Compaction/Safeguard: preserve recent turns verbatim

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

@@ -10,6 +10,7 @@ export type CompactionSafeguardRuntimeValue = {
    * (extensionRunner.initialize() is never called in that path).
    */
   model?: Model<Api>;
+  recentTurnsPreserve?: number;
 };
 
 const registry = createSessionManagerRuntimeRegistry<CompactionSafeguardRuntimeValue>();

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

@@ -11,6 +11,9 @@ import compactionSafeguardExtension, { __testing } from "./compaction-safeguard.
 const {
   collectToolFailures,
   formatToolFailuresSection,
+  splitPreservedRecentTurns,
+  formatPreservedTurnsSection,
+  resolveRecentTurnsPreserve,
   computeAdaptiveChunkRatio,
   isOversizedForSummary,
   BASE_CHUNK_RATIO,
@@ -363,6 +366,42 @@ describe("compaction-safeguard runtime registry", () => {
   });
 });
 
+describe("compaction-safeguard recent-turn preservation", () => {
+  it("preserves the most recent user/assistant messages", () => {
+    const messages: AgentMessage[] = [
+      { role: "user", content: "older ask", timestamp: 1 },
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "older answer" }],
+        timestamp: 2,
+      } as unknown as AgentMessage,
+      { role: "user", content: "recent ask", timestamp: 3 },
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "recent answer" }],
+        timestamp: 4,
+      } as unknown as AgentMessage,
+    ];
+
+    const split = splitPreservedRecentTurns({
+      messages,
+      recentTurnsPreserve: 1,
+    });
+
+    expect(split.preservedMessages).toHaveLength(2);
+    expect(split.summarizableMessages).toHaveLength(2);
+    expect(formatPreservedTurnsSection(split.preservedMessages)).toContain(
+      "## Recent turns preserved verbatim",
+    );
+  });
+
+  it("clamps preserve count into a safe range", () => {
+    expect(resolveRecentTurnsPreserve(undefined)).toBe(3);
+    expect(resolveRecentTurnsPreserve(-1)).toBe(0);
+    expect(resolveRecentTurnsPreserve(99)).toBe(12);
+  });
+});
+
 describe("compaction-safeguard extension model fallback", () => {
   it("uses runtime.model when ctx.model is undefined (compact.ts workflow)", async () => {
     // This test verifies the root-cause fix: when extensionRunner.initialize() is not called

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

@@ -28,6 +28,9 @@ const TURN_PREFIX_INSTRUCTIONS =
   " early progress, and any details needed to understand the retained suffix.";
 const MAX_TOOL_FAILURES = 8;
 const MAX_TOOL_FAILURE_CHARS = 240;
+const DEFAULT_RECENT_TURNS_PRESERVE = 3;
+const MAX_RECENT_TURNS_PRESERVE = 12;
+const MAX_RECENT_TURN_TEXT_CHARS = 600;
 
 type ToolFailure = {
   toolCallId: string;
@@ -36,6 +39,18 @@ type ToolFailure = {
   meta?: string;
 };
 
+function clampNonNegativeInt(value: unknown, fallback: number): number {
+  const normalized = typeof value === "number" && Number.isFinite(value) ? value : fallback;
+  return Math.max(0, Math.floor(normalized));
+}
+
+function resolveRecentTurnsPreserve(value: unknown): number {
+  return Math.min(
+    MAX_RECENT_TURNS_PRESERVE,
+    clampNonNegativeInt(value, DEFAULT_RECENT_TURNS_PRESERVE),
+  );
+}
+
 function normalizeFailureText(text: string): string {
   return text.replace(/\s+/g, " ").trim();
 }
@@ -158,6 +173,87 @@ function formatFileOperations(readFiles: string[], modifiedFiles: string[]): str
   return `\n\n${sections.join("\n\n")}`;
 }
 
