diff --git a/src/app/api/fitbit/callback/route.ts b/src/app/api/fitbit/callback/route.ts
new file mode 100644
index 000000000..a20d4280e
--- /dev/null
+++ b/src/app/api/fitbit/callback/route.ts
@@ -0,0 +1,176 @@
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { annotate, getEvent } from "@/lib/logging/context";
+import { auditLog } from "@/lib/auth/audit";
+import { encrypt } from "@/lib/crypto";
+import {
+  exchangeCode,
+  fetchProfile,
+  resolveFitbitUserId,
+  FITBIT_OAUTH_SCOPE,
+} from "@/lib/fitbit/client";
+import { getUserFitbitCredentials } from "@/lib/fitbit/credentials";
+import { FITBIT_OAUTH_STATE_COOKIE } from "@/lib/fitbit/oauth-state";
+import { FITBIT_BACKFILL_QUEUE } from "@/lib/jobs/fitbit-backfill";
+import { getGlobalBoss } from "@/lib/jobs/boss-instance";
+import { markReconnected } from "@/lib/integrations/status";
+import { Prisma } from "@/generated/prisma/client";
+import { NextRequest, NextResponse } from "next/server";
+import { timingSafeEqual } from "node:crypto";
+
+/**
+ * OAuth callback from Google for Fitbit / Google Health (v1.12.0). Mirrors the
+ * WHOOP callback: the `state` param is a random base64url nonce keyed against
+ * the `FitbitOAuthState` ledger. The in-flight user is resolved via the row's
+ * `userId` (never parsed from the cookie value) and cross-checked against the
+ * session. The row is consumed (deleted) atomically on every exit branch so a
+ * replay of the same nonce fails the second time.
+ *
+ * Reason tags distinguish the four post-delete branches for the audit trail:
+ * `csrf1` (URL/cookie mismatch, short-circuit before delete), `replay` (P2025 —
+ * nonce already consumed), `expired` (valid row, TTL elapsed), and `cross_user`
+ * (valid row, session userId mismatch).
+ *
+ * On success: exchange the code, fetch the Google Health profile for the
+ * external `fitbitUserId`, persist the encrypted `FitbitConnection`, clear any
+ * prior reauth state, and enqueue the self-converging history backfill.
+ */
+const ERR = (reason: string) =>
+  NextResponse.redirect(
+    new URL(
+      `/settings/integrations?fitbit=error&reason=${reason}`,
+      process.env.NEXT_PUBLIC_APP_URL!,
+    ),
+  );
+
+export const GET = apiHandler(async (request: NextRequest) => {
+  const { user } = await requireAuth();
+  annotate({ action: { name: "fitbit.callback" } });
+
+  const { searchParams } = new URL(request.url);
+  const code = searchParams.get("code");
+  const state = searchParams.get("state");
+  const storedState = request.cookies.get(FITBIT_OAUTH_STATE_COOKIE)?.value;
+
+  // CSRF leg 1: URL state must match the cookie state byte-for-byte
+  // (timing-safe). Runs BEFORE the atomic delete so a probe with arbitrary
+  // state values can't grief legitimate ledger rows.
+  if (
+    !state ||
+    !storedState ||
+    state.length !== storedState.length ||
+    !timingSafeEqual(Buffer.from(state), Buffer.from(storedState))
+  ) {
+    annotate({ meta: { reason: "csrf1" } });
+    return ERR("csrf1");
+  }
+
+  // CSRF leg 2: atomically consume the ledger row. `delete` returns the row on
+  // success so the `expiresAt` + `userId` checks run against the consumed
+  // payload. P2025 means the nonce was already consumed (replay) or never
+  // existed. Atomic at the Postgres row level: two concurrent callbacks with the
+  // same nonce can't both pass before either deletes.
+  let stateRow: { userId: string; expiresAt: Date } | null = null;
+  try {
+    stateRow = await prisma.fitbitOAuthState.delete({
+      where: { nonce: state },
+    });
+  } catch (err) {
+    if (
+      err instanceof Prisma.PrismaClientKnownRequestError &&
+      err.code === "P2025"
+    ) {
+      annotate({ meta: { reason: "replay" } });
+      return ERR("replay");
+    }
+    const errName = err instanceof Error ? err.name : "unknown";
+    getEvent()?.addWarning(`oauth-state-delete failed: ${errName}`);
+    annotate({ meta: { reason: "state" } });
+    return ERR("state");
+  }
+
+  if (stateRow.expiresAt <= new Date()) {
+    annotate({ meta: { reason: "expired" } });
+    return ERR("expired");
+  }
+  if (stateRow.userId !== user.id) {
+    annotate({ meta: { reason: "cross_user" } });
+    return ERR("cross_user");
+  }
+
+  if (!code) {
+    return ERR("nocode");
+  }
+
+  try {
+    const creds = await getUserFitbitCredentials(user.id);
+    if (!creds) {
+      return ERR("nocreds");
+    }
+
+    const tokens = await exchangeCode(code, creds);
+    const profile = await fetchProfile(tokens.access_token);
+    const fitbitUserId = resolveFitbitUserId(profile);
+    const expiresAt = new Date(Date.now() + tokens.expires_in * 1000);
+
+    // The initial code exchange always returns a refresh token (the connect
+    // route forces `prompt=consent` + `access_type=offline`). Fall back to the
+    // existing stored token defensively if Google ever omits it on a re-consent.
+    await prisma.fitbitConnection.upsert({
+      where: { userId: user.id },
+      update: {
+        fitbitUserId,
+        accessToken: encrypt(tokens.access_token),
+        tokenExpiresAt: expiresAt,
+        scope: tokens.scope ?? FITBIT_OAUTH_SCOPE,
+        backfillCompletedAt: null,
+        ...(tokens.refresh_token
+          ? { refreshToken: encrypt(tokens.refresh_token) }
+          : {}),
+      },
+      create: {
+        userId: user.id,
+        fitbitUserId,
+        accessToken: encrypt(tokens.access_token),
+        refreshToken: encrypt(tokens.refresh_token ?? ""),
+        tokenExpiresAt: expiresAt,
+        scope: tokens.scope ?? FITBIT_OAUTH_SCOPE,
+      },
+    });
+
+    await auditLog("fitbit.connect", {
+      userId: user.id,
+      details: { fitbitUserId },
+    });
+
+    // Re-completing OAuth clears any prior reauth-required state.
+    await markReconnected(user.id, "fitbit");
+
+    // Enqueue the self-converging history backfill. Best-effort: the boot-time
+    // discovery query (`backfillCompletedAt IS NULL`) is the safety net, so a
+    // missing boss instance here doesn't strand the connection.
+    const boss = getGlobalBoss();
+    if (boss) {
+      await boss
+        .send(FITBIT_BACKFILL_QUEUE, {
+          userId: user.id,
+          enqueuedAt: new Date().toISOString(),
+        })
+        .catch((err) =>
+          getEvent()?.addWarning(`fitbit-backfill enqueue failed: ${err}`),
+        );
+    }
+
+    const response = NextResponse.redirect(
+      new URL(
+        "/settings/integrations?fitbit=connected",
+        process.env.NEXT_PUBLIC_APP_URL!,
+      ),
+    );
+    response.cookies.delete(FITBIT_OAUTH_STATE_COOKIE);
+    return response;
+  } catch (err) {
+    getEvent()?.setError(err);
+    return ERR("token");
+  }
+});
diff --git a/src/app/api/fitbit/connect/route.ts b/src/app/api/fitbit/connect/route.ts
new file mode 100644
index 000000000..93a30fb5a
--- /dev/null
+++ b/src/app/api/fitbit/connect/route.ts
@@ -0,0 +1,90 @@
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { apiError } from "@/lib/api-response";
+import { prisma } from "@/lib/db";
+import { annotate, getEvent } from "@/lib/logging/context";
+import { checkRateLimit } from "@/lib/rate-limit";
+import { getAuthorizationUrl } from "@/lib/fitbit/client";
+import { getUserFitbitCredentials } from "@/lib/fitbit/credentials";
+import {
+  FITBIT_OAUTH_STATE_COOKIE,
+  FITBIT_OAUTH_STATE_TTL_MS,
+  mintFitbitOAuthStateNonce,
+} from "@/lib/fitbit/oauth-state";
+import { NextRequest, NextResponse } from "next/server";
+import { shouldEmitSecureCookie } from "@/lib/auth/secure-cookie";
+
+/**
+ * Redirect the user to the Google OAuth consent page for Fitbit / Google Health
+ * (v1.12.0).
