diff --git a/Dockerfile b/Dockerfile
index 1324920..bdac9eb 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,6 +1,6 @@
 FROM node:22-alpine
 WORKDIR /app
-COPY package.json index.js metrics.js claude-usage.js ./
+COPY package.json index.js metrics.js claude-usage.js claude-usage-history.js ./
 EXPOSE 3001
 USER node
 CMD ["node", "index.js"]
diff --git a/__tests__/auth.test.js b/__tests__/auth.test.js
index e5549c7..b52e11a 100644
--- a/__tests__/auth.test.js
+++ b/__tests__/auth.test.js
@@ -26,6 +26,9 @@ const ROUTES = [
   "/defenseurs/state",
   "/reports/scans",
   "/hosts",
+  // #55. GET /claude-usage is not listed: POST /claude-usage exists, so the
+  // "404 on POST" row below does not hold for it (claude-usage.test.js covers it).
+  "/claude-usage/history",
 ];
 
 // Ingest token, distinct from the read token on purpose: a read token must not
@@ -83,6 +86,7 @@ beforeEach(async () => {
   process.env.DEFENSEURS_FINDINGS_STATE_PATH = path.join(tmpDir, "findings-state.json");
   process.env.HOSTS_DIR = path.join(tmpDir, "hosts");
   process.env.HOSTS_ALLOWED_IDS = "thinkpad";
+  process.env.CLAUDE_USAGE_PATH = path.join(tmpDir, "claude-usage", "claude-usage.json");
   // Configured on purpose: an unset ingest token answers 503 before the header
   // is ever read, which would hide the 401 these tests are here to pin.
   process.env.HOSTS_INGEST_TOKEN = INGEST_TOKEN;
@@ -98,6 +102,10 @@ afterEach(async () => {
   fs.rmSync(tmpDir, { recursive: true, force: true });
 });
 
+afterAll(() => {
+  delete process.env.CLAUDE_USAGE_PATH;
+});
+
 describe("auth gate — missing Authorization header", () => {
   test.each(ROUTES)("401 on GET %s with no Authorization header", async (route) => {
     const { status, body } = await request(route, { auth: null });
@@ -306,6 +314,12 @@ describe("routing", () => {
     expect(body.error).toBe("Not found");
   });
 
+  test("404 (not 401) on POST /claude-usage/history without Authorization", async () => {
+    const { status, body } = await request("/claude-usage/history", { method: "POST", auth: null });
+    expect(status).toBe(404);
+    expect(body.error).toBe("Not found");
+  });
+
   test("404 (not 401) on POST /health without Authorization", async () => {
     const { status, body } = await request("/health", { method: "POST", auth: null });
     expect(status).toBe(404);
diff --git a/__tests__/ci.test.js b/__tests__/ci.test.js
index 09a27a5..995c256 100644
--- a/__tests__/ci.test.js
+++ b/__tests__/ci.test.js
@@ -342,7 +342,7 @@ describe("test-curl.sh", () => {
       expect(proxied).toEqual([]);
       expect(code, stdout + stderr).toBe(0);
       // Every case ran against the API: a count, so a script cut short fails.
-      expect(stdout).toMatch(/=== Results: 45 passed, 0 failed ===/);
+      expect(stdout).toMatch(/=== Results: 51 passed, 0 failed ===/);
     } finally {
       await new Promise((resolve) => proxy.close(resolve));
     }
@@ -361,7 +361,7 @@ describe("test-curl.sh", () => {
 
