diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 2b659d8cf..2c8dabc78 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -68,6 +68,12 @@ jobs:
     # fail on a Windows runner for reasons unrelated to these fixes.
     # The wiki-worker suites mock execFileSync entirely (no real spawning,
     # no POSIX-only paths), so they run cleanly on windows-latest.
+    #
+    # client-os.test.ts is here for a different reason: it is the only suite
+    # whose result DEPENDS on running on real Windows. It asserts the unstubbed
+    # process.platform maps to the OS name the backend records, so this leg is
+    # what proves win32 -> "windows" on a genuine Windows machine instead of a
+    # redefined property (PLA-498).
     name: Windows smoke (spawn + hook dedup + wiki-worker)
     runs-on: windows-latest
     steps:
@@ -88,7 +94,7 @@ jobs:
         run: npm install
 
       - name: Run Windows-relevant suites
-        run: npx vitest run tests/shared/spawn-detached.test.ts tests/cli/install-helpers.test.ts tests/codex/codex-wiki-worker.test.ts tests/cursor/cursor-wiki-worker.test.ts tests/pi/pi-wiki-worker.test.ts tests/hermes/hermes-wiki-worker.test.ts
+        run: npx vitest run tests/shared/spawn-detached.test.ts tests/cli/install-helpers.test.ts tests/codex/codex-wiki-worker.test.ts tests/cursor/cursor-wiki-worker.test.ts tests/pi/pi-wiki-worker.test.ts tests/hermes/hermes-wiki-worker.test.ts tests/claude-code/client-os.test.ts
 
   test:
     name: Typecheck and Test
diff --git a/src/commands/auth.ts b/src/commands/auth.ts
index 5990c6524..9fc8d6446 100644
--- a/src/commands/auth.ts
+++ b/src/commands/auth.ts
@@ -5,6 +5,7 @@
 