+ *
+ * Mirrors the WHOOP connect route: a fully-random base64url state nonce backed
+ * by a 10-minute `FitbitOAuthState` ledger row carries the `(nonce → userId)`
+ * mapping, so the user id never travels in the OAuth `state` param (which can
+ * land in request logs / network captures). The httpOnly + Secure cookie
+ * carries JUST the nonce; the callback resolves the user via the row's `userId`.
+ *
+ * Rate-limited per user (10 calls / 60 s) so a logged-in session can't spam
+ * ledger rows for the full 10-min TTL window. Both the rate-limit and
+ * create-failure paths redirect (not JSON) because the entry point is a browser
+ * navigation — a 429 envelope would surface as a blank page.
+ */
+const CONNECT_RATE_LIMIT = 10;
+const CONNECT_WINDOW_MS = 60_000;
+
+export const GET = apiHandler(async (req: NextRequest) => {
+  const { user } = await requireAuth();
+  annotate({ action: { name: "fitbit.connect" } });
+
+  const rl = await checkRateLimit(
+    `fitbit:connect:${user.id}`,
+    CONNECT_RATE_LIMIT,
+    CONNECT_WINDOW_MS,
+  );
+  if (!rl.allowed) {
+    annotate({ action: { name: "fitbit.connect.rate_limited" } });
+    return NextResponse.redirect(
+      new URL(
+        "/settings/integrations?fitbit=error&reason=rate_limited",
+        req.url,
+      ),
+    );
+  }
+
+  const creds = await getUserFitbitCredentials(user.id);
+  if (!creds) {
+    return apiError(
+      "Please configure your Google OAuth Client ID and Client Secret in Settings first.",
+      400,
+    );
+  }
+
+  const nonce = mintFitbitOAuthStateNonce();
+  try {
+    await prisma.fitbitOAuthState.create({
+      data: {
+        nonce,
+        userId: user.id,
+        expiresAt: new Date(Date.now() + FITBIT_OAUTH_STATE_TTL_MS),
+      },
+    });
+  } catch (err) {
+    getEvent()?.setError(err);
+    annotate({ action: { name: "fitbit.connect.create_failed" } });
+    return NextResponse.redirect(
+      new URL("/settings/integrations?fitbit=error&reason=connect", req.url),
+    );
+  }
+
+  const url = getAuthorizationUrl(nonce, creds);
+
+  const response = NextResponse.redirect(url);
+  response.cookies.set(FITBIT_OAUTH_STATE_COOKIE, nonce, {
+    httpOnly: true,
+    secure: shouldEmitSecureCookie(),
+    sameSite: "lax",
+    maxAge: Math.floor(FITBIT_OAUTH_STATE_TTL_MS / 1000),
+    path: "/",
+  });
+
+  return response;
+});
diff --git a/src/app/api/fitbit/credentials/route.ts b/src/app/api/fitbit/credentials/route.ts
new file mode 100644
index 000000000..b2cabfee1
--- /dev/null
+++ b/src/app/api/fitbit/credentials/route.ts
@@ -0,0 +1,79 @@
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { apiSuccess, apiError, safeJson } from "@/lib/api-response";
+import { annotate } from "@/lib/logging/context";
+import { encrypt } from "@/lib/crypto";
+import { fitbitCredentialsSchema } from "@/lib/validations/fitbit";
+import { NextRequest } from "next/server";
+import { z } from "zod/v4";
+
+/**
+ * Check whether the user has Fitbit / Google Health BYO-key credentials stored
+ * (v1.12.0).
+ */
+export const GET = apiHandler(async () => {
+  const { user } = await requireAuth();
+  annotate({ action: { name: "fitbit.credentials.get" } });
+
+  const dbUser = await prisma.user.findUnique({
+    where: { id: user.id },
+    select: {
+      fitbitClientIdEncrypted: true,
+      fitbitClientSecretEncrypted: true,
+    },
+  });
+
+  return apiSuccess({
+    hasCredentials:
+      !!dbUser?.fitbitClientIdEncrypted &&
+      !!dbUser?.fitbitClientSecretEncrypted,
+  });
+});
+
+/**
+ * Save Fitbit / Google Health API credentials (encrypted at rest).
+ */
+export const PUT = apiHandler(async (request: NextRequest) => {
+  const { user } = await requireAuth();
+  annotate({ action: { name: "fitbit.credentials.update" } });
+
+  const { data: body, error: jsonError } = await safeJson(request);
+  if (jsonError) return jsonError;
+
+  const result = z.safeParse(fitbitCredentialsSchema, body);
+  if (!result.success) {
+    return apiError("Client ID and Client Secret are required", 422);
+  }
+
+  await prisma.user.update({
+    where: { id: user.id },
+    data: {
+      fitbitClientIdEncrypted: encrypt(result.data.clientId),
+      fitbitClientSecretEncrypted: encrypt(result.data.clientSecret),
+    },
+  });
+
+  return apiSuccess({ updated: true });
+});
+
+/**
+ * Delete Fitbit credentials and the active connection.
+ */
+export const DELETE = apiHandler(async () => {
+  const { user } = await requireAuth();
+  annotate({ action: { name: "fitbit.credentials.delete" } });
+
+  await prisma.fitbitConnection
+    .delete({ where: { userId: user.id } })
+    .catch(() => {});
+
+  await prisma.user.update({
+    where: { id: user.id },
+    data: {
+      fitbitClientIdEncrypted: null,
+      fitbitClientSecretEncrypted: null,
+    },
+  });
+
+  return apiSuccess({ deleted: true });
+});
diff --git a/src/lib/fitbit/__tests__/client.test.ts b/src/lib/fitbit/__tests__/client.test.ts
new file mode 100644
index 000000000..9afec3831
--- /dev/null
+++ b/src/lib/fitbit/__tests__/client.test.ts
@@ -0,0 +1,162 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import {
+  FITBIT_API_BASE,
+  FITBIT_OAUTH_SCOPE,
+  exchangeCode,
+  fetchProfile,
+  getAuthorizationUrl,
+  refreshAccessToken,
+  resolveFitbitUserId,
+} from "../client";
+
+const CREDS = { clientId: "cid", clientSecret: "csecret" };
+
+/** Stub global fetch with a queue of `{ status, body }` responses. */
+function installFetchMock(pages: Array<{ status: number; body: unknown }>) {
+  let i = 0;
+  const fetchMock = vi.fn(async () => {
+    const page = pages[Math.min(i, pages.length - 1)]!;
+    i += 1;
+    return {
+      status: page.status,
+      json: async () => page.body,
+    };
+  });
+  vi.stubGlobal("fetch", fetchMock);
+  return fetchMock;
+}
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+  vi.restoreAllMocks();
+});
+
+describe("getAuthorizationUrl", () => {
+  it("builds the Google authorize URL with offline access + consent prompt", () => {
+    const url = getAuthorizationUrl("nonce123", CREDS);
+    expect(url).toContain("accounts.google.com/o/oauth2/v2/auth");
+    expect(url).toContain("response_type=code");
+    expect(url).toContain("client_id=cid");
+    expect(url).toContain("state=nonce123");
+    expect(url).toContain("access_type=offline");
+    expect(url).toContain("prompt=consent");
+    // URLSearchParams encodes the space-separated scope; compare the parsed
+    // `scope` param back to the canonical constant.
