diff --git a/server/db/schema.sql b/server/db/schema.sql
index 9edc0d9..d46e57b 100644
--- a/server/db/schema.sql
+++ b/server/db/schema.sql
@@ -197,6 +197,41 @@ CREATE TABLE IF NOT EXISTS mobile_refresh_tokens (
   INDEX idx_mrt_expires (expires_at)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 
+-- Mobile SSO authorization bridge (M9). Two short-lived, self-pruning tables that
+-- bridge a browser SSO redirect flow to the native app. They carry the app↔website
+-- PKCE + CSRF state (a SECOND PKCE layer, distinct from the website↔IdP PKCE the
+-- sso_tx cookie already carries) and the one-time code the app trades for bearer
+-- tokens. No secret is stored in the clear: code_challenge is a hash by construction
+-- and the authorization code is stored as a sha256 hash only (same pattern as
+-- mobile_refresh_tokens / user_invites / password_resets). See docs BACKEND_DESIGN §3/§4.
+CREATE TABLE IF NOT EXISTS mobile_auth_sessions (
+  id             INT AUTO_INCREMENT PRIMARY KEY,
+  session_id     CHAR(36)     NOT NULL UNIQUE,        -- uuid; carried inside the signed sso_tx (mode 'mobile')
+  provider       VARCHAR(40)  NOT NULL,               -- provider id, validated enabled at /start
+  code_challenge VARCHAR(255) NOT NULL,               -- app-supplied PKCE S256 challenge (base64url)
+  redirect_uri   VARCHAR(255) NOT NULL,               -- app callback; EXACT-match against the allowlist
+  state          VARCHAR(255) NOT NULL,               -- app-generated opaque CSRF value, echoed to the app
+  status         ENUM('pending','completed','consumed') NOT NULL DEFAULT 'pending',
+  user_id        INT          NULL,                   -- set once SSO resolves the account
+  created_at     DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  expires_at     DATETIME NOT NULL,                   -- ~10 min (one redirect round-trip incl. TOTP)
+  used_at        DATETIME NULL,                       -- stamped at exchange
+  CONSTRAINT fk_mas_user FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE,
+  INDEX idx_mas_expires (expires_at)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS mobile_auth_codes (
+  id          INT AUTO_INCREMENT PRIMARY KEY,
+  code_hash   CHAR(64)     NOT NULL UNIQUE,           -- sha256 hex of the opaque >=128-bit code
+  user_id     INT          NOT NULL,
+  session_id  CHAR(36)     NOT NULL,                  -- owning mobile_auth_sessions.session_id (ties code→PKCE challenge)
+  created_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  expires_at  DATETIME NOT NULL,                      -- very short (~5 min)
+  used_at     DATETIME NULL,                          -- set on first successful exchange (single use)
+  CONSTRAINT fk_mac_user FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE,
+  INDEX idx_mac_expires (expires_at)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
 -- Denylist of revoked web/cookie session tokens, keyed on the JWT `jti` minted
 -- per session in createSession. A single logout adds this session's jti here;
 -- requireAuth rejects any token whose jti is present. Rows self-expire: expires_at
diff --git a/server/src/auth/ssoState.js b/server/src/auth/ssoState.js
index 9bbf2d5..e4e5203 100644
--- a/server/src/auth/ssoState.js
+++ b/server/src/auth/ssoState.js
@@ -70,10 +70,15 @@ function verifyTx(txToken, stateNonce) {
 // session: `stage: 'totp'` makes session validation reject it (same marker the
 // local TOTP challenge uses), and `kind: 'sso_totp'` both reinforces that and
 // scopes it to the SSO completion endpoint.
-function createTotpPending({ userId, provider, authMethod, returnTo }) {
+//
+// `mobileSessionId` is present only for a mobile SSO bridge flow (mode 'mobile'):
+// it threads the bridge session through the TOTP form so that, on a correct code,
+// the completion mints an authorization code + deep-links back to the app instead
+// of setting a web session cookie. Absent for ordinary web SSO.
+function createTotpPending({ userId, provider, authMethod, returnTo, mobileSessionId }) {
   return token.signToken(
     { id: userId }, // subject only; identity is re-loaded fresh when the code is verified
-    { stage: 'totp', kind: 'sso_totp', provider, authMethod, returnTo },
+    { stage: 'totp', kind: 'sso_totp', provider, authMethod, returnTo, mobileSessionId },
     { expiresIn: TOTP_TTL },
   )
 }
diff --git a/server/src/middleware/rateLimit.js b/server/src/middleware/rateLimit.js
index fa00683..b1c71f3 100644
--- a/server/src/middleware/rateLimit.js
+++ b/server/src/middleware/rateLimit.js
@@ -2,13 +2,18 @@ const rateLimit = require('express-rate-limit')
 
 const log = require('../utils/logger')('ratelimit')
 
-function makeLimiter({ windowMs, max, label, message }) {
+function makeLimiter({ windowMs, max, label, message, keyGenerator, validate }) {
   return rateLimit({
     windowMs,
     max,
     standardHeaders: true,
     legacyHeaders: false,
     message: { message },
+    // Default key is the client IP; callers can widen it (e.g. IP + provider).
+    ...(keyGenerator ? { keyGenerator } : {}),
+    // Custom keyGenerators that fold in req.ip trip v7's IPv6 fallback validator;
+    // callers pass `validate` to scope that off just for their limiter.
+    ...(validate !== undefined ? { validate } : {}),
     handler: (req, res, next, options) => {
       log.warn(`${label} rate limit exceeded`, { ip: req.ip, path: req.originalUrl })
       res.status(options.statusCode).json(options.message)
@@ -71,6 +76,29 @@ const ssoStartLimiter = makeLimiter({
   message: 'Too many sign-in attempts. Please try again later.',
 })
 
+// Mobile SSO bridge — throttle /start per IP AND per provider: each call spawns a
+// mobile_auth_sessions row, so without a per-provider dimension /start is a cheap
+// way to spam rows for one provider from many-but-few IPs. Generous for real users
+// (a login is a handful of taps). `validate:{ip:false}` scopes off v7's IPv6
+// fallback check, which fires only because our key folds in req.ip.
+const mobileSsoStartLimiter = makeLimiter({
+  windowMs: 15 * 60 * 1000,
+  max: 20,
+  label: 'mobile-sso-start',
+  message: 'Too many sign-in attempts. Please try again later.',
+  keyGenerator: (req) => `${req.ip}:${req.query && req.query.provider ? req.query.provider : ''}`,
+  validate: { ip: false },
+})
+
+// Mobile SSO bridge — throttle /exchange per IP. The code is single-use, PKCE-bound
+// and short-lived, but cap redemption attempts anyway to blunt guessing.
+const mobileSsoExchangeLimiter = makeLimiter({
+  windowMs: 15 * 60 * 1000,
+  max: 30,
+  label: 'mobile-sso-exchange',
+  message: 'Too many attempts. Please try again later.',
+})
+
 // Password-reset requests per IP. Each one can send email, so cap tighter than
 // login to blunt email-bombing and enumeration timing probes. The endpoint always
 // returns a generic success regardless of match, so honest users never see this.
@@ -97,6 +125,8 @@ module.exports = {
   contactLimiter,
   mobileRefreshLimiter,
   ssoStartLimiter,
+  mobileSsoStartLimiter,
+  mobileSsoExchangeLimiter,
   passwordResetRequestLimiter,
   passwordResetConfirmLimiter,
 }
diff --git a/server/src/model/mobileAuthBridge/mobileAuthBridge.db.js b/server/src/model/mobileAuthBridge/mobileAuthBridge.db.js
new file mode 100644
index 0000000..2ca462e
--- /dev/null
+++ b/server/src/model/mobileAuthBridge/mobileAuthBridge.db.js
@@ -0,0 +1,101 @@
+const { query } = require('../../utils/db')
+
+// SQL for the mobile SSO authorization bridge (mobile_auth_sessions +
+// mobile_auth_codes). The authorization code is stored as a sha256 hash only
+// (hashing is done in the .model layer, mirroring mobileSessions); the PKCE
+// code_challenge is a hash by construction and is stored as-supplied. Both
+// tables self-prune on expiry (pruneExpired). See docs BACKEND_DESIGN §3/§4.
+
+// ── mobile_auth_sessions ───────────────────────────────────────────────────
+
+// Insert a pending bridge session. expiresAt is a JS Date (or ms epoch).
