diff --git a/src/main.js b/src/main.js
index 49048f84..091279b4 100644
--- a/src/main.js
+++ b/src/main.js
@@ -63,6 +63,9 @@ export async function bootstrap(app, env) {
 
   if (app.token && !isTokenExpired(app.token, 0)) {
     ss.removeItem('oauth_shared_sql'); // consumed
+    // Resolve config first so the header shows the real CH identity (the
+    // ch_auth=basic username, not the raw email claim) on first paint.
+    await app.ensureConfig();
     app.renderApp();
   } else {
     app.showLogin(callbackError);
diff --git a/src/net/oauth-config.js b/src/net/oauth-config.js
index fd71532d..199efa86 100644
--- a/src/net/oauth-config.js
+++ b/src/net/oauth-config.js
@@ -30,9 +30,15 @@ function normalizeEntry(e) {
     // or 'access_token' (audience-gated CH).
     bearer: e.bearer === 'access_token' ? 'access_token' : 'id_token',
     // How the token reaches ClickHouse: 'bearer' (default; Authorization: Bearer
-    // <jwt>) or 'basic' (Authorization: Basic base64(email:jwt), for OSS CH
+    // <jwt>) or 'basic' (Authorization: Basic base64(user:jwt), for OSS CH
     // behind a verifier such as ch-jwt-verify).
     chAuth: e.ch_auth === 'basic' ? 'basic' : 'bearer',
+    // For ch_auth=basic, which JWT claim becomes the Basic username (= the CH
+    // user the verifier must return). Empty → default chain (email →
+    // preferred_username → sub). Set e.g. 'nickname' when an IdP must map to a
+    // CH username distinct from another IdP's (avoids same-name collisions —
+    // e.g. a token-directory Bearer user vs. a static http user on Antalya CH).
+    basicUserClaim: e.basic_user_claim || '',
     // Extra params merged into /authorize (e.g. Auth0 { organization: 'org_…' }).
     authorizeParams: e.authorize_params && typeof e.authorize_params === 'object'
       ? e.authorize_params
diff --git a/src/ui/app.js b/src/ui/app.js
index 2bb21351..5ad4723e 100644
--- a/src/ui/app.js
+++ b/src/ui/app.js
@@ -70,10 +70,16 @@ export function createApp(env = {}) {
   app.host = () => loc.host || 'clickhouse';
   app.activeTab = () => activeTab(app.state);
   app.isSignedIn = () => !!app.token && !isTokenExpired(app.token, 0);
-  app.email = () => {
-    const p = decodeJwtPayload(app.token);
-    return p.email || p.preferred_username || p.sub || '';
-  };
+  // The CH-facing identity for the current token — what currentUser() will be:
+  // for ch_auth=basic it's the Basic username (honouring basicUserClaim); for
+  // bearer it's the email the token-processor keys on. Shared by authHeader and
+  // the header display so the UI never shows a different claim than CH sees.
+  function chUsername(p) {
+    return (app.chAuth === 'basic' && app.basicUserClaim && p[app.basicUserClaim])
+      || p.email || p.preferred_username || p.sub || '';
+  }
+  app.chUsername = chUsername;
+  app.email = () => chUsername(decodeJwtPayload(app.token));
 
   function setTokens(id, refresh) {
     app.token = id;
@@ -137,10 +143,12 @@ export function createApp(env = {}) {
   // (OSS + a verifier like ch-jwt-verify, where the JWT is the Basic password
   // and the username is the token's email). Resolved from config by ensureConfig.
   app.chAuth = 'bearer';
+  // Which claim becomes the Basic username (per-IdP, from config). Empty → the
+  // default chain. Lets one IdP map to a CH username distinct from another's.
+  app.basicUserClaim = '';
   function authHeader(token) {
     if (app.chAuth !== 'basic') return 'Bearer ' + token;
-    const p = decodeJwtPayload(token);
-    const user = p.email || p.preferred_username || p.sub || '';
+    const user = chUsername(decodeJwtPayload(token));
     return 'Basic ' + btoa(unescape(encodeURIComponent(user + ':' + token)));
   }
   const chCtx = {
@@ -160,6 +168,7 @@ export function createApp(env = {}) {
     try {
       const cfg = await resolveConfig();
       app.chAuth = cfg.chAuth;
+      app.basicUserClaim = cfg.basicUserClaim || '';
       return cfg;
     } catch {
       return null;
diff --git a/tests/unit/app.test.js b/tests/unit/app.test.js
index 5090f761..69d7739f 100644
--- a/tests/unit/app.test.js
+++ b/tests/unit/app.test.js
@@ -648,6 +648,31 @@ describe('exhaustive controller coverage', () => {
     expect(decodeURIComponent(escape(atob(auth.slice(6))))).toMatch(/^me@example\.com:/);
   });
 