+    const scope = new URL(url).searchParams.get("scope");
+    expect(scope).toBe(FITBIT_OAUTH_SCOPE);
+    expect(FITBIT_OAUTH_SCOPE).toContain("googlehealth.profile.readonly");
+  });
+
+  it("requests exactly the four launch Restricted read bundles", () => {
+    const scopes = FITBIT_OAUTH_SCOPE.split(" ");
+    expect(scopes).toHaveLength(4);
+    expect(scopes).toContain(
+      "https://www.googleapis.com/auth/googlehealth.activity_and_fitness.readonly",
+    );
+    expect(scopes).toContain(
+      "https://www.googleapis.com/auth/googlehealth.health_metrics_and_measurements.readonly",
+    );
+    expect(scopes).toContain(
+      "https://www.googleapis.com/auth/googlehealth.sleep.readonly",
+    );
+    // ECG / nutrition / location are deliberately omitted from the launch set.
+    expect(FITBIT_OAUTH_SCOPE).not.toContain("ecg");
+    expect(FITBIT_OAUTH_SCOPE).not.toContain("nutrition");
+    expect(FITBIT_OAUTH_SCOPE).not.toContain("location");
+  });
+});
+
+describe("token exchange + refresh", () => {
+  it("exchanges an authorization code for a token pair via Basic auth", async () => {
+    const fetchMock = installFetchMock([
+      {
+        status: 200,
+        body: {
+          access_token: "at",
+          refresh_token: "rt",
+          expires_in: 3600,
+          scope: "https://www.googleapis.com/auth/googlehealth.sleep.readonly",
+        },
+      },
+    ]);
+    const tok = await exchangeCode("code", CREDS);
+    expect(tok.access_token).toBe("at");
+    expect(tok.refresh_token).toBe("rt");
+    expect(tok.expires_in).toBe(3600);
+
+    const [url, init] = fetchMock.mock.calls[0] as unknown as [
+      string,
+      { headers: Record<string, string>; body: string },
+    ];
+    expect(url).toContain("oauth2.googleapis.com/token");
+    expect(init.body).toContain("grant_type=authorization_code");
+    // Confidential client credentials ride in the Basic-auth header.
+    const expected = `Basic ${Buffer.from("cid:csecret").toString("base64")}`;
+    expect(init.headers.Authorization).toBe(expected);
+  });
+
+  it("refreshes WITHOUT re-sending scope (Google preserves the grant)", async () => {
+    const fetchMock = installFetchMock([
+      {
+        status: 200,
+        body: { access_token: "at2", expires_in: 3600 },
+      },
+    ]);
+    const tok = await refreshAccessToken("rt1", CREDS);
+    expect(tok.access_token).toBe("at2");
+    const [, init] = fetchMock.mock.calls[0] as unknown as [
+      string,
+      { body: string },
+    ];
+    expect(init.body).toContain("grant_type=refresh_token");
+    expect(init.body).toContain("refresh_token=rt1");
+    // Google preserves the original grant — no scope param is re-sent.
+    expect(init.body).not.toContain("scope=");
+  });
+
+  it("returns an absent refresh_token unchanged (Google does not rotate)", async () => {
+    installFetchMock([
+      { status: 200, body: { access_token: "at3", expires_in: 3600 } },
+    ]);
+    const tok = await refreshAccessToken("rt1", CREDS);
+    expect(tok.access_token).toBe("at3");
+    expect(tok.refresh_token).toBeUndefined();
+  });
+
+  it("throws a classified FitbitApiError on a 401 token response", async () => {
+    installFetchMock([{ status: 401, body: { error: "invalid_grant" } }]);
+    await expect(exchangeCode("bad", CREDS)).rejects.toMatchObject({
+      name: "FitbitApiError",
+      classification: "reauth_required",
+    });
+  });
+});
+
+describe("fetchProfile + resolveFitbitUserId", () => {
+  it("fetches the profile from the Google Health base", async () => {
+    const fetchMock = installFetchMock([
+      { status: 200, body: { name: "users/abc123" } },
+    ]);
+    const profile = await fetchProfile("at");
+    expect(profile.name).toBe("users/abc123");
+    const [url] = fetchMock.mock.calls[0] as unknown as [string];
+    expect(url).toBe(`${FITBIT_API_BASE}/users/me/profile`);
+  });
+
+  it("throws a classified error on a 403 profile read", async () => {
+    installFetchMock([{ status: 403, body: {} }]);
+    await expect(fetchProfile("at")).rejects.toMatchObject({
+      name: "FitbitApiError",
+      classification: "reauth_required",
+    });
+  });
+
+  it("resolves the external user id from a users/{id} resource name", () => {
+    expect(resolveFitbitUserId({ name: "users/abc123" })).toBe("abc123");
+  });
+
+  it("falls back to a bare id, then to 'me'", () => {
+    expect(resolveFitbitUserId({ id: "xyz" })).toBe("xyz");
+    expect(resolveFitbitUserId({})).toBe("me");
+  });
+});
diff --git a/src/lib/fitbit/__tests__/oauth-state.test.ts b/src/lib/fitbit/__tests__/oauth-state.test.ts
new file mode 100644
index 000000000..1406cf203
--- /dev/null
+++ b/src/lib/fitbit/__tests__/oauth-state.test.ts
@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+import {
+  FITBIT_OAUTH_STATE_COOKIE,
+  FITBIT_OAUTH_STATE_TTL_MS,
+  mintFitbitOAuthStateNonce,
+} from "../oauth-state";
+
+describe("Fitbit OAuth state", () => {
+  it("pins the cookie name and 10-minute TTL", () => {
+    expect(FITBIT_OAUTH_STATE_COOKIE).toBe("fitbit_state");
+    expect(FITBIT_OAUTH_STATE_TTL_MS).toBe(10 * 60 * 1000);
+  });
+
+  it("mints a 22-char base64url nonce (128 bits, no padding)", () => {
+    const nonce = mintFitbitOAuthStateNonce();
+    expect(nonce).toHaveLength(22);
+    expect(nonce).toMatch(/^[A-Za-z0-9_-]{22}$/);
+  });
+
+  it("mints unique nonces", () => {
+    const set = new Set(
+      Array.from({ length: 100 }, () => mintFitbitOAuthStateNonce()),
+    );
+    expect(set.size).toBe(100);
+  });
+});
diff --git a/src/lib/fitbit/__tests__/response-classifier.test.ts b/src/lib/fitbit/__tests__/response-classifier.test.ts
new file mode 100644
index 000000000..f98ed0b52
--- /dev/null
+++ b/src/lib/fitbit/__tests__/response-classifier.test.ts
@@ -0,0 +1,100 @@
+import { describe, expect, it } from "vitest";
+import {
+  FitbitApiError,
+  classifyFitbitError,
+  classifyFitbitResponse,
+} from "../response-classifier";
+
+describe("classifyFitbitResponse", () => {
+  it("classifies 2xx as success", () => {
+    for (const status of [200, 201, 204, 299]) {
+      const v = classifyFitbitResponse(status);
+      expect(v.classification).toBe("success");
+      expect(v.httpStatus).toBe(status);
+    }
+  });
+
+  it("classifies 429 as transient (rate-limit)", () => {
+    const v = classifyFitbitResponse(429);
+    expect(v.classification).toBe("transient");
+    expect(v.reason).toBe("http_429");
+  });
+
+  it("classifies 401 and 403 as reauth_required", () => {
+    expect(classifyFitbitResponse(401).classification).toBe("reauth_required");
+    expect(classifyFitbitResponse(403).classification).toBe("reauth_required");
+  });
+
+  it("classifies other 4xx as persistent", () => {
+    for (const status of [400, 404, 422]) {
+      expect(classifyFitbitResponse(status).classification).toBe("persistent");
+    }
+  });
+
+  it("classifies 5xx as transient", () => {
+    for (const status of [500, 502, 503, 504]) {
+      expect(classifyFitbitResponse(status).classification).toBe("transient");
+    }
+  });
+
+  it("defaults an unrecognised status to transient", () => {
+    expect(classifyFitbitResponse(308).classification).toBe("transient");
+  });
+});
+
+describe("FitbitApiError", () => {
+  it("carries the verdict and caps the message at 1024 chars", () => {
+    const err = new FitbitApiError({
+      verb: "exchangeCode",
+      classification: "reauth_required",
+      httpStatus: 401,
+      reason: "http_401",
+      upstreamError: "invalid_grant",
+    });
+    expect(err.name).toBe("FitbitApiError");
+    expect(err.classification).toBe("reauth_required");
+    expect(err.httpStatus).toBe(401);
+    expect(err.upstreamError).toBe("invalid_grant");
+    expect(err.message).toContain("invalid_grant");
+    expect(err.message.length).toBeLessThanOrEqual(1024);
+  });
+});
+
+describe("classifyFitbitError", () => {
+  it("reads the classification off a FitbitApiError", () => {
+    const err = new FitbitApiError({
+      verb: "refreshAccessToken",
+      classification: "persistent",
+      httpStatus: 400,
+      reason: "http_400",
+    });
+    expect(classifyFitbitError(err)).toBe("persistent");
+  });
+
+  it("parses the status out of a legacy message shape", () => {
+    expect(classifyFitbitError(new Error("Fitbit fetchProfile error: 401"))).toBe(
+      "reauth_required",
+    );
+    expect(classifyFitbitError(new Error("Fitbit sync error: 500"))).toBe(
+      "transient",
+    );
+  });
+
+  it("defaults a non-Fitbit error to transient", () => {
+    expect(classifyFitbitError(new Error("network down"))).toBe("transient");
+    expect(classifyFitbitError("boom")).toBe("transient");
+  });
+
+  it("treats a 401 on the token endpoint as reauth (no invalid_grant special-case)", () => {
+    // Google signals a revoked grant with a 401 status, not a 400 invalid_grant
+    // body — the status-driven classifier already buckets it as reauth.
