diff --git a/packages/sdk/src/chat-handle.test.ts b/packages/sdk/src/chat-handle.test.ts
new file mode 100644
index 0000000..cf161de
--- /dev/null
+++ b/packages/sdk/src/chat-handle.test.ts
@@ -0,0 +1,349 @@
+/**
+ * Unit tests for `ChatHandle` — exercise the parts that are easy to get subtly
+ * wrong with no live harness:
+ *
+ *   - Usage aggregation: tokens always SUM; cost respects `costSemantic`
+ *     (cumulative → MAX, delta → SUM, mixed → prefer cumulative).
+ *   - The dual interface: `for await` yields raw events; `await handle`
+ *     resolves to a ChatResult. Both consume the same stream once.
+ *   - Permission callback contract: `onPermissionRequest` is called with the
+ *     exact args from the event, and the resolved decision is POSTed back to
+ *     the harness. Default decision is "allow".
+ *   - Lifecycle: `onComplete` fires after `ca_session_ended`; `drain()`
+ *     throws when the stream closes without an end event; `result()` is
+ *     memoized.
+ *
+ * Everything is mocked — no network, no harness — so the suite runs in <50ms.
+ */
+import { describe, it, expect, vi } from "vitest";
+import type { HarnessEvent } from "@open-gitagent/protocol";
+import { ChatHandle } from "./chat-handle.js";
+import type { PermissionDecision } from "./types.js";
+
+const SESS = "sess_test_01";
+const HARNESS = "http://localhost:9999";
+
+/** Tiny helper: turn a fixed list of events into an AsyncIterable<HarnessEvent>. */
+async function* iter(events: HarnessEvent[]): AsyncIterable<HarnessEvent> {
+  for (const ev of events) yield ev;
+}
+
+/** Build a ChatHandle wired to a deterministic event stream + a fetch mock. */
+function makeHandle(
+  events: HarnessEvent[],
+  extra: {
+    onPermissionRequest?: (
+      callId: string,
+      toolName: string,
+      input: unknown,
+      risk?: "low" | "medium" | "high" | "destructive",
+    ) => Promise<PermissionDecision> | PermissionDecision;
+    onComplete?: () => Promise<void> | void;
+  } = {},
+): { handle: ChatHandle; fetchImpl: ReturnType<typeof vi.fn> } {
+  const fetchImpl = vi.fn(async () =>
+    new Response("", { status: 200, headers: { "content-type": "application/json" } }),
+  );
+  const handle = new ChatHandle({
+    sessionIdPromise: Promise.resolve(SESS),
+    events: iter(events),
+    harnessUrlPromise: Promise.resolve(HARNESS),
+    fetchImpl: fetchImpl as unknown as typeof fetch,
+    ...extra,
+  });
+  return { handle, fetchImpl };
+}
+
+const endedEvent = (): HarnessEvent =>
+  ({ kind: "ca_session_ended", sessionId: SESS, reason: "complete" } as HarnessEvent);
+
+describe("ChatHandle — usage aggregation", () => {
+  it("sums input/output tokens across snapshots", async () => {
+    const { handle } = makeHandle([
+      { kind: "ca_usage_snapshot", sessionId: SESS, inputTokens: 100, outputTokens: 30 } as HarnessEvent,
+      { kind: "ca_usage_snapshot", sessionId: SESS, inputTokens: 50, outputTokens: 12 } as HarnessEvent,
+      endedEvent(),
+    ]);
+    const result = await handle;
+    expect(result.usage.inputTokens).toBe(150);
+    expect(result.usage.outputTokens).toBe(42);
+  });
+
+  it("sums cache creation + cache read tokens", async () => {
+    const { handle } = makeHandle([
+      { kind: "ca_usage_snapshot", sessionId: SESS, cacheCreationInputTokens: 200, cacheReadInputTokens: 800 } as HarnessEvent,
+      { kind: "ca_usage_snapshot", sessionId: SESS, cacheCreationInputTokens: 50, cacheReadInputTokens: 100 } as HarnessEvent,
+      endedEvent(),
+    ]);
+    const result = await handle;
+    expect(result.usage.cacheCreationInputTokens).toBe(250);
+    expect(result.usage.cacheReadInputTokens).toBe(900);
+  });
+
+  it("cost: cumulative semantic → takes the MAX across snapshots", async () => {
+    // claude-agent-sdk emits a running cumulative total; we should latch onto
+    // the largest value seen (typically the last one).
+    const { handle } = makeHandle([
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.01, costSemantic: "cumulative" } as HarnessEvent,
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.04, costSemantic: "cumulative" } as HarnessEvent,
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.03, costSemantic: "cumulative" } as HarnessEvent,
+      endedEvent(),
+    ]);
+    const result = await handle;
+    expect(result.usage.costUsd).toBe(0.04);
+  });
+
+  it("cost: delta semantic → SUMs per-message values", async () => {
+    // gitclaw emits per-message deltas; we should add them up.