       for (const { code, stdout, stderr } of results) {
         expect(code, stdout + stderr).toBe(0);
-        expect(stdout).toMatch(/=== Results: 45 passed, 0 failed ===/);
+        expect(stdout).toMatch(/=== Results: 51 passed, 0 failed ===/);
       }
     } finally {
       for (const port of ports) expect(await released(port)).toBe(true);
diff --git a/__tests__/claude-usage-history.test.js b/__tests__/claude-usage-history.test.js
new file mode 100644
index 0000000..8e0c04b
--- /dev/null
+++ b/__tests__/claude-usage-history.test.js
@@ -0,0 +1,581 @@
+const http = require("node:http");
+const fs = require("node:fs");
+const os = require("node:os");
+const path = require("node:path");
+const { spawnSync } = require("node:child_process");
+
+// Claude usage history (issue #55): the closed days of the snapshot, kept past
+// its 30-day window in one file per month under <dir of CLAUDE_USAGE_PATH>/history/.
+//
+// Pinned here:
+//   * the pure helpers of claude-usage-history.js: the two merge rules, the
+//     grouping by the month of the date string, the whitelist re-read of a
+//     month file, and the range rules of the read route;
+//   * the archive written by POST /claude-usage: every day but the last, the
+//     latest snapshot winning, an unreadable month never overwritten, and an
+//     archive failure that never changes the 204;
+//   * GET /claude-usage/history: defaults, bounds, degraded months;
+//   * the one-shot import command, run as a real process: the archived date
+//     wins, and an invalid import file writes nothing.
+
+const TOKEN = "test-token";
+const CLAUDE_TOKEN = "test-claude-token";
+const MODULE = path.join(__dirname, "..", "claude-usage-history.js");
+const FROZEN_NOW = Date.UTC(2026, 9, 8, 19, 0, 0); // 2026-10-08
+
+let tmpDir;
+let usagePath;
+let historyPath;
+let server;
+let baseUrl;
+
+function day(date, output = 1) {
+  return {
+    date,
+    models: { "claude-opus-5-5": { input: 1, output, cacheCreation: 0, cacheRead: 0, requests: 1 } },
+  };
+}
+
+function snapshotWith(days) {
+  return {
+    schemaVersion: 1,
+    generatedAt: "2026-10-02T19:00:00.000Z",
+    timeZone: "America/Toronto",
+    limits: { status: "ok", fetchedAt: "2026-10-02T19:00:00.000Z", items: [] },
+    days,
+    projects: [{ name: "proj", work: 3, cacheRead: 4, requests: 1 }],
+  };
+}
+
+function monthFile(month) {
+  return path.join(historyPath, `${month}.json`);
+}
+
+function readMonthFile(month) {
+  return JSON.parse(fs.readFileSync(monthFile(month), "utf-8"));
+}
+
+function writeMonthFile(month, content) {
+  fs.mkdirSync(historyPath, { recursive: true });
+  fs.writeFileSync(monthFile(month), typeof content === "string" ? content : JSON.stringify(content));
+}
+
+function freshHistory() {
+  delete require.cache[require.resolve("../claude-usage-history.js")];
+  delete require.cache[require.resolve("../claude-usage.js")];
+  return require("../claude-usage-history.js");
+}
+
+function startServer() {
+  for (const mod of ["../index.js", "../claude-usage.js", "../claude-usage-history.js"]) {
+    delete require.cache[require.resolve(mod)];
+  }
+  const { handler } = require("../index.js");
+  server = http.createServer(handler);
+  return new Promise((resolve) => {
+    server.listen(0, () => {
+      baseUrl = `http://127.0.0.1:${server.address().port}`;
+      resolve();
+    });
+  });
+}
+
+function stopServer() {
+  return new Promise((resolve) => {
+    if (!server) return resolve();
+    server.close(() => resolve());
+  });
+}
+
+async function request(route, { method = "GET", auth = `Bearer ${TOKEN}`, body } = {}) {
+  const headers = {};
+  if (auth) headers.Authorization = auth;
+  if (body !== undefined) headers["Content-Type"] = "application/json";
+  const res = await fetch(`${baseUrl}${route}`, { method, headers, body });
+  const parsed = await res.json().catch(() => ({}));
+  return { status: res.status, body: parsed };
+}
+
+function push(days) {
+  return request("/claude-usage", {
+    method: "POST",
+    auth: `Bearer ${CLAUDE_TOKEN}`,
+    body: JSON.stringify(snapshotWith(days)),
+  });
+}
+
+async function history(query = "") {
+  const realNow = Date.now;
+  Date.now = () => FROZEN_NOW;
+  try {
+    return await request(`/claude-usage/history${query}`);
+  } finally {
+    Date.now = realNow;
+  }
+}
+
+// Every file under dir, relative, sorted: what a case may and may not touch.
+function tree(dir) {
+  if (!fs.existsSync(dir)) return [];
+  return fs
+    .readdirSync(dir, { recursive: true })
+    .map(String)
+    .sort();
+}
+
+beforeEach(async () => {
+  tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "vps-health-history-test-"));
+  usagePath = path.join(tmpDir, "claude-usage", "claude-usage.json");
+  historyPath = path.join(tmpDir, "claude-usage", "history");
+
+  process.env.HEALTH_TOKEN = TOKEN;
+  process.env.HOSTS_INGEST_TOKEN = "test-ingest-token";
+  process.env.CLAUDE_USAGE_INGEST_TOKEN = CLAUDE_TOKEN;
+  process.env.CLAUDE_USAGE_PATH = usagePath;
+  process.env.HOSTS_DIR = path.join(tmpDir, "hosts");
+  process.env.HOSTS_ALLOWED_IDS = "thinkpad";
+  process.env.REPORTS_DIR = path.join(tmpDir, "reports");
+  process.env.DEFENSEURS_AGENTS_MAP_PATH = path.join(tmpDir, "agents-map.json");
+  process.env.DEFENSEURS_STATUS_PATH = path.join(tmpDir, "status.json");
+  process.env.DEFENSEURS_FINDINGS_STATE_PATH = path.join(tmpDir, "findings-state.json");
+  // Closed port: no request may reach the real IdP.
+  process.env.LOGTO_HEALTH_URL = "http://127.0.0.1:1/oidc/.well-known/openid-configuration";
+
+  await startServer();
+});
+
+afterEach(async () => {
+  vi.restoreAllMocks();
+  await stopServer();
+  fs.rmSync(tmpDir, { recursive: true, force: true });
+});
+
+afterAll(() => {
+  delete process.env.CLAUDE_USAGE_INGEST_TOKEN;
+  delete process.env.CLAUDE_USAGE_PATH;
+});
+
+// --- Pure helpers -----------------------------------------------------------
+
+describe("month arithmetic", () => {
+  test("addMonths crosses years both ways", () => {
+    const { addMonths } = freshHistory();
+    expect(addMonths("2026-10", -11)).toBe("2025-11");
+    expect(addMonths("2026-12", 1)).toBe("2027-01");
+    expect(addMonths("2026-01", -1)).toBe("2025-12");
+    expect(addMonths("2026-01", -36)).toBe("2023-01");
+  });
+
+  test("monthSpan counts both ends", () => {
+    const { monthSpan } = freshHistory();
+    expect(monthSpan("2026-01", "2026-01")).toBe(1);
+    expect(monthSpan("2026-01", "2026-12")).toBe(12);
+    expect(monthSpan("2024-01", "2026-12")).toBe(36);
+  });
+});
+
+describe("mergeDays", () => {
+  test("the latest value wins by default, result sorted, inputs untouched", () => {
+    const { mergeDays } = freshHistory();
+    const existing = [day("2026-09-02", 1), day("2026-09-01", 1)];
+    const incoming = [day("2026-09-03", 3), day("2026-09-02", 2)];
+    const before = JSON.stringify([existing, incoming]);
+    const merged = mergeDays(existing, incoming);
+    expect(merged.map((d) => [d.date, d.models["claude-opus-5-5"].output])).toEqual([
+      ["2026-09-01", 1],
+      ["2026-09-02", 2],
+      ["2026-09-03", 3],
+    ]);
+    expect(JSON.stringify([existing, incoming])).toBe(before);
+  });
+
+  test("preferExisting keeps an archived date and adds only the missing ones", () => {
+    const { mergeDays } = freshHistory();
+    const merged = mergeDays([day("2026-09-02", 1)], [day("2026-09-02", 9), day("2026-09-01", 9)], {
+      preferExisting: true,
+    });
+    expect(merged.map((d) => [d.date, d.models["claude-opus-5-5"].output])).toEqual([
+      ["2026-09-01", 9],
+      ["2026-09-02", 1],
+    ]);
+  });
+
+  test("a date repeated in the incoming list: the later entry wins", () => {
+    const { mergeDays } = freshHistory();
+    const merged = mergeDays([], [day("2026-09-01", 1), day("2026-09-01", 2)]);
+    expect(merged).toEqual([day("2026-09-01", 2)]);
+  });
+});
+
+describe("groupByMonth", () => {
+  test("groups by the YYYY-MM prefix of the date string", () => {
+    const { groupByMonth } = freshHistory();
+    const months = groupByMonth([day("2026-10-31"), day("2026-11-01"), day("2026-10-01")]);
+    expect([...months.keys()]).toEqual(["2026-10", "2026-11"]);
+    expect(months.get("2026-10").map((d) => d.date)).toEqual(["2026-10-31", "2026-10-01"]);
+  });
+});
+
+describe("sanitizeMonthFile — the snapshot's whitelist on the way back", () => {
+  test("a clean file comes back identical", () => {
+    const { sanitizeMonthFile } = freshHistory();
+    const file = { schemaVersion: 1, month: "2026-09", days: [day("2026-09-01"), day("2026-09-02")] };
+    expect(sanitizeMonthFile(file, "2026-09")).toEqual(file.days);
+  });
+
+  test("unknown fields, __proto__, invalid values and foreign dates are dropped", () => {
+    const { sanitizeMonthFile } = freshHistory();
+    const raw =
+      '{"schemaVersion":1,"month":"2026-09","extra":"x","days":[' +
+      '{"date":"2026-09-01","extra":"<script>","models":{"__proto__":{"input":1},"claude-opus-5-5":{"input":-1,"output":2,"polluted":true}}},' +
+      '{"date":"2026-10-01","models":{}},' +
+      '{"date":"2026-09-31","models":{}},' +
+      '"not a day",' +
+      '{"__proto__":{"polluted":true},"date":"2026-09-02","models":{}}' +
+      "]}";
+    const days = sanitizeMonthFile(JSON.parse(raw), "2026-09");
+    expect(days).toEqual([
+      { date: "2026-09-01", models: { "claude-opus-5-5": { output: 2 } } },
+      { date: "2026-09-02", models: {} },
+    ]);
+    expect({}.polluted).toBeUndefined();
+    expect(Object.keys(days[0].models)).toEqual(["claude-opus-5-5"]);
+  });
+
+  test.each([
+    ["an array", []],
+    ["null", null],
+    ["another schemaVersion", { schemaVersion: 2, month: "2026-09", days: [] }],
+    ["no schemaVersion", { month: "2026-09", days: [] }],
+    ["another month", { schemaVersion: 1, month: "2026-08", days: [] }],
+    ["days not an array", { schemaVersion: 1, month: "2026-09", days: {} }],
+  ])("off-format as a whole (%s) -> null", (_label, parsed) => {
+    const { sanitizeMonthFile } = freshHistory();
+    expect(sanitizeMonthFile(parsed, "2026-09")).toBeNull();
+  });
+});
+
+describe("parseRange", () => {
+  const ctx = { lastArchived: "2026-09", currentMonth: "2026-10" };
+
+  test.each([
+    ["nothing archived: 12 months up to the current one", null, null, { lastArchived: null, currentMonth: "2026-10" }, { from: "2025-11", to: "2026-10" }],
+    ["12 months up to the last archived month", null, null, ctx, { from: "2025-10", to: "2026-09" }],
+    ["to alone: 12 months up to it", null, "2026-03", ctx, { from: "2025-04", to: "2026-03" }],
+    ["from alone: up to the last archived month", "2026-01", null, ctx, { from: "2026-01", to: "2026-09" }],
+    ["from alone, past the last archived month: to = from", "2026-12", null, ctx, { from: "2026-12", to: "2026-12" }],
+    ["exactly 36 months", "2024-01", "2026-12", ctx, { from: "2024-01", to: "2026-12" }],
+  ])("%s", (_label, from, to, context, expected) => {
+    const { parseRange } = freshHistory();
+    expect(parseRange(from, to, context)).toEqual(expected);
+  });
+
+  test.each([
+    ["37 months", "2023-12", "2026-12"],
+    ["from alone, 37 months before the last archived month", "2023-09", null],
+    ["from after to", "2026-09", "2026-08"],
+    ["month 13", "2026-13", null],
+    ["month 00", "2026-00", null],
+    ["a one-digit month", "2026-1", null],
+    ["a full date", "2026-09-01", null],
+    ["an empty value", "", null],
+    ["a traversal", "../etc", null],
+    ["a bad to", null, "2026-9"],
+  ])("error on %s", (_label, from, to) => {
+    const { parseRange } = freshHistory();
+    expect(parseRange(from, to, ctx).error).toEqual(expect.any(String));
+  });
+});
+
+// --- POST /claude-usage writes the archive ----------------------------------
+
+describe("POST /claude-usage — the archive", () => {
+  test("every day but the last is archived, mode 600, no temp file", async () => {
+    expect((await push([day("2026-09-29"), day("2026-09-30"), day("2026-10-01")])).status).toBe(204);
+    expect(fs.readdirSync(historyPath)).toEqual(["2026-09.json"]);
+    expect(readMonthFile("2026-09")).toEqual({
+      schemaVersion: 1,
+      month: "2026-09",
+      days: [day("2026-09-29"), day("2026-09-30")],
+    });
+    expect(fs.statSync(monthFile("2026-09")).mode & 0o777).toBe(0o600);
+  });
+
+  test("a one-day snapshot archives nothing and creates no directory", async () => {
+    expect((await push([day("2026-10-01")])).status).toBe(204);
+    expect(fs.existsSync(historyPath)).toBe(false);
+  });
+
+  test("a second identical POST leaves the archive byte for byte identical", async () => {
+    const days = [day("2026-09-29"), day("2026-09-30"), day("2026-10-01"), day("2026-10-02")];
+    await push(days);
+    const before = [fs.readFileSync(monthFile("2026-09")), fs.readFileSync(monthFile("2026-10"))];
+    await push(days);
+    expect([fs.readFileSync(monthFile("2026-09")), fs.readFileSync(monthFile("2026-10"))]).toEqual(before);
+    expect(fs.readdirSync(historyPath).sort()).toEqual(["2026-09.json", "2026-10.json"]);
+  });
+
+  test("a later snapshot carrying a day with other values replaces it", async () => {
+    await push([day("2026-09-29", 1), day("2026-09-30", 1)]);
+    await push([day("2026-09-29", 7), day("2026-09-30", 1)]);
+    expect(readMonthFile("2026-09").days).toEqual([day("2026-09-29", 7)]);
+  });
+
+  test("a day that left the window keeps its archived value", async () => {
+    await push([day("2026-09-28", 5), day("2026-09-29", 5), day("2026-09-30", 5)]);
+    await push([day("2026-09-29", 6), day("2026-09-30", 6), day("2026-10-01", 6)]);
