diff --git a/tests/lib/state.test.ts b/tests/lib/state.test.ts
new file mode 100644
index 0000000..4aa6988
--- /dev/null
+++ b/tests/lib/state.test.ts
@@ -0,0 +1,122 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { mkdirSync, writeFileSync, readFileSync, existsSync, rmSync } from "fs";
+import { join } from "path";
+
+// Point PROJECT_DIR to a temp directory before importing state
+const TEST_DIR = join(import.meta.dirname ?? __dirname, "..", ".tmp-state-test");
+vi.stubEnv("CLAUDE_PROJECT_DIR", TEST_DIR);
+
+// Dynamic import so env is set first
+const { loadState, saveState, appendLog, readLog, now, STATE_DIR } = await import(
+  "../../src/lib/state.js"
+);
+
+describe("lib/state", () => {
+  beforeEach(() => {
+    // Clean slate
+    if (existsSync(TEST_DIR)) {
+      rmSync(TEST_DIR, { recursive: true });
+    }
+    mkdirSync(TEST_DIR, { recursive: true });
+  });
+
+  afterEach(() => {
+    if (existsSync(TEST_DIR)) {
+      rmSync(TEST_DIR, { recursive: true });
+    }
+  });
+
+  describe("loadState", () => {
+    it("returns empty object when file does not exist", () => {
+      expect(loadState("nonexistent")).toEqual({});
+    });
+
+    it("returns parsed JSON when file exists", () => {
+      mkdirSync(STATE_DIR, { recursive: true });
+      writeFileSync(join(STATE_DIR, "test.json"), JSON.stringify({ a: 1 }));
+      expect(loadState("test")).toEqual({ a: 1 });
+    });
+
+    it("returns empty object on corrupt JSON", () => {
+      mkdirSync(STATE_DIR, { recursive: true });
+      writeFileSync(join(STATE_DIR, "bad.json"), "{not json");
+      expect(loadState("bad")).toEqual({});
+    });
+  });
+
+  describe("saveState", () => {
+    it("creates state dir and writes JSON", () => {
+      saveState("mystate", { foo: "bar" });
+      const content = readFileSync(join(STATE_DIR, "mystate.json"), "utf-8");
+      expect(JSON.parse(content)).toEqual({ foo: "bar" });
+    });
+
+    it("overwrites existing state", () => {
+      saveState("overwrite", { v: 1 });
+      saveState("overwrite", { v: 2 });
+      expect(loadState("overwrite")).toEqual({ v: 2 });
+    });
+  });
+
+  describe("appendLog / readLog", () => {
+    it("appends entries and reads them back", () => {
+      appendLog("test.jsonl", { id: 1 });
+      appendLog("test.jsonl", { id: 2 });
+      appendLog("test.jsonl", { id: 3 });
+      const entries = readLog("test.jsonl");
+      expect(entries).toHaveLength(3);
+      expect(entries[0]).toEqual({ id: 1 });
+      expect(entries[2]).toEqual({ id: 3 });
+    });
+
+    it("readLog returns empty array for missing file", () => {
+      expect(readLog("missing.jsonl")).toEqual([]);
+    });
+
+    it("readLog supports lastN parameter", () => {
+      for (let i = 0; i < 10; i++) {
+        appendLog("many.jsonl", { i });
+      }
+      const last3 = readLog("many.jsonl", 3);
+      expect(last3).toHaveLength(3);
+      expect(last3[0]).toEqual({ i: 7 });
+      expect(last3[2]).toEqual({ i: 9 });
+    });
+
+    it("readLog skips corrupt lines gracefully", () => {
+      mkdirSync(STATE_DIR, { recursive: true });
+      const logFile = join(STATE_DIR, "corrupt.jsonl");
+      writeFileSync(logFile, '{"a":1}\nbroken line\n{"b":2}\n');
+      const entries = readLog("corrupt.jsonl");
+      expect(entries).toHaveLength(2);
+      expect(entries[0]).toEqual({ a: 1 });
+      expect(entries[1]).toEqual({ b: 2 });
+    });
+
+    it("rotates log file when it exceeds 5MB", () => {
+      mkdirSync(STATE_DIR, { recursive: true });
+      const logFile = join(STATE_DIR, "big.jsonl");
+      // Write a 5MB+ file
+      const bigLine = JSON.stringify({ data: "x".repeat(1000) }) + "\n";
+      const count = Math.ceil((5 * 1024 * 1024) / bigLine.length) + 1;
+      writeFileSync(logFile, bigLine.repeat(count));
+
+      // Next append should trigger rotation
+      appendLog("big.jsonl", { after: "rotation" });
+
+      expect(existsSync(logFile + ".old")).toBe(true);
+      // The new file should only have the latest entry
+      const entries = readLog("big.jsonl");
+      expect(entries).toHaveLength(1);
+      expect(entries[0]).toEqual({ after: "rotation" });
+    });
+  });
+
+  describe("now", () => {
+    it("returns a valid ISO timestamp", () => {
+      const ts = now();
+      expect(() => new Date(ts)).not.toThrow();
+      expect(new Date(ts).toISOString()).toBe(ts);
+    });
+  });
+});