+    const { handle } = makeHandle([
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.01, costSemantic: "delta" } as HarnessEvent,
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.02, costSemantic: "delta" } as HarnessEvent,
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.005, costSemantic: "delta" } as HarnessEvent,
+      endedEvent(),
+    ]);
+    const result = await handle;
+    expect(result.usage.costUsd).toBeCloseTo(0.035, 5);
+  });
+
+  it("cost: undefined semantic is treated as cumulative (defensive)", async () => {
+    const { handle } = makeHandle([
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.07 } as HarnessEvent,
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.04 } as HarnessEvent,
+      endedEvent(),
+    ]);
+    const result = await handle;
+    // MAX, not SUM
+    expect(result.usage.costUsd).toBe(0.07);
+  });
+
+  it("cost: mixed cumulative + delta in one turn → prefer cumulative (no double-count)", async () => {
+    // Defensive case — shouldn't happen in practice but the SDK has explicit
+    // handling and a documented preference. Pin the behavior.
+    const { handle } = makeHandle([
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.10, costSemantic: "cumulative" } as HarnessEvent,
+      { kind: "ca_usage_snapshot", sessionId: SESS, costUsd: 0.02, costSemantic: "delta" } as HarnessEvent,
+      endedEvent(),
+    ]);
+    const result = await handle;
+    expect(result.usage.costUsd).toBe(0.10);
+  });
+
+  it("no cost snapshots → costUsd is undefined", async () => {
+    const { handle } = makeHandle([
+      { kind: "ca_usage_snapshot", sessionId: SESS, inputTokens: 100, outputTokens: 30 } as HarnessEvent,
+      endedEvent(),
+    ]);
+    const result = await handle;
+    expect(result.usage.costUsd).toBeUndefined();
+    expect(result.usage.inputTokens).toBe(100);
+  });
+
+  it("getUsage() returns the same rollup mid-stream and after drain", async () => {
+    const { handle } = makeHandle([
+      { kind: "ca_usage_snapshot", sessionId: SESS, inputTokens: 10, costUsd: 0.01, costSemantic: "cumulative" } as HarnessEvent,
+      { kind: "sdk_message", sessionId: SESS, payload: {} } as HarnessEvent,
+      { kind: "ca_usage_snapshot", sessionId: SESS, inputTokens: 5, costUsd: 0.05, costSemantic: "cumulative" } as HarnessEvent,
+      endedEvent(),
+    ]);
+    let midSnapshotInput = 0;
+    for await (const ev of handle) {
+      if (ev.kind === "sdk_message") {
+        midSnapshotInput = handle.getUsage().inputTokens;
+      }
+    }
+    // After the first snapshot, before the second.
+    expect(midSnapshotInput).toBe(10);
+    // After drain — second snapshot has been folded in.
+    expect(handle.getUsage().inputTokens).toBe(15);
+    expect(handle.getUsage().costUsd).toBe(0.05);
+  });
+});
+
+describe("ChatHandle — dual iteration interface", () => {
+  it("`for await` yields every event in order", async () => {
+    const { handle } = makeHandle([
+      { kind: "sdk_message", sessionId: SESS, payload: "a" } as HarnessEvent,
+      { kind: "sdk_message", sessionId: SESS, payload: "b" } as HarnessEvent,
+      endedEvent(),
+    ]);
+    const kinds: string[] = [];
+    for await (const ev of handle) kinds.push(ev.kind);
+    expect(kinds).toEqual(["sdk_message", "sdk_message", "ca_session_ended"]);
+  });
+
+  it("`await handle` drains to a ChatResult", async () => {
+    const { handle } = makeHandle([
+      { kind: "sdk_message", sessionId: SESS, payload: { role: "assistant" } } as HarnessEvent,
+      endedEvent(),
+    ]);
+    const result = await handle;
+    expect(result.sessionId).toBe(SESS);
+    expect(result.messages).toHaveLength(1);
+    expect(result.ended.reason).toBe("complete");
+  });
+
+  it("result() is memoized — calling twice doesn't re-drain", async () => {
+    // Build a generator that only yields once; if drain() re-iterated, the
+    // second call would hang or throw. Memoization should give us the
+    // cached promise on call #2.