+    expect(readMonthFile("2026-09").days).toEqual([day("2026-09-28", 5), day("2026-09-29", 6), day("2026-09-30", 6)]);
+  });
+
+  test("a snapshot straddling two months writes two files, each day in its own", async () => {
+    await push([day("2026-09-29"), day("2026-09-30"), day("2026-10-01"), day("2026-10-02"), day("2026-10-03")]);
+    expect(readMonthFile("2026-09").days.map((d) => d.date)).toEqual(["2026-09-29", "2026-09-30"]);
+    expect(readMonthFile("2026-10").days.map((d) => d.date)).toEqual(["2026-10-01", "2026-10-02"]);
+  });
+
+  test("the excluded day is the last element sent, and a repeated date resolves to its later entry", async () => {
+    // Out of order and with a repeat: the agent never sends this, the rule
+    // must still be the one written down.
+    await push([day("2026-09-30", 1), day("2026-09-29", 1), day("2026-09-30", 2), day("2026-09-28", 1)]);
+    expect(readMonthFile("2026-09").days).toEqual([day("2026-09-29", 1), day("2026-09-30", 2)]);
+  });
+
+  test("an unreadable month: 204, snapshot written, that month untouched, the other written, logged without content", async () => {
+    writeMonthFile("2026-09", "{corrupt");
+    const error = vi.spyOn(console, "error").mockImplementation(() => {});
+    const res = await push([day("2026-09-29"), day("2026-09-30"), day("2026-10-01"), day("2026-10-02")]);
+    expect(res.status).toBe(204);
+    expect(JSON.parse(fs.readFileSync(usagePath, "utf-8")).days).toHaveLength(4);
+    expect(fs.readFileSync(monthFile("2026-09"), "utf-8")).toBe("{corrupt");
+    expect(readMonthFile("2026-10").days.map((d) => d.date)).toEqual(["2026-10-01"]);
+    const lines = error.mock.calls.map((args) => args.join(" "));
+    expect(lines).toEqual(["[claude-usage] history 2026-09 skipped: unreadable file left untouched (parse_error)"]);
+  });
+
+  test("an off-format month (another schemaVersion) is not overwritten either", async () => {
+    const foreign = JSON.stringify({ schemaVersion: 2, month: "2026-09", days: [day("2026-09-01")] });
+    writeMonthFile("2026-09", foreign);
+    vi.spyOn(console, "error").mockImplementation(() => {});
+    expect((await push([day("2026-09-29"), day("2026-09-30")])).status).toBe(204);
+    expect(fs.readFileSync(monthFile("2026-09"), "utf-8")).toBe(foreign);
+  });
+
+  test("history/ cannot be created: 204, snapshot written, logged", async () => {
+    fs.mkdirSync(path.dirname(usagePath), { recursive: true });
+    fs.writeFileSync(historyPath, "a file where the directory should be");
+    const error = vi.spyOn(console, "error").mockImplementation(() => {});
+    const res = await push([day("2026-09-29"), day("2026-09-30")]);
+    expect(res.status).toBe(204);
+    expect(JSON.parse(fs.readFileSync(usagePath, "utf-8")).days).toHaveLength(2);
+    const lines = error.mock.calls.map((args) => args.join(" "));
+    expect(lines).toHaveLength(1);
+    expect(lines[0]).toMatch(/^\[claude-usage\] history not written \(E[A-Z]+\)$/);
+  });
+
+  test("a month whose rename fails: 204, its temp file removed, the other month written, logged", async () => {
+    const realRename = fs.renameSync;
+    vi.spyOn(fs, "renameSync").mockImplementation((from, to) => {
+      if (String(to).endsWith("2026-09.json")) {
+        throw Object.assign(new Error("simulated"), { code: "EIO" });
+      }
+      return realRename(from, to);
+    });
+    const error = vi.spyOn(console, "error").mockImplementation(() => {});
+    const res = await push([day("2026-09-30"), day("2026-10-01"), day("2026-10-02")]);
+    expect(res.status).toBe(204);
+    expect(fs.readdirSync(historyPath)).toEqual(["2026-10.json"]);
+    expect(error.mock.calls.map((args) => args.join(" "))).toEqual(["[claude-usage] history 2026-09 not written (EIO)"]);
+  });
+
+  test("a rejected POST archives nothing", async () => {
+    const res = await request("/claude-usage", {
+      method: "POST",
+      auth: `Bearer ${CLAUDE_TOKEN}`,
+      body: JSON.stringify({ ...snapshotWith([day("2026-09-29"), day("2026-09-30")]), schemaVersion: 2 }),
+    });
+    expect(res.status).toBe(400);
+    expect(fs.existsSync(historyPath)).toBe(false);
+  });
+
+  test("a snapshot that cannot be written archives nothing", async () => {
+    fs.mkdirSync(path.join(usagePath, "occupied"), { recursive: true });
+    vi.spyOn(console, "error").mockImplementation(() => {});
+    expect((await push([day("2026-09-29"), day("2026-09-30")])).status).toBe(500);
+    expect(fs.existsSync(historyPath)).toBe(false);
+  });
+});
+
+// --- GET /claude-usage/history ----------------------------------------------
+
+describe("GET /claude-usage/history", () => {
+  test("nothing archived: 200, 12 months up to the current one, no day", async () => {
+    const res = await history();
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual({ from: "2025-11", to: "2026-10", days: [] });
+  });
+
+  test("defaults to 12 months up to the last archived month, days sorted across files", async () => {
+    writeMonthFile("2026-08", { schemaVersion: 1, month: "2026-08", days: [day("2026-08-31")] });
+    writeMonthFile("2026-09", { schemaVersion: 1, month: "2026-09", days: [day("2026-09-01"), day("2026-09-02")] });
+    writeMonthFile("2025-09", { schemaVersion: 1, month: "2025-09", days: [day("2025-09-30")] });
+    const res = await history();
+    expect(res.body).toEqual({
+      from: "2025-10",
+      to: "2026-09",
+      days: [day("2026-08-31"), day("2026-09-01"), day("2026-09-02")],
+    });
+  });
+
+  test("from and to select the months, an absent month is simply empty", async () => {
+    writeMonthFile("2026-06", { schemaVersion: 1, month: "2026-06", days: [day("2026-06-15")] });
+    writeMonthFile("2026-09", { schemaVersion: 1, month: "2026-09", days: [day("2026-09-01")] });
+    const res = await history("?from=2026-06&to=2026-08");
+    expect(res.body).toEqual({ from: "2026-06", to: "2026-08", days: [day("2026-06-15")] });
+  });
+
+  test("an unreadable month is left out and logged, the others are served", async () => {
+    writeMonthFile("2026-08", "{corrupt");
+    writeMonthFile("2026-09", { schemaVersion: 1, month: "2026-09", days: [day("2026-09-01")] });
+    const error = vi.spyOn(console, "error").mockImplementation(() => {});
+    const res = await history("?from=2026-08&to=2026-09");
+    expect(res.status).toBe(200);
+    expect(res.body.days).toEqual([day("2026-09-01")]);
+    expect(error.mock.calls.map((args) => args.join(" "))).toEqual([
+      "[claude-usage] history 2026-08 unreadable, left out (parse_error)",
+    ]);
+  });
+
+  test("the files are re-sanitized on the way out", async () => {
+    writeMonthFile(
+      "2026-09",
+      '{"schemaVersion":1,"month":"2026-09","days":[{"date":"2026-09-01","extra":1,"models":{"__proto__":{"input":1},"claude-opus-5-5":{"output":-1,"input":3}}},{"date":"2026-08-31","models":{}}]}',
+    );
+    const res = await history("?from=2026-09&to=2026-09");
+    expect(res.body.days).toEqual([{ date: "2026-09-01", models: { "claude-opus-5-5": { input: 3 } } }]);
+  });
+
+  test("a stray file name that is not a month is not the default end", async () => {
+    writeMonthFile("2026-09", { schemaVersion: 1, month: "2026-09", days: [day("2026-09-01")] });
+    writeMonthFile("2026-13", "{}");
+    fs.writeFileSync(path.join(historyPath, ".2026-12.json.1.2.tmp"), "{}");
+    expect((await history()).body.to).toBe("2026-09");
+  });
+
+  test.each([
+    ["?from=2026-13"],
+    ["?to=2026-9"],
+    ["?from=2026-09&to=2026-08"],
+    ["?from=2023-12&to=2026-12"],
+    ["?from=../../etc"],
+    ["?from="],
+  ])("400 on %s", async (query) => {
+    const res = await history(query);
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/^Bad request: /);
+  });
+
+  test("36 months exactly is served", async () => {
+    expect((await history("?from=2024-01&to=2026-12")).status).toBe(200);
+  });
+
+  test("the read token only: no token, the claude ingest token -> 401", async () => {
+    expect((await request("/claude-usage/history", { auth: null })).status).toBe(401);
+    expect((await request("/claude-usage/history", { auth: `Bearer ${CLAUDE_TOKEN}` })).status).toBe(401);
+  });
+
+  test("POST /claude-usage/history -> 404, before any auth", async () => {
+    expect((await request("/claude-usage/history", { method: "POST", auth: null })).status).toBe(404);
+  });
+
+  test("POST then GET: the closed days come back", async () => {
+    await push([day("2026-09-30"), day("2026-10-01"), day("2026-10-02")]);
+    const res = await history("?from=2026-09&to=2026-10");
+    expect(res.body.days).toEqual([day("2026-09-30"), day("2026-10-01")]);
+  });
+});
+
+// --- Import command ---------------------------------------------------------
+
+describe("node claude-usage-history.js --import", () => {
+  function runImport(args, { input } = {}) {
+    return spawnSync(process.execPath, [MODULE, ...args], {
+      env: { PATH: process.env.PATH, CLAUDE_USAGE_PATH: usagePath },
+      input,
+      encoding: "utf-8",
+    });
+  }
+
+  function importFile(content) {
+    const file = path.join(tmpDir, "backfill.json");
+    fs.writeFileSync(file, typeof content === "string" ? content : JSON.stringify(content));
+    return file;
+  }
+
+  test("an archived date is never replaced, absent dates are added, only history/ changes", async () => {
+    writeMonthFile("2026-09", { schemaVersion: 1, month: "2026-09", days: [day("2026-09-02", 1)] });
+    const file = importFile({ days: [day("2026-08-31", 9), day("2026-09-01", 9), day("2026-09-02", 9)] });
+    const before = tree(tmpDir).filter((name) => !name.startsWith(`claude-usage${path.sep}history`));
+
+    const result = runImport(["--import", file]);
+    expect(result.stderr).toBe("");
+    expect(result.status).toBe(0);
+    expect(result.stdout).toBe("import: 3 day(s) read, 2 added, 1 already archived, 2 month(s) written\n");
+    expect(readMonthFile("2026-09").days).toEqual([day("2026-09-01", 9), day("2026-09-02", 1)]);
+    expect(readMonthFile("2026-08").days).toEqual([day("2026-08-31", 9)]);
+    expect(fs.statSync(monthFile("2026-08")).mode & 0o777).toBe(0o600);
+    expect(fs.readdirSync(historyPath).sort()).toEqual(["2026-08.json", "2026-09.json"]);
+    expect(tree(tmpDir).filter((name) => !name.startsWith(`claude-usage${path.sep}history`))).toEqual(before);
+  });
+
+  test("reads standard input on `-`", () => {
+    const result = runImport(["--import", "-"], { input: JSON.stringify({ days: [day("2026-07-01")] }) });
+    expect(result.status).toBe(0);
+    expect(readMonthFile("2026-07").days).toEqual([day("2026-07-01")]);
+  });
+
+  test.each([
+    ["not JSON", "{nope"],
+    ["no days", JSON.stringify({})],
+    ["days not an array", JSON.stringify({ days: {} })],
+    ["an invalid day", JSON.stringify({ days: [day("2026-07-01"), { date: "2026-02-31", models: {} }] })],
+    ["a repeated date", JSON.stringify({ days: [day("2026-07-01"), day("2026-07-01", 2)] })],
+  ])("an invalid import file (%s) writes nothing, exit 1", (_label, content) => {
+    const file = importFile(content);
+    const before = tree(tmpDir);
+    const result = runImport(["--import", file]);
+    expect(result.status).toBe(1);
+    expect(result.stderr).toBe("import: invalid import file, nothing written\n");
+    expect(tree(tmpDir)).toEqual(before);
+  });
+
+  test("an unreadable month is skipped and left as is, the others are written, exit 1", () => {
+    writeMonthFile("2026-09", "{corrupt");
+    const file = importFile({ days: [day("2026-08-31"), day("2026-09-01")] });
+    const result = runImport(["--import", file]);
+    expect(result.status).toBe(1);
+    expect(result.stderr).toBe("import: month 2026-09 skipped, unreadable (parse_error)\n");
+    expect(fs.readFileSync(monthFile("2026-09"), "utf-8")).toBe("{corrupt");
+    expect(readMonthFile("2026-08").days).toEqual([day("2026-08-31")]);
+  });
+
+  test("a missing import file: exit 1, nothing written", () => {
+    const result = runImport(["--import", path.join(tmpDir, "absent.json")]);
+    expect(result.status).toBe(1);
+    expect(result.stderr).toBe("import: cannot read the import file (ENOENT)\n");
+    expect(fs.existsSync(historyPath)).toBe(false);
+  });
+
+  test.each([[[]], [["--import"]], [["--other", "x"]]])("usage error on %j: exit 2", (args) => {
+    const result = runImport(args);
+    expect(result.status).toBe(2);
+    expect(fs.existsSync(historyPath)).toBe(false);
+  });
+});
+
+describe("the server image", () => {
+  test("the Dockerfile COPY line ships claude-usage-history.js", () => {
+    const dockerfile = fs.readFileSync(path.join(__dirname, "..", "Dockerfile"), "utf-8");
+    const copy = dockerfile.split("\n").find((line) => /^\s*COPY\s/.test(line));
+    expect(copy.trim().split(/\s+/)).toContain("claude-usage-history.js");
+  });
+});
diff --git a/__tests__/claude-usage.test.js b/__tests__/claude-usage.test.js
index ec08e58..b9c59f8 100644
--- a/__tests__/claude-usage.test.js
+++ b/__tests__/claude-usage.test.js
@@ -725,8 +725,9 @@ describe("POST /claude-usage — payload", () => {
     // A client-supplied receivedAt is ignored: the server stamps it.
     expect(new Date(receivedAt).getTime()).toBeGreaterThanOrEqual(before - 1000);
     expect(fs.statSync(usagePath).mode & 0o777).toBe(0o600);
-    // Atomic write: no temp file left beside the snapshot.
-    expect(fs.readdirSync(path.dirname(usagePath))).toEqual(["claude-usage.json"]);
+    // Atomic write: no temp file left beside the snapshot. history/ holds the
+    // closed day of this two-day snapshot (#55, claude-usage-history.test.js).
+    expect(fs.readdirSync(path.dirname(usagePath)).sort()).toEqual(["claude-usage.json", "history"]);
   });
 
