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(