+    const err = new FitbitApiError({
+      verb: "refreshAccessToken",
+      classification: "reauth_required",
+      httpStatus: 401,
+      reason: "http_401",
+      upstreamError: "invalid_grant",
+    });
+    expect(classifyFitbitError(err)).toBe("reauth_required");
+  });
+});
diff --git a/src/lib/fitbit/__tests__/sync.test.ts b/src/lib/fitbit/__tests__/sync.test.ts
new file mode 100644
index 000000000..db7ee54f8
--- /dev/null
+++ b/src/lib/fitbit/__tests__/sync.test.ts
@@ -0,0 +1,194 @@
+/**
+ * v1.12.0 — Fitbit token-management tests (mocked). Covers `getValidToken`:
+ *   - the NO-ROTATION refresh branch (refresh token preserved when the response
+ *     omits it; overwritten only when a fresh one is returned);
+ *   - the stored-token fast path when not near expiry;
+ *   - a reauth failure recorded when credentials are missing on refresh.
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+const { prismaMock, refreshAccessTokenMock, recordSyncFailure } = vi.hoisted(
+  () => ({
+    prismaMock: {
+      fitbitConnection: {
+        findUnique: vi.fn(),
+        update: vi.fn(),
+      },
+    },
+    refreshAccessTokenMock: vi.fn(),
+    recordSyncFailure: vi.fn<(...a: unknown[]) => Promise<void>>(async () => {}),
+  }),
+);
+
+vi.mock("@/lib/db", () => ({ prisma: prismaMock }));
+
+vi.mock("@/lib/crypto", () => ({
+  encrypt: (s: string) => `enc(${s})`,
+  decrypt: (s: string) => s.replace(/^enc\(|\)$/g, ""),
+}));
+
+vi.mock("../client", async (orig) => {
+  const actual = await orig<typeof import("../client")>();
+  return {
+    ...actual,
+    refreshAccessToken: (...a: unknown[]) => refreshAccessTokenMock(...a),
+  };
+});
+
+vi.mock("../credentials", () => ({
+  getUserFitbitCredentials: vi.fn(async () => ({
+    clientId: "cid",
+    clientSecret: "csecret",
+  })),
+}));
+
+vi.mock("@/lib/integrations/status", () => ({
+  recordSyncFailure: (...a: unknown[]) => recordSyncFailure(...a),
+}));
+
+import {
+  classificationToFailureKind,
+  getValidToken,
+} from "../sync";
+
+beforeEach(() => {
+  vi.clearAllMocks();
+});
+
+afterEach(() => {
+  vi.clearAllMocks();
+});
+
+describe("getValidToken — non-rotating refresh", () => {
+  it("persists the new access token and KEEPS the stored refresh token when the response omits one", async () => {
+    prismaMock.fitbitConnection.findUnique.mockResolvedValue({
+      id: "conn1",
+      fitbitUserId: "abc123",
+      accessToken: "enc(old-access)",
+      refreshToken: "enc(old-refresh)",
+      // Expired so the refresh path fires.
+      tokenExpiresAt: new Date(Date.now() - 1000),
+    });
+    // Google does NOT return a refresh_token on a routine refresh.
+    refreshAccessTokenMock.mockResolvedValue({
+      access_token: "new-access",
+      expires_in: 3600,
+    });
+    prismaMock.fitbitConnection.update.mockResolvedValue({});
+
+    const result = await getValidToken("user1");
+
+    expect(result?.accessToken).toBe("new-access");
+    expect(refreshAccessTokenMock).toHaveBeenCalledWith("old-refresh", {
+      clientId: "cid",
+      clientSecret: "csecret",
+    });
+    const updateArg = prismaMock.fitbitConnection.update.mock.calls[0]![0];
+    expect(updateArg.data.accessToken).toBe("enc(new-access)");
+    // The stored refresh token must NOT be touched when the response omits one.
+    expect(updateArg.data).not.toHaveProperty("refreshToken");
+  });
+
+  it("overwrites the stored refresh token only when the response carries a fresh one", async () => {
+    prismaMock.fitbitConnection.findUnique.mockResolvedValue({
+      id: "conn1",
+      fitbitUserId: "abc123",
+      accessToken: "enc(old-access)",
+      refreshToken: "enc(old-refresh)",
+      tokenExpiresAt: new Date(Date.now() - 1000),
+    });
+    refreshAccessTokenMock.mockResolvedValue({
+      access_token: "new-access",
+      refresh_token: "rotated-refresh",
+      expires_in: 3600,
+    });
+    prismaMock.fitbitConnection.update.mockResolvedValue({});
+
+    await getValidToken("user1");
+
+    const updateArg = prismaMock.fitbitConnection.update.mock.calls[0]![0];
+    expect(updateArg.data.accessToken).toBe("enc(new-access)");
+    expect(updateArg.data.refreshToken).toBe("enc(rotated-refresh)");
+  });
+
+  it("returns the stored token without refresh when not near expiry", async () => {
+    prismaMock.fitbitConnection.findUnique.mockResolvedValue({
+      id: "conn1",
+      fitbitUserId: "abc123",
+      accessToken: "enc(live-access)",
+      refreshToken: "enc(live-refresh)",
+      tokenExpiresAt: new Date(Date.now() + 60 * 60 * 1000),
+    });
+
+    const result = await getValidToken("user1");
+
+    expect(result?.accessToken).toBe("live-access");
+    expect(refreshAccessTokenMock).not.toHaveBeenCalled();
+    expect(prismaMock.fitbitConnection.update).not.toHaveBeenCalled();
+  });
+
+  it("returns null when there is no connection", async () => {
+    prismaMock.fitbitConnection.findUnique.mockResolvedValue(null);
+    expect(await getValidToken("user1")).toBeNull();
+  });
+
+  it("records a reauth failure when credentials are missing on refresh", async () => {
+    const { getUserFitbitCredentials } = await import("../credentials");
+    (
+      getUserFitbitCredentials as ReturnType<typeof vi.fn>
+    ).mockResolvedValueOnce(null);
+    prismaMock.fitbitConnection.findUnique.mockResolvedValue({
+      id: "conn1",
+      fitbitUserId: "abc123",
+      accessToken: "enc(a)",
+      refreshToken: "enc(r)",
+      tokenExpiresAt: new Date(Date.now() - 1000),
+    });
+
+    const result = await getValidToken("user1");
+
+    expect(result).toBeNull();
+    expect(recordSyncFailure).toHaveBeenCalledWith(
+      expect.objectContaining({
+        integration: "fitbit",
+        kind: "reauth_required",
+      }),
+    );
+  });
+
+  it("records a classified failure and returns null when the refresh itself fails", async () => {
+    prismaMock.fitbitConnection.findUnique.mockResolvedValue({
+      id: "conn1",
+      fitbitUserId: "abc123",
+      accessToken: "enc(a)",
+      refreshToken: "enc(r)",
+      tokenExpiresAt: new Date(Date.now() - 1000),
+    });
+    refreshAccessTokenMock.mockRejectedValue(
+      new Error("Fitbit refreshAccessToken error: 401"),
+    );
+
+    const result = await getValidToken("user1");
+
+    expect(result).toBeNull();
+    expect(recordSyncFailure).toHaveBeenCalledWith(
+      expect.objectContaining({
+        integration: "fitbit",
+        kind: "reauth_required",
+      }),
+    );
+  });
+});
+
+describe("classificationToFailureKind", () => {
+  it("maps each classification onto a FailureKind", () => {
+    expect(classificationToFailureKind("reauth_required")).toBe(
+      "reauth_required",
+    );
+    expect(classificationToFailureKind("persistent")).toBe("persistent");
+    expect(classificationToFailureKind("transient")).toBe("transient");
+    // A success has no failure kind; surface it as transient (contract-bug
+    // anomaly is still recorded).