   test("a second push replaces the first", async () => {
diff --git a/__tests__/writes.test.js b/__tests__/writes.test.js
index 499ceb6..6b7c6ac 100644
--- a/__tests__/writes.test.js
+++ b/__tests__/writes.test.js
@@ -205,11 +205,19 @@ function validClaudeUsage() {
     schemaVersion: 1,
     timeZone: "America/Toronto",
     limits: { status: "ok", fetchedAt: "2026-09-30T07:30:00.000Z", items: [] },
-    days: [{ date: "2026-09-30", models: { "claude-opus-5-5": { input: 1, output: 2, requests: 1 } } }],
+    // Two days: POST archives the first one into history/ (#55).
+    days: [
+      { date: "2026-09-29", models: { "claude-opus-5-5": { input: 1, output: 1, requests: 1 } } },
+      { date: "2026-09-30", models: { "claude-opus-5-5": { input: 1, output: 2, requests: 1 } } },
+    ],
     projects: [{ name: "proj", work: 3, cacheRead: 4, requests: 1 }],
   };
 }
 
+function historyFixturePath(month) {
+  return path.join(tmpDir, "claude-usage", "history", `${month}.json`);
+}
+
 function claudeUsageFixturePath() {
   return path.join(tmpDir, "claude-usage", "claude-usage.json");
 }
@@ -295,6 +303,12 @@ function writeFixtures() {
   // not under it, so a write there is checked against its own rule.
   writeJson(claudeUsageFixturePath(), { ...validClaudeUsage(), receivedAt: new Date().toISOString() });
   process.env.CLAUDE_USAGE_PATH = claudeUsageFixturePath();
+  // An archived month (#55), so GET /claude-usage/history runs its read path.
+  writeJson(historyFixturePath("2026-09"), {
+    schemaVersion: 1,
+    month: "2026-09",
+    days: [{ date: "2026-09-28", models: {} }],
+  });
   process.env.CLAUDE_USAGE_INGEST_TOKEN = CLAUDE_TOKEN;
   // Closed port: /health must never reach the real IdP.
   process.env.LOGTO_HEALTH_URL = "http://127.0.0.1:1/oidc/.well-known/openid-configuration";
@@ -352,6 +366,7 @@ describe("fs writes — the API writes only under HOSTS_DIR (#36)", () => {
       },
     ],
     ["/claude-usage", (body) => expect(body.snapshot.projects).toHaveLength(1)],
+    ["/claude-usage/history", (body) => expect(body.days).toEqual([{ date: "2026-09-28", models: {} }])],
   ];
 
