diff --git a/tests/lib/state.test.ts b/tests/lib/state.test.ts
new file mode 100644
index 0000000..18a7258
--- /dev/null
+++ b/tests/lib/state.test.ts
@@ -0,0 +1,149 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { mkdirSync, rmSync, readFileSync, writeFileSync, existsSync, statSync } from "fs";
+import { join } from "path";
+
+// We need to override PROJECT_DIR before importing state, so we mock files.ts
+const TEST_DIR = join(import.meta.dirname ?? __dirname, ".tmp-state-test");
+const TEST_STATE_DIR = join(TEST_DIR, ".claude", "preflight-state");
+
+import { vi } from "vitest";
+
+vi.mock("../../src/lib/files.js", () => ({
+  PROJECT_DIR: join(import.meta.dirname ?? __dirname, ".tmp-state-test"),
+}));
+
+// Import after mock
+const { loadState, saveState, appendLog, readLog, now, STATE_DIR } = await import("../../src/lib/state.js");
+
+describe("state module", () => {
+  beforeEach(() => {
+    // Clean slate
+    rmSync(TEST_DIR, { recursive: true, force: true });
+  });
+
+  afterEach(() => {
+    rmSync(TEST_DIR, { recursive: true, force: true });
+  });
+
+  describe("loadState", () => {
+    it("returns empty object for missing file", () => {
+      expect(loadState("nonexistent")).toEqual({});
+    });
+
+    it("returns empty object for corrupt JSON", () => {
+      mkdirSync(TEST_STATE_DIR, { recursive: true });
+      writeFileSync(join(TEST_STATE_DIR, "bad.json"), "not json{{{");
+      expect(loadState("bad")).toEqual({});
+    });
+
+    it("loads valid JSON state", () => {
+      mkdirSync(TEST_STATE_DIR, { recursive: true });
+      const data = { foo: "bar", count: 42 };
+      writeFileSync(join(TEST_STATE_DIR, "good.json"), JSON.stringify(data));
+      expect(loadState("good")).toEqual(data);
+    });
+  });
+
+  describe("saveState", () => {
+    it("creates state dir and file", () => {
+      expect(existsSync(TEST_STATE_DIR)).toBe(false);
+      saveState("test", { hello: "world" });
+      expect(existsSync(TEST_STATE_DIR)).toBe(true);
+      const content = JSON.parse(readFileSync(join(TEST_STATE_DIR, "test.json"), "utf-8"));
+      expect(content).toEqual({ hello: "world" });
+    });
+
+    it("overwrites existing state", () => {
+      saveState("overwrite", { v: 1 });
+      saveState("overwrite", { v: 2 });
+      expect(loadState("overwrite")).toEqual({ v: 2 });
+    });
+  });
+
+  describe("appendLog", () => {
+    it("creates log file and appends entries", () => {
+      appendLog("test.jsonl", { action: "first" });
+      appendLog("test.jsonl", { action: "second" });
+
+      const content = readFileSync(join(TEST_STATE_DIR, "test.jsonl"), "utf-8");
+      const lines = content.trim().split("\n");
+      expect(lines).toHaveLength(2);
+      expect(JSON.parse(lines[0])).toEqual({ action: "first" });
+      expect(JSON.parse(lines[1])).toEqual({ action: "second" });
+    });
+
+    it("rotates when file exceeds 5MB", () => {
+      mkdirSync(TEST_STATE_DIR, { recursive: true });
+      const logPath = join(TEST_STATE_DIR, "big.jsonl");
+      // Create a file just over 5MB
+      const bigLine = JSON.stringify({ data: "x".repeat(1000) }) + "\n";
+      const lines = Math.ceil((5 * 1024 * 1024 + 1) / bigLine.length);
+      writeFileSync(logPath, bigLine.repeat(lines));
+
+      expect(statSync(logPath).size).toBeGreaterThan(5 * 1024 * 1024);
+
+      // Appending should trigger rotation
+      appendLog("big.jsonl", { action: "after-rotate" });
+
+      // Old file should exist as backup
+      expect(existsSync(logPath + ".old")).toBe(true);
+      // New file should only have the one new entry
+      const newContent = readFileSync(logPath, "utf-8").trim();
+      expect(newContent.split("\n")).toHaveLength(1);
+      expect(JSON.parse(newContent)).toEqual({ action: "after-rotate" });
+    });
+  });
+
+  describe("readLog", () => {
+    it("returns empty array for missing file", () => {
+      expect(readLog("missing.jsonl")).toEqual([]);
+    });
+
+    it("reads all entries", () => {
+      appendLog("read.jsonl", { n: 1 });
+      appendLog("read.jsonl", { n: 2 });
+      appendLog("read.jsonl", { n: 3 });
+
+      const entries = readLog("read.jsonl");
+      expect(entries).toHaveLength(3);
+      expect(entries[2]).toEqual({ n: 3 });
+    });
+
+    it("returns last N entries when specified", () => {
+      appendLog("tail.jsonl", { n: 1 });
+      appendLog("tail.jsonl", { n: 2 });
+      appendLog("tail.jsonl", { n: 3 });
+      appendLog("tail.jsonl", { n: 4 });
+
+      const last2 = readLog("tail.jsonl", 2);
+      expect(last2).toHaveLength(2);
+      expect(last2[0]).toEqual({ n: 3 });
+      expect(last2[1]).toEqual({ n: 4 });
+    });
+
+    it("skips corrupt lines gracefully", () => {
+      mkdirSync(TEST_STATE_DIR, { recursive: true });
+      const logPath = join(TEST_STATE_DIR, "mixed.jsonl");
+      writeFileSync(logPath, '{"a":1}\ngarbage\n{"b":2}\n');
+
+      const entries = readLog("mixed.jsonl");
+      expect(entries).toHaveLength(2);
+      expect(entries[0]).toEqual({ a: 1 });
+      expect(entries[1]).toEqual({ b: 2 });
+    });
+
+    it("returns empty array for empty file", () => {
+      mkdirSync(TEST_STATE_DIR, { recursive: true });
+      writeFileSync(join(TEST_STATE_DIR, "empty.jsonl"), "");
+      expect(readLog("empty.jsonl")).toEqual([]);
+    });
+  });
+
+  describe("now", () => {
+    it("returns a valid ISO timestamp", () => {
+      const ts = now();
+      expect(() => new Date(ts)).not.toThrow();
+      expect(new Date(ts).toISOString()).toBe(ts);
+    });
+  });
+});