diff --git a/src/agents/harness/hook-history.ts b/src/agents/harness/hook-history.ts
new file mode 100644
index 00000000000..19f52630509
--- /dev/null
+++ b/src/agents/harness/hook-history.ts
@@ -0,0 +1,21 @@
+export const MAX_AGENT_HOOK_HISTORY_MESSAGES = 100;
+
+export function limitAgentHookHistoryMessages(
+  messages: readonly unknown[],
+  maxMessages = MAX_AGENT_HOOK_HISTORY_MESSAGES,
+): unknown[] {
+  if (maxMessages <= 0) {
+    return [];
+  }
+  return messages.slice(-maxMessages);
+}
+
+export function buildAgentHookConversationMessages(params: {
+  historyMessages?: readonly unknown[];
+  currentTurnMessages?: readonly unknown[];
+}): unknown[] {
+  return [
+    ...limitAgentHookHistoryMessages(params.historyMessages ?? []),
+    ...(params.currentTurnMessages ?? []),
+  ];
+}