+  it('ch_auth=basic with basic_user_claim maps the Basic username to that claim', async () => {
+    const tok = jwt({ email: 'me@example.com', nickname: 'BorisT', exp: Math.floor(Date.now() / 1000) + 3600 });
+    const e = env({
+      window: fakeWin(),
+      sessionStorage: memSession({ oauth_id_token: tok }),
+      fetch: makeFetch([
+        [(u) => /config\.json/.test(u), resp({ json: { issuer: 'https://accounts.google.com', client_id: 'cid', ch_auth: 'basic', basic_user_claim: 'nickname' } })],
+        [(u) => /openid-configuration/.test(u), resp({ json: { authorization_endpoint: 'https://a', token_endpoint: 'https://t' } })],
+        [(u, sql) => /SELECT 1/.test(sql), resp({ body: streamBody(['{"row":{}}\n']) })],
+      ]),
+    });
+    const app = createApp(e);
+    app.renderApp();
+    await app.ensureConfig();
+    expect(app.basicUserClaim).toBe('nickname');
+    app.activeTab().sql = 'SELECT 1';
+    await app.actions.run();
+    const q = e.fetch.mock.calls.find((c) => c[1] && c[1].body === 'SELECT 1');
+    const auth = q[1].headers.Authorization;
+    // username segment is the nickname claim, not the email
+    expect(decodeURIComponent(escape(atob(auth.slice(6))))).toMatch(/^BorisT:/);
+    // the header identity matches the CH user (nickname), not the email claim
+    expect(app.email()).toBe('BorisT');
+  });
+
   it('shows and dismisses the auth-failure banner', () => {
     const app = createApp(env());
     app.renderApp();
diff --git a/tests/unit/main.test.js b/tests/unit/main.test.js
index ed7318f2..61002c46 100644
--- a/tests/unit/main.test.js
+++ b/tests/unit/main.test.js
@@ -12,6 +12,7 @@ function fakeApp(over = {}) {
     token: null,
     state: { tabs: [{ id: 't1', sql: '', name: 'Untitled' }] },
     loadConfig: vi.fn(async () => ({ clientId: 'c', tokenUri: 'https://t', clientSecret: '' })),
+    ensureConfig: vi.fn(async () => ({})),
     setTokens: vi.fn(function (id) { this.token = id; }),
     renderApp: vi.fn(),
     showLogin: vi.fn(),
diff --git a/tests/unit/oauth-config.test.js b/tests/unit/oauth-config.test.js
index 7661dfee..11fc5865 100644
--- a/tests/unit/oauth-config.test.js
+++ b/tests/unit/oauth-config.test.js
@@ -32,6 +32,7 @@ describe('loadConfigDoc', () => {
       id: 'accounts.google.com', label: 'accounts.google.com',
       issuer: 'https://accounts.google.com', clientId: 'cid', clientSecret: 'sek',
       audience: 'aud', bearer: 'id_token', chAuth: 'bearer', authorizeParams: {},
+      basicUserClaim: '',
     }]);
     expect(f.mock.calls[0][0]).toBe('/sql/config.json');
   });
@@ -59,14 +60,16 @@ describe('loadConfigDoc', () => {
     expect(idp.bearer).toBe('id_token');
     expect(idp.chAuth).toBe('bearer');
     expect(idp.authorizeParams).toEqual({});
+    expect(idp.basicUserClaim).toBe('');
   });
-  it('honours ch_auth=basic, bearer=access_token, and an authorize_params object', async () => {
+  it('honours ch_auth=basic, bearer=access_token, basic_user_claim, and an authorize_params object', async () => {
     const [idp] = await docOf({
       issuer: 'https://i', client_id: 'c', ch_auth: 'basic', bearer: 'access_token',
-      authorize_params: { organization: 'org_x' },
+      basic_user_claim: 'nickname', authorize_params: { organization: 'org_x' },
     });
     expect(idp.chAuth).toBe('basic');
     expect(idp.bearer).toBe('access_token');
+    expect(idp.basicUserClaim).toBe('nickname');
     expect(idp.authorizeParams).toEqual({ organization: 'org_x' });
   });
   it('ignores a non-object authorize_params and an unknown bearer', async () => {