refactor(agent): snapshot harness turn state

packages/agent/src/agent-loop.ts

@@ -153,13 +153,15 @@ function createAgentStream(): EventStream<AgentEvent, AgentMessage[]> {
  * Main loop logic shared by agentLoop and agentLoopContinue.
  */
 async function runLoop(
-	currentContext: AgentContext,
+	initialContext: AgentContext,
 	newMessages: AgentMessage[],
-	config: AgentLoopConfig,
+	initialConfig: AgentLoopConfig,
 	signal: AbortSignal | undefined,
 	emit: AgentEventSink,
 	streamFn?: StreamFn,
 ): Promise<void> {
+	let currentContext = initialContext;
+	let config = initialConfig;
 	let firstTurn = true;
 	// Check for steering messages at start (user may have typed while waiting)
 	let pendingMessages: AgentMessage[] = (await config.getSteeringMessages?.()) || [];
@@ -215,6 +217,27 @@ async function runLoop(
 
 			await emit({ type: "turn_end", message, toolResults });
 
+			const nextTurnContext = {
+				message,
+				toolResults,
+				context: currentContext,
+				newMessages,
+			};
+			const nextTurnSnapshot = await config.prepareNextTurn?.(nextTurnContext);
+			if (nextTurnSnapshot) {
+				currentContext = nextTurnSnapshot.context ?? currentContext;
+				config = {
+					...config,
+					model: nextTurnSnapshot.model ?? config.model,
+					reasoning:
+						nextTurnSnapshot.thinkingLevel === undefined
+							? config.reasoning
+							: nextTurnSnapshot.thinkingLevel === "off"
+								? undefined
+								: nextTurnSnapshot.thinkingLevel,
+				};
+			}
+
 			if (
 				await config.shouldStopAfterTurn?.({
 					message,