fix(agent): delivered checkpoint reminder to active loop

packages/coding-agent/src/session/agent-session.ts

@@ -2344,11 +2344,11 @@ export class AgentSession {
 
 	/**
 	 * Builds the transient checkpoint-active reminder for a successful
-	 * checkpoint tool result, or undefined otherwise. The reminder is appended
-	 * to agent.state synchronously in the message_end handler (before any
-	 * await) so the next provider call within the same tool loop sees it, and
-	 * is persisted via #persistMessageEnd. Because the entry sits after the
-	 * checkpoint entry, the rewind branch cut drops it from the active path.
+	 * checkpoint tool result, or undefined otherwise. The reminder is queued as
+	 * steering synchronously in the message_end handler (before any await), so
+	 * the agent loop folds it into the next provider call and persists it through
+	 * its normal custom-message event. Because the entry sits after the checkpoint
+	 * entry, the rewind branch cut drops it from the active path.
 	 */
 	#checkpointActiveReminderFor(
 		message: AgentMessage,
@@ -2527,14 +2527,12 @@ export class AgentSession {
 			this.agent.appendMessage(interruptedThinkingMessage);
 		}
 
-		// Same pre-await visibility requirement as the interrupted-thinking
-		// message: agent-core invokes message_end listeners fire-and-forget, so
-		// the next provider call can start before any awaited session-event
-		// emission or persistence below settles. The checkpoint-active reminder
-		// must already be in agent.state for that call — append it synchronously
-		// here and persist it later (see #persistMessageEnd). It sits after the
-		// checkpoint entry, so the rewind branch cut
-		// (branchWithSummary(checkpointEntryId)) drops it from the active path
+		// message_end listeners are fire-and-forget, and the agent loop runs against
+		// a context cloned at prompt start. Appending only to agent.state would not
+		// reach the next provider call in this tool loop. Queue the reminder as
+		// steering before any await so the loop drains it at the next step boundary;
+		// its normal custom-message event persists it after the checkpoint entry,
+		// allowing the rewind branch cut to drop it from the active path.
 		const checkpointReminder =
 			event.type === "message_end" && event.message.role === "toolResult"
 				? this.#checkpointActiveReminderFor(event.message)
@@ -2555,7 +2553,7 @@ export class AgentSession {
 			};
 			this.#pendingRewindReport = undefined;
 			this.#lastCompletedRewind = undefined;
-			this.agent.appendMessage(checkpointReminder);
+			this.agent.steer(checkpointReminder);
 		}
 
 		const messageEndPersistence =
@@ -2677,15 +2675,6 @@ export class AgentSession {
 					interruptedThinkingMessage.attribution,
 				);
 			}
-			if (checkpointReminder) {
-				this.sessionManager.appendCustomMessageEntry(
-					checkpointReminder.customType,
-					checkpointReminder.content,
-					checkpointReminder.display,
-					checkpointReminder.details,
-					checkpointReminder.attribution,
-				);
-			}
 			// Other message types (bashExecution, compactionSummary, branchSummary) are persisted elsewhere
 
 			if (event.message.role === "assistant") {

packages/coding-agent/test/agent-session-checkpoint-rewind-branch.test.ts

@@ -301,9 +301,10 @@ describe("AgentSession checkpoint rewind branch context", () => {
 		while (
 			!session.messages.some(
 				message => message.role === "custom" && message.customType === "checkpoint-active-reminder",
-			)
+			) ||
+			mock.calls.length < 2
 		) {
-			if (Date.now() > deadline) throw new Error("checkpoint-active-reminder never appeared");
+			if (Date.now() > deadline) throw new Error("checkpoint reminder/provider call never appeared");
 			await Bun.sleep(10);
 		}
 		const reminder = session.messages.find(
@@ -312,6 +313,11 @@ describe("AgentSession checkpoint rewind branch context", () => {
 		);
 		expect(reminder).toBeDefined();
 		expect(reminder?.content).toContain("MUST `rewind` before yielding");
+		const activeCall = mock.calls[1];
+		expect(activeCall).toBeDefined();
+		expect(
+			activeCall?.context.messages.some(message => messageText(message).includes("Exploration checkpoint active.")),
+		).toBe(true);
 		// #checkpointState is set synchronously with the reminder (pre-await), so an
 		// immediate rewind would find an active checkpoint, not "No active checkpoint".
 		expect(session.getCheckpointState()).toBeDefined();