+function extractMessageText(message: AgentMessage): string {
+  const content = (message as { content?: unknown }).content;
+  if (typeof content === "string") {
+    return content.trim();
+  }
+  if (!Array.isArray(content)) {
+    return "";
+  }
+  const parts: string[] = [];
+  for (const block of content) {
+    if (!block || typeof block !== "object") {
+      continue;
+    }
+    const text = (block as { text?: unknown }).text;
+    if (typeof text === "string" && text.trim().length > 0) {
+      parts.push(text.trim());
+    }
+  }
+  return parts.join("\n").trim();
+}
+
+function splitPreservedRecentTurns(params: {
+  messages: AgentMessage[];
+  recentTurnsPreserve: number;
+}): { summarizableMessages: AgentMessage[]; preservedMessages: AgentMessage[] } {
+  const preserveTurns = Math.min(
+    MAX_RECENT_TURNS_PRESERVE,
+    clampNonNegativeInt(params.recentTurnsPreserve, 0),
+  );
+  if (preserveTurns <= 0) {
+    return { summarizableMessages: params.messages, preservedMessages: [] };
+  }
+  const preserveMessages = preserveTurns * 2;
+  const candidateIndexes: number[] = [];
+  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") {
+      candidateIndexes.push(i);
+    }
+    if (candidateIndexes.length >= preserveMessages) {
+      break;
+    }
+  }
+  if (candidateIndexes.length === 0) {
+    return { summarizableMessages: params.messages, preservedMessages: [] };
+  }
+  const preservedIndexSet = new Set(candidateIndexes);
+  const summarizableMessages = params.messages.filter((_, idx) => !preservedIndexSet.has(idx));
+  const preservedMessages = params.messages
+    .filter((_, idx) => preservedIndexSet.has(idx))
+    .filter((msg) => {
+      const role = (msg as { role?: unknown }).role;
+      return role === "user" || role === "assistant";
+    });
+  return { summarizableMessages, preservedMessages };
+}
+
+function formatPreservedTurnsSection(messages: AgentMessage[]): string {
+  if (messages.length === 0) {
+    return "";
+  }
+  const lines = messages
+    .map((message) => {
+      const role = message.role === "assistant" ? "Assistant" : "User";
+      const text = extractMessageText(message);
+      if (!text) {
+        return null;
+      }
+      const trimmed =
+        text.length > MAX_RECENT_TURN_TEXT_CHARS
+          ? `${text.slice(0, MAX_RECENT_TURN_TEXT_CHARS)}...`
+          : text;
+      return `- ${role}: ${trimmed}`;
+    })
+    .filter((line): line is string => Boolean(line));
+  if (lines.length === 0) {
+    return "";
+  }
+  return `\n\n## Recent turns preserved verbatim\n${lines.join("\n")}`;
+}
+
 /**
  * Read and format critical workspace context for compaction summary.
  * Extracts "Session Startup" and "Red Lines" from AGENTS.md.
@@ -240,6 +336,7 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
       const contextWindowTokens = runtime?.contextWindowTokens ?? modelContextWindow;
       const turnPrefixMessages = preparation.turnPrefixMessages ?? [];
       let messagesToSummarize = preparation.messagesToSummarize;
+      const recentTurnsPreserve = resolveRecentTurnsPreserve(runtime?.recentTurnsPreserve);
 
       const maxHistoryShare = runtime?.maxHistoryShare ?? 0.5;
 
@@ -309,6 +406,16 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
         }
       }
 
+      const {
+        summarizableMessages: summaryTargetMessages,
+        preservedMessages: preservedRecentMessages,
+      } = splitPreservedRecentTurns({
+        messages: messagesToSummarize,
+        recentTurnsPreserve,
+      });
+      messagesToSummarize = summaryTargetMessages;
+      const preservedTurnsSection = formatPreservedTurnsSection(preservedRecentMessages);
+
       // Use adaptive chunk ratio based on message sizes, reserving headroom for
       // the summarization prompt, system prompt, previous summary, and reasoning budget
       // that generateSummary adds on top of the serialized conversation chunk.
@@ -351,6 +458,7 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
         });
         summary = `${historySummary}\n\n---\n\n**Turn Context (split turn):**\n\n${prefixSummary}`;
       }
+      summary += preservedTurnsSection;
 
       summary += toolFailureSection;
       summary += fileOpsSummary;
@@ -383,6 +491,9 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
 export const __testing = {
   collectToolFailures,
   formatToolFailuresSection,
+  splitPreservedRecentTurns,
+  formatPreservedTurnsSection,
+  resolveRecentTurnsPreserve,
   computeAdaptiveChunkRatio,
   isOversizedForSummary,
   BASE_CHUNK_RATIO,