+    expect(classificationToFailureKind("success")).toBe("transient");
+  });
+});
diff --git a/src/lib/fitbit/client.ts b/src/lib/fitbit/client.ts
new file mode 100644
index 000000000..e6bfaea28
--- /dev/null
+++ b/src/lib/fitbit/client.ts
@@ -0,0 +1,235 @@
+/**
+ * Fitbit / Google Health API client — OAuth half (v1.12.0).
+ * Docs: https://developers.google.com/health (re-verify at build — the Google
+ * Health API is young; v4, post-Fitbit-Web-API).
+ *
+ * Mirrors the WHOOP client structure (`src/lib/whoop/client.ts`): hand-rolled
+ * fetch over `safeFetch` (no SDK), an OAuth handshake (`getAuthorizationUrl` /
+ * `exchangeCode` / `refreshAccessToken`), and a single profile fetch for the
+ * connection's external user id. The data-fetch half (paginated `dataPoints`
+ * walker + per-type mappers + `FITBIT_FIELD_MAP`) lands in a later wave.
+ *
+ * KEY DELTA vs WHOOP — Google does NOT rotate refresh tokens. WHOOP invalidates
+ * the prior refresh token on every refresh; Google's refresh tokens are stable
+ * (long-lived in production, 7-day expiry only in Testing mode). So
+ * `refreshAccessToken` may return a token response WITHOUT a `refresh_token`,
+ * and the sync layer keeps the stored one in that case (see `getValidToken` in
+ * `sync.ts`).
+ *
+ * Confidential web-server client: HealthLog holds the client secret
+ * server-side, so PKCE is omitted (matches WHOOP). Client credentials are sent
+ * to the token endpoint via HTTP Basic auth (RFC 6749 §2.3.1), the way Google's
+ * token endpoint accepts a confidential client.
+ */
+import { getEvent } from "@/lib/logging/context";
+import { safeFetch } from "@/lib/safe-fetch";
+import { FitbitApiError, classifyFitbitResponse } from "./response-classifier";
+
+export const FITBIT_API_BASE = "https://health.googleapis.com/v4";
+const GOOGLE_OAUTH_AUTH_URL = "https://accounts.google.com/o/oauth2/v2/auth";
+const GOOGLE_OAUTH_TOKEN_URL = "https://oauth2.googleapis.com/token";
+
+export interface FitbitCredentials {
+  clientId: string;
+  clientSecret: string;
+}
+
+function getRedirectUri(): string {
+  return (
+    process.env.FITBIT_REDIRECT_URI ??
+    `${process.env.NEXT_PUBLIC_APP_URL}/api/fitbit/callback`
+  );
+}
+
+/**
+ * OAuth scopes HealthLog requests (space-separated on the wire). The four
+ * launch Restricted read bundles cover every v1.12.0 metric; ECG / nutrition /
+ * location / IRN are deliberately omitted to keep the Restricted-scope CASA
+ * review surface minimal. Every scope is Restricted → the operator's OAuth
+ * client needs Google brand verification + an annual CASA assessment before it
+ * leaves Testing mode.
+ */
+export const FITBIT_OAUTH_SCOPE = [
+  "https://www.googleapis.com/auth/googlehealth.activity_and_fitness.readonly",
+  "https://www.googleapis.com/auth/googlehealth.health_metrics_and_measurements.readonly",
+  "https://www.googleapis.com/auth/googlehealth.sleep.readonly",
+  "https://www.googleapis.com/auth/googlehealth.profile.readonly",
+].join(" ");
+
+/**
+ * Generate the Google OAuth authorization URL (a browser redirect, not a
+ * fetch). `state` is the opaque CSRF nonce minted by `oauth-state.ts`.
+ *
+ * `access_type=offline` + `prompt=consent` are Google's requirement to receive
+ * a refresh token (the equivalent of WHOOP's `offline` scope); `prompt=consent`
+ * forces the consent screen so a re-connect always returns a fresh refresh
+ * token even if the user previously granted.
+ */
+export function getAuthorizationUrl(
+  state: string,
+  creds: FitbitCredentials,
+): string {
+  const params = new URLSearchParams({
+    response_type: "code",
+    client_id: creds.clientId,
+    redirect_uri: getRedirectUri(),
+    scope: FITBIT_OAUTH_SCOPE,
+    access_type: "offline",
+    prompt: "consent",
+    state,
+  });
+  return `${GOOGLE_OAUTH_AUTH_URL}?${params}`;
+}
+
+export interface FitbitTokenResponse {
+  access_token: string;
+  /**
+   * Present on the initial code exchange and whenever Google issues a new
+   * refresh token; ABSENT on a routine refresh because Google does not rotate.
+   * The sync layer keeps the stored token when this is missing.
+   */
+  refresh_token?: string;
+  expires_in: number;
+  scope?: string;
+  token_type?: string;
+}
+
+/** Basic-auth header carrying the confidential client credentials. */
+function basicAuthHeader(creds: FitbitCredentials): string {
+  const raw = `${creds.clientId}:${creds.clientSecret}`;
+  return `Basic ${Buffer.from(raw, "utf8").toString("base64")}`;
+}
+
+async function postToken(
+  params: URLSearchParams,
+  creds: FitbitCredentials,
+  verb: string,
+): Promise<FitbitTokenResponse> {
+  const start = performance.now();
+  const res = await safeFetch(GOOGLE_OAUTH_TOKEN_URL, {
+    method: "POST",
+    headers: {
+      "Content-Type": "application/x-www-form-urlencoded",
+      Authorization: basicAuthHeader(creds),
+    },
+    body: params.toString(),
+  });
+
+  const json = await res.json().catch(() => null);
+  const verdict = classifyFitbitResponse(res.status);
+  getEvent()?.addExternalCall({
+    service: "fitbit",
+    method: verb,
+    duration_ms: Math.round(performance.now() - start),
+    status: res.status,
+    error: verdict.classification === "success" ? undefined : verdict.reason,
+  });
+  if (verdict.classification !== "success") {
+    throw new FitbitApiError({
+      verb,
+      classification: verdict.classification,
+      httpStatus: verdict.httpStatus,
+      reason: verdict.reason,
+      upstreamError: typeof json?.error === "string" ? json.error : undefined,
+    });
+  }
+  return json as FitbitTokenResponse;
+}
+
+/** Exchange an authorization code for the initial token pair. */
+export async function exchangeCode(
+  code: string,
+  creds: FitbitCredentials,
+): Promise<FitbitTokenResponse> {
+  return postToken(
+    new URLSearchParams({
+      grant_type: "authorization_code",
+      code,
+      client_id: creds.clientId,
+      redirect_uri: getRedirectUri(),
+    }),
+    creds,
+    "exchangeCode",
+  );
+}
+
+/**
+ * Refresh an expired access token. Google does NOT rotate refresh tokens — the
+ * response carries a fresh `access_token` + `expires_in` but usually omits
+ * `refresh_token`. The caller persists the new access token + expiry and keeps
+ * the stored refresh token unless a new one is returned. The original scope is
+ * preserved by Google, so no `scope` param is re-sent.
