diff --git a/src/index.ts b/src/index.ts
index a2788a7..7b08550 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -59,6 +59,9 @@ export type {
   ImageContent,
   MessageContentItem,
   SendMessage,
+  // List messages API
+  ListMessagesOptions,
+  ListMessagesResult,
   // Tool types
   AgentTool,
   AgentToolResult,
diff --git a/src/session.ts b/src/session.ts
index e8084bd..e9856b1 100644
--- a/src/session.ts
+++ b/src/session.ts
@@ -21,6 +21,8 @@ import type {
   SendMessage,
   AnyAgentTool,
   ExecuteExternalToolRequest,
+  ListMessagesOptions,
+  ListMessagesResult,
 } from "./types.js";
 import {
   isHeadlessAutoAllowTool,
@@ -47,6 +49,13 @@ export class Session implements AsyncDisposable {
   private pumpPromise: Promise<void> | null = null;
   private pumpClosed = false;
   private droppedStreamMessages = 0;
+  // Waiters for SDK-initiated control requests (e.g., listMessages).
+  // Keyed by request_id; pump resolves the matching waiter when it sees
+  // a control_response with that request_id instead of queuing it as a stream msg.
+  private controlResponseWaiters = new Map<
+    string,
+    (response: { subtype: string; response?: unknown; error?: string }) => void
+  >();
 
   constructor(
     private options: InternalSessionOptions = {}
@@ -254,6 +263,23 @@ export class Session implements AsyncDisposable {
         continue;
       }
 
+      // Route control_response to a registered waiter (e.g., from listMessages).
+      // Unmatched control_responses are logged and dropped — they never reach the stream.
+      if (wireMsg.type === "control_response") {
+        const respMsg = wireMsg as unknown as {
+          response: { subtype: string; request_id?: string; response?: unknown; error?: string };
+        };
+        const requestId = respMsg.response?.request_id;
+        if (requestId && this.controlResponseWaiters.has(requestId)) {
+          const resolve = this.controlResponseWaiters.get(requestId)!;
+          this.controlResponseWaiters.delete(requestId);
+          resolve(respMsg.response);
+        } else {
+          sessionLog("pump", `DROPPED unmatched control_response: request_id=${requestId ?? "N/A"}`);
+        }
+        continue;
+      }
+
       const sdkMsg = this.transformMessage(wireMsg);
       if (sdkMsg) {
         this.enqueueStreamMessage(sdkMsg);
@@ -332,6 +358,11 @@ export class Session implements AsyncDisposable {
       resolve(msg);
     }
     this.streamResolvers = [];
+    // Also cancel any in-flight control request waiters (e.g., listMessages)
+    for (const resolve of this.controlResponseWaiters.values()) {
+      resolve({ subtype: "error", error: "session closed" });
+    }
+    this.controlResponseWaiters.clear();
   }
 
   /**
@@ -538,6 +569,65 @@ export class Session implements AsyncDisposable {
     });
   }
 
+  /**
+   * Fetch a page of conversation messages via the CLI control protocol.
+   *
+   * The session must be initialized before calling this method.
+   * Safe to call concurrently with an active stream() — the pump routes
+   * matching control_response messages to this waiter without touching the
+   * stream queue.
+   */
+  async listMessages(options: ListMessagesOptions = {}): Promise<ListMessagesResult> {
+    if (!this.initialized) {
+      throw new Error("Session must be initialized before calling listMessages()");
+    }
+
+    const requestId = `list-${Date.now()}-${Math.random().toString(36).slice(2, 7)}`;
+
+    const responsePromise = new Promise<{
+      subtype: string;
+      response?: unknown;
+      error?: string;
+    }>((resolve) => {
+      this.controlResponseWaiters.set(requestId, resolve);
+    });
+
+    await this.transport.write({
+      type: "control_request",
+      request_id: requestId,
+      request: {
+        subtype: "list_messages",
+        ...(options.conversationId ? { conversation_id: options.conversationId } : {}),
+        ...(options.before ? { before: options.before } : {}),
+        ...(options.after ? { after: options.after } : {}),
+        ...(options.order ? { order: options.order } : {}),
+        ...(options.limit !== undefined ? { limit: options.limit } : {}),
+      },
+    });
+
+    // Race against session close (pump sets pumpClosed and resolves all waiters with null)
+    const resp = await responsePromise;
+
+    if (!resp) {
+      throw new Error("Session closed before listMessages response arrived");
+    }
+    if (resp.subtype === "error") {
+      throw new Error(resp.error ?? "listMessages failed");
+    }
+
+    const payload = resp.response as {
+      messages?: unknown[];
+      next_before?: string | null;
+      has_more?: boolean;
+    } | undefined;
+
+    return {
+      messages: payload?.messages ?? [],
+      nextBefore: payload?.next_before ?? null,
+      hasMore: payload?.has_more ?? false,
+    };
+  }
+
   /**
    * Close the session
    */
diff --git a/src/tests/list-messages.test.ts b/src/tests/list-messages.test.ts
new file mode 100644
index 0000000..3a65053
--- /dev/null
+++ b/src/tests/list-messages.test.ts
@@ -0,0 +1,396 @@
+/**
+ * Unit tests for listMessages() SDK layer.