   test.each(READ_ROUTES)("GET %s writes nothing, not even after the response", async (route, expectContent) => {
@@ -397,6 +412,9 @@ describe("fs writes — the API writes only under HOSTS_DIR (#36)", () => {
     const writes = drain();
     expect(writes.map(({ fn }) => fn)).toEqual(expect.arrayContaining(["writeFileSync", "renameSync"]));
     expect(violations(writes, path.dirname(process.env.CLAUDE_USAGE_PATH))).toEqual([]);
+    // The archive (#55) is part of this surface, and the case proves it ran:
+    // a rename onto the month file of the closed day.
+    expect(writes.some(({ fn, targets }) => fn === "renameSync" && targets.includes(historyFixturePath("2026-09")))).toBe(true);
     expect(foreignCommands()).toEqual([]);
   });
 
@@ -419,6 +437,11 @@ describe("fs writes — the API writes only under HOSTS_DIR (#36)", () => {
     const { status, body } = await send("/claude-usage");
     expect(status).toBe(200);
     expect(body).toEqual({ status: "no_data" });
+    // Same for a corrupt month of the archive, and for a refused range (#55).
+    real.writeFileSync(historyFixturePath("2026-09"), "{corrupt");
+    const history = await send("/claude-usage/history?from=2026-09&to=2026-09");
+    expect([history.status, history.body]).toEqual([200, { from: "2026-09", to: "2026-09", days: [] }]);
+    expect((await send("/claude-usage/history?from=2026-13")).status).toBe(400);
 