+ */
+export async function refreshAccessToken(
+  refreshToken: string,
+  creds: FitbitCredentials,
+): Promise<FitbitTokenResponse> {
+  return postToken(
+    new URLSearchParams({
+      grant_type: "refresh_token",
+      refresh_token: refreshToken,
+      client_id: creds.clientId,
+    }),
+    creds,
+    "refreshAccessToken",
+  );
+}
+
+/**
+ * The Google Health profile carries the external user id used as the
+ * connection's `fitbitUserId`. Mirrors WHOOP's `fetchProfile` (a single GET,
+ * not a paginated collection).
+ */
+export interface FitbitProfile {
+  /**
+   * External user identifier. Google returns `me`-relative profile data; the
+   * stable id is surfaced under `name` (a `users/{id}` resource name) or `id`
+   * depending on the API surface — both are captured here and resolved at the
+   * call site. Re-verify the exact field against a live account at the data-sync
+   * wave.
+   */
+  name?: string;
+  id?: string;
+}
+
+export async function fetchProfile(
+  accessToken: string,
+): Promise<FitbitProfile> {
+  const start = performance.now();
+  const res = await safeFetch(`${FITBIT_API_BASE}/users/me/profile`, {
+    method: "GET",
+    headers: { Authorization: `Bearer ${accessToken}` },
+  });
+  const json = (await res.json().catch(() => null)) as FitbitProfile | null;
+  const verdict = classifyFitbitResponse(res.status);
+  getEvent()?.addExternalCall({
+    service: "fitbit",
+    method: "fetchProfile",
+    duration_ms: Math.round(performance.now() - start),
+    status: res.status,
+    error: verdict.classification === "success" ? undefined : verdict.reason,
+  });
+  if (verdict.classification !== "success") {
+    throw new FitbitApiError({
+      verb: "fetchProfile",
+      classification: verdict.classification,
+      httpStatus: verdict.httpStatus,
+      reason: verdict.reason,
+    });
+  }
+  return (json ?? {}) as FitbitProfile;
+}
+
+/**
+ * Resolve the stable external user id from a Google Health profile. The
+ * `users/{id}` resource name is the canonical anchor; fall back to a bare `id`
+ * or "me" so the connection always persists a non-empty `fitbitUserId`.
+ */
+export function resolveFitbitUserId(profile: FitbitProfile): string {
+  if (profile.name) {
+    const tail = profile.name.split("/").pop();
+    if (tail) return tail;
+  }
+  if (profile.id) return profile.id;
+  return "me";
+}
diff --git a/src/lib/fitbit/credentials.ts b/src/lib/fitbit/credentials.ts
new file mode 100644
index 000000000..6725daef8
--- /dev/null
+++ b/src/lib/fitbit/credentials.ts
@@ -0,0 +1,38 @@
+/**
+ * Helper to retrieve per-user Fitbit / Google Health OAuth credentials from the
+ * database. Mirrors `src/lib/whoop/credentials.ts`.
+ *
+ * Fitbit ships per-user BYO-keys: each self-hoster registers their own Google
+ * Cloud OAuth client and pastes the client id/secret into Settings, stored
+ * encrypted on `User` (the Restricted-scope brand verification + annual CASA
+ * assessment is per-OAuth-client, so a single shared app is unworkable for a
+ * multi-operator product).
+ */
+import { prisma } from "@/lib/db";
+import { decrypt } from "@/lib/crypto";
+import type { FitbitCredentials } from "./client";
+
+/**
+ * Fetch and decrypt the user's Fitbit API credentials.
+ * Returns null if the user has not configured credentials.
+ */
+export async function getUserFitbitCredentials(
+  userId: string,
+): Promise<FitbitCredentials | null> {
+  const user = await prisma.user.findUnique({
+    where: { id: userId },
+    select: {
+      fitbitClientIdEncrypted: true,
+      fitbitClientSecretEncrypted: true,
+    },
+  });
+
+  if (!user?.fitbitClientIdEncrypted || !user?.fitbitClientSecretEncrypted) {
+    return null;
+  }
+
+  return {
+    clientId: decrypt(user.fitbitClientIdEncrypted),
+    clientSecret: decrypt(user.fitbitClientSecretEncrypted),
+  };
+}
diff --git a/src/lib/fitbit/oauth-state.ts b/src/lib/fitbit/oauth-state.ts
new file mode 100644
index 000000000..2985c722f
--- /dev/null
+++ b/src/lib/fitbit/oauth-state.ts
@@ -0,0 +1,34 @@
+/**
+ * v1.12.0 — shared constants + mint helper for the Fitbit / Google Health OAuth
+ * state nonce ledger. Mirrors `src/lib/whoop/oauth-state.ts`.
+ *
+ * The nonce is random + opaque; a short-lived `FitbitOAuthState` row carries
+ * the `(nonce → userId)` mapping so the user id never leaks into request logs
+ * or network captures via the OAuth `state` param. The connect route, the
+ * callback route, and the cleanup cron all read these constants so the
+ * contract has one source of truth and the unit tests can pin it without
+ * importing either route file.
+ */
+import { randomBytes } from "node:crypto";
+
+/**
+ * Cookie name carrying the in-flight state nonce. httpOnly + Secure +
+ * sameSite:lax at the connect route (mirrors the WHOOP cookie shape).
+ */
+export const FITBIT_OAUTH_STATE_COOKIE = "fitbit_state" as const;
+
+/**
+ * 10-minute TTL on the state row. Matches the cookie `maxAge`. Long enough to
+ * cover a user approving the Google consent prompt; short enough that an
+ * abandoned handshake doesn't strand a row for hours.
+ */
+export const FITBIT_OAUTH_STATE_TTL_MS = 10 * 60 * 1000;
+
+/**
+ * Mint a fresh state nonce. 16 random bytes → 22 base64url chars without
+ * padding — 128 bits of entropy, aligned with the OAuth 2.0 §10.10
+ * "at least 128 bits" recommendation.
+ */
+export function mintFitbitOAuthStateNonce(): string {
+  return randomBytes(16).toString("base64url");
+}
diff --git a/src/lib/fitbit/response-classifier.ts b/src/lib/fitbit/response-classifier.ts
new file mode 100644
index 000000000..ca90b8290
--- /dev/null
+++ b/src/lib/fitbit/response-classifier.ts
@@ -0,0 +1,160 @@
+/**
+ * Off-response classifier for Fitbit / Google Health API replies (v1.12.0).
+ *
+ * Mirrors the WHOOP classifier (`src/lib/whoop/response-classifier.ts`) but
+ * speaks the Google Health wire. Like WHOOP, Google signals failure through the
+ * HTTP status code itself (no `status` field embedded in a 200 body), so this
+ * classifier is HTTP-status-driven.
+ *
+ * Classification buckets:
+ *
+ *   - `success`         → HTTP 2xx
+ *   - `transient`       → retryable: HTTP 429 (rate-limited — honour
+ *                         `Retry-After` for backoff at the call site), 5xx
+ *                         upstream outages, 3xx CDN/network glitches, and an
+ *                         empty / off-spec body where we never reached the
+ *                         envelope.
+ *   - `reauth_required` → HTTP 401 / 403 — the access token is rejected or the
+ *                         grant was revoked; the user must redo OAuth. (The
+ *                         per-data-class 403 soft-skip lives in `sync.ts`'s
+ *                         `isCollectionForbidden`, not here, so a Restricted
+ *                         scope granted independently doesn't park the whole
+ *                         connection.)