+ *
+ * Covers:
+ * 1. ListMessagesOptions / ListMessagesResult type shapes
+ * 2. controlResponseWaiters routing (pump mock) — concurrent, error, close cleanup
+ * 3. includePartialMessages flag forwarding to CLI args
+ * 4. Waiter resolution while a stream is active (concurrent safety)
+ * 5. Close / error waiter cleanup — no hanging promises
+ *
+ * Real end-to-end tests with a live CLI are in the manual smoke suite.
+ */
+import { describe, expect, test } from "bun:test";
+import type { ListMessagesOptions, ListMessagesResult } from "../types.js";
+
+// ─────────────────────────────────────────────────────────────────────────────
+// 1. Type shapes
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("ListMessagesOptions type", () => {
+  test("accepts all optional fields", () => {
+    const opts: ListMessagesOptions = {
+      conversationId: "conv-123",
+      before: "msg-abc",
+      after: "msg-xyz",
+      order: "desc",
+      limit: 50,
+    };
+    expect(opts.conversationId).toBe("conv-123");
+    expect(opts.limit).toBe(50);
+  });
+
+  test("accepts empty options object", () => {
+    const opts: ListMessagesOptions = {};
+    expect(opts.conversationId).toBeUndefined();
+    expect(opts.limit).toBeUndefined();
+  });
+
+  test("order can be asc or desc", () => {
+    const asc: ListMessagesOptions = { order: "asc" };
+    const desc: ListMessagesOptions = { order: "desc" };
+    expect(asc.order).toBe("asc");
+    expect(desc.order).toBe("desc");
+  });
+});
+
+describe("ListMessagesResult type", () => {
+  test("well-formed success result with messages", () => {
+    const result: ListMessagesResult = {
+      messages: [{ id: "msg-1", message_type: "user_message" }],
+      nextBefore: "msg-1",
+      hasMore: false,
+    };
+    expect(result.messages).toHaveLength(1);
+    expect(result.hasMore).toBe(false);
+    expect(result.nextBefore).toBe("msg-1");
+  });
+
+  test("empty page — nextBefore is null", () => {
+    const result: ListMessagesResult = {
+      messages: [],
+      nextBefore: null,
+      hasMore: false,
+    };
+    expect(result.messages).toHaveLength(0);
+    expect(result.nextBefore).toBeNull();
+  });
+
+  test("partial page — hasMore is true", () => {
+    const result: ListMessagesResult = {
+      messages: new Array(50).fill({ id: "x" }),
+      nextBefore: "msg-50",
+      hasMore: true,
+    };
+    expect(result.hasMore).toBe(true);
+    expect(result.nextBefore).toBe("msg-50");
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// 2. controlResponseWaiters routing
+// ─────────────────────────────────────────────────────────────────────────────
+
+/** Minimal pump simulator — mirrors the routing logic in session.ts. */
+function makePump() {
+  const waiters = new Map<
+    string,
+    (resp: { subtype: string; response?: unknown; error?: string }) => void
+  >();
+
+  function route(wireMsg: {
+    type: string;
+    response?: {
+      subtype: string;
+      request_id?: string;
+      response?: unknown;
+      error?: string;
+    };
+  }): boolean {
+    if (wireMsg.type !== "control_response") return false;
+    const requestId = wireMsg.response?.request_id;
+    if (requestId && waiters.has(requestId)) {
+      const resolve = waiters.get(requestId)!;
+      waiters.delete(requestId);
+      resolve(wireMsg.response!);
+      return true;
+    }
+    return false;
+  }
+
+  /** Simulate session.close() clearing all waiters with an error. */
+  function closeAll() {
+    for (const resolve of waiters.values()) {
+      resolve({ subtype: "error", error: "session closed" });
+    }
+    waiters.clear();
+  }
+
+  return { waiters, route, closeAll };
+}
+
+describe("controlResponseWaiters routing", () => {
+  test("routes matching control_response to waiter and removes it", async () => {
+    const { waiters, route } = makePump();
+
+    const promise = new Promise<{ subtype: string; response?: unknown }>(
+      (res) => { waiters.set("list_001", res); }
+    );
+
+    const handled = route({
+      type: "control_response",
+      response: {
+        subtype: "success",
+        request_id: "list_001",
+        response: { messages: [], has_more: false },
+      },
+    });
+
+    expect(handled).toBe(true);
+    const resp = await promise;
+    expect(resp.subtype).toBe("success");
+    expect(waiters.size).toBe(0); // waiter consumed
+  });
+
+  test("drops unmatched control_response (no registered waiter)", () => {
+    const { waiters, route } = makePump();
+
+    const handled = route({
+      type: "control_response",
+      response: { subtype: "success", request_id: "unknown_id" },
+    });
+
+    expect(handled).toBe(false);