     await settle();
     expect(drain()).toEqual([]);
diff --git a/claude-usage-history.js b/claude-usage-history.js
new file mode 100644
index 0000000..402b4c8
--- /dev/null
+++ b/claude-usage-history.js
@@ -0,0 +1,315 @@
+// Claude usage history (issue #55): the closed days of the Claude usage
+// snapshot, kept past its 30-day window, one file per month.
+//
+// Layout: <dir of CLAUDE_USAGE_PATH>/history/YYYY-MM.json, each file
+//   { "schemaVersion": 1, "month": "YYYY-MM", "days": [{ "date", "models" }] }
+// with the days sorted oldest first, in the very shape of a `days` entry of the
+// snapshot (same per-day cleaner, exported by claude-usage.js).
+//
+// Two writers, two merge rules:
+//   - POST /claude-usage (index.js): the LATEST SNAPSHOT WINS. Every day the
+//     snapshot carries, except the last one (today, still open), replaces the
+//     archived value. A day leaving the window keeps the last value it had,
+//     the one the agent froze at D+2.
+//   - the one-shot import below (`--import`): the ARCHIVED DATE WINS. The live
+//     archive is the reference; a backfill only fills the dates it lacks.
+//
+// A month file that cannot be read back is never overwritten: it may hold days
+// that left the window long ago, and rewriting it from one snapshot would lose
+// them. That month is skipped and reported; the other months are written.
+//
+// No config and no environment read outside the CLI block at the bottom: every
+// function takes the history directory as an argument. Dates are grouped by
+// the YYYY-MM prefix of their string, never by a clock computation.
+
+const fs = require("node:fs");
+const path = require("node:path");
+const { sanitizeDay } = require("./claude-usage.js");
+
+const HISTORY_SCHEMA_VERSION = 1;
+const MONTH_RE = /^\d{4}-(0[1-9]|1[0-2])$/;
+const MONTH_FILE_RE = /^\d{4}-(0[1-9]|1[0-2])\.json$/;
+// GET /claude-usage/history: default span, and the widest one it serves.
+const DEFAULT_MONTHS = 12;
+const MAX_MONTHS = 36;
+
+function isPlainObject(value) {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}
+
+function isMonth(value) {
+  return typeof value === "string" && MONTH_RE.test(value);
+}
+
+function monthOf(date) {
+  return date.slice(0, 7);
+}
+
+// Calendar arithmetic on the two numbers of the string, no Date involved.
+function addMonths(month, n) {
+  const index = Number(month.slice(0, 4)) * 12 + Number(month.slice(5, 7)) - 1 + n;
+  const year = Math.floor(index / 12);
+  return `${String(year).padStart(4, "0")}-${String((index % 12) + 1).padStart(2, "0")}`;
+}
+
+// Inclusive: 2026-01 to 2026-12 spans 12 months.
+function monthSpan(from, to) {
+  const index = (m) => Number(m.slice(0, 4)) * 12 + Number(m.slice(5, 7));
+  return index(to) - index(from) + 1;
+}
+
+function byDate(a, b) {
+  return a.date < b.date ? -1 : a.date > b.date ? 1 : 0;
+}
+
+// Replace or add each incoming date (preferExisting: false, the POST rule), or
+// add only the dates not there yet (preferExisting: true, the import rule).
+// Returns a new array sorted oldest first; neither input is modified.
+function mergeDays(existing, incoming, { preferExisting = false } = {}) {
+  const merged = new Map();
+  for (const day of existing) merged.set(day.date, day);
+  for (const day of incoming) {
+    if (preferExisting && merged.has(day.date)) continue;
+    merged.set(day.date, day);
+  }
+  return [...merged.values()].sort(byDate);
+}
+
+// YYYY-MM -> days of that month, in input order.
+function groupByMonth(days) {
+  const months = new Map();
+  for (const day of days) {
+    const month = monthOf(day.date);
+    if (!months.has(month)) months.set(month, []);
+    months.get(month).push(day);
+  }
+  return months;
+}
+
+// A parsed month file, re-read through the snapshot's whitelist: the file
+// lives on a writable bind-mount and is no more trusted than a payload.
+// Returns the clean days, or null when the file is off-format as a whole (it
+// is then "unreadable": served without it, and never overwritten). A single
+// invalid day, or one dated outside the month, is dropped; on a repeated
+// date, the later entry wins.
+function sanitizeMonthFile(parsed, month) {
+  if (!isPlainObject(parsed)) return null;
+  if (parsed.schemaVersion !== HISTORY_SCHEMA_VERSION) return null;
+  if (parsed.month !== month || !Array.isArray(parsed.days)) return null;
+  const days = [];
+  for (const entry of parsed.days) {
+    const day = sanitizeDay(entry);
+    if (day !== undefined && monthOf(day.date) === month) days.push(day);
+  }
+  return mergeDays([], days);
+}
+
+function historyDir(claudeUsagePath) {
+  return path.join(path.dirname(claudeUsagePath), "history");
+}
+
+function monthPath(dir, month) {
+  return path.join(dir, `${month}.json`);
+}
+
+// { state: "absent" } | { state: "ok", days } | { state: "unreadable", code }
+function readMonth(dir, month) {
+  let parsed;
+  try {
+    parsed = JSON.parse(fs.readFileSync(monthPath(dir, month), "utf-8"));
+  } catch (err) {
+    if (err.code === "ENOENT") return { state: "absent" };
+    return { state: "unreadable", code: err.code || "parse_error" };
+  }
+  const days = sanitizeMonthFile(parsed, month);
+  if (days === null) return { state: "unreadable", code: "off_format" };
+  return { state: "ok", days };
+}
+
+// Same atomic pattern as writeClaudeUsage() in index.js: temp file in the SAME
+// directory (pid-suffixed, so two containers of a rolling redeploy never share
+// one), mode 600, then renameSync. A concurrent read sees the old month or the
+// new one, never a truncated JSON. No sweep covers these temp files: an orphan
+// is harmless, the reader only opens YYYY-MM.json.
+function writeMonth(dir, month, days) {
+  const finalPath = monthPath(dir, month);
+  const tmpPath = path.join(dir, `.${month}.json.${process.pid}.${Date.now()}.tmp`);
+  const record = { schemaVersion: HISTORY_SCHEMA_VERSION, month, days };
+  try {
+    fs.writeFileSync(tmpPath, JSON.stringify(record), { mode: 0o600 });
+    fs.renameSync(tmpPath, finalPath);
+  } catch (err) {
+    try {
+      fs.unlinkSync(tmpPath);
+    } catch {
+      // Best effort — the temp file may never have been created.
+    }
+    throw err;
+  }
+}
+
+// Merge `days` into their month files. Each month stands alone: an unreadable
+// one is skipped (never overwritten), a failed write is reported, and the
+// other months go on. Only a failure to create the directory throws.
+//
+// Returns { written, skipped, failed, added, kept }: month lists, plus the
+// number of dates added and, under preferExisting, of dates already archived.
+function archiveDays(dir, days, { preferExisting = false } = {}) {
+  const report = { written: [], skipped: [], failed: [], added: 0, kept: 0 };
+  if (!days.length) return report;
+  fs.mkdirSync(dir, { recursive: true });
+  for (const [month, incoming] of [...groupByMonth(days)].sort(([a], [b]) => (a < b ? -1 : 1))) {
+    const current = readMonth(dir, month);
+    if (current.state === "unreadable") {
+      report.skipped.push({ month, code: current.code });
+      continue;
+    }
+    const existing = current.state === "ok" ? current.days : [];
+    const known = new Set(existing.map((day) => day.date));
+    const merged = mergeDays(existing, incoming, { preferExisting });
+    try {
+      writeMonth(dir, month, merged);
+    } catch (err) {
+      report.failed.push({ month, code: err.code || "write_error" });
+      continue;
+    }
+    report.written.push(month);
+    for (const day of incoming) {
+      if (!known.has(day.date)) report.added += 1;
+      else if (preferExisting) report.kept += 1;
+    }
+  }
+  return report;
+}
+
+// The most recent month that has a file, or null (no directory, no file).
+function lastArchivedMonth(dir) {
+  let names;
+  try {
+    names = fs.readdirSync(dir);
+  } catch {
+    return null;
+  }
+  const months = names.filter((name) => MONTH_FILE_RE.test(name)).map((name) => name.slice(0, 7));
+  return months.length ? months.sort().at(-1) : null;
+}
+
+// Query string of GET /claude-usage/history -> { from, to } or { error }.
+// Defaults: `to` is the month of the last archived day (the current month when
+// nothing is archived yet), or `from` itself when `from` is later; `from` is
+// DEFAULT_MONTHS months back from `to`, `to` included.
+function parseRange(fromRaw, toRaw, { lastArchived, currentMonth }) {
+  if (fromRaw !== null && !isMonth(fromRaw)) return { error: "from must be YYYY-MM" };
+  if (toRaw !== null && !isMonth(toRaw)) return { error: "to must be YYYY-MM" };
+  let to = toRaw;
+  if (to === null) {
+    to = lastArchived || currentMonth;
+    if (fromRaw !== null && fromRaw > to) to = fromRaw;
+  }
+  const from = fromRaw !== null ? fromRaw : addMonths(to, -(DEFAULT_MONTHS - 1));
+  if (from > to) return { error: "from must not be after to" };
+  if (monthSpan(from, to) > MAX_MONTHS) return { error: `range must not exceed ${MAX_MONTHS} months` };
+  return { from, to };
+}
+
+// Every archived day from `from` to `to` (both YYYY-MM, inclusive), oldest
+// first. An absent month contributes nothing; an unreadable one is listed in
+// `unreadable` and contributes nothing either.
+function readHistory(dir, from, to) {
+  const days = [];
+  const unreadable = [];
+  for (let month = from; month <= to; month = addMonths(month, 1)) {
+    const current = readMonth(dir, month);
+    if (current.state === "ok") days.push(...current.days);
+    else if (current.state === "unreadable") unreadable.push({ month, code: current.code });
+  }
+  return { days, unreadable };
+}
+
+// Import file of the one-shot backfill: { days: [...] }. Strict, since it is
+// written once by hand: any entry that is not a clean day, or a repeated date,
+// refuses the whole file. Returns the days, or null.
+function parseImport(raw) {
+  let parsed;
+  try {
+    parsed = JSON.parse(raw);
+  } catch {
+    return null;
+  }
+  if (!isPlainObject(parsed) || !Array.isArray(parsed.days)) return null;
+  const days = [];
+  const seen = new Set();
+  for (const entry of parsed.days) {
+    const day = sanitizeDay(entry);
+    if (day === undefined || seen.has(day.date)) return null;
+    seen.add(day.date);
+    days.push(day);
+  }
+  return days;
+}
+
+// `node claude-usage-history.js --import <file|->`, run inside the container
+// (docker exec, as `node`): merges a backfill into the live archive, the
+// archived dates winning. Prints counts only, never a day's content.
+function runImport(argv, env, io) {
+  if (argv.length !== 2 || argv[0] !== "--import") {
+    io.err("usage: node claude-usage-history.js --import <file|->");
+    return 2;
+  }
+  let raw;
+  try {
+    raw = argv[1] === "-" ? fs.readFileSync(0, "utf-8") : fs.readFileSync(argv[1], "utf-8");
+  } catch (err) {
+    io.err(`import: cannot read the import file (${err.code || "read_error"})`);
+    return 1;
+  }
+  const days = parseImport(raw);
+  if (days === null) {
+    io.err("import: invalid import file, nothing written");
+    return 1;
+  }
+  const dir = historyDir(env.CLAUDE_USAGE_PATH || "/data/claude-usage/claude-usage.json");
+  let report;
+  try {
+    report = archiveDays(dir, days, { preferExisting: true });
+  } catch (err) {
+    io.err(`import: cannot create the history directory (${err.code || "error"})`);
+    return 1;
+  }
+  io.out(
+    `import: ${days.length} day(s) read, ${report.added} added, ${report.kept} already archived, ` +
+      `${report.written.length} month(s) written`,
+  );
+  for (const { month, code } of report.skipped) io.err(`import: month ${month} skipped, unreadable (${code})`);
+  for (const { month, code } of report.failed) io.err(`import: month ${month} not written (${code})`);
+  return report.skipped.length || report.failed.length ? 1 : 0;
+}
+
+if (require.main === module) {
+  process.exitCode = runImport(process.argv.slice(2), process.env, {
+    out: (line) => console.log(line),
+    err: (line) => console.error(line),
+  });
+}
+
+module.exports = {
+  isMonth,
+  monthOf,
+  addMonths,
+  monthSpan,
+  mergeDays,
+  groupByMonth,
+  sanitizeMonthFile,
+  historyDir,
+  readMonth,
+  writeMonth,
+  archiveDays,
+  lastArchivedMonth,
+  parseRange,
+  readHistory,
+  parseImport,
+  runImport,
+  HISTORY_SCHEMA_VERSION,
+  DEFAULT_MONTHS,
+  MAX_MONTHS,
+};
diff --git a/claude-usage.js b/claude-usage.js
index bdeb207..3e19569 100644
--- a/claude-usage.js
+++ b/claude-usage.js
@@ -113,16 +113,24 @@ function sanitizeModels(value) {
   return out;
 }
 