 import { deeplakeClientHeader } from "../utils/client-header.js";
 import { hivemindInstallIDHeader } from "./install-id.js";
+import { hivemindOsHeader } from "../utils/client-os.js";
 import { openInBrowser } from "../dashboard/open.js";
 import {
   type Credentials,
@@ -59,6 +60,7 @@ async function apiGet(path: string, token: string, apiUrl: string, orgId?: strin
     Authorization: `Bearer ${token}`,
     "Content-Type": "application/json",
     ...deeplakeClientHeader(),
+    ...hivemindOsHeader(),
   };
   if (orgId) headers["X-Activeloop-Org-Id"] = orgId;
   const resp = await fetch(`${apiUrl}${path}`, { headers });
@@ -71,6 +73,7 @@ async function apiPost(path: string, body: unknown, token: string, apiUrl: strin
     Authorization: `Bearer ${token}`,
     "Content-Type": "application/json",
     ...deeplakeClientHeader(),
+    ...hivemindOsHeader(),
   };
   if (orgId) headers["X-Activeloop-Org-Id"] = orgId;
   const resp = await fetch(`${apiUrl}${path}`, { method: "POST", headers, body: JSON.stringify(body) });
@@ -83,6 +86,7 @@ async function apiDelete(path: string, token: string, apiUrl: string, orgId?: st
     Authorization: `Bearer ${token}`,
     "Content-Type": "application/json",
     ...deeplakeClientHeader(),
+    ...hivemindOsHeader(),
   };
   if (orgId) headers["X-Activeloop-Org-Id"] = orgId;
   const resp = await fetch(`${apiUrl}${path}`, { method: "DELETE", headers });
@@ -115,6 +119,7 @@ export async function requestDeviceCode(apiUrl = DEFAULT_API_URL, ref?: string):
     headers: {
       "Content-Type": "application/json",
       ...deeplakeClientHeader(),
+      ...hivemindOsHeader(),
       ...hivemindInstallIDHeader(),
       ...hivemindReferrerHeader(ref),
       ...signupFlowHeader(),
@@ -130,6 +135,7 @@ export async function pollForToken(deviceCode: string, apiUrl = DEFAULT_API_URL)
     headers: {
       "Content-Type": "application/json",
       ...deeplakeClientHeader(),
+      ...hivemindOsHeader(),
       ...hivemindInstallIDHeader(),
       // The backend resolves/creates the user on this poll (trackDeviceFlowAuth),
       // so the flow header must ride along here too — the /auth/device/code
diff --git a/src/utils/client-os.ts b/src/utils/client-os.ts
new file mode 100644
index 000000000..6d087a557
--- /dev/null
+++ b/src/utils/client-os.ts
@@ -0,0 +1,50 @@
+/**
+ * X-Hivemind-OS header helper.
+ *
+ * The deeplake-api backend records the client OS on the signup analytics
+ * events and forwards it to the marketing CRM, so "did this lead install on
+ * Windows?" has an answer. `process.platform` is the only truthful source:
+ * the server would see its own GOOS, and the CLI's HTTP client sends no OS
+ * token in its User-Agent.
+ *
+ * Normalized here so the wire value is already the vocabulary the CRM reads.
+ * The backend normalizes again on ingest (normalizeOS in auth_analytics.go)
+ * because two other producers exist and disagree — hivemind.ps1 sends
+ * "Windows" and hivemind.sh sends raw `uname -s` — so the canonical mapping
+ * has to live at the backend boundary regardless. Sending the canonical name
+ * from here keeps this CLI from being a third dialect.
+ *
+ * Platforms we do not ship an installer for omit the header entirely — an
+ * absent property is more honest than a bucket named "other", and it matches
+ * how the install-id header degrades.
+ *
+ * Header, not a dimension on X-Deeplake-Client: that header's value is a
+ * parsed contract (product, optionally product/version) and overloading it
+ * would break ParseClientHeader on the backend.
+ */
+
+export const HIVEMIND_OS_HEADER = "X-Hivemind-OS";
+
+// Node platform -> the vocabulary the backend allowlists. Deliberately only
+// the three we publish install routes for (hivemind.sh, hivemind.ps1).
+const OS_NAMES: Record<string, string> = {
+  darwin: "macos",
+  win32: "windows",
+  linux: "linux",
+};
+
+/** Returns "macos" | "windows" | "linux", or "" on any other platform. */
+export function hivemindOsValue(): string {
+  return OS_NAMES[process.platform] ?? "";
+}
+
+/**
+ * Returns `{ "X-Hivemind-OS": "<os>" }` for spreading into a headers object,
+ * or `{}` on a platform we do not ship for. Same shape and same
+ * never-throws contract as deeplakeClientHeader() / hivemindInstallIDHeader().
+ */
+export function hivemindOsHeader(): Record<string, string> {
+  const os = hivemindOsValue();
+  if (!os) return {};
+  return { [HIVEMIND_OS_HEADER]: os };
+}
diff --git a/tests/claude-code/auth.test.ts b/tests/claude-code/auth.test.ts
index f7148fce4..2426ac882 100644
--- a/tests/claude-code/auth.test.ts
+++ b/tests/claude-code/auth.test.ts
@@ -15,6 +15,7 @@ const fetchMock = vi.fn();
 const saveCredentialsMock = vi.fn();
 const loadCredentialsMock = vi.fn();
 const installIDHeaderMock = vi.fn();
+const osHeaderMock = vi.fn();
 
 vi.stubGlobal("fetch", fetchMock);
 vi.mock("../../src/commands/auth-creds.js", () => ({
@@ -32,6 +33,12 @@ vi.mock("../../src/utils/client-header.js", () => ({
 vi.mock("../../src/commands/install-id.js", () => ({
   hivemindInstallIDHeader: () => installIDHeaderMock(),
 }));
+// Mock the OS header for the same reason as install-id: the real helper reads
+// process.platform, so without this every assertion below would depend on which
+// machine CI runs on. client-os.test.ts covers the real mapping.
+vi.mock("../../src/utils/client-os.js", () => ({
+  hivemindOsHeader: () => osHeaderMock(),
+}));
 
 async function importAuth() {
   vi.resetModules();
@@ -47,6 +54,8 @@ beforeEach(() => {
   saveCredentialsMock.mockReset();
   loadCredentialsMock.mockReset();
   installIDHeaderMock.mockReset();
+  osHeaderMock.mockReset();
+  osHeaderMock.mockReturnValue({});
   // Default: install-id helper returns the empty object, so the header
   // is omitted (matches the graceful-degradation path). Individual tests
   // override this to assert the happy path.
@@ -110,6 +119,31 @@ describe("requestDeviceCode", () => {
     expect(init.headers["X-Hivemind-Install-Id"]).toBe("uuid-from-helper");
   });
 