+    expect(waiters.size).toBe(0);
+  });
+
+  test("routes error subtype to waiter", async () => {
+    const { waiters, route } = makePump();
+
+    const promise = new Promise<{ subtype: string; error?: string }>(
+      (res) => { waiters.set("list_002", res); }
+    );
+
+    route({
+      type: "control_response",
+      response: {
+        subtype: "error",
+        request_id: "list_002",
+        error: "conversation not found",
+      },
+    });
+
+    const resp = await promise;
+    expect(resp.subtype).toBe("error");
+    expect(resp.error).toContain("conversation not found");
+  });
+
+  test("concurrent waiters for different request_ids resolve independently", async () => {
+    const { waiters, route } = makePump();
+
+    const p1 = new Promise<{ subtype: string; response?: unknown }>(
+      (res) => waiters.set("req_A", res)
+    );
+    const p2 = new Promise<{ subtype: string; response?: unknown }>(
+      (res) => waiters.set("req_B", res)
+    );
+
+    // Deliver in reverse order — both should resolve to their own response
+    route({
+      type: "control_response",
+      response: { subtype: "success", request_id: "req_B", response: { messages: [1] } },
+    });
+    route({
+      type: "control_response",
+      response: { subtype: "success", request_id: "req_A", response: { messages: [2] } },
+    });
+
+    const [rA, rB] = await Promise.all([p1, p2]);
+    expect(rA.subtype).toBe("success");
+    expect(rB.subtype).toBe("success");
+    expect(waiters.size).toBe(0);
+  });
+
+  test("non-control_response messages are ignored (pass-through)", () => {
+    const { waiters, route } = makePump();
+    waiters.set("req_X", () => { throw new Error("should not be called"); });
+
+    const handled = route({ type: "assistant_message" });
+    expect(handled).toBe(false);
+    expect(waiters.size).toBe(1); // waiter still registered
+  });
+
+  test("control_response without request_id is dropped", () => {
+    const { waiters, route } = makePump();
+    waiters.set("req_Y", () => { throw new Error("should not be called"); });
+
+    const handled = route({
+      type: "control_response",
+      response: { subtype: "success" /* no request_id */ },
+    });
+    expect(handled).toBe(false);
+    expect(waiters.size).toBe(1);
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// 3. includePartialMessages arg forwarding
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("includePartialMessages arg forwarding", () => {
+  /**
+   * Simulate transport.ts buildArgs() to verify the flag is included.
+   * The actual transport builds the args array and passes it to spawn().
+   */
+  function buildArgs(options: {
+    agentId?: string;
+    conversationId?: string;
+    includePartialMessages?: boolean;
+  }): string[] {
+    const args: string[] = ["--output", "stream-json"];
+    if (options.agentId) args.push("--agent", options.agentId);
+    if (options.conversationId) args.push("--conv", options.conversationId);
+    if (options.includePartialMessages) args.push("--include-partial-messages");
+    return args;
+  }
+
+  test("flag absent when includePartialMessages is false", () => {
+    const args = buildArgs({ agentId: "agent-1", includePartialMessages: false });
+    expect(args).not.toContain("--include-partial-messages");
+  });
+
+  test("flag absent when includePartialMessages is undefined", () => {
+    const args = buildArgs({ agentId: "agent-1" });
+    expect(args).not.toContain("--include-partial-messages");
+  });
+
+  test("flag present when includePartialMessages is true", () => {
+    const args = buildArgs({ agentId: "agent-1", includePartialMessages: true });
+    expect(args).toContain("--include-partial-messages");
+  });
+
+  test("flag position is after other args (no disruption)", () => {
+    const args = buildArgs({
+      agentId: "agent-1",
+      conversationId: "conv-abc",
+      includePartialMessages: true,
+    });
+    // Other args still present
+    expect(args).toContain("--agent");
+    expect(args).toContain("agent-1");
+    expect(args).toContain("--conv");
+    expect(args).toContain("conv-abc");
+    expect(args).toContain("--include-partial-messages");
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// 4. Waiter while stream is active
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("listMessages waiter while stream active", () => {
+  /**
+   * The key invariant: a listMessages call can be in-flight at the same time
+   * as a stream is running.  The pump should route stream events to stream
+   * consumers and control_responses to listMessages waiters independently —
+   * no interference.