+async function insertSession({ sessionId, provider, codeChallenge, redirectUri, state, expiresAt }) {
+  const res = await query(
+    `INSERT INTO mobile_auth_sessions (session_id, provider, code_challenge, redirect_uri, state, expires_at)
+     VALUES (?, ?, ?, ?, ?, ?)`,
+    [sessionId, provider, codeChallenge, redirectUri, state, new Date(expiresAt)],
+  )
+  return res.insertId
+}
+
+// Look up a bridge session by its opaque session_id. Returns the row or null.
+async function getSession(sessionId) {
+  const rows = await query('SELECT * FROM mobile_auth_sessions WHERE session_id = ? LIMIT 1', [sessionId])
+  return rows[0] || null
+}
+
+// Mark a still-pending, unexpired session `completed` and attach the resolved
+// user. Guards on status + expiry so a replayed callback can't re-complete a
+// consumed/expired session. Returns rows changed (0 = not eligible).
+async function completeSession(sessionId, userId) {
+  const res = await query(
+    `UPDATE mobile_auth_sessions
+        SET status = 'completed', user_id = ?
+      WHERE session_id = ? AND status = 'pending' AND expires_at > NOW()`,
+    [userId, sessionId],
+  )
+  return Number(res.affectedRows || 0)
+}
+
+// Mark a session `consumed` after a successful token exchange (stamps used_at).
+async function consumeSession(sessionId) {
+  const res = await query(
+    "UPDATE mobile_auth_sessions SET status = 'consumed', used_at = NOW() WHERE session_id = ?",
+    [sessionId],
+  )
+  return Number(res.affectedRows || 0)
+}
+
+// ── mobile_auth_codes ──────────────────────────────────────────────────────
+
+// Insert a freshly minted authorization code (by hash).
+async function insertCode({ codeHash, userId, sessionId, expiresAt }) {
+  const res = await query(
+    `INSERT INTO mobile_auth_codes (code_hash, user_id, session_id, expires_at)
+     VALUES (?, ?, ?, ?)`,
+    [codeHash, userId, sessionId, new Date(expiresAt)],
+  )
+  return res.insertId
+}
+
+// Return an authorization-code row only if it is still redeemable: not used and
+// not expired. Returns the row (incl. user_id, session_id) or null.
+async function findValidCode(codeHash) {
+  const rows = await query(
+    `SELECT * FROM mobile_auth_codes
+      WHERE code_hash = ? AND used_at IS NULL AND expires_at > NOW()
+      LIMIT 1`,
+    [codeHash],
+  )
+  return rows[0] || null
+}
+
+// Atomically mark a code used (single-use gate). Only affects a not-yet-used row,
+// so a concurrent double-redeem sees affectedRows = 0 on the loser. Returns rows
+// changed.
+async function markCodeUsed(codeHash) {
+  const res = await query(
+    'UPDATE mobile_auth_codes SET used_at = NOW() WHERE code_hash = ? AND used_at IS NULL',
+    [codeHash],
+  )
+  return Number(res.affectedRows || 0)
+}
+
+// Housekeeping: drop long-dead rows from both bridge tables (expired, or codes
+// already used). Keeps the tables from growing without bound. Returns rows removed.
+async function pruneExpired() {
+  const a = await query('DELETE FROM mobile_auth_codes WHERE expires_at < NOW() OR used_at IS NOT NULL')
+  const b = await query("DELETE FROM mobile_auth_sessions WHERE expires_at < NOW() OR status = 'consumed'")
+  return Number(a.affectedRows || 0) + Number(b.affectedRows || 0)
+}
+
+module.exports = {
+  insertSession,
+  getSession,
+  completeSession,
+  consumeSession,
+  insertCode,
+  findValidCode,
+  markCodeUsed,
+  pruneExpired,
+}
diff --git a/server/src/model/mobileAuthBridge/mobileAuthBridge.model.js b/server/src/model/mobileAuthBridge/mobileAuthBridge.model.js
new file mode 100644
index 0000000..136f914
--- /dev/null
+++ b/server/src/model/mobileAuthBridge/mobileAuthBridge.model.js
@@ -0,0 +1,102 @@
+// ── Mobile SSO authorization bridge — logic layer ──────────────────────────
+//
+// Bridges the browser SSO redirect flow to the native app. A `/start` seeds a
+// `mobile_auth_sessions` row carrying the app-supplied PKCE challenge, the app's
+// CSRF `state`, and the exact callback URI. The SSO callback (once the account is
+// resolved) mints a single-use authorization CODE tied to that session; the app
+// redeems the code + its PKCE verifier at `/exchange` for bearer tokens.
+//
+// Secrets: the raw authorization code is generated here and returned ONCE to the
+// caller (to place in the redirect URL); only its sha256 hash is persisted — the
+// raw code never touches the DB. The PKCE code_challenge is already a hash.
+
+const crypto = require('crypto')
+
+const db = require('./mobileAuthBridge.db')
+
+// TTLs. A start→callback→exchange round-trip is quick, but the callback may route
+// through the TOTP form, so give the session a little room; the code itself is
+// very short-lived.
+const SESSION_TTL_MS = 10 * 60 * 1000 // 10 min — one redirect round-trip incl. TOTP
+const CODE_TTL_MS = 5 * 60 * 1000 //     5 min — the app redeems immediately
+
+// sha256 hex of a raw token — the persisted representation of an authorization code.
+function hashCode(raw) {
+  return crypto.createHash('sha256').update(String(raw)).digest('hex')
+}
+
+// Create a pending bridge session. Returns { sessionId } (opaque uuid) which the
+// caller embeds in the signed sso_tx so the callback can find this row. Best-effort
+// prune keeps the table trimmed without a cron (same approach as revoked_sessions).
+async function startSession({ provider, codeChallenge, redirectUri, state, now = Date.now() }) {
+  const sessionId = crypto.randomUUID()
+  await db.insertSession({
+    sessionId,
+    provider,
+    codeChallenge,
+    redirectUri,
+    state,
+    expiresAt: new Date(now + SESSION_TTL_MS),
+  })
+  db.pruneExpired().catch(() => {}) // opportunistic; never blocks the flow
+  return { sessionId }
+}
+
+// Fetch a bridge session row by session_id (or null).
+async function getSession(sessionId) {
+  if (!sessionId) return null
+  return db.getSession(sessionId)
+}
+
+// Mint the one-time authorization code for a resolved account. Marks the owning
+// session `completed`; returns { code } (the RAW code, to place in the redirect
+// URL) or null if the session isn't pending/eligible. 256 bits of entropy — well
+// above the ≥128-bit floor — url-safe and opaque.
+async function issueAuthCode({ sessionId, userId, now = Date.now() }) {
+  const completed = await db.completeSession(sessionId, userId)
+  if (!completed) return null // not pending / expired / already used
+  const code = crypto.randomBytes(32).toString('base64url')
+  await db.insertCode({
+    codeHash: hashCode(code),
+    userId,
+    sessionId,
+    expiresAt: new Date(now + CODE_TTL_MS),
+  })
+  db.pruneExpired().catch(() => {})
+  return { code }
+}
+
+// Look up a redeemable code row (unused + unexpired) for a raw code, or null.
+async function findRedeemableCode(rawCode) {
+  if (!rawCode) return null
+  return db.findValidCode(hashCode(rawCode))
+}
+
+// Atomically burn a code (single-use). Returns true iff THIS call was the one that
+// marked it used — a concurrent double-redeem gets false on the loser.
+async function consumeCode(rawCode) {
+  const changed = await db.markCodeUsed(hashCode(rawCode))
+  return changed > 0
+}
+
+// Mark a session fully consumed after a successful exchange.
+async function finishSession(sessionId) {
+  return db.consumeSession(sessionId)
+}
+
+// Drop expired/used rows from both tables.
+async function pruneExpired() {
+  return db.pruneExpired()
+}
+
+module.exports = {
+  SESSION_TTL_MS,
+  CODE_TTL_MS,
+  startSession,
+  getSession,
+  issueAuthCode,
+  findRedeemableCode,
+  consumeCode,
+  finishSession,
+  pruneExpired,
+}
diff --git a/server/src/router/v1/auth/mobile.routes.js b/server/src/router/v1/auth/mobile.routes.js
index 6b5a55e..d5459f0 100644
--- a/server/src/router/v1/auth/mobile.routes.js
+++ b/server/src/router/v1/auth/mobile.routes.js
@@ -6,9 +6,15 @@ const { requireAuth } = require('../../../auth/session.middleware')
 const { loginLimiter, mobileRefreshLimiter } = require('../../../middleware/rateLimit')
 const { slowLogin, backoffGuard } = require('../../../middleware/loginProtection')
 const validate = require('../../../middleware/validate')
+const mobileSsoRouter = require('./mobileSso.routes')
 
 const mobileRouter = express.Router()
 
+// Native SSO authorization bridge (M9) — /auth/mobile/sso/{start,exchange}.