+  it("includes X-Hivemind-OS header on the device-code request", async () => {
+    osHeaderMock.mockReturnValueOnce({ "X-Hivemind-OS": "windows" });
+    fetchMock.mockResolvedValueOnce(
+      ok({ device_code: "d", user_code: "u", verification_uri: "v", verification_uri_complete: "vc", expires_in: 1, interval: 1 }),
+    );
+    const { requestDeviceCode } = await importAuth();
+    await requestDeviceCode("https://api.example");
+    const init = fetchMock.mock.calls[0][1];
+    expect(init.headers["X-Hivemind-OS"]).toBe("windows");
+  });
+
+  it("omits X-Hivemind-OS on an unsupported platform without dropping the other headers", async () => {
+    osHeaderMock.mockReturnValueOnce({});
+    installIDHeaderMock.mockReturnValueOnce({ "X-Hivemind-Install-Id": "uuid-x" });
+    fetchMock.mockResolvedValueOnce(
+      ok({ device_code: "d", user_code: "u", verification_uri: "v", verification_uri_complete: "vc", expires_in: 1, interval: 1 }),
+    );
+    const { requestDeviceCode } = await importAuth();
+    await requestDeviceCode("https://api.example");
+    const init = fetchMock.mock.calls[0][1];
+    expect(init.headers["X-Hivemind-OS"]).toBeUndefined();
+    expect(init.headers["X-Hivemind-Install-Id"]).toBe("uuid-x");
+    expect(init.headers["X-Deeplake-Client"]).toBe("hivemind/test");
+  });
+
   it("omits X-Hivemind-Install-Id header when install-id helper returns empty (graceful-degrade path)", async () => {
     // beforeEach default already sets installIDHeaderMock to return {} — make it explicit here for clarity.
     installIDHeaderMock.mockReturnValueOnce({});
@@ -230,6 +264,24 @@ describe("listOrgs / listWorkspaces", () => {
     expect(init.method).toBeUndefined();
   });
 
+  // Regression guard for why the OS header rides the authenticated helpers at
+  // all. signup_completed fires from whichever request provisions the user: if
+  // that is not /auth/device/token, it is the CLI's first authenticated call,
+  // which is GET /me inside saveCredentialsFromToken (listOrgs follows it).
+  // Both go through apiGet, so asserting on listOrgs covers the shared helper —
+  // but if /me ever moves to its own fetch, it must carry the header too, or it
+  // consumes the one-time signup capture without an OS and nothing later can
+  // repair it.
+  it("apiGet forwards X-Hivemind-OS so a middleware-path signup still sees it", async () => {
+    osHeaderMock.mockReturnValueOnce({ "X-Hivemind-OS": "macos" });
+    fetchMock.mockResolvedValueOnce(ok([{ id: "o1", name: "acme" }]));
+    const { listOrgs } = await importAuth();
+    await listOrgs("tok", "https://api.example");
+    const init = fetchMock.mock.calls[0][1];
+    expect(init.headers["X-Hivemind-OS"]).toBe("macos");
+    expect(init.headers.Authorization).toBe("Bearer tok");
+  });
+
   it("listOrgs returns [] when API gives a non-array body", async () => {
     fetchMock.mockResolvedValueOnce(ok({ unexpected: true }));
     const { listOrgs } = await importAuth();
@@ -745,6 +797,25 @@ describe("saveCredentialsFromToken — org-pinning", () => {
     delete process.env.HIVEMIND_ORG_ID;
   });
 