+   */
+  test("control_response for listMessages does not discard stream messages", async () => {
+    const { waiters, route } = makePump();
+
+    // Register a listMessages waiter
+    const listPromise = new Promise<{ subtype: string; response?: unknown }>(
+      (res) => { waiters.set("concurrent_list", res); }
+    );
+
+    // Simulate stream events arriving first
+    const streamEvents = [
+      { type: "assistant_message", content: "hello" },
+      { type: "tool_call_message" },
+    ];
+    for (const ev of streamEvents) {
+      // Stream events are not control_response, so pump returns false
+      const handled = route(ev as Parameters<typeof route>[0]);
+      expect(handled).toBe(false);
+    }
+    expect(waiters.size).toBe(1); // listMessages waiter still waiting
+
+    // Now the control_response arrives
+    route({
+      type: "control_response",
+      response: {
+        subtype: "success",
+        request_id: "concurrent_list",
+        response: { messages: [{ id: "m1" }], has_more: false },
+      },
+    });
+
+    const resp = await listPromise;
+    expect(resp.subtype).toBe("success");
+    expect(waiters.size).toBe(0);
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// 5. Close / error waiter cleanup
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("waiter cleanup on session close", () => {
+  test("close cancels in-flight listMessages waiter with error", async () => {
+    const { waiters, closeAll } = makePump();
+
+    const listPromise = new Promise<{ subtype: string; error?: string }>(
+      (res) => { waiters.set("inflight_req", res); }
+    );
+
+    // Session closes before response arrives
+    closeAll();
+
+    const resp = await listPromise;
+    expect(resp.subtype).toBe("error");
+    expect(resp.error).toBe("session closed");
+    expect(waiters.size).toBe(0); // map drained
+  });
+
+  test("close cancels multiple concurrent waiters", async () => {
+    const { waiters, closeAll } = makePump();
+
+    const p1 = new Promise<{ subtype: string; error?: string }>(
+      (res) => { waiters.set("req_1", res); }
+    );
+    const p2 = new Promise<{ subtype: string; error?: string }>(
+      (res) => { waiters.set("req_2", res); }
+    );
+    const p3 = new Promise<{ subtype: string; error?: string }>(
+      (res) => { waiters.set("req_3", res); }
+    );
+
+    closeAll();
+
+    const results = await Promise.all([p1, p2, p3]);
+    for (const r of results) {
+      expect(r.subtype).toBe("error");
+      expect(r.error).toBe("session closed");
+    }
+    expect(waiters.size).toBe(0);
+  });
+
+  test("close is idempotent — second close does not throw", () => {
+    const { waiters, closeAll } = makePump();
+    waiters.set("req", (r) => { void r; });
+
+    closeAll();
+    expect(() => closeAll()).not.toThrow(); // already empty, should be safe
+    expect(waiters.size).toBe(0);
+  });
+
+  test("waiter registered after close is never resolved (guard by caller)", async () => {
+    // This verifies that if a caller checks initialized before calling listMessages,
+    // a late waiter registration doesn't silently hang.
+    // We model this as: waiter registered, then close fires immediately.
+    const { waiters, closeAll } = makePump();
+
+    let resolved = false;
+    const listPromise = new Promise<{ subtype: string; error?: string }>((res) => {
+      waiters.set("late_req", res);
+      // Close fires synchronously before any response can arrive
+      closeAll();
+      resolved = true;
+    });
+
+    const resp = await listPromise;
+    expect(resolved).toBe(true);
+    expect(resp.subtype).toBe("error");
+    expect(resp.error).toBe("session closed");
+  });
+});
diff --git a/src/tests/transport-args.test.ts b/src/tests/transport-args.test.ts
new file mode 100644
index 0000000..e8ebe60
--- /dev/null
+++ b/src/tests/transport-args.test.ts
@@ -0,0 +1,263 @@
+/**
+ * Tests for buildCliArgs() — the real production function that builds
+ * the CLI argument array passed to spawn().
+ *
+ * These tests exercise the actual transport code path, not a local replica.
+ * SubprocessTransport.buildArgs() is a thin delegation to buildCliArgs(),
+ * so testing buildCliArgs() directly covers the production spawn args.
+ */
+import { describe, expect, test } from "bun:test";
+import { buildCliArgs } from "../transport.js";
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Baseline: every invocation includes these two pairs
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("buildCliArgs — baseline args", () => {
+  test("always includes --output-format stream-json and --input-format stream-json", () => {
+    const args = buildCliArgs({});
+    expect(args).toContain("--output-format");
+    expect(args).toContain("stream-json");
+    expect(args).toContain("--input-format");
+    const outIdx = args.indexOf("--output-format");
+    expect(args[outIdx + 1]).toBe("stream-json");
+    const inIdx = args.indexOf("--input-format");
+    expect(args[inIdx + 1]).toBe("stream-json");
+  });
+
+  test("minimum invocation (empty options) produces exactly the two baseline pairs", () => {
+    const args = buildCliArgs({});
+    expect(args).toEqual([
+      "--output-format", "stream-json",
+      "--input-format", "stream-json",
+    ]);
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// includePartialMessages — the flag that started this investigation
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("buildCliArgs — includePartialMessages", () => {
+  test("flag absent when includePartialMessages is undefined", () => {
+    const args = buildCliArgs({ agentId: "agent-1" });
+    expect(args).not.toContain("--include-partial-messages");
+  });
