refactor(agent): snapshot harness turn state

packages/agent/test/agent-loop.test.ts

@@ -894,6 +894,79 @@ describe("agentLoop with AgentMessage", () => {
 		expect(parallelObserved).toBe(true);
 	});
 
+	it("should use prepareNextTurn snapshot before continuing", async () => {
+		const toolSchema = Type.Object({ value: Type.String() });
+		const tool: AgentTool<typeof toolSchema, { value: string }> = {
+			name: "echo",
+			label: "Echo",
+			description: "Echo tool",
+			parameters: toolSchema,
+			async execute(_toolCallId, params) {
+				return {
+					content: [{ type: "text", text: `echoed: ${params.value}` }],
+					details: { value: params.value },
+				};
+			},
+		};
+		const context: AgentContext = {
+			systemPrompt: "first prompt",
+			messages: [],
+			tools: [tool],
+		};
+		let convertedSecondTurnSystemPrompt = "";
+		let prepared = false;
+		const config: AgentLoopConfig = {
+			model: createModel(),
+			convertToLlm: identityConverter,
+			prepareNextTurn: async ({ context: currentContext }) => {
+				if (prepared) return undefined;
+				prepared = true;
+				return {
+					context: {
+						systemPrompt: "second prompt",
+						messages: currentContext.messages.slice(),
+						tools: currentContext.tools,
+					},
+				};
+			},
+		};
+
+		let llmCalls = 0;
+		const stream = agentLoop([createUserMessage("echo something")], context, config, undefined, (_model, ctx) => {
+			llmCalls++;
+			if (llmCalls === 2) {
+				convertedSecondTurnSystemPrompt = ctx.systemPrompt ?? "";
+			}
+			const mockStream = new MockAssistantStream();
+			queueMicrotask(() => {
+				if (llmCalls === 1) {
+					mockStream.push({
+						type: "done",
+						reason: "toolUse",
+						message: createAssistantMessage(
+							[{ type: "toolCall", id: "tool-1", name: "echo", arguments: { value: "hello" } }],
+							"toolUse",
+						),
+					});
+				} else {
+					mockStream.push({
+						type: "done",
+						reason: "stop",
+						message: createAssistantMessage([{ type: "text", text: "done" }]),
+					});
+				}
+			});
+			return mockStream;
+		});
+
+		for await (const _event of stream) {
+			// consume
+		}
+
+		expect(llmCalls).toBe(2);
+		expect(convertedSecondTurnSystemPrompt).toBe("second prompt");
+	});
+
 	it("should stop after the current turn when shouldStopAfterTurn returns true", async () => {
 		const toolSchema = Type.Object({ value: Type.String() });
 		const executed: string[] = [];