+// Additive alongside the credential login below; reuses the website SSO flow and
+// terminates in the same bearer tokens.
+mobileRouter.use('/sso', mobileSsoRouter)
+
 // Mobile login is a credential surface too, so it sits behind the SAME guards as
 // web login (cheapest rejection first): per-IP backoff → progressive slowdown →
 // hard rate cap.
diff --git a/server/src/router/v1/auth/mobileSso.controller.js b/server/src/router/v1/auth/mobileSso.controller.js
new file mode 100644
index 0000000..06f1a41
--- /dev/null
+++ b/server/src/router/v1/auth/mobileSso.controller.js
@@ -0,0 +1,151 @@
+// ── Mobile SSO authorization bridge — start + exchange ─────────────────────
+//
+// The native app's "Sign in with Google/Discord" without shipping any OAuth
+// secret. This controller owns the two app-facing endpoints; the SSO redirect
+// mechanics and the callback branch live in sso.controller (reused, not
+// duplicated), and the bridge state lives in the mobileAuthBridge model.
+//
+//   GET  /auth/mobile/sso/start     → seed a bridge session, reuse the SSO redirect
+//   POST /auth/mobile/sso/exchange  → code + PKCE verifier → mobile bearer tokens
+//
+// Two PKCE layers are in play (see docs BACKEND_DESIGN §4): Layer A (website↔IdP,
+// handled entirely inside the reused SSO flow) and Layer B (app↔website, verified
+// here at /exchange). Do not conflate them.
+
+const crypto = require('crypto')
+
+const users = require('../../../model/users/users.model')
+const activity = require('../../../model/activity/activity.model')
+const mobileSessions = require('../../../model/mobileSessions/mobileSessions.model')
+const mobileBridge = require('../../../model/mobileAuthBridge/mobileAuthBridge.model')
+const sessionService = require('../../../auth/session.service')
+const ssoState = require('../../../auth/ssoState')
+const ssoController = require('./sso.controller')
+
+const log = require('../../../utils/logger')('auth-mobile-sso')
+
+// Exact-match allowlist of app callback URIs. Default is the one fixed
+// application-owned custom scheme; HTTPS App Link URIs can be appended per shard
+// later (see docs/android/APP_LINKS.md). EXACT match only — never a prefix match
+// (prefix matching on custom schemes is a known open-redirect vector on mobile).
+const REDIRECT_ALLOWLIST = new Set(
+  (process.env.MOBILE_AUTH_REDIRECT_URIS || 'runicgateway://auth/callback')
+    .split(',')
+    .map((s) => s.trim())
+    .filter(Boolean),
+)
+
+const PROVIDER_ID_RE = /^[a-z0-9-]+$/
+
+// Append query params to an (already-allowlisted) app callback URI.
+function appDeepLink(redirectUri, params) {
+  const sep = redirectUri.includes('?') ? '&' : '?'
+  const qs = Object.entries(params)
+    .map(([k, v]) => `${k}=${encodeURIComponent(v)}`)
+    .join('&')
+  return `${redirectUri}${sep}${qs}`
+}
+
+// Constant-time string compare (equal-length guard first).
+function safeEqual(a, b) {
+  const bufA = Buffer.from(String(a))
+  const bufB = Buffer.from(String(b))
+  return bufA.length === bufB.length && crypto.timingSafeEqual(bufA, bufB)
+}
+
+// GET /auth/mobile/sso/start?provider&code_challenge&state&redirect_uri
+// Opened by the app in a Custom Tab. Validates the request, seeds a bridge
+// session carrying the app's PKCE challenge + state + callback, then reuses the
+// existing SSO redirect (tagged mode:'mobile') to hand off to the IdP.
+async function start(req, res) {
+  const { provider, code_challenge: codeChallenge, state, redirect_uri: redirectUri } = req.query
+  try {
+    // redirect_uri must be exactly one of the registered app callbacks. Validate
+    // it FIRST — everything else can only be surfaced to the app by redirecting
+    // to a trusted callback, so an untrusted one is a hard 400 (no redirect).
+    if (!REDIRECT_ALLOWLIST.has(redirectUri)) {
+      log.warn('mobile sso start: redirect_uri not in allowlist', { ip: req.ip })
+      return res.status(400).json({ message: 'Unrecognized redirect URI.' })
+    }
+    if (!PROVIDER_ID_RE.test(provider || '')) {
+      return res.redirect(appDeepLink(redirectUri, { error: 'invalid_provider', state }))
+    }
+
+    const { sessionId } = await mobileBridge.startSession({ provider, codeChallenge, redirectUri, state })
+    const ok = await ssoController.redirectToIdp(req, res, provider, {
+      mode: 'mobile',
+      mobileSessionId: sessionId,
+    })
+    if (!ok) {
+      log.warn('mobile sso start: provider unavailable', { provider })
+      return res.redirect(appDeepLink(redirectUri, { error: 'provider_unavailable', state }))
+    }
+    // redirectToIdp already issued the 302 on success.
+  } catch (err) {
+    log.error('mobile sso start', err)
+    // We validated redirect_uri above, so it's safe to bounce the error to the app.
+    return res.redirect(appDeepLink(redirectUri, { error: 'server_error', state }))
+  }
+}
+
+// POST /auth/mobile/sso/exchange  { code, code_verifier }
+// Trades the one-time authorization code (+ its PKCE verifier) for the SAME mobile
+// access + refresh pair as /auth/mobile/login. Single-use, PKCE-bound.
+async function exchange(req, res) {
+  const { code, code_verifier: codeVerifier } = req.body
+  try {
+    const codeRow = await mobileBridge.findRedeemableCode(code)
+    if (!codeRow) {
+      log.warn('mobile sso exchange: code unknown/expired/used', { ip: req.ip })
+      return res.status(401).json({ message: 'Invalid or expired authorization code.' })
+    }
+    const sess = await mobileBridge.getSession(codeRow.session_id)
+    if (!sess) {
+      return res.status(401).json({ message: 'Invalid or expired authorization code.' })
+    }
+
+    // PKCE Layer B: the app proves it holds the verifier for the challenge it
+    // registered at /start. Check BEFORE burning the code so a caller lacking the
+    // verifier (e.g. a callback interceptor) can't consume a legitimate code.
+    if (!safeEqual(ssoState.codeChallengeFor(codeVerifier || ''), sess.code_challenge)) {
+      log.warn('mobile sso exchange: PKCE verifier mismatch', { ip: req.ip })
+      return res.status(401).json({ message: 'PKCE verification failed.' })
+    }
+
+    // Atomic single-use gate: only the winner of a concurrent double-redeem
+    // proceeds; a losing/replayed attempt sees false here.
+    if (!(await mobileBridge.consumeCode(code))) {
+      log.warn('mobile sso exchange: code already used', { ip: req.ip })
+      return res.status(401).json({ message: 'Invalid or expired authorization code.' })
+    }
+
+    const user = await users.getById(codeRow.user_id) // fresh row; 401 if the account vanished
+    if (!user) {
+      return res.status(401).json({ message: 'Invalid or expired authorization code.' })
+    }
+    await mobileBridge.finishSession(codeRow.session_id)
+
+    const meta = sessionService.sessionMeta(req)
+    const out = sessionService.createMobileSession(user, meta)
+    await mobileSessions.store({
+      userId: user.id,
+      tokenHash: out.refreshHash,
+      deviceHash: out.deviceHash,
+      userAgent: out.userAgent,
+      expiresAt: out.refreshExpiresAt,
+    })
+    await activity.log({ req, userId: user.id, action: 'auth.mobile.login', detail: { sso: sess.provider } })
+    log.info('mobile sso exchange success', { id: user.id, provider: sess.provider, ip: req.ip })
+    return res.json({
+      accessToken: out.accessToken,
+      refreshToken: out.refreshToken,
+      expiresIn: out.expiresIn,
+      user: { id: user.id, username: user.username, role: user.role },
+    })
+  } catch (err) {
+    log.error('mobile sso exchange', err)
+    return res.status(500).json({ message: 'Internal Server Error' })
+  }
+}
+
+module.exports = { start, exchange }
diff --git a/server/src/router/v1/auth/mobileSso.routes.js b/server/src/router/v1/auth/mobileSso.routes.js
new file mode 100644
index 0000000..8f76ec0
--- /dev/null
+++ b/server/src/router/v1/auth/mobileSso.routes.js
@@ -0,0 +1,55 @@
+const express = require('express')
+const { body, query } = require('express-validator')
+
+const { start, exchange } = require('./mobileSso.controller')
+const { mobileSsoStartLimiter, mobileSsoExchangeLimiter } = require('../../../middleware/rateLimit')
+const validate = require('../../../middleware/validate')
+
+// Mobile SSO authorization bridge (M9). Mounted at /auth/mobile/sso. Native
+// "Sign in with Google/Discord" that reuses the website's SSO flow and terminates
+// in the existing mobile bearer tokens — no OAuth secret ever ships in the app.