+// One entry of `days`, or undefined when it has no usable date. Also re-reads
+// the monthly history files (claude-usage-history.js, #55): an archived day
+// goes through the very whitelist of the snapshot it came from.
+function sanitizeDay(entry) {
+  if (!isPlainObject(entry)) return undefined;
+  const date = cleanDayDate(entry.date);
+  if (date === undefined) return undefined;
+  const day = { date };
+  put(day, "models", sanitizeModels(entry.models));
+  return day;
+}
+
 function sanitizeDays(value) {
   if (!Array.isArray(value)) return undefined;
   const days = [];
   for (const entry of value) {
-    if (!isPlainObject(entry)) continue;
-    const date = cleanDayDate(entry.date);
-    if (date === undefined) continue;
-    const day = { date };
-    put(day, "models", sanitizeModels(entry.models));
-    days.push(day);
+    const day = sanitizeDay(entry);
+    if (day !== undefined) days.push(day);
   }
   return days.slice(-MAX_DAYS);
 }
@@ -183,6 +191,7 @@ function sanitizeClaudeUsage(payload) {
 
 module.exports = {
   sanitizeClaudeUsage,
+  sanitizeDay,
   isIsoTimestamp,
   CLAUDE_USAGE_MAX_BODY_BYTES,
   SCHEMA_VERSION,
diff --git a/index.js b/index.js
index 97c0715..c7c29d7 100644
--- a/index.js
+++ b/index.js
@@ -14,6 +14,13 @@ const path = require("node:path");
 const { createHash, timingSafeEqual } = require("node:crypto");
 const { collectMetrics } = require("./metrics.js");
 const { sanitizeClaudeUsage, isIsoTimestamp, CLAUDE_USAGE_MAX_BODY_BYTES } = require("./claude-usage.js");
+const {
+  historyDir,
+  archiveDays,
+  lastArchivedMonth,
+  parseRange,
+  readHistory,
+} = require("./claude-usage-history.js");
 
 const PORT = parseInt(process.env.PORT || "3001", 10);
 const TOKEN = process.env.HEALTH_TOKEN;
@@ -85,6 +92,10 @@ const CLAUDE_USAGE_INGEST_TOKEN = process.env.CLAUDE_USAGE_INGEST_TOKEN;
 const CLAUDE_USAGE_PATH =
   process.env.CLAUDE_USAGE_PATH || "/data/claude-usage/claude-usage.json";
 
