Compaction/Safeguard: bound preserved tail fallback

src/agents/pi-extensions/compaction-safeguard.test.ts

@@ -559,6 +559,68 @@ describe("compaction-safeguard recent-turn preservation", () => {
     expect(section).not.toContain("[non-text content]");
   });
 
+  it("caps preserved tail when user turns are below preserve target", () => {
+    const messages: AgentMessage[] = [
+      { role: "user", content: "single user prompt", timestamp: 1 },
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "assistant-1" }],
+        timestamp: 2,
+      } as unknown as AgentMessage,
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "assistant-2" }],
+        timestamp: 3,
+      } as unknown as AgentMessage,
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "assistant-3" }],
+        timestamp: 4,
+      } as unknown as AgentMessage,
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "assistant-4" }],
+        timestamp: 5,
+      } as unknown as AgentMessage,
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "assistant-5" }],
+        timestamp: 6,
+      } as unknown as AgentMessage,
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "assistant-6" }],
+        timestamp: 7,
+      } as unknown as AgentMessage,
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "assistant-7" }],
+        timestamp: 8,
+      } as unknown as AgentMessage,
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "assistant-8" }],
+        timestamp: 9,
+      } as unknown as AgentMessage,
+    ];
+
+    const split = splitPreservedRecentTurns({
+      messages,
+      recentTurnsPreserve: 3,
+    });
+
+    // preserve target is 3 turns -> fallback should cap at 6 role messages
+    expect(split.preservedMessages).toHaveLength(6);
+    expect(
+      split.preservedMessages.some(
+        (msg) =>
+          msg.role === "user" && (msg as { content?: unknown }).content === "single user prompt",
+      ),
+    ).toBe(true);
+    expect(formatPreservedTurnsSection(split.preservedMessages)).toContain("assistant-8");
+    expect(formatPreservedTurnsSection(split.preservedMessages)).not.toContain("assistant-2");
+  });
+
   it("clamps preserve count into a safe range", () => {
     expect(resolveRecentTurnsPreserve(undefined)).toBe(3);
     expect(resolveRecentTurnsPreserve(-1)).toBe(0);

src/agents/pi-extensions/compaction-safeguard.ts

@@ -228,26 +228,6 @@ function formatNonTextPlaceholder(content: unknown): string | null {
   return `[non-text content: ${parts.join(", ")}]`;
 }
 
-function findPreservedStartIndexByTurnBoundary(
-  messages: AgentMessage[],
-  preserveTurns: number,
-): number {
-  let seenUsers = 0;
-  let earliestSelectedUserIndex = -1;
-  for (let i = messages.length - 1; i >= 0; i -= 1) {
-    const role = (messages[i] as { role?: unknown }).role;
-    if (role !== "user") {
-      continue;
-    }
-    seenUsers += 1;
-    earliestSelectedUserIndex = i;
-    if (seenUsers >= preserveTurns) {
-      return i;
-    }
-  }
-  return earliestSelectedUserIndex;
-}
-
 function splitPreservedRecentTurns(params: {
   messages: AgentMessage[];
   recentTurnsPreserve: number;
@@ -259,22 +239,42 @@ function splitPreservedRecentTurns(params: {
   if (preserveTurns <= 0) {
     return { summarizableMessages: params.messages, preservedMessages: [] };
   }
-  const boundaryStartIndex = findPreservedStartIndexByTurnBoundary(params.messages, preserveTurns);
+  const conversationIndexes: number[] = [];
+  const userIndexes: number[] = [];
+  for (let i = 0; i < params.messages.length; i += 1) {
+    const role = (params.messages[i] as { role?: unknown }).role;
+    if (role === "user" || role === "assistant") {
+      conversationIndexes.push(i);
+      if (role === "user") {
+        userIndexes.push(i);
+      }
+    }
+  }
+  if (conversationIndexes.length === 0) {
+    return { summarizableMessages: params.messages, preservedMessages: [] };
+  }
+
   const preservedIndexSet = new Set<number>();
-  if (boundaryStartIndex >= 0) {
-    for (let i = boundaryStartIndex; i < params.messages.length; i += 1) {
-      const role = (params.messages[i] as { role?: unknown }).role;
-      if (role === "user" || role === "assistant") {
-        preservedIndexSet.add(i);
+  if (userIndexes.length >= preserveTurns) {
+    const boundaryStartIndex = userIndexes[userIndexes.length - preserveTurns] ?? -1;
+    if (boundaryStartIndex >= 0) {
+      for (const index of conversationIndexes) {
+        if (index >= boundaryStartIndex) {
+          preservedIndexSet.add(index);
+        }
       }
     }
   } else {
     const fallbackMessageCount = preserveTurns * 2;
-    for (let i = params.messages.length - 1; i >= 0; i -= 1) {
-      const role = (params.messages[i] as { role?: unknown }).role;
-      if (role === "user" || role === "assistant") {
-        preservedIndexSet.add(i);
+    for (const userIndex of userIndexes) {
+      preservedIndexSet.add(userIndex);
+    }
+    for (let i = conversationIndexes.length - 1; i >= 0; i -= 1) {
+      const index = conversationIndexes[i];
+      if (index === undefined) {
+        continue;
       }
+      preservedIndexSet.add(index);
       if (preservedIndexSet.size >= fallbackMessageCount) {
         break;
       }