+// Provider discovery reuses GET /auth/providers; refresh/logout reuse the existing
+// /auth/mobile/{refresh,logout}. See docs BACKEND_DESIGN §4 + docs/android/PLAN.md §9.
+const mobileSsoRouter = express.Router()
+
+// GET /auth/mobile/sso/start — opened by the app in a Custom Tab; 302s to the IdP.
+mobileSsoRouter.get(
+  '/start',
+  // #swagger.tags = ['Auth · Mobile']
+  // #swagger.summary = 'Begin native SSO login (redirect to the IdP)'
+  // #swagger.description = 'Opened by the Android app in a Custom Tab. Validates the provider is enabled and the redirect_uri is an exact match of a registered app callback, seeds a short-lived bridge session carrying the app PKCE challenge + state, and 302-redirects into the existing website SSO flow. On success the callback redirects to `redirect_uri?code=…&state=…` (a one-time code, never a token). Errors are surfaced to the app as `redirect_uri?error=…&state=…`.'
+  // #swagger.parameters['provider'] = { in: 'query', required: true, schema: { type: 'string' }, description: 'Provider id from GET /auth/providers (e.g. google, discord).' }
+  // #swagger.parameters['code_challenge'] = { in: 'query', required: true, schema: { type: 'string' }, description: 'App-generated PKCE S256 challenge (base64url).' }
+  // #swagger.parameters['state'] = { in: 'query', required: true, schema: { type: 'string' }, description: 'App-generated opaque CSRF value, echoed on the callback for the app to verify.' }
+  // #swagger.parameters['redirect_uri'] = { in: 'query', required: true, schema: { type: 'string' }, description: 'The app callback; must EXACTLY match a registered value (default runicgateway://auth/callback).' }
+  /* #swagger.responses[302] = { description: 'Redirect to the identity provider (or back to the app callback on error)' } */
+  /* #swagger.responses[400] = { description: 'Unrecognized redirect URI or validation error', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  /* #swagger.responses[429] = { description: 'Too many attempts (rate limited)', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  mobileSsoStartLimiter,
+  query('provider').isString().trim().isLength({ min: 1, max: 40 }),
+  query('code_challenge').isString().trim().isLength({ min: 20, max: 255 }),
+  query('state').isString().trim().isLength({ min: 8, max: 255 }),
+  query('redirect_uri').isString().trim().isLength({ min: 1, max: 255 }),
+  validate,
+  start,
+)
+
+// POST /auth/mobile/sso/exchange — code + PKCE verifier → mobile bearer tokens.
+mobileSsoRouter.post(
+  '/exchange',
+  // #swagger.tags = ['Auth · Mobile']
+  // #swagger.summary = 'Exchange an SSO authorization code for mobile tokens'
+  // #swagger.description = 'Redeems the single-use authorization code returned to the app callback, together with the PKCE code_verifier, for the SAME access + refresh pair as /auth/mobile/login. The code is single-use and PKCE-bound: a wrong verifier, an expired/used code, or a reused code all fail 401.'
+  /* #swagger.requestBody = { required: true, content: { "application/json": { schema: { $ref: "#/components/schemas/MobileSsoExchangeRequest" } } } } */
+  /* #swagger.responses[200] = { description: 'Access + refresh tokens', content: { "application/json": { schema: { $ref: "#/components/schemas/MobileTokenResponse" } } } } */
+  /* #swagger.responses[400] = { description: 'Validation error', content: { "application/json": { schema: { $ref: "#/components/schemas/ValidationError" } } } } */
+  /* #swagger.responses[401] = { description: 'Invalid/expired/used code or failed PKCE verification', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  /* #swagger.responses[429] = { description: 'Too many attempts (rate limited)', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
+  mobileSsoExchangeLimiter,
+  body('code').isString().trim().isLength({ min: 20, max: 255 }),
+  body('code_verifier').isString().trim().isLength({ min: 20, max: 255 }),
+  validate,
+  exchange,
+)
+
+module.exports = mobileSsoRouter
diff --git a/server/src/router/v1/auth/sso.controller.js b/server/src/router/v1/auth/sso.controller.js
index 6bc1509..3d3f1cf 100644
--- a/server/src/router/v1/auth/sso.controller.js
+++ b/server/src/router/v1/auth/sso.controller.js
@@ -18,6 +18,7 @@ const userIdentities = require('../../../model/userIdentities/userIdentities.mod
 const settings = require('../../../model/settings/settings.model')
 const registry = require('../../../auth/providers/registry')
 const sessionService = require('../../../auth/session.service')
+const mobileBridge = require('../../../model/mobileAuthBridge/mobileAuthBridge.model')
 const ssoState = require('../../../auth/ssoState')
 const token = require('../../../auth/token')
 const totp = require('../../../utils/totp')
@@ -96,6 +97,27 @@ async function listProviders(req, res) {
   }
 }
 
+// Shared IdP redirect: validate the provider is usable, mint the SSO tx (carrying
+// any extra `txData`, e.g. mode/linkUserId/returnTo, or the mobile bridge's
+// mode:'mobile' + mobileSessionId), set the httpOnly tx cookie, and 302 to the
+// provider authorize URL. Returns true on redirect; false means the provider is
+// unavailable and the caller renders its own failure (web pages redirect to an
+// error; the mobile bridge surfaces it to the app). Used by both web start
+// (beginFlow) and the mobile bridge start (routes/mobileSso.controller).
+async function redirectToIdp(req, res, providerId, txData = {}) {
+  const row = await authProviders.getWithSecret(providerId)
+  if (!row || !row.enabled || !registry.validateConfig(row).valid) return false
+  const provider = registry.instantiate(row)
+  const tx = ssoState.createTx({ provider: providerId, ...txData })
+  res.cookie(ssoState.TX_COOKIE, tx.txToken, txCookieOptions(req))
+  const url = provider.getAuthorizationUrl(tx.nonce, {
+    redirectUri: redirectUriFor(req, providerId),
+    codeChallenge: tx.codeChallenge,
+  })
+  res.redirect(url)
+  return true
+}
+
 // Shared start for both login and link. `mode` ∈ 'login' | 'link'. For link,
 // requireAuth has already run so req.user is the account to attach the identity to.
 async function beginFlow(req, res, mode) {
@@ -105,26 +127,17 @@ async function beginFlow(req, res, mode) {
   const failUrl = mode === 'link' ? accountError('error', portal) : loginError('error', portal)
   try {
     if (!PROVIDER_ID_RE.test(providerId)) return res.redirect(failUrl)
-    const row = await authProviders.getWithSecret(providerId)
-    if (!row || !row.enabled || !registry.validateConfig(row).valid) {
+    const ok = await redirectToIdp(req, res, providerId, {
+      mode,
+      linkUserId: mode === 'link' ? req.user.id : undefined,
+      returnTo: returnTo || undefined,
+    })
+    if (!ok) {
       log.warn('sso start: provider unavailable', { provider: providerId, mode })
       return res.redirect(
         mode === 'link' ? accountError('unavailable', portal) : loginError('unavailable', portal),
       )
     }
-    const provider = registry.instantiate(row)
-    const tx = ssoState.createTx({
-      provider: providerId,
-      mode,
-      linkUserId: mode === 'link' ? req.user.id : undefined,
-      returnTo: returnTo || undefined,
-    })
-    res.cookie(ssoState.TX_COOKIE, tx.txToken, txCookieOptions(req))
-    const url = provider.getAuthorizationUrl(tx.nonce, {
-      redirectUri: redirectUriFor(req, providerId),
-      codeChallenge: tx.codeChallenge,
-    })
-    return res.redirect(url)
   } catch (err) {
     log.error('sso start', err)
     return res.redirect(failUrl)
@@ -167,6 +180,7 @@ async function callback(req, res) {
       codeVerifier: tx.verifier,
     })
     if (tx.mode === 'link') return finishLink(req, res, providerId, tx, profile)
+    if (tx.mode === 'mobile') return finishMobileLogin(req, res, providerId, row.kind, tx, profile)
     return finishLogin(req, res, providerId, row.kind, tx, profile)
   } catch (err) {
     log.error('sso callback', err)
@@ -266,6 +280,100 @@ async function finishLogin(req, res, providerId, kind, tx, profile) {
   return res.redirect(sanitizeReturn(tx.returnTo) || homePath(portal))
 }
 
+// ── Mobile SSO bridge (mode 'mobile') ──────────────────────────────────────
+// The mobile flow ends by handing the app a deep link carrying a one-time
+// authorization code (never a token) + the app's original `state`. redirect_uri
+// came from the exact-match allowlist at /start, so appending our params is safe.
