feat: add configurable DM history limit

src/agents/pi-embedded-runner.test.ts

@@ -11,6 +11,7 @@ import {
   applyGoogleTurnOrderingFix,
   buildEmbeddedSandboxInfo,
   createSystemPromptOverride,
+  limitHistoryTurns,
   runEmbeddedPiAgent,
   splitSdkTools,
 } from "./pi-embedded-runner.js";
@@ -279,6 +280,96 @@ describe("applyGoogleTurnOrderingFix", () => {
   });
 });
 
+describe("limitHistoryTurns", () => {
+  const makeMessages = (roles: ("user" | "assistant")[]): AgentMessage[] =>
+    roles.map((role, i) => ({
+      role,
+      content: [{ type: "text", text: `message ${i}` }],
+    }));
+
+  it("returns all messages when limit is undefined", () => {
+    const messages = makeMessages(["user", "assistant", "user", "assistant"]);
+    expect(limitHistoryTurns(messages, undefined)).toBe(messages);
+  });
+
+  it("returns all messages when limit is 0", () => {
+    const messages = makeMessages(["user", "assistant", "user", "assistant"]);
+    expect(limitHistoryTurns(messages, 0)).toBe(messages);
+  });
+
+  it("returns all messages when limit is negative", () => {
+    const messages = makeMessages(["user", "assistant", "user", "assistant"]);
+    expect(limitHistoryTurns(messages, -1)).toBe(messages);
+  });
+
+  it("returns empty array when messages is empty", () => {
+    expect(limitHistoryTurns([], 5)).toEqual([]);
+  });
+
+  it("keeps all messages when fewer user turns than limit", () => {
+    const messages = makeMessages(["user", "assistant", "user", "assistant"]);
+    expect(limitHistoryTurns(messages, 10)).toBe(messages);
+  });
+
+  it("limits to last N user turns", () => {
+    const messages = makeMessages([
+      "user",
+      "assistant",
+      "user",
+      "assistant",
+      "user",
+      "assistant",
+    ]);
+    const limited = limitHistoryTurns(messages, 2);
+    expect(limited.length).toBe(4);
+    expect(limited[0].content).toEqual([{ type: "text", text: "message 2" }]);
+  });
+
+  it("handles single user turn limit", () => {
+    const messages = makeMessages([
+      "user",
+      "assistant",
+      "user",
+      "assistant",
+      "user",
+      "assistant",
+    ]);
+    const limited = limitHistoryTurns(messages, 1);
+    expect(limited.length).toBe(2);
+    expect(limited[0].content).toEqual([{ type: "text", text: "message 4" }]);
+    expect(limited[1].content).toEqual([{ type: "text", text: "message 5" }]);
+  });
+
+  it("handles messages with multiple assistant responses per user turn", () => {
+    const messages = makeMessages([
+      "user",
+      "assistant",
+      "assistant",
+      "user",
+      "assistant",
+    ]);
+    const limited = limitHistoryTurns(messages, 1);
+    expect(limited.length).toBe(2);
+    expect(limited[0].role).toBe("user");
+    expect(limited[1].role).toBe("assistant");
+  });
+
+  it("preserves message content integrity", () => {
+    const messages: AgentMessage[] = [
+      { role: "user", content: [{ type: "text", text: "first" }] },
+      {
+        role: "assistant",
+        content: [{ type: "toolCall", id: "1", name: "bash", arguments: {} }],
+      },
+      { role: "user", content: [{ type: "text", text: "second" }] },
+      { role: "assistant", content: [{ type: "text", text: "response" }] },
+    ];
+    const limited = limitHistoryTurns(messages, 1);
+    expect(limited[0].content).toEqual([{ type: "text", text: "second" }]);
+    expect(limited[1].content).toEqual([{ type: "text", text: "response" }]);
+  });
+});
+
 describe("runEmbeddedPiAgent", () => {
   it("writes models.json into the provided agentDir", async () => {
     const agentDir = await fs.mkdtemp(

src/agents/pi-embedded-runner.ts

@@ -413,6 +413,38 @@ async function sanitizeSessionHistory(params: {
   }).messages;
 }
 
+/**
+ * Limits conversation history to the last N user turns (and their associated
+ * assistant responses). This reduces token usage for long-running DM sessions.
+ *
+ * @param messages - The full message history
+ * @param limit - Max number of user turns to keep (undefined = no limit)
+ * @returns Messages trimmed to the last `limit` user turns
+ */
+export function limitHistoryTurns(
+  messages: AgentMessage[],
+  limit: number | undefined,
+): AgentMessage[] {
+  if (!limit || limit <= 0 || messages.length === 0) return messages;
+
+  // Count user messages from the end, find cutoff point
+  let userCount = 0;
+  let lastUserIndex = messages.length;
+
+  for (let i = messages.length - 1; i >= 0; i--) {
+    if (messages[i].role === "user") {
+      userCount++;
+      if (userCount > limit) {
+        // We exceeded the limit; keep from the last valid user turn onwards
+        return messages.slice(lastUserIndex);
+      }
+      lastUserIndex = i;
+    }
+  }
+  // Fewer than limit user turns, keep all
+  return messages;
+}
+
 const ACTIVE_EMBEDDED_RUNS = new Map<string, EmbeddedPiQueueHandle>();
 type EmbeddedRunWaiter = {
   resolve: (ended: boolean) => void;
@@ -1026,8 +1058,12 @@ export async function compactEmbeddedPiSession(params: {
               sessionId: params.sessionId,
             });
             const validated = validateGeminiTurns(prior);
-            if (validated.length > 0) {
+            const limited = limitHistoryTurns(
-              session.agent.replaceMessages(validated);
+              validated,
+              params.config?.session?.dmHistoryLimit,
+            );
+            if (limited.length > 0) {
+              session.agent.replaceMessages(limited);
             }
             const result = await session.compact(params.customInstructions);
             return {
@@ -1417,8 +1453,12 @@ export async function runEmbeddedPiAgent(params: {
               sessionId: params.sessionId,
             });
             const validated = validateGeminiTurns(prior);
-            if (validated.length > 0) {
+            const limited = limitHistoryTurns(
-              session.agent.replaceMessages(validated);
+              validated,
+              params.config?.session?.dmHistoryLimit,
+            );
+            if (limited.length > 0) {
+              session.agent.replaceMessages(limited);
             }
           } catch (err) {
             session.dispose();

src/config/types.ts

@@ -57,6 +57,7 @@ export type SessionConfig = {
   resetTriggers?: string[];
   idleMinutes?: number;
   heartbeatIdleMinutes?: number;
+  dmHistoryLimit?: number;
   store?: string;
   typingIntervalSeconds?: number;
   typingMode?: TypingMode;

src/config/zod-schema.ts

@@ -630,6 +630,7 @@ const SessionSchema = z
     resetTriggers: z.array(z.string()).optional(),
     idleMinutes: z.number().int().positive().optional(),
     heartbeatIdleMinutes: z.number().int().positive().optional(),
+    dmHistoryLimit: z.number().int().positive().optional(),
     store: z.string().optional(),
     typingIntervalSeconds: z.number().int().positive().optional(),
     typingMode: z