+  // Pins the request that actually matters. GET /me is the CLI's FIRST
+  // authenticated call, so it is the one that can trigger the middleware's
+  // one-time signup capture. If /me ever moves off apiGet to its own fetch
+  // without the OS header, it consumes that capture with no OS and no later
+  // request can repair it — this asserts on /me by URL, not on the helper.
+  it("sends X-Hivemind-OS on the first authenticated call, GET /me", async () => {
+    osHeaderMock.mockReturnValue({ "X-Hivemind-OS": "windows" });
+    const token = makeToken({ org_id: "o1", user_id: "u1" });
+    fetchMock
+      .mockResolvedValueOnce(ok({ id: "u1", name: "Alice" }))
+      .mockResolvedValueOnce(ok([{ id: "o1", name: "acme" }]));
+    const { saveCredentialsFromToken } = await importAuth();
+    await saveCredentialsFromToken(token, "https://api.example", { skipTokenMint: true });
+
+    const [meUrl, meInit] = fetchMock.mock.calls[0];
+    expect(meUrl).toBe("https://api.example/me");
+    expect(meInit.headers["X-Hivemind-OS"]).toBe("windows");
+  });
+
   it("skipTokenMint=true honors the org_id claim from the token JWT (multi-org user)", async () => {
     const token = makeToken({ org_id: "o2", user_id: "u1" });
     fetchMock
diff --git a/tests/claude-code/client-os.test.ts b/tests/claude-code/client-os.test.ts
new file mode 100644
index 000000000..65c53990f
--- /dev/null
+++ b/tests/claude-code/client-os.test.ts
@@ -0,0 +1,101 @@
+import { describe, it, expect, afterEach } from "vitest";
+import { hivemindOsValue, hivemindOsHeader, HIVEMIND_OS_HEADER } from "../../src/utils/client-os.js";
+
+/**
+ * Source-level tests for src/utils/client-os.ts.
+ *
+ * process.platform is a read-only accessor, so each case redefines it and the
+ * afterEach restores the real descriptor. The module reads process.platform
+ * per call (no module-level capture), which is what makes this work against a
+ * single static import — same reason install-id.ts resolves its paths lazily.
+ */
+
+const REAL_PLATFORM = Object.getOwnPropertyDescriptor(process, "platform")!;
+// Captured before any stubbing, so the real-platform test below cannot be
+// fooled by a leaked redefinition from an earlier case.
+const REAL_PLATFORM_NAME = process.platform;
+
+function setPlatform(value: string): void {
+  Object.defineProperty(process, "platform", { value, configurable: true });
+}
+
+afterEach(() => {
+  Object.defineProperty(process, "platform", REAL_PLATFORM);
+});
+
+describe("hivemindOsValue", () => {
+  it("maps the three platforms we publish an installer for", () => {
+    setPlatform("darwin");
+    expect(hivemindOsValue()).toBe("macos");
+    setPlatform("win32");
+    expect(hivemindOsValue()).toBe("windows");
+    setPlatform("linux");
+    expect(hivemindOsValue()).toBe("linux");
+  });
+
+  it("returns empty on a platform we do not ship for, rather than inventing a bucket", () => {
+    setPlatform("freebsd");
+    expect(hivemindOsValue()).toBe("");
+    setPlatform("aix");
+    expect(hivemindOsValue()).toBe("");
+  });
+
+  // Asserted value by value, not "is it in the allowlist": the loose form passes
+  // even when a platform maps to the WRONG allowed name — sunos returning
+  // "linux" would look fine — and the backend would then record a confident lie.
+  it("maps every platform to its exact value, supported or not", () => {
+    for (const [platform, want] of Object.entries({
+      darwin: "macos",
+      win32: "windows",
+      linux: "linux",
+      freebsd: "",
+      sunos: "",
+      android: "",
+    })) {
+      setPlatform(platform);
+      expect(hivemindOsValue()).toBe(want);
+    }
+  });
+});
+
+// The only assertion here that runs against a REAL machine rather than a
+// redefined property. It is why this file is in ci.yaml's windows-smoke suite
+// list: on windows-latest it proves win32 -> "windows" on genuine Windows,
+// which is the CLI half of what PLA-498's acceptance asks for and what we
+// otherwise had no machine to check.
+describe("the platform this process actually runs on", () => {
+  it("maps the real platform, unstubbed", () => {
+    const expected: Record<string, string> = { darwin: "macos", win32: "windows", linux: "linux" };
+    expect(hivemindOsValue()).toBe(expected[REAL_PLATFORM_NAME] ?? "");
+  });
+
+  // toBeTruthy() would accept any non-empty string, so on the Windows runner the
+  // header could read "macos" and this would still pass — defeating the only
+  // reason this file runs on windows-latest at all.
+  it("emits the canonical value for the real platform", () => {
+    const expected: Record<string, string> = { darwin: "macos", win32: "windows", linux: "linux" };
+    const want = expected[REAL_PLATFORM_NAME];
+    if (!want) {
+      expect(hivemindOsHeader()).toEqual({});
+      return;
+    }
+    expect(hivemindOsHeader()[HIVEMIND_OS_HEADER]).toBe(want);
+  });
+});
+
+describe("hivemindOsHeader", () => {
+  it("returns a spreadable single-entry object on a supported platform", () => {
+    setPlatform("win32");
+    expect(hivemindOsHeader()).toEqual({ [HIVEMIND_OS_HEADER]: "windows" });
+  });
+
+  it("returns {} on an unsupported platform so spreading omits the header entirely", () => {
+    setPlatform("freebsd");
+    expect(hivemindOsHeader()).toEqual({});
+    // Spreading {} must leave a headers object untouched — the graceful-degrade
+    // contract the install-id header follows.
+    expect({ "Content-Type": "application/json", ...hivemindOsHeader() }).toEqual({
+      "Content-Type": "application/json",
+    });
+  });
+});