+ *   - `persistent`      → any other 4xx (400, 404, 422, …) — a malformed
+ *                         request or a contract mismatch that retrying without
+ *                         operator intervention will not fix.
+ *
+ * The classifier is conservative: anything it doesn't recognise defaults to
+ * `transient` so a single unknown response doesn't hard-disable the
+ * integration. The 3-strike admin-alert ladder in `recordSyncFailure` already
+ * catches the "keeps happening" case for recurring transients.
+ *
+ * Unlike WHOOP, there is no `invalid_grant` token-body special-case: Google
+ * signals a revoked refresh token with a 401 on the token endpoint, which the
+ * status-driven classifier already buckets as `reauth_required`.
+ */
+
+/** Outcome buckets for a Fitbit response. Same shape as WHOOP. */
+export type FitbitClassification =
+  | "success"
+  | "transient"
+  | "reauth_required"
+  | "persistent";
+
+export interface ClassifiedFitbitResponse {
+  classification: FitbitClassification;
+  /** The HTTP status code that drove the verdict (undefined if none seen). */
+  httpStatus: number | undefined;
+  /** Short human-readable label for logs / audit details. */
+  reason: string;
+}
+
+/**
+ * Classify a single Fitbit / Google Health API response by its HTTP status.
+ *
+ * @param httpStatus  The HTTP status from fetch.
+ */
+export function classifyFitbitResponse(
+  httpStatus: number,
+): ClassifiedFitbitResponse {
+  if (httpStatus >= 200 && httpStatus < 300) {
+    return { classification: "success", httpStatus, reason: "ok" };
+  }
+  // 429 is rate-limit — honour `Retry-After` for backoff at the call site, but
+  // the verdict is a plain transient so the next sync retries.
+  if (httpStatus === 429) {
+    return { classification: "transient", httpStatus, reason: "http_429" };
+  }
+  // 401/403 — the bearer token is rejected or the grant revoked. The user has
+  // to reconnect; do not retry silently. (A per-data-class 403 soft-skip is
+  // applied in sync.ts before this verdict drives a connection-wide reauth.)
+  if (httpStatus === 401 || httpStatus === 403) {
+    return {
+      classification: "reauth_required",
+      httpStatus,
+      reason: `http_${httpStatus}`,
+    };
+  }
+  // Any other 4xx is a malformed request / contract mismatch — persistent.
+  if (httpStatus >= 400 && httpStatus < 500) {
+    return {
+      classification: "persistent",
+      httpStatus,
+      reason: `http_${httpStatus}`,
+    };
+  }
+  // 5xx upstream outage — retryable.
+  if (httpStatus >= 500 && httpStatus < 600) {
+    return {
+      classification: "transient",
+      httpStatus,
+      reason: `http_${httpStatus}`,
+    };
+  }
+  // 3xx (a CDN / redirect glitch — safeFetch uses redirect:"manual", so a
+  // surfaced 3xx is unexpected) and everything else defaults to transient.
+  return {
+    classification: "transient",
+    httpStatus,
+    reason: `http_${httpStatus}_unknown`,
+  };
+}
+
+/**
+ * Typed Error subclass carrying the classification verdict so downstream
+ * `try/catch` blocks can branch without re-parsing the message. Mirrors
+ * `WhoopApiError`.
+ */
+export class FitbitApiError extends Error {
+  readonly classification: FitbitClassification;
+  readonly httpStatus: number | undefined;
+  readonly reason: string;
+  readonly verb: string;
+  /**
+   * The OAuth `error` code from a token-endpoint failure body (e.g.
+   * `invalid_grant`). Captured for the audit trail; the classifier does not
+   * branch on it (Google signals a revoked grant with a 401 status).
+   */
+  readonly upstreamError: string | undefined;
+
+  constructor(opts: {
+    verb: string;
+    classification: FitbitClassification;
+    httpStatus: number | undefined;
+    reason: string;
+    upstreamError?: string;
+  }) {
+    const statusLabel =
+      typeof opts.httpStatus === "number" ? opts.httpStatus : "?";
+    const errSegment = opts.upstreamError ? ` - ${opts.upstreamError}` : "";
+    // Cap the message at 1024 chars in the constructor (same bound as
+    // WhoopApiError) so a misbehaving upstream returning a multi-MB error body
+    // can't bloat an AuditLog row downstream.
+    const raw = `Fitbit ${opts.verb} error: ${statusLabel}${errSegment}`;
+    super(raw.slice(0, 1024));
+    this.name = "FitbitApiError";
+    this.verb = opts.verb;
+    this.classification = opts.classification;
+    this.httpStatus = opts.httpStatus;
+    this.reason = opts.reason;
+    this.upstreamError = opts.upstreamError;
+  }
+}
+
+/**
+ * Read the classification from any caught error. Returns `transient` for a
+ * non-`FitbitApiError` input so a call site surfacing a plain `Error` retries
+ * rather than permanently disabling the integration. Falls back to parsing the
+ * HTTP status out of the legacy `"Fitbit <verb> error: <status>"` message shape
+ * for callers that lose the original prototype across a pg-boss retry.
+ */
+export function classifyFitbitError(err: unknown): FitbitClassification {
+  if (err instanceof FitbitApiError) return err.classification;
+
+  const msg = err instanceof Error ? err.message : String(err);
+  const m = /Fitbit\s+\w+\s+error:\s*(\d+)/.exec(msg);
+  if (!m) return "transient";
+  const status = Number.parseInt(m[1]!, 10);
+  if (!Number.isFinite(status)) return "transient";
+  return classifyFitbitResponse(status).classification;
+}
diff --git a/src/lib/fitbit/sync.ts b/src/lib/fitbit/sync.ts
new file mode 100644
index 000000000..d9844c48f
--- /dev/null
+++ b/src/lib/fitbit/sync.ts
@@ -0,0 +1,158 @@
+/**
+ * Fitbit / Google Health sync service (v1.12.0).
+ *
+ * This wave (OAuth + credentials) lands the token-management half:
+ *   - `getValidToken` decrypts the stored token, refreshes at
+ *     `tokenExpiresAt - 5 min`, persists the new access token + expiry, and —
+ *     UNLIKE WHOOP — only overwrites the stored refresh token when the response
+ *     carries a fresh one (Google does not rotate refresh tokens).
+ *   - `recordFitbitSyncFailure` / `classificationToFailureKind` map a
+ *     classified API error onto the shared integration-status ledger.
+ *
+ * The per-resource data sync (`upsertFitbitMeasurements`, `syncUserFitbit`, the
+ * collection walkers, and the 403 soft-skip orchestration) lands in a later
+ * wave and extends this file.
+ */
+import { prisma } from "@/lib/db";
+import { encrypt, decrypt } from "@/lib/crypto";
+import { getEvent } from "@/lib/logging/context";
+import {
+  recordSyncFailure,
+  type FailureKind,
+} from "@/lib/integrations/status";
+import { refreshAccessToken } from "./client";
+import { getUserFitbitCredentials } from "./credentials";
+import {
+  FitbitApiError,
+  classifyFitbitError,
+  type FitbitClassification,
+} from "./response-classifier";
+
+/** Refresh the access token this many ms before `tokenExpiresAt`. */
+const TOKEN_REFRESH_BUFFER_MS = 5 * 60 * 1000;
+
+export interface FitbitTokenInfo {
+  accessToken: string;
+  connection: { id: string; fitbitUserId: string };
+}
+
+/**
+ * Resolve a valid Fitbit access token for a user, refreshing if it is within
+ * the 5-minute expiry buffer. Returns null when there is no connection, no
+ * credentials, or the refresh fails (the failure is recorded so scheduled syncs
+ * back off).
+ *
+ * KEY DELTA vs WHOOP: Google does not rotate refresh tokens. On refresh, persist
+ * the new access token + expiry and overwrite the stored `refreshToken` ONLY
+ * when the response carries a fresh one — otherwise keep the existing refresh
+ * token so the next refresh still authenticates.