+    let yields = 0;
+    async function* once(): AsyncIterable<HarnessEvent> {
+      if (yields > 0) throw new Error("re-iteration would dead-lock");
+      yields++;
+      yield endedEvent();
+    }
+    const handle = new ChatHandle({
+      sessionIdPromise: Promise.resolve(SESS),
+      events: once(),
+      harnessUrlPromise: Promise.resolve(HARNESS),
+      fetchImpl: (async () => new Response("", { status: 200 })) as unknown as typeof fetch,
+    });
+    const a = await handle.result();
+    const b = await handle.result();
+    expect(a).toBe(b);
+  });
+
+  it("drain() throws if the stream closes without ca_session_ended", async () => {
+    const { handle } = makeHandle([
+      { kind: "sdk_message", sessionId: SESS, payload: "stranded" } as HarnessEvent,
+      // no ca_session_ended
+    ]);
+    await expect(handle.result()).rejects.toThrow(/ca_session_ended/);
+  });
+});
+
+describe("ChatHandle — permission callback", () => {
+  it("calls onPermissionRequest with event args + POSTs the decision", async () => {
+    const onPermissionRequest = vi.fn(async () => ({ decision: "allow" as const }));
+    const { handle, fetchImpl } = makeHandle(
+      [
+        {
+          kind: "ca_permission_request",
+          sessionId: SESS,
+          callId: "call_42",
+          toolName: "Bash",
+          input: { command: "ls" },
+          risk: "low",
+        } as HarnessEvent,
+        endedEvent(),
+      ],
+      { onPermissionRequest },
+    );
+
+    await handle.result();
+
+    // Hook fired exactly once with the exact event payload (minus discriminator).
+    expect(onPermissionRequest).toHaveBeenCalledOnce();
+    expect(onPermissionRequest).toHaveBeenCalledWith(
+      "call_42",
+      "Bash",
+      { command: "ls" },
+      "low",
+    );
+
+    // Decision was POSTed back to the harness at the right URL shape.
+    expect(fetchImpl).toHaveBeenCalledWith(
+      `${HARNESS}/v1/sessions/${SESS}/permission/call_42`,
+      expect.objectContaining({
+        method: "POST",
+        headers: expect.objectContaining({ "Content-Type": "application/json" }),
+      }),
+    );
+  });
+
+  it("default decision is `allow` when no onPermissionRequest is provided", async () => {
+    const { handle, fetchImpl } = makeHandle([
+      {
+        kind: "ca_permission_request",
+        sessionId: SESS,
+        callId: "call_1",
+        toolName: "Read",
+        input: { path: "/etc/hostname" },
+      } as HarnessEvent,
+      endedEvent(),
+    ]);
+    await handle.result();
+
+    // The first call should be the permission POST. Body says allow.
+    const firstCall = fetchImpl.mock.calls[0];
+    expect(firstCall[0]).toBe(`${HARNESS}/v1/sessions/${SESS}/permission/call_1`);
+    const body = JSON.parse(firstCall[1].body);
+    expect(body.decision).toBe("allow");
+  });
+
+  it("honors a custom deny decision", async () => {
+    const onPermissionRequest = vi.fn(async () => ({
+      decision: "deny" as const,
+      reason: "test refusal",
+    }));
+    const { handle, fetchImpl } = makeHandle(
+      [
+        {
+          kind: "ca_permission_request",
+          sessionId: SESS,
+          callId: "call_9",
+          toolName: "Bash",
+          input: { command: "rm -rf /" },
+          risk: "destructive",
+        } as HarnessEvent,
+        endedEvent(),
+      ],
+      { onPermissionRequest },
+    );
+
+    await handle.result();
+
+    const body = JSON.parse(fetchImpl.mock.calls[0][1].body);
+    expect(body.decision).toBe("deny");
+    expect(body.reason).toBe("test refusal");
+  });
+});
+
+describe("ChatHandle — lifecycle", () => {
+  it("fires onComplete after ca_session_ended (once)", async () => {
+    const onComplete = vi.fn();
+    const { handle } = makeHandle(
+      [
+        { kind: "sdk_message", sessionId: SESS, payload: "x" } as HarnessEvent,
+        endedEvent(),
+      ],
+      { onComplete },
+    );
+    await handle.result();
+    expect(onComplete).toHaveBeenCalledOnce();
+  });
+
+  it("does NOT fire onComplete if stream never ends", async () => {
+    const onComplete = vi.fn();
+    const { handle } = makeHandle(
+      [{ kind: "sdk_message", sessionId: SESS, payload: "x" } as HarnessEvent],
+      { onComplete },
+    );
+    // Drain throws (no end), but we still verify the hook didn't fire.
+    await expect(handle.result()).rejects.toThrow();
+    expect(onComplete).not.toHaveBeenCalled();
+  });
+
+  it("collects every sdk_message payload into result.messages, in order", async () => {
+    const payloads = [
+      { type: "assistant", text: "hi" },
+      { type: "tool_use", name: "Bash" },
+      { type: "assistant", text: "done" },
+    ];
+    const { handle } = makeHandle([
+      ...payloads.map((p) => ({ kind: "sdk_message", sessionId: SESS, payload: p }) as HarnessEvent),
+      endedEvent(),
+    ]);
+    const result = await handle.result();
+    expect(result.messages).toEqual(payloads);
+  });
+
+  it("cancel() POSTs /cancel for the right session", async () => {
+    const { handle, fetchImpl } = makeHandle([endedEvent()]);
+    await handle.cancel();
+    expect(fetchImpl).toHaveBeenCalledWith(
+      `${HARNESS}/v1/sessions/${SESS}/cancel`,
+      expect.objectContaining({ method: "POST" }),
+    );
+  });
+});