+
+  test("flag absent when includePartialMessages is false", () => {
+    const args = buildCliArgs({ agentId: "agent-1", includePartialMessages: false });
+    expect(args).not.toContain("--include-partial-messages");
+  });
+
+  test("flag present when includePartialMessages is true", () => {
+    const args = buildCliArgs({ agentId: "agent-1", includePartialMessages: true });
+    expect(args).toContain("--include-partial-messages");
+  });
+
+  test("flag appears after conversation/agent args, not before", () => {
+    const args = buildCliArgs({
+      agentId: "agent-1",
+      conversationId: "conv-abc",
+      includePartialMessages: true,
+    });
+    const convIdx = args.indexOf("--conversation");
+    const flagIdx = args.indexOf("--include-partial-messages");
+    expect(convIdx).toBeGreaterThan(-1);
+    expect(flagIdx).toBeGreaterThan(convIdx);
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Conversation / agent routing
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("buildCliArgs — conversation and agent routing", () => {
+  test("conversationId → --conversation flag (agent auto-derived from conv)", () => {
+    const args = buildCliArgs({ conversationId: "conv-xyz" });
+    const idx = args.indexOf("--conversation");
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe("conv-xyz");
+    expect(args).not.toContain("--agent");
+  });
+
+  test("agentId only → --agent flag, no --new or --default", () => {
+    const args = buildCliArgs({ agentId: "agent-1" });
+    const idx = args.indexOf("--agent");
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe("agent-1");
+    expect(args).not.toContain("--new");
+    expect(args).not.toContain("--default");
+  });
+
+  test("agentId + newConversation → --agent + --new", () => {
+    const args = buildCliArgs({ agentId: "agent-1", newConversation: true });
+    expect(args).toContain("--agent");
+    expect(args).toContain("--new");
+    expect(args).not.toContain("--default");
+  });
+
+  test("agentId + defaultConversation → --agent + --default", () => {
+    const args = buildCliArgs({ agentId: "agent-1", defaultConversation: true });
+    expect(args).toContain("--agent");
+    expect(args).toContain("--default");
+    expect(args).not.toContain("--new");
+  });
+
+  test("createOnly → --new-agent", () => {
+    const args = buildCliArgs({ createOnly: true });
+    expect(args).toContain("--new-agent");
+  });
+
+  test("newConversation without agentId → --new (LRU agent)", () => {
+    const args = buildCliArgs({ newConversation: true });
+    expect(args).toContain("--new");
+    expect(args).not.toContain("--agent");
+  });
+
+  test("conversationId takes priority over agentId (conv wins)", () => {
+    // When conversationId is set, --conversation is used and --agent is skipped
+    const args = buildCliArgs({ conversationId: "conv-abc", agentId: "agent-1" });
+    expect(args).toContain("--conversation");
+    expect(args).not.toContain("--agent");
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Model and embedding
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("buildCliArgs — model and embedding", () => {
+  test("model option → -m flag", () => {
+    const args = buildCliArgs({ model: "claude-sonnet-4" });
+    const idx = args.indexOf("-m");
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe("claude-sonnet-4");
+  });
+
+  test("no model → no -m flag", () => {
+    const args = buildCliArgs({});
+    expect(args).not.toContain("-m");
+  });
+
+  test("embedding option → --embedding flag", () => {
+    const args = buildCliArgs({ embedding: "text-embedding-3" });
+    const idx = args.indexOf("--embedding");
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe("text-embedding-3");
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Permission mode
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("buildCliArgs — permission mode", () => {
+  test("bypassPermissions → --yolo", () => {
+    const args = buildCliArgs({ permissionMode: "bypassPermissions" });
+    expect(args).toContain("--yolo");
+    expect(args).not.toContain("--permission-mode");
+  });
+
+  test("other non-default modes → --permission-mode <mode>", () => {
+    const args = buildCliArgs({ permissionMode: "acceptEdits" });
+    const idx = args.indexOf("--permission-mode");
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe("acceptEdits");
+  });
+
+  test("default mode → no permission flag", () => {
+    const args = buildCliArgs({ permissionMode: "default" });
+    expect(args).not.toContain("--permission-mode");
+    expect(args).not.toContain("--yolo");
+  });
+
+  test("no permissionMode → no permission flag", () => {
+    const args = buildCliArgs({});
+    expect(args).not.toContain("--permission-mode");
+    expect(args).not.toContain("--yolo");
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// System prompt
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("buildCliArgs — system prompt", () => {
+  test("preset string → --system <name>", () => {
+    const args = buildCliArgs({ systemPrompt: "letta-claude" });
+    const idx = args.indexOf("--system");
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe("letta-claude");
+  });
+
+  test("custom string → --system-custom <text>", () => {
+    const args = buildCliArgs({ systemPrompt: "you are a helpful bot" });
+    const idx = args.indexOf("--system-custom");
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe("you are a helpful bot");
+    expect(args).not.toContain("--system");
+  });
+
+  test("preset object → --system <preset> + --system-append <text>", () => {
+    const args = buildCliArgs({
+      systemPrompt: { type: "preset", preset: "default", append: "extra context" },