+// Closed days kept past the 30-day window (#55): one file per month, on the
+// same mount as the snapshot. Layout and merge rules in claude-usage-history.js.
+const CLAUDE_USAGE_HISTORY_DIR = historyDir(CLAUDE_USAGE_PATH);
+
 const claudeStaleRaw = parseInt(process.env.CLAUDE_USAGE_STALE_SECONDS || "", 10);
 const CLAUDE_USAGE_STALE_SECONDS =
   Number.isFinite(claudeStaleRaw) && claudeStaleRaw > 0 ? claudeStaleRaw : 2700;
@@ -791,13 +802,35 @@ async function handleClaudeUsageIngest(req, res) {
 
   try {
     writeClaudeUsage(record);
-    res.removeHeader("Content-Type");
-    res.writeHead(204);
-    res.end();
   } catch (err) {
     console.error("[claude-usage] failed to persist snapshot:", err.message);
     res.writeHead(500);
     res.end(JSON.stringify({ error: "Internal error" }));
+    return;
+  }
+
+  // The snapshot first, the archive second: an archive failure never costs
+  // the main data, and never changes the 204.
+  archiveClosedDays(record.days || []);
+  res.removeHeader("Content-Type");
+  res.writeHead(204);
+  res.end();
+}
+
+// Every day of the snapshot except the last one (today, still open), the
+// latest snapshot winning (#55). Months and error codes only in the log, never
+// a day's content.
+function archiveClosedDays(days) {
+  try {
+    const report = archiveDays(CLAUDE_USAGE_HISTORY_DIR, days.slice(0, -1));
+    for (const { month, code } of report.skipped) {
+      console.error(`[claude-usage] history ${month} skipped: unreadable file left untouched (${code})`);
+    }
+    for (const { month, code } of report.failed) {
+      console.error(`[claude-usage] history ${month} not written (${code})`);
+    }
+  } catch (err) {
+    console.error(`[claude-usage] history not written (${err.code || "error"})`);
   }
 }
 
@@ -825,6 +858,31 @@ function handleClaudeUsageRead(req, res) {
   }));
 }
 
+// Archived days, `from` to `to` months inclusive (#55). Defaults and bounds in
+// parseRange(); an unreadable month is left out and logged, never fatal.
+function handleClaudeUsageHistory(req, res, ctx) {
+  const range = parseRange(
+    ctx.parsedUrl.searchParams.get("from"),
+    ctx.parsedUrl.searchParams.get("to"),
+    {
+      lastArchived: lastArchivedMonth(CLAUDE_USAGE_HISTORY_DIR),
+      currentMonth: new Date(Date.now()).toISOString().slice(0, 7),
+    },
+  );
+  if (range.error) {
+    res.writeHead(400);
+    res.end(JSON.stringify({ error: `Bad request: ${range.error}` }));
+    return;
+  }
+
+  const { days, unreadable } = readHistory(CLAUDE_USAGE_HISTORY_DIR, range.from, range.to);
+  for (const { month, code } of unreadable) {
+    console.error(`[claude-usage] history ${month} unreadable, left out (${code})`);
+  }
+  res.writeHead(200);
+  res.end(JSON.stringify({ from: range.from, to: range.to, days }));
+}
+
 // --- Routing ----------------------------------------------------------------
 
 // Route descriptors. `tokenKind` is the ONLY thing that varies between the read
@@ -840,6 +898,7 @@ const ROUTES = [
   { method: "GET", path: "/hosts", tokenKind: "read", handle: handleHostsList },
   { method: "POST", pattern: HOST_ROUTE_RE, tokenKind: "ingest", handle: handleHostIngest },
   { method: "GET", path: "/claude-usage", tokenKind: "read", handle: handleClaudeUsageRead },
+  { method: "GET", path: "/claude-usage/history", tokenKind: "read", handle: handleClaudeUsageHistory },
   { method: "POST", path: "/claude-usage", tokenKind: "claude-ingest", handle: handleClaudeUsageIngest },
 ];
 
diff --git a/test-curl.sh b/test-curl.sh
index 8460fc6..dd9a2d7 100755
--- a/test-curl.sh
+++ b/test-curl.sh
@@ -65,6 +65,8 @@ TOKEN="test-token-123"
 # only meaningful because the two values cannot be confused. Read from the
 # environment, two inherited values that happened to be equal would break them.
 INGEST_TOKEN="test-ingest-token-456"
+# Third token, for POST /claude-usage (#48), fixed for the same reason.
+CLAUDE_TOKEN="test-claude-token-789"
 TMP_DIR="$(mktemp -d -t vps-health-api.XXXXXX)"
 SERVER_PID=""
 NOINGEST_PID=""
@@ -202,6 +204,8 @@ HOSTS_DIR="$TMP_DIR/hosts" \
 DEFENSEURS_FINDINGS_STATE_PATH="$TMP_DIR/findings-state.json" \
 HOSTS_ALLOWED_IDS="thinkpad,workbench" \
 HOSTS_INGEST_TOKEN="$INGEST_TOKEN" \
+CLAUDE_USAGE_PATH="$TMP_DIR/claude-usage/claude-usage.json" \
+CLAUDE_USAGE_INGEST_TOKEN="$CLAUDE_TOKEN" \
 LOGTO_HEALTH_URL="http://127.0.0.1:1/never" \
 node "$(dirname "$0")/index.js" >/dev/null 2>&1 &
 SERVER_PID=$!
@@ -551,6 +555,25 @@ code=$(http_code "$BASE_URL/defenseurs/state")
 code=$(http_code -H "Authorization: Bearer $INGEST_TOKEN" "$BASE_URL/defenseurs/state")
 [[ "$code" == "401" ]] && pass "GET /defenseurs/state with ingest token -> 401" || fail "GET /defenseurs/state ingest token -> got $code"
 
+# Case 33 : GET /claude-usage/history (#55), the archive of the closed days.
+code=$(http_code "$BASE_URL/claude-usage/history")
+[[ "$code" == "401" ]] && pass "GET /claude-usage/history no-auth -> 401" || fail "GET /claude-usage/history no-auth -> got $code"
+code=$(http_code -H "Authorization: Bearer $CLAUDE_TOKEN" "$BASE_URL/claude-usage/history")
+[[ "$code" == "401" ]] && pass "GET /claude-usage/history with the claude ingest token -> 401" || fail "history with ingest token -> got $code"
+code=$(http_code -H "Authorization: Bearer $TOKEN" "$BASE_URL/claude-usage/history?from=2026-13")
+[[ "$code" == "400" ]] && pass "GET /claude-usage/history?from=2026-13 -> 400" || fail "history bad from -> got $code"
+# A two-day snapshot: the first day is closed and lands in history/2026-09.json.
+printf '%s' '{"schemaVersion":1,"days":[{"date":"2026-09-29","models":{"claude-opus-5-5":{"input":1,"output":2,"requests":1}}},{"date":"2026-09-30","models":{}}]}' \
+  > "$TMP_DIR/claude-usage.json"
+code=$(http_code -X POST -H "Authorization: Bearer $CLAUDE_TOKEN" -H "Content-Type: application/json" \
+  --data-binary @"$TMP_DIR/claude-usage.json" "$BASE_URL/claude-usage")
+[[ "$code" == "204" ]] && pass "POST /claude-usage two-day snapshot -> 204" || fail "POST /claude-usage -> got $code"
+body=$(http_body -H "Authorization: Bearer $TOKEN" "$BASE_URL/claude-usage/history?from=2026-09&to=2026-09")
+dates=$(echo "$body" | node -e 'let s="";process.stdin.on("data",d=>s+=d).on("end",()=>{console.log(JSON.parse(s).days.map(d=>d.date).join(","));});')
+[[ "$dates" == "2026-09-29" ]] && pass "GET /claude-usage/history serves the closed day only" || fail "history after push -> $dates"
+mode=$(node -e 'console.log((require("fs").statSync(process.argv[1]).mode & 0o777).toString(8))' "$TMP_DIR/claude-usage/history/2026-09.json")
+[[ "$mode" == "600" ]] && pass "history month file written in mode 600" || fail "history file mode -> $mode"
+
 echo
 echo "=== Results: $PASS passed, $FAIL failed ==="
 [[ "$FAIL" == "0" ]] || exit 1