheckpoint);

const batch0 = [makeUser("h1"), makeToolResult("c1", "r1")];

await callTransform(agent, batch0);

const batch1 = [...batch0, makeUser("h2"), makeToolResult("c2", "r2")];

await callTransform(agent, batch1);

expect(onAfterTurnCheckpoint).toHaveBeenCalledTimes(1);

expect(onAfterTurnCheckpoint).toHaveBeenCalledWith(batch1.length);

});

it("skips afterTurn and assemble when messages have not changed", async () => {

const agent = makeGuardableAgent();

const engine = makeMockEngine();