+    });
+    expect(args).toContain("--system");
+    expect(args).toContain("default");
+    expect(args).toContain("--system-append");
+    expect(args).toContain("extra context");
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Tools and tags
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("buildCliArgs — tools and tags", () => {
+  test("allowedTools → --allowedTools joined with comma", () => {
+    const args = buildCliArgs({ allowedTools: ["Read", "Write", "Bash"] });
+    const idx = args.indexOf("--allowedTools");
+    expect(args[idx + 1]).toBe("Read,Write,Bash");
+  });
+
+  test("disallowedTools → --disallowedTools joined with comma", () => {
+    const args = buildCliArgs({ disallowedTools: ["EnterPlanMode"] });
+    const idx = args.indexOf("--disallowedTools");
+    expect(args[idx + 1]).toBe("EnterPlanMode");
+  });
+
+  test("tags → --tags joined with comma", () => {
+    const args = buildCliArgs({ tags: ["production", "v2"] });
+    const idx = args.indexOf("--tags");
+    expect(args[idx + 1]).toBe("production,v2");
+  });
+
+  test("empty tags array → no --tags flag", () => {
+    const args = buildCliArgs({ tags: [] });
+    expect(args).not.toContain("--tags");
+  });
+});
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Memory filesystem
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe("buildCliArgs — memfs", () => {
+  test("memfs=true → --memfs", () => {
+    const args = buildCliArgs({ memfs: true });
+    expect(args).toContain("--memfs");
+    expect(args).not.toContain("--no-memfs");
+  });
+
+  test("memfs=false → --no-memfs", () => {
+    const args = buildCliArgs({ memfs: false });
+    expect(args).toContain("--no-memfs");
+    expect(args).not.toContain("--memfs");
+  });
+
+  test("memfs undefined → neither flag", () => {
+    const args = buildCliArgs({});
+    expect(args).not.toContain("--memfs");
+    expect(args).not.toContain("--no-memfs");
+  });
+});
diff --git a/src/transport.ts b/src/transport.ts
index cf786bf..658e01a 100644
--- a/src/transport.ts
+++ b/src/transport.ts
@@ -13,6 +13,177 @@ function sdkLog(tag: string, ...args: unknown[]) {
   if (process.env.DEBUG_SDK) console.error(`[SDK-Transport] [${tag}]`, ...args);
 }
 
+/**
+ * Build the CLI argument array for a given set of session options.
+ *
+ * Exported as a pure function for testing — this IS the real production code
+ * path. SubprocessTransport.buildArgs() delegates here.
+ */
+export function buildCliArgs(options: InternalSessionOptions): string[] {
+  const args: string[] = [
+    "--output-format",
+    "stream-json",
+    "--input-format",
+    "stream-json",
+  ];
+
+  // Conversation and agent handling
+  if (options.conversationId) {
+    args.push("--conversation", options.conversationId);
+  } else if (options.agentId) {
+    args.push("--agent", options.agentId);
+    if (options.newConversation) {
+      args.push("--new");
+    } else if (options.defaultConversation) {
+      args.push("--default");
+    }
+  } else if (options.createOnly) {
+    args.push("--new-agent");
+  } else if (options.newConversation) {
+    args.push("--new");
+  }
+
+  // Model
+  if (options.model) {
+    args.push("-m", options.model);
+  }
+
+  // Partial message streaming
+  if (options.includePartialMessages) {
+    args.push("--include-partial-messages");
+  }
+
+  // Embedding model
+  if (options.embedding) {
+    args.push("--embedding", options.embedding);
+  }
+
+  // System prompt configuration
+  if (options.systemPrompt !== undefined) {
+    if (typeof options.systemPrompt === "string") {
+      const validPresets = [
+        "default",
+        "letta-claude",
+        "letta-codex",
+        "letta-gemini",
+        "claude",
+        "codex",
+        "gemini",
+      ];
+      if (validPresets.includes(options.systemPrompt)) {
+        args.push("--system", options.systemPrompt);
+      } else {
+        args.push("--system-custom", options.systemPrompt);
+      }
+    } else {
+      args.push("--system", options.systemPrompt.preset);
+      if (options.systemPrompt.append) {
+        args.push("--system-append", options.systemPrompt.append);
+      }
+    }
+  }
+
+  // Memory blocks (only for new agents)
+  if (options.memory !== undefined && !options.agentId) {
+    if (options.memory.length === 0) {
+      args.push("--init-blocks", "");
+    } else {
+      const presetNames: string[] = [];
+      const memoryBlocksJson: Array<
+        | { label: string; value: string }
+        | { blockId: string }
+      > = [];
+
+      for (const item of options.memory) {
+        if (typeof item === "string") {
+          presetNames.push(item);
+        } else if ("blockId" in item) {
+          memoryBlocksJson.push(item as { blockId: string });
+        } else {
+          memoryBlocksJson.push(item as { label: string; value: string });
+        }
+      }
+
+      if (memoryBlocksJson.length > 0) {
+        args.push("--memory-blocks", JSON.stringify(memoryBlocksJson));
+        if (presetNames.length > 0) {
+          console.warn(
+            "[letta-code-sdk] Using custom memory blocks. " +
+            `Preset blocks are ignored when custom blocks are provided: ${presetNames.join(", ")}`
+          );
+        }
+      } else if (presetNames.length > 0) {
+        args.push("--init-blocks", presetNames.join(","));
+      }
+    }
+  }
+
+  // Convenience props for block values (only for new agents)
+  if (!options.agentId) {
+    if (options.persona !== undefined) {
+      args.push("--block-value", `persona=${options.persona}`);
+    }
+    if (options.human !== undefined) {
+      args.push("--block-value", `human=${options.human}`);