+
+function appDeepLink(redirectUri, params) {
+  const sep = redirectUri.includes('?') ? '&' : '?'
+  const qs = Object.entries(params)
+    .map(([k, v]) => `${k}=${encodeURIComponent(v)}`)
+    .join('&')
+  return `${redirectUri}${sep}${qs}`
+}
+const appError = (sess, code) => appDeepLink(sess.redirect_uri, { error: code, state: sess.state })
+
+// Mint the one-time auth code for a resolved account and return the app success
+// deep link (or null if the bridge session was no longer pending — e.g. expired
+// or already used). Shared by the direct callback and the TOTP-completion path so
+// the "issue code + record login + audit" logic lives in one place. Does not touch
+// res, so either caller can 302 (callback) or JSON-wrap it (TOTP fetch).
+async function mintMobileAuthLink(req, sess, user, providerId, viaTotp) {
+  const issued = await mobileBridge.issueAuthCode({ sessionId: sess.session_id, userId: user.id })
+  if (!issued) {
+    log.warn('mobile sso: auth code not issued (session not pending)', { provider: providerId, id: user.id })
+    return null
+  }
+  await users.recordLogin(user.id, req.ip)
+  await activity.log({
+    req,
+    userId: user.id,
+    action: 'auth.sso.login',
+    detail: { provider: providerId, mobile: true, totp: viaTotp || undefined },
+  })
+  log.info('mobile sso login success', { provider: providerId, id: user.id, ip: req.ip, totp: !!viaTotp })
+  return appDeepLink(sess.redirect_uri, { code: issued.code, state: sess.state })
+}
+
+// Mobile variant of finishLogin: identical account-resolution policy (link-only
+// with opt-in provisioning, status gate, TOTP), but a success mints a one-time
+// code and 302s to the app callback instead of setting a session cookie. A 2FA
+// account is routed through the same web TOTP form (carrying the bridge session)
+// and completes in finishSsoTotp — the second factor is never bypassed.
+async function finishMobileLogin(req, res, providerId, kind, tx, profile) {
+  const sess = await mobileBridge.getSession(tx.mobileSessionId)
+  if (!sess || sess.status !== 'pending' || new Date(sess.expires_at).getTime() <= Date.now()) {
+    log.warn('mobile sso callback: bridge session invalid/expired', { provider: providerId })
+    // Without a valid session we can't trust a redirect_uri — fail generically.
+    if (sess) return res.redirect(appError(sess, 'session_expired'))
+    return res
+      .status(400)
+      .json({ message: 'This sign-in session is invalid or has expired. Please try again from the app.' })
+  }
+
+  let user
+  const identity = await userIdentities.findByProviderSubject(providerId, profile.subject)
+  if (identity) {
+    user = await users.getById(identity.user_id)
+    if (!user) return res.redirect(appError(sess, 'not_linked'))
+  } else {
+    const mode = await settings.getRegistrationMode()
+    if (mode !== 'sso' && mode !== 'both') {
+      log.warn('mobile sso login refused: no linked account', { provider: providerId })
+      return res.redirect(appError(sess, 'not_linked'))
+    }
+    user = await provisionSsoPlayer(req, providerId, profile)
+    if (!user) return res.redirect(appError(sess, 'error'))
+  }
+
+  if (user.status && user.status !== 'active') {
+    log.warn('mobile sso login refused: inactive account', { provider: providerId, id: user.id, status: user.status })
+    return res.redirect(appError(sess, 'disabled'))
+  }
+
+  const authMethod = sessionService.AUTH_METHODS.includes(kind) ? kind : 'sso'
+
+  if (needsTotp(user)) {
+    // Same second-factor gate as web: stage a signed pending-TOTP cookie (now
+    // carrying the bridge session) and route the Custom Tab through the player
+    // TOTP form. finishSsoTotp completes the mobile flow on a correct code.
+    const pending = ssoState.createTotpPending({
+      userId: user.id,
+      provider: providerId,
+      authMethod,
+      returnTo: '/account',
+      mobileSessionId: sess.session_id,
+    })
+    res.cookie(ssoState.TOTP_COOKIE, pending, totpCookieOptions(req))
+    log.info('mobile sso login: awaiting TOTP', { provider: providerId, id: user.id, ip: req.ip })
+    return res.redirect(`${loginPath('account')}?sso_totp=1`)
+  }
+
+  const link = await mintMobileAuthLink(req, sess, user, providerId, false)
+  return res.redirect(link || appError(sess, 'error'))
+}
+
 // POST /auth/sso/totp — second factor for an SSO login whose account has TOTP on.
 // Reads the staged pending-TOTP cookie, verifies the authenticator code, then
 // mints the full session. Mirrors auth.controller.loginTotp: a wrong code is a
@@ -293,9 +401,25 @@ async function finishSsoTotp(req, res) {
       return res.status(403).json({ message: 'This account is not active. Contact an administrator.' })
     }
 
-    // Second factor satisfied — clear the staged cookie and issue the real session.
+    // Second factor satisfied — clear the staged cookie.
     res.clearCookie(ssoState.TOTP_COOKIE, token.cookieOptions(req))
     loginProtection.recordSuccess(req.ip)
+
+    // Mobile SSO bridge: instead of a web session, mint the one-time auth code and
+    // return a deep link for the app to redeem. The second factor is now complete,
+    // so the code is issued no earlier than an ordinary web session would be.
+    if (pending.mobileSessionId) {
+      const sess = await mobileBridge.getSession(pending.mobileSessionId)
+      if (!sess || sess.status !== 'pending' || new Date(sess.expires_at).getTime() <= Date.now()) {
+        return res.status(401).json({ message: 'Your sign-in session expired. Please sign in again from the app.' })
+      }
+      const link = await mintMobileAuthLink(req, sess, user, pending.provider, true)
+      if (!link) {
+        return res.status(409).json({ message: 'This sign-in session was already used. Please sign in again from the app.' })
+      }
+      return res.json({ redirect: link })
+    }
+
     const authMethod = sessionService.AUTH_METHODS.includes(pending.authMethod) ? pending.authMethod : 'sso'
     const { token: sessionToken } = sessionService.createSession(user, authMethod)
     token.setAuthCookie(req, res, sessionToken)
@@ -330,4 +454,15 @@ async function finishLink(req, res, providerId, tx, profile) {
   return res.redirect(`${accountPath(portal)}?linked=${providerId}`)
 }
 
-module.exports = { listProviders, start, linkStart, callback, beginFlow, finishLogin, finishSsoTotp, finishLink }
+module.exports = {
+  listProviders,
+  start,
+  linkStart,
+  callback,
+  beginFlow,
+  redirectToIdp,
+  finishLogin,
+  finishMobileLogin,
+  finishSsoTotp,
+  finishLink,
+}
diff --git a/server/src/server.js b/server/src/server.js
index 28e95a2..f9890a3 100644
--- a/server/src/server.js
+++ b/server/src/server.js
@@ -13,6 +13,7 @@ const { ensureSchema, close } = require('./utils/db')
 const { seedDefaults, createInitialAdminFromEnv } = require('../db/seed')
 const settings = require('./model/settings/settings.model')
 const revokedSessions = require('./model/revokedSessions/revokedSessions.model')
+const mobileAuthBridge = require('./model/mobileAuthBridge/mobileAuthBridge.model')
 const createLogger = require('./utils/logger')
 const { evaluateBotInternalKey } = require('./utils/botInternalKey')
 const brand = require('./config/brand')
@@ -67,6 +68,15 @@ async function start() {
     log.warn('revoked-session prune failed', { error: err.message })
   }
 
+  // Same treatment for the mobile SSO bridge tables (also pruned opportunistically
+  // on each bridge write). Boot-time sweep catches rows orphaned by a crash.
