diff --git a/tests/state.test.ts b/tests/state.test.ts
new file mode 100644
index 0000000..b3c432e
--- /dev/null
+++ b/tests/state.test.ts
@@ -0,0 +1,139 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { mkdtempSync, writeFileSync, readFileSync, existsSync, mkdirSync, rmSync } from "fs";
+import { join } from "path";
+import { tmpdir } from "os";
+
+let tempDir: string;
+let stateDir: string;
+
+// Mock files.js to return our temp dir
+vi.mock("../src/lib/files.js", () => {
+  // Use a placeholder that we'll override per test via the module
+  return {
+    PROJECT_DIR: "/tmp/preflight-mock-placeholder",
+  };
+});
+
+describe("state lib", () => {
+  beforeEach(async () => {
+    tempDir = mkdtempSync(join(tmpdir(), "preflight-state-test-"));
+    stateDir = join(tempDir, ".claude", "preflight-state");
+
+    // Update the mock value
+    const filesModule = await import("../src/lib/files.js");
+    (filesModule as any).PROJECT_DIR = tempDir;
+
+    // Reset the state module so STATE_DIR picks up new PROJECT_DIR
+    vi.resetModules();
+  });
+
+  afterEach(() => {
+    rmSync(tempDir, { recursive: true, force: true });
+  });
+
+  async function getState() {
+    // Re-mock before re-importing
+    vi.doMock("../src/lib/files.js", () => ({
+      PROJECT_DIR: tempDir,
+    }));
+    return await import("../src/lib/state.js");
+  }
+
+  describe("loadState", () => {
+    it("returns empty object when file does not exist", async () => {
+      const { loadState } = await getState();
+      expect(loadState("nonexistent")).toEqual({});
+    });
+
+    it("returns parsed JSON when file exists", async () => {
+      mkdirSync(stateDir, { recursive: true });
+      writeFileSync(join(stateDir, "test.json"), JSON.stringify({ foo: "bar" }));
+      const { loadState } = await getState();
+      expect(loadState("test")).toEqual({ foo: "bar" });
+    });
+
+    it("returns empty object for corrupt JSON", async () => {
+      mkdirSync(stateDir, { recursive: true });
+      writeFileSync(join(stateDir, "bad.json"), "not json{{{");
+      const { loadState } = await getState();
+      expect(loadState("bad")).toEqual({});
+    });
+  });
+
+  describe("saveState", () => {
+    it("creates state dir and writes file", async () => {
+      const { saveState } = await getState();
+      saveState("mystate", { count: 42 });
+      const data = JSON.parse(readFileSync(join(stateDir, "mystate.json"), "utf-8"));
+      expect(data).toEqual({ count: 42 });
+    });
+
+    it("overwrites existing state", async () => {
+      const { saveState } = await getState();
+      saveState("mystate", { v: 1 });
+      saveState("mystate", { v: 2 });
+      const data = JSON.parse(readFileSync(join(stateDir, "mystate.json"), "utf-8"));
+      expect(data).toEqual({ v: 2 });
+    });
+  });
+
+  describe("appendLog / readLog", () => {
+    it("appends entries and reads them back", async () => {
+      const { appendLog, readLog } = await getState();
+      appendLog("test.jsonl", { action: "a" });
+      appendLog("test.jsonl", { action: "b" });
+      appendLog("test.jsonl", { action: "c" });
+
+      const entries = readLog("test.jsonl");
+      expect(entries).toHaveLength(3);
+      expect(entries[0]).toEqual({ action: "a" });
+      expect(entries[2]).toEqual({ action: "c" });
+    });
+
+    it("readLog returns empty array for missing file", async () => {
+      const { readLog } = await getState();
+      expect(readLog("nope.jsonl")).toEqual([]);
+    });
+
+    it("readLog with lastN returns only last N entries", async () => {
+      const { appendLog, readLog } = await getState();
+      appendLog("log.jsonl", { n: 1 });
+      appendLog("log.jsonl", { n: 2 });
+      appendLog("log.jsonl", { n: 3 });
+      appendLog("log.jsonl", { n: 4 });
+
+      const last2 = readLog("log.jsonl", 2);
+      expect(last2).toHaveLength(2);
+      expect(last2[0]).toEqual({ n: 3 });
+      expect(last2[1]).toEqual({ n: 4 });
+    });
+
+    it("skips corrupt lines in log", async () => {
+      const { readLog } = await getState();
+      mkdirSync(stateDir, { recursive: true });
+      writeFileSync(
+        join(stateDir, "mixed.jsonl"),
+        '{"ok":true}\nnot json\n{"also":"ok"}\n'
+      );
+      const entries = readLog("mixed.jsonl");
+      expect(entries).toHaveLength(2);
+      expect(entries[0]).toEqual({ ok: true });
+      expect(entries[1]).toEqual({ also: "ok" });
+    });
+
+    it("readLog returns empty for empty file", async () => {
+      const { readLog } = await getState();
+      mkdirSync(stateDir, { recursive: true });
+      writeFileSync(join(stateDir, "empty.jsonl"), "");
+      expect(readLog("empty.jsonl")).toEqual([]);
+    });
+  });
+
+  describe("now", () => {
+    it("returns a valid ISO string", async () => {
+      const { now } = await getState();
+      const ts = now();
+      expect(new Date(ts).toISOString()).toBe(ts);
+    });
+  });
+});