+    }
+  }
+
+  // Permission mode
+  if (options.permissionMode === "bypassPermissions") {
+    args.push("--yolo");
+  } else if (options.permissionMode && options.permissionMode !== "default") {
+    args.push("--permission-mode", options.permissionMode);
+  }
+
+  // Allowed / disallowed tools
+  if (options.allowedTools) {
+    args.push("--allowedTools", options.allowedTools.join(","));
+  }
+  if (options.disallowedTools) {
+    args.push("--disallowedTools", options.disallowedTools.join(","));
+  }
+
+  // Tags
+  if (options.tags && options.tags.length > 0) {
+    args.push("--tags", options.tags.join(","));
+  }
+
+  // Memory filesystem
+  if (options.memfs === true) {
+    args.push("--memfs");
+  } else if (options.memfs === false) {
+    args.push("--no-memfs");
+  }
+
+  // Skills sources
+  if (options.skillSources !== undefined) {
+    const sources = [...new Set(options.skillSources)];
+    if (sources.length === 0) {
+      args.push("--no-skills");
+    } else {
+      args.push("--skill-sources", sources.join(","));
+    }
+  }
+
+  // Session context reminder toggle
+  if (options.systemInfoReminder === false) {
+    args.push("--no-system-info-reminder");
+  }
+
+  // Sleeptime / reflection settings
+  if (options.sleeptime?.trigger !== undefined) {
+    args.push("--reflection-trigger", options.sleeptime.trigger);
+  }
+  if (options.sleeptime?.behavior !== undefined) {
+    args.push("--reflection-behavior", options.sleeptime.behavior);
+  }
+  if (options.sleeptime?.stepCount !== undefined) {
+    args.push("--reflection-step-count", String(options.sleeptime.stepCount));
+  }
+
+  return args;
+}
+
 export class SubprocessTransport {
   private process: ChildProcess | null = null;
   private stdout: Interface | null = null;
@@ -218,199 +389,11 @@ export class SubprocessTransport {
   }
 
   private buildArgs(): string[] {
-    const args: string[] = [
-      "--output-format",
-      "stream-json",
-      "--input-format",
-      "stream-json",
-    ];
-
-    // Note: All validation happens in validateInternalSessionOptions() called from Session constructor
-
-    // Conversation and agent handling
-    if (this.options.conversationId) {
-      // Resume specific conversation (derives agent automatically)
-      args.push("--conversation", this.options.conversationId);
-    } else if (this.options.agentId) {
-      // Resume existing agent
-      args.push("--agent", this.options.agentId);
-      if (this.options.newConversation) {
-        // Create new conversation on this agent
-        args.push("--new");
-      } else if (this.options.defaultConversation) {
-        // Use agent's default conversation explicitly
-        args.push("--default");
-      }
-    } else if (this.options.createOnly) {
-      // createAgent() - explicitly create new agent
-      args.push("--new-agent");
-    } else if (this.options.newConversation) {
-      // createSession() without agentId - LRU agent + new conversation
-      args.push("--new");
-    }
-    // else: no agent flags = default behavior (LRU agent, default conversation)
-
-    // Model
-    if (this.options.model) {
-      args.push("-m", this.options.model);
-    }
-
-    // Partial message streaming (token-level stream_event chunks)
-    if (this.options.includePartialMessages) {
-      args.push("--include-partial-messages");
-    }
-
-    // Embedding model
-    if (this.options.embedding) {
-      args.push("--embedding", this.options.embedding);
-    }
-
-    // System prompt configuration
-    if (this.options.systemPrompt !== undefined) {
-      if (typeof this.options.systemPrompt === "string") {
-        // Check if it's a valid preset name or custom string
-        const validPresets = [
-          "default",
-          "letta-claude",
-          "letta-codex",
-          "letta-gemini",
-          "claude",
-          "codex",
-          "gemini",
-        ];
-        if (validPresets.includes(this.options.systemPrompt)) {
-          // Preset name → --system
-          args.push("--system", this.options.systemPrompt);
-        } else {
-          // Custom string → --system-custom
-          args.push("--system-custom", this.options.systemPrompt);
-        }
-      } else {
-        // Preset object → --system (+ optional --system-append)
-        args.push("--system", this.options.systemPrompt.preset);
-        if (this.options.systemPrompt.append) {
-          args.push("--system-append", this.options.systemPrompt.append);
-        }
-      }
-    }
-
-    // Memory blocks (only for new agents)
-    if (this.options.memory !== undefined && !this.options.agentId) {
-      if (this.options.memory.length === 0) {
-        // Empty array → no memory blocks (just core)
-        args.push("--init-blocks", "");
-      } else {
-        // Separate preset names from custom/reference blocks
-        const presetNames: string[] = [];
-        const memoryBlocksJson: Array<
-          | { label: string; value: string }
-          | { blockId: string }
-        > = [];
-
-        for (const item of this.options.memory) {
-          if (typeof item === "string") {
-            // Preset name
-            presetNames.push(item);
-          } else if ("blockId" in item) {
-            // Block reference - pass to --memory-blocks
-            memoryBlocksJson.push(item as { blockId: string });
-          } else {
-            // CreateBlock
-            memoryBlocksJson.push(item as { label: string; value: string });
-          }
-        }
-
-        // NOTE: When custom blocks are provided via --memory-blocks, they define the complete
-        // memory configuration. Preset blocks (--init-blocks) cannot be mixed with custom blocks.