+  try {
+    const pruned = await mobileAuthBridge.pruneExpired()
+    if (pruned) log.info(`pruned ${pruned} expired mobile-auth-bridge row(s)`)
+  } catch (err) {
+    log.warn('mobile-auth-bridge prune failed', { error: err.message })
+  }
+
   const mode = await settings.get('site_mode')
   log.info(`site mode: ${String(mode || 'live').toUpperCase()}`)
 
diff --git a/server/swagger/swagger-output.json b/server/swagger/swagger-output.json
index 19b90e4..e743f9a 100644
--- a/server/swagger/swagger-output.json
+++ b/server/swagger/swagger-output.json
@@ -894,6 +894,142 @@
         }
       }
     },
+    "/api/v1/auth/mobile/sso/start": {
+      "get": {
+        "tags": [
+          "Auth · Mobile"
+        ],
+        "summary": "Begin native SSO login (redirect to the IdP)",
+        "description": "Opened by the Android app in a Custom Tab. Validates the provider is enabled and the redirect_uri is an exact match of a registered app callback, seeds a short-lived bridge session carrying the app PKCE challenge + state, and 302-redirects into the existing website SSO flow. On success the callback redirects to `redirect_uri?code=…&state=…` (a one-time code, never a token). Errors are surfaced to the app as `redirect_uri?error=…&state=…`.",
+        "parameters": [
+          {
+            "name": "provider",
+            "in": "query",
+            "required": true,
+            "schema": {
+              "type": "string"
+            },
+            "description": "Provider id from GET /auth/providers (e.g. google, discord)."
+          },
+          {
+            "name": "code_challenge",
+            "in": "query",
+            "required": true,
+            "schema": {
+              "type": "string"
+            },
+            "description": "App-generated PKCE S256 challenge (base64url)."
+          },
+          {
+            "name": "state",
+            "in": "query",
+            "required": true,
+            "schema": {
+              "type": "string"
+            },
+            "description": "App-generated opaque CSRF value, echoed on the callback for the app to verify."
+          },
+          {
+            "name": "redirect_uri",
+            "in": "query",
+            "required": true,
+            "schema": {
+              "type": "string"
+            },
+            "description": "The app callback; must EXACTLY match a registered value (default runicgateway://auth/callback)."
+          }
+        ],
+        "responses": {
+          "302": {
+            "description": "Redirect to the identity provider (or back to the app callback on error)"
+          },
+          "400": {
+            "description": "Unrecognized redirect URI or validation error",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/Error"
+                }
+              }
+            }
+          },
+          "429": {
+            "description": "Too many attempts (rate limited)",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/Error"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
+    "/api/v1/auth/mobile/sso/exchange": {
+      "post": {
+        "tags": [
+          "Auth · Mobile"
+        ],
+        "summary": "Exchange an SSO authorization code for mobile tokens",
+        "description": "Redeems the single-use authorization code returned to the app callback, together with the PKCE code_verifier, for the SAME access + refresh pair as /auth/mobile/login. The code is single-use and PKCE-bound: a wrong verifier, an expired/used code, or a reused code all fail 401.",
+        "responses": {
+          "200": {
+            "description": "Access + refresh tokens",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/MobileTokenResponse"
+                }
+              }
+            }
+          },
+          "400": {
+            "description": "Validation error",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ValidationError"
+                }
+              }
+            }
+          },
+          "401": {
+            "description": "Invalid/expired/used code or failed PKCE verification",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/Error"
+                }
+              }
+            }
+          },
+          "429": {
+            "description": "Too many attempts (rate limited)",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/Error"
+                }
+              }
+            }
+          },
+          "500": {
+            "description": "Internal Server Error"
+          }
+        },
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/MobileSsoExchangeRequest"
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/v1/auth/providers": {
       "get": {
         "tags": [
@@ -1096,6 +1232,9 @@
           "403": {
             "description": "Forbidden"
           },
+          "409": {
+            "description": "Conflict"
+          },
           "429": {
             "description": "Too many attempts (rate limited / backoff)",
             "content": {
@@ -11115,6 +11254,56 @@
           }
         }
       },
+      "MobileSsoExchangeRequest": {
+        "type": "object",
+        "properties": {
+          "type": {
+            "type": "string",
+            "example": "object"
+          },
+          "required": {
+            "type": "array",
+            "example": [
+              "code",
+              "code_verifier"
+            ],
+            "items": {
+              "type": "string"
+            }
+          },
+          "properties": {
+            "type": "object",
+            "properties": {
+              "code": {
+                "type": "object",
+                "properties": {
+                  "type": {
+                    "type": "string",
+                    "example": "string"
+                  },
+                  "description": {
+                    "type": "string",
+                    "example": "The single-use authorization code returned to the app callback."
+                  }
+                }
+              },
+              "code_verifier": {
+                "type": "object",
+                "properties": {
+                  "type": {
+                    "type": "string",
+                    "example": "string"
+                  },
+                  "description": {
+                    "type": "string",
+                    "example": "The PKCE verifier for the challenge sent to /auth/mobile/sso/start."
+                  }
+                }
+              }
+            }
+          }
+        }
+      },
       "Message": {
         "type": "object",
         "properties": {
diff --git a/server/swagger/swagger.js b/server/swagger/swagger.js
index c31ec07..870a2fd 100644
--- a/server/swagger/swagger.js
+++ b/server/swagger/swagger.js
@@ -185,6 +185,20 @@ const doc = {
           all: { type: 'boolean', description: 'Revoke every session for the user.', example: false },
         },
       },
+      MobileSsoExchangeRequest: {
+        type: 'object',
+        required: ['code', 'code_verifier'],
+        properties: {
+          code: {
+            type: 'string',
+            description: 'The single-use authorization code returned to the app callback.',
+          },
+          code_verifier: {
+            type: 'string',
+            description: 'The PKCE verifier for the challenge sent to /auth/mobile/sso/start.',
+          },
+        },
+      },
       Message: {
         type: 'object',
         properties: { message: { type: 'string', example: 'Logged out.' } },
diff --git a/server/test/mobileAuthBridge.model.test.js b/server/test/mobileAuthBridge.model.test.js
new file mode 100644
index 0000000..47dec8e
--- /dev/null
+++ b/server/test/mobileAuthBridge.model.test.js
@@ -0,0 +1,83 @@
+// Pure-logic tests for the mobile SSO bridge MODEL. The .db layer is stubbed
+// (plain object exports → mutable in-process), so these are DB-free and only
+// exercise the model's gating, hashing, and code-generation logic.
+process.env.JWT_SECRET = process.env.JWT_SECRET || 'test-secret'
+process.env.DB_HOST = '127.0.0.1'
+process.env.DB_PORT = '59999'
+
+const { test, beforeEach, after } = require('node:test')
+const assert = require('node:assert/strict')
+const crypto = require('crypto')
+
+const bridge = require('../src/model/mobileAuthBridge/mobileAuthBridge.model')
+const bridgeDb = require('../src/model/mobileAuthBridge/mobileAuthBridge.db')
+const db = require('../src/utils/db')
+
+after(() => db.close())
+
+const sha256 = (s) => crypto.createHash('sha256').update(String(s)).digest('hex')
+
+let calls
+beforeEach(() => {
+  calls = { insertSession: [], insertCode: [], completeSession: [], markCodeUsed: [], findValidCode: [] }
+  bridgeDb.insertSession = async (a) => { calls.insertSession.push(a); return 1 }
+  bridgeDb.insertCode = async (a) => { calls.insertCode.push(a); return 1 }
+  bridgeDb.completeSession = async (sid, uid) => { calls.completeSession.push([sid, uid]); return 1 }
+  bridgeDb.consumeSession = async () => 1
+  bridgeDb.getSession = async () => null
+  bridgeDb.findValidCode = async (h) => { calls.findValidCode.push(h); return { code_hash: h, user_id: 9, session_id: 's' } }
+  bridgeDb.markCodeUsed = async () => 1
+  bridgeDb.pruneExpired = async () => 0
+})
+
+test('startSession generates a uuid session_id and persists the row', async () => {
+  const { sessionId } = await bridge.startSession({
+    provider: 'google', codeChallenge: 'chal', redirectUri: 'runicgateway://auth/callback', state: 'st',
+  })
+  assert.match(sessionId, /^[0-9a-f-]{36}$/)
+  assert.equal(calls.insertSession.length, 1)
+  assert.equal(calls.insertSession[0].sessionId, sessionId)
+  assert.equal(calls.insertSession[0].provider, 'google')
+  assert.ok(calls.insertSession[0].expiresAt instanceof Date)
+  assert.ok(calls.insertSession[0].expiresAt.getTime() > Date.now())
+})
+
+test('issueAuthCode: completes the session, stores only the code HASH, returns the raw code once', async () => {
+  const out = await bridge.issueAuthCode({ sessionId: 's-1', userId: 42 })
+  assert.ok(out && typeof out.code === 'string')
+  // >=128-bit, url-safe, opaque.