+ */
+export async function getValidToken(
+  userId: string,
+): Promise<FitbitTokenInfo | null> {
+  const connection = await prisma.fitbitConnection.findUnique({
+    where: { userId },
+  });
+  if (!connection) return null;
+
+  const accessToken = decrypt(connection.accessToken);
+  const refreshToken = decrypt(connection.refreshToken);
+
+  if (
+    connection.tokenExpiresAt.getTime() - TOKEN_REFRESH_BUFFER_MS <
+    Date.now()
+  ) {
+    try {
+      const creds = await getUserFitbitCredentials(userId);
+      if (!creds) {
+        getEvent()?.addWarning(
+          `No Fitbit credentials found for user ${userId} during token refresh`,
+        );
+        await recordSyncFailure({
+          userId,
+          integration: "fitbit",
+          kind: "reauth_required",
+          message: "Fitbit credentials missing — token refresh skipped",
+          errorCode: "credentials_missing",
+        });
+        return null;
+      }
+
+      const newTokens = await refreshAccessToken(refreshToken, creds);
+      const expiresAt = new Date(Date.now() + newTokens.expires_in * 1000);
+
+      // Google does NOT rotate refresh tokens: a routine refresh returns a new
+      // access token but usually omits `refresh_token`. Persist the new access
+      // token + expiry and only overwrite the stored refresh token when the
+      // response actually carries a fresh one — otherwise keep the existing one.
+      await prisma.fitbitConnection.update({
+        where: { id: connection.id },
+        data: {
+          accessToken: encrypt(newTokens.access_token),
+          tokenExpiresAt: expiresAt,
+          ...(newTokens.refresh_token
+            ? { refreshToken: encrypt(newTokens.refresh_token) }
+            : {}),
+        },
+      });
+
+      return {
+        accessToken: newTokens.access_token,
+        connection: {
+          id: connection.id,
+          fitbitUserId: connection.fitbitUserId,
+        },
+      };
+    } catch (err) {
+      getEvent()?.addWarning(
+        `Fitbit token refresh failed for user ${userId}: ${err}`,
+      );
+      await recordFitbitSyncFailure(userId, err);
+      return null;
+    }
+  }
+
+  return {
+    accessToken,
+    connection: {
+      id: connection.id,
+      fitbitUserId: connection.fitbitUserId,
+    },
+  };
+}
+
+/**
+ * Map a Fitbit response classification onto a `FailureKind` and record it.
+ * Shared by the token-refresh path and (later) every per-resource catch block.
+ */
+export async function recordFitbitSyncFailure(
+  userId: string,
+  err: unknown,
+): Promise<void> {
+  const message = err instanceof Error ? err.message : String(err);
+  await recordSyncFailure({
+    userId,
+    integration: "fitbit",
+    kind: classificationToFailureKind(classifyFitbitError(err)),
+    message,
+    errorCode:
+      err instanceof FitbitApiError ? err.httpStatus?.toString() : undefined,
+  });
+}
+
+export function classificationToFailureKind(
+  classification: FitbitClassification,
+): FailureKind {
+  switch (classification) {
+    case "reauth_required":
+      return "reauth_required";
+    case "persistent":
+      return "persistent";
+    case "transient":
+      return "transient";
+    case "success":
+      // A caller asking for the FailureKind of a success is a contract bug;
+      // surface it as transient so the audit log still records the anomaly.
+      return "transient";
+  }
+}
diff --git a/src/lib/jobs/fitbit-backfill.ts b/src/lib/jobs/fitbit-backfill.ts
new file mode 100644
index 000000000..1aed63144
--- /dev/null
+++ b/src/lib/jobs/fitbit-backfill.ts
@@ -0,0 +1,94 @@
+/**
+ * v1.12.0 — pg-boss queue + boot-time self-converging backfill for newly
+ * connected Fitbit / Google Health accounts. Modelled on the WHOOP backfill
+ * (`src/lib/jobs/whoop-backfill.ts`): a discovery query enqueues one job per
+ * connection that has NOT yet been backfilled, and the pass is idempotent
+ * across reboots (the predicate `backfill_completed_at IS NULL` drops a
+ * connection once its backfill finishes).
+ *
+ * The per-user backfill handler (`runFitbitBackfillForUser`) and the
+ * reminder-worker registration land in the data-sync wave; this module exports
+ * the queue contract + the discovery enqueue so the OAuth callback can request
+ * a backfill on connect.
+ *
+ * The queue name MUST be registered in `allQueues` in
+ * `src/lib/jobs/reminder-worker.ts` or pg-boss never provisions it and the boot
+ * enqueue silently never drains (the v1.4.37 dead-queue class).
+ */
+import { prisma } from "@/lib/db";
+import { getGlobalBoss } from "@/lib/jobs/boss-instance";
+
+export const FITBIT_BACKFILL_QUEUE = "fitbit-backfill";
+
+/**
+ * Serial concurrency — a backfill walks years of history for one account and is
+ * rate-bounded by Google's per-app quota; concurrency-1 keeps it from crowding
+ * the request pool, matching `WHOOP_BACKFILL_CONCURRENCY`.
+ */
+export const FITBIT_BACKFILL_CONCURRENCY = 1;
+
+export interface FitbitBackfillPayload {
+  userId: string;
+  enqueuedAt: string;
+}
+
+/**
+ * Boot-time discovery. Finds every Fitbit connection not yet backfilled
+ * (`backfill_completed_at IS NULL`) and enqueues one backfill job per account.
+ *
+ * Idempotent across reboots: once a connection's backfill completes,
+ * `backfillCompletedAt` is set and the predicate drops it from the discovery
+ * set. pg-boss `singletonKey` coalesces duplicate sends so a fast restart while
+ * a job is queued doesn't double up.
+ *
+ * Best-effort: errors are returned through the result value so the worker boot
+ * never fails because of a backfill miss.
+ */
+export async function enqueueBootTimeFitbitBackfill(): Promise<{
+  enqueued: number;
+  skipped: number;
+  error: string | null;
+}> {
+  const boss = getGlobalBoss();
+  if (!boss) {
+    return { enqueued: 0, skipped: 0, error: null };
+  }
+
+  try {
+    const connections = await prisma.fitbitConnection.findMany({
+      where: { backfillCompletedAt: null },
+      select: { userId: true },
+    });
+
+    if (connections.length === 0) {
+      return { enqueued: 0, skipped: 0, error: null };
+    }
+
+    let enqueued = 0;
+    let skipped = 0;
+    for (const { userId } of connections) {
+      const payload: FitbitBackfillPayload = {
+        userId,
+        enqueuedAt: new Date().toISOString(),
+      };
+      const jobId = await boss.send(FITBIT_BACKFILL_QUEUE, payload, {
+        retryLimit: 3,
+        retryDelay: 60,
+        retryBackoff: true,
+        singletonKey: `fitbit-backfill|${userId}`,
+      });
+      if (jobId) {
+        enqueued += 1;
+      } else {
+        skipped += 1;
+      }
+    }
+    return { enqueued, skipped, error: null };
+  } catch (err) {
+    return {
+      enqueued: 0,
+      skipped: 0,
+      error: err instanceof Error ? err.message : String(err),
+    };
+  }
+}
diff --git a/src/lib/validations/fitbit.ts b/src/lib/validations/fitbit.ts
new file mode 100644
index 000000000..e6a5224b1
--- /dev/null
+++ b/src/lib/validations/fitbit.ts
@@ -0,0 +1,15 @@
+import { z } from "zod/v4";
+
+/**
+ * Per-user Fitbit / Google Health BYO-key credentials. Each self-hoster
+ * registers their own Google Cloud OAuth client (the Restricted-scope brand
+ * verification + CASA assessment is per-OAuth-client, so a single shared app is
+ * unworkable for a multi-operator product) and pastes the client id/secret into
+ * Settings. Stored encrypted on `User`.
+ */
+export const fitbitCredentialsSchema = z.object({
+  clientId: z.string().min(1).max(200),
+  clientSecret: z.string().min(1).max(200),
+});
+
+export type FitbitCredentialsInput = z.infer<typeof fitbitCredentialsSchema>;