-        if (memoryBlocksJson.length > 0) {
-          // Use custom blocks only
-          args.push("--memory-blocks", JSON.stringify(memoryBlocksJson));
-          if (presetNames.length > 0) {
-            console.warn(
-              "[letta-code-sdk] Using custom memory blocks. " +
-              `Preset blocks are ignored when custom blocks are provided: ${presetNames.join(", ")}`
-            );
-          }
-        } else if (presetNames.length > 0) {
-          // Use presets only
-          args.push("--init-blocks", presetNames.join(","));
-        }
-      }
-    }
-
-    // Convenience props for block values (only for new agents)
-    if (!this.options.agentId) {
-      if (this.options.persona !== undefined) {
-        args.push("--block-value", `persona=${this.options.persona}`);
-      }
-      if (this.options.human !== undefined) {
-        args.push("--block-value", `human=${this.options.human}`);
-      }
-    }
-
-    // Permission mode
-    if (this.options.permissionMode === "bypassPermissions") {
-      // Keep using alias for backwards compatibility
-      args.push("--yolo");
-    } else if (
-      this.options.permissionMode &&
-      this.options.permissionMode !== "default"
-    ) {
-      args.push("--permission-mode", this.options.permissionMode);
-    }
-
-    // Allowed tools
-    if (this.options.allowedTools) {
-      args.push("--allowedTools", this.options.allowedTools.join(","));
-    }
-    if (this.options.disallowedTools) {
-      args.push("--disallowedTools", this.options.disallowedTools.join(","));
-    }
-
-    // Tags
-    if (this.options.tags && this.options.tags.length > 0) {
-      args.push("--tags", this.options.tags.join(","));
-    }
-
-    // Memory filesystem
-    if (this.options.memfs === true) {
-      args.push("--memfs");
-    } else if (this.options.memfs === false) {
-      args.push("--no-memfs");
-    }
-
-    // Skills sources
-    if (this.options.skillSources !== undefined) {
-      const sources = [...new Set(this.options.skillSources)];
-      if (sources.length === 0) {
-        args.push("--no-skills");
-      } else {
-        args.push("--skill-sources", sources.join(","));
-      }
-    }
-
-    // Session context reminder toggle
-    if (this.options.systemInfoReminder === false) {
-      args.push("--no-system-info-reminder");
-    }
-
-    // Sleeptime/reflection settings
-    if (this.options.sleeptime?.trigger !== undefined) {
-      args.push("--reflection-trigger", this.options.sleeptime.trigger);
-    }
-    if (this.options.sleeptime?.behavior !== undefined) {
-      args.push("--reflection-behavior", this.options.sleeptime.behavior);
-    }
-    if (this.options.sleeptime?.stepCount !== undefined) {
-      args.push(
-        "--reflection-step-count",
-        String(this.options.sleeptime.stepCount),
-      );
-    }
-
-    return args;
+    return buildCliArgs(this.options);
   }
 
+
+
   private async findCli(): Promise<string> {
     // Try multiple resolution strategies
     const { existsSync } = await import("node:fs");
diff --git a/src/types.ts b/src/types.ts
index 08ba141..a6fc96e 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -492,6 +492,38 @@ export type SDKMessage =
   | SDKResultMessage
   | SDKStreamEventMessage;
 
+// ═══════════════════════════════════════════════════════════════
+// LIST MESSAGES API
+// ═══════════════════════════════════════════════════════════════
+
+/**
+ * Options for session.listMessages().
+ */
+export interface ListMessagesOptions {
+  /** Explicit conversation ID (e.g. "conv-123"). If omitted, uses agent default. */
+  conversationId?: string;
+  /** Return messages before this message ID (cursor for older pages). */
+  before?: string;
+  /** Return messages after this message ID (cursor for newer pages). */
+  after?: string;
+  /** Sort order. Defaults to "desc" (newest first). */
+  order?: "asc" | "desc";
+  /** Max messages per page. Defaults to 50. */
+  limit?: number;
+}
+
+/**
+ * Result from session.listMessages().
+ * `messages` are raw Letta API message objects in the requested order.
+ */
+export interface ListMessagesResult {
+  messages: unknown[];
+  /** ID of the oldest message in this page; use as `before` for the next page. */
+  nextBefore?: string | null;
+  /** Whether more pages exist in the requested direction. */
+  hasMore?: boolean;
+}
+
 // ═══════════════════════════════════════════════════════════════
 // EXTERNAL TOOL PROTOCOL TYPES
 // ═══════════════════════════════════════════════════════════════