+  assert.ok(out.code.length >= 22, 'at least 128 bits of base64url entropy')
+  assert.match(out.code, /^[A-Za-z0-9_-]+$/)
+  // Session was marked completed for THIS user.
+  assert.deepEqual(calls.completeSession[0], ['s-1', 42])
+  // Only the sha256 hash of the code is persisted; the raw code never is.
+  assert.equal(calls.insertCode[0].codeHash, sha256(out.code))
+  assert.equal(calls.insertCode[0].userId, 42)
+  assert.equal(calls.insertCode[0].sessionId, 's-1')
+})
+
+test('issueAuthCode returns null (mints no code) when the session is not pending/eligible', async () => {
+  bridgeDb.completeSession = async () => 0 // not pending / expired / already used
+  const out = await bridge.issueAuthCode({ sessionId: 's-1', userId: 42 })
+  assert.equal(out, null)
+  assert.equal(calls.insertCode.length, 0, 'no code row when the session could not be completed')
+})
+
+test('findRedeemableCode looks up by the code HASH, not the raw code', async () => {
+  const row = await bridge.findRedeemableCode('raw-code-value')
+  assert.equal(calls.findValidCode[0], sha256('raw-code-value'))
+  assert.equal(row.user_id, 9)
+})
+
+test('consumeCode is single-use: true only when a row was actually flipped', async () => {
+  bridgeDb.markCodeUsed = async () => 1
+  assert.equal(await bridge.consumeCode('c'), true)
+  bridgeDb.markCodeUsed = async () => 0 // already used / raced
+  assert.equal(await bridge.consumeCode('c'), false)
+})
+
+test('two issued codes are distinct (fresh entropy each time)', async () => {
+  const a = await bridge.issueAuthCode({ sessionId: 's', userId: 1 })
+  const b = await bridge.issueAuthCode({ sessionId: 's', userId: 1 })
+  assert.notEqual(a.code, b.code)
+})
diff --git a/server/test/mobileSsoBridge.test.js b/server/test/mobileSsoBridge.test.js
new file mode 100644
index 0000000..ff43fc5
--- /dev/null
+++ b/server/test/mobileSsoBridge.test.js
@@ -0,0 +1,264 @@
+// Mobile SSO authorization bridge — controller/integration tests. Models are
+// stubbed (mutable object exports), so these are DB-free and exercise the bridge
+// start/exchange handlers plus the mode:'mobile' branches added to sso.controller.
+process.env.JWT_SECRET = process.env.JWT_SECRET || 'test-secret'
+process.env.SECRET_ENC_KEY = process.env.SECRET_ENC_KEY || 'unit-test-enc-key'
+process.env.DB_HOST = '127.0.0.1'
+process.env.DB_PORT = '59999'
+
+const { test, beforeEach, after } = require('node:test')
+const assert = require('node:assert/strict')
+
+const mobileSso = require('../src/router/v1/auth/mobileSso.controller')
+const ssoCtrl = require('../src/router/v1/auth/sso.controller')
+const ssoState = require('../src/auth/ssoState')
+const token = require('../src/auth/token')
+const bridge = require('../src/model/mobileAuthBridge/mobileAuthBridge.model')
+const mobileSessions = require('../src/model/mobileSessions/mobileSessions.model')
+const users = require('../src/model/users/users.model')
+const activity = require('../src/model/activity/activity.model')
+const authProviders = require('../src/model/authProviders/authProviders.model')
+const userIdentities = require('../src/model/userIdentities/userIdentities.model')
+const settings = require('../src/model/settings/settings.model')
+const registry = require('../src/auth/providers/registry')
+const totp = require('../src/utils/totp')
+const db = require('../src/utils/db')
+
+after(() => db.close())
+
+const CALLBACK = 'runicgateway://auth/callback'
+const PROFILE = { subject: 'sub-1', email: 'alice@example.com', name: 'Alice' }
+const SESSION = {
+  session_id: 'sess-1',
+  status: 'pending',
+  provider: 'google',
+  redirect_uri: CALLBACK,
+  state: 'st-abc',
+  code_challenge: ssoState.codeChallengeFor('verifier-xyz'),
+  expires_at: new Date(Date.now() + 5 * 60 * 1000),
+}
+
+let logged
+beforeEach(() => {
+  logged = []
+  activity.log = async (e) => { logged.push(e) }
+  authProviders.getWithSecret = async () => ({ id: 'google', kind: 'google', enabled: 1, client_id: 'c', client_secret_enc: 'e' })
+  registry.instantiate = () => ({ handleCallback: async () => ({ ...PROFILE }) })
+  registry.validateConfig = () => ({ valid: true })
+  userIdentities.findByProviderSubject = async () => ({ user_id: 7 })
+  settings.getRegistrationMode = async () => 'disabled'
+  users.getById = async (id) => ({ id, username: 'alice', role: 'player' })
+  users.recordLogin = async () => {}
+  mobileSessions.store = async () => 1
+  bridge.getSession = async () => ({ ...SESSION })
+  bridge.issueAuthCode = async () => ({ code: 'RAWCODE' })
+  bridge.startSession = async () => ({ sessionId: 'sess-1' })
+  bridge.findRedeemableCode = async () => ({ user_id: 7, session_id: 'sess-1' })
+  bridge.consumeCode = async () => true
+  bridge.finishSession = async () => 1
+})
+
+function mockRes() {
+  return {
+    statusCode: 200, redirectedTo: null, body: null, cookies: {}, cleared: [],
+    status(c) { this.statusCode = c; return this },
+    json(b) { this.body = b; return this },
+    redirect(u) { this.redirectedTo = u; return this },
+    cookie(n, v) { this.cookies[n] = v; return this },
+    clearCookie(n) { this.cleared.push(n); return this },
+  }
+}
+
+// ── /start ─────────────────────────────────────────────────────────────────
+
+test('start: an un-allowlisted redirect_uri is a hard 400 (no redirect, exact-match only)', async () => {
+  const res = mockRes()
+  await mobileSso.start(
+    { query: { provider: 'google', code_challenge: 'c', state: 's', redirect_uri: 'evil://auth/callback' }, ip: '1.2.3.4' },
+    res,
+  )
+  assert.equal(res.statusCode, 400)
+  assert.equal(res.redirectedTo, null)
+})
+
+test('start: a prefix of the allowlisted URI is rejected (not a prefix match)', async () => {
+  const res = mockRes()
+  await mobileSso.start(
+    { query: { provider: 'google', code_challenge: 'c', state: 's', redirect_uri: 'runicgateway://auth/callback.evil.com' }, ip: '1.2.3.4' },
+    res,
+  )
+  assert.equal(res.statusCode, 400)
+})
+
+test('start: seeds a bridge session and reuses the SSO redirect tagged mode:mobile', async () => {
+  let startArgs = null
+  let txData = null
+  bridge.startSession = async (a) => { startArgs = a; return { sessionId: 'sess-99' } }
+  const saved = ssoCtrl.redirectToIdp
+  ssoCtrl.redirectToIdp = async (req, res, provider, data) => { txData = { provider, ...data }; res.redirect('https://idp/authorize'); return true }
+  try {
+    const res = mockRes()
+    await mobileSso.start(
+      { query: { provider: 'google', code_challenge: 'chal', state: 'st', redirect_uri: CALLBACK }, ip: '1.2.3.4' },
+      res,
+    )
+    assert.equal(res.redirectedTo, 'https://idp/authorize')
+    assert.equal(startArgs.redirectUri, CALLBACK)
+    assert.equal(startArgs.codeChallenge, 'chal')
+    assert.equal(txData.provider, 'google')
+    assert.equal(txData.mode, 'mobile')
+    assert.equal(txData.mobileSessionId, 'sess-99')
+  } finally {
+    ssoCtrl.redirectToIdp = saved
+  }
+})
+
+test('start: a disabled/unavailable provider surfaces the error to the app callback', async () => {
+  const saved = ssoCtrl.redirectToIdp
+  ssoCtrl.redirectToIdp = async () => false
+  try {
+    const res = mockRes()
+    await mobileSso.start(
+      { query: { provider: 'google', code_challenge: 'chal', state: 'st', redirect_uri: CALLBACK }, ip: '1.2.3.4' },
+      res,
+    )
+    assert.equal(res.redirectedTo, `${CALLBACK}?error=provider_unavailable&state=st`)
+  } finally {
+    ssoCtrl.redirectToIdp = saved
+  }
+})
+
+// ── /exchange ────────────────────────────────────────────────────────────────
+
+function exchangeReq(body) {
+  return { body, ip: '127.0.0.1', headers: { 'user-agent': 'Android' } }
+}
+
+test('exchange: valid code + PKCE verifier → mobile bearer tokens (same shape as login)', async () => {
+  const res = mockRes()
+  await mobileSso.exchange(exchangeReq({ code: 'RAWCODE', code_verifier: 'verifier-xyz' }), res)
+  assert.equal(res.statusCode, 200)
+  assert.ok(res.body.accessToken, 'access token issued')
+  assert.ok(res.body.refreshToken, 'refresh token issued')
+  assert.equal(res.body.user.id, 7)
+  // The issued access token validates as a mobile session.
+  const s = require('../src/auth/session.service').validateBearerToken(res.body.accessToken)
+  assert.equal(s.authMethod, 'mobile')
+  assert.equal(logged.at(-1).action, 'auth.mobile.login')
+  assert.equal(logged.at(-1).detail.sso, 'google')
+})
+
+test('exchange: unknown/expired/used code → 401', async () => {
+  bridge.findRedeemableCode = async () => null
+  const res = mockRes()
+  await mobileSso.exchange(exchangeReq({ code: 'nope', code_verifier: 'verifier-xyz' }), res)
+  assert.equal(res.statusCode, 401)
+})
+
+test('exchange: wrong PKCE verifier → 401 and the code is NOT consumed', async () => {
+  let consumed = false
+  bridge.consumeCode = async () => { consumed = true; return true }
+  const res = mockRes()
+  await mobileSso.exchange(exchangeReq({ code: 'RAWCODE', code_verifier: 'WRONG' }), res)
+  assert.equal(res.statusCode, 401)
+  assert.equal(consumed, false, 'a failed PKCE check must not burn a legitimate code')
+})
+
+test('exchange: a reused (already-consumed) code → 401 (single use)', async () => {
+  bridge.consumeCode = async () => false // lost the single-use race / replay
+  const res = mockRes()
+  await mobileSso.exchange(exchangeReq({ code: 'RAWCODE', code_verifier: 'verifier-xyz' }), res)
+  assert.equal(res.statusCode, 401)
+})
+
+test('exchange: vanished user → 401', async () => {
+  users.getById = async () => null
+  const res = mockRes()
+  await mobileSso.exchange(exchangeReq({ code: 'RAWCODE', code_verifier: 'verifier-xyz' }), res)
+  assert.equal(res.statusCode, 401)
+})
+
+// ── callback (mode:'mobile') via sso.controller ──────────────────────────────
+
+function callbackReq(tx) {
+  return {
+    params: { provider: 'google' },
+    cookies: { [ssoState.TX_COOKIE]: tx.txToken },
+    query: { state: tx.nonce, code: 'idp-code' },
+    ip: '127.0.0.1', protocol: 'http', get: () => 'localhost', headers: {},
+  }
+}
+
+test('callback (mobile): linked account → deep link with a one-time code + echoed state, NO cookie', async () => {
+  const tx = ssoState.createTx({ provider: 'google', mode: 'mobile', mobileSessionId: 'sess-1' })
+  const res = mockRes()
+  await ssoCtrl.callback(callbackReq(tx), res)
+  assert.equal(res.redirectedTo, `${CALLBACK}?code=RAWCODE&state=st-abc`)
+  assert.equal(res.cookies[token.COOKIE_NAME], undefined, 'no web session cookie on a mobile flow')
+  assert.equal(logged.at(-1).action, 'auth.sso.login')
+  assert.equal(logged.at(-1).detail.mobile, true)
+})
+
+test('callback (mobile): unlinked account with registration closed → error deep link (link-only)', async () => {
+  userIdentities.findByProviderSubject = async () => null
+  const tx = ssoState.createTx({ provider: 'google', mode: 'mobile', mobileSessionId: 'sess-1' })
+  const res = mockRes()
+  await ssoCtrl.callback(callbackReq(tx), res)
+  assert.equal(res.redirectedTo, `${CALLBACK}?error=not_linked&state=st-abc`)
+  assert.equal(logged.length, 0)
+})
+
+test('callback (mobile): a 2FA account is routed through the TOTP form, code NOT yet issued', async () => {
+  users.getById = async (id) => ({ id, username: 'alice', role: 'player', totp_enabled: 1 })
+  let issued = false
+  bridge.issueAuthCode = async () => { issued = true; return { code: 'RAWCODE' } }
+  const tx = ssoState.createTx({ provider: 'google', mode: 'mobile', mobileSessionId: 'sess-1' })
+  const res = mockRes()
+  await ssoCtrl.callback(callbackReq(tx), res)
+  assert.equal(res.redirectedTo, '/account/login?sso_totp=1')
+  assert.ok(res.cookies[ssoState.TOTP_COOKIE], 'pending-TOTP cookie staged')
+  assert.equal(issued, false, 'no auth code before the second factor passes')
+  // The staged challenge carries the bridge session so completion can deep-link back.
+  const pending = ssoState.verifyTotpPending(res.cookies[ssoState.TOTP_COOKIE])
+  assert.equal(pending.mobileSessionId, 'sess-1')
+})
+
+test('callback (mobile): an invalid/expired bridge session fails without leaking a redirect', async () => {
+  bridge.getSession = async () => null
+  const tx = ssoState.createTx({ provider: 'google', mode: 'mobile', mobileSessionId: 'gone' })
+  const res = mockRes()
+  await ssoCtrl.callback(callbackReq(tx), res)
+  assert.equal(res.statusCode, 400)
+  assert.equal(res.redirectedTo, null)
+})
+
+// ── finishSsoTotp (mode:'mobile') ────────────────────────────────────────────
+
+function totpReq(pending, code) {
+  return {
+    cookies: pending ? { [ssoState.TOTP_COOKIE]: pending } : {},
+    body: { code }, ip: '127.0.0.1', protocol: 'http', get: () => 'localhost', headers: {},
+  }
+}
+
+test('finishSsoTotp (mobile): correct code → JSON { redirect } deep link, no cookie', async () => {
+  users.getRawById = async (id) => ({ id, username: 'alice', role: 'player', totp_enabled: 1, totp_secret: 'S' })
+  totp.verifyCode = () => true
+  const pending = ssoState.createTotpPending({ userId: 7, provider: 'google', authMethod: 'google', returnTo: '/account', mobileSessionId: 'sess-1' })
+  const res = mockRes()
+  await ssoCtrl.finishSsoTotp(totpReq(pending, '123456'), res)
+  assert.equal(res.body.redirect, `${CALLBACK}?code=RAWCODE&state=st-abc`)
+  assert.equal(res.cookies[token.COOKIE_NAME], undefined, 'mobile 2FA completion sets no web session cookie')
+  assert.ok(res.cleared.includes(ssoState.TOTP_COOKIE))
+  assert.equal(logged.at(-1).detail.totp, true)
+})
+
+test('finishSsoTotp (mobile): expired bridge session → 401', async () => {
+  users.getRawById = async (id) => ({ id, username: 'alice', role: 'player', totp_enabled: 1, totp_secret: 'S' })
+  totp.verifyCode = () => true
+  bridge.getSession = async () => null
+  const pending = ssoState.createTotpPending({ userId: 7, provider: 'google', authMethod: 'google', mobileSessionId: 'gone' })
+  const res = mockRes()
+  await ssoCtrl.finishSsoTotp(totpReq(pending, '123456'), res)
+  assert.equal(res.statusCode, 401)
+})