diff --git a/api/server/routes/oauth.js b/api/server/routes/oauth.js
index 5302158031..d5e4c939ea 100644
--- a/api/server/routes/oauth.js
+++ b/api/server/routes/oauth.js
@@ -4,7 +4,13 @@ const passport = require('passport');
 const { randomState } = require('openid-client');
 const { logger } = require('@librechat/data-schemas');
 const { ErrorTypes } = require('librechat-data-provider');
-const { createSetBalanceConfig } = require('@librechat/api');
+const {
+  buildOAuthFailureLog,
+  createOpenIDCallbackAuthenticator,
+  createSetBalanceConfig,
+  getOAuthFailureMessage,
+  redirectToAuthFailure,
+} = require('@librechat/api');
 const { checkDomainAllowed, loginLimiter, logHeaders } = require('~/server/middleware');
 const { createOAuthHandler } = require('~/server/controllers/auth/oauth');
 const { findBalanceByUser, upsertBalanceFields } = require('~/models');
@@ -23,19 +29,35 @@ const domains = {
   server: process.env.DOMAIN_SERVER,
 };
 
+const authFailureRedirectOptions = {
+  clientDomain: domains.client,
+  authFailedError: ErrorTypes.AUTH_FAILED,
+};
+
 router.use(logHeaders);
 router.use(loginLimiter);
 
 const oauthHandler = createOAuthHandler();
+const authenticateOpenIDCallback = createOpenIDCallbackAuthenticator({
+  passport,
+  logger,
+  ...authFailureRedirectOptions,
+});
 
 router.get('/error', (req, res) => {
   /** A single error message is pushed by passport when authentication fails. */
-  const errorMessage = req.session?.messages?.pop() || 'Unknown OAuth error';
-  logger.error('Error in OAuth authentication:', {
-    message: errorMessage,
-  });
+  const errorMessage = getOAuthFailureMessage(req);
+  logger.warn(
+    '[OAuth] Authentication failed',
+    buildOAuthFailureLog({
+      provider: 'unknown',
+      req,
+      info: { message: errorMessage },
+      defaultMessage: errorMessage,
+    }),
+  );
 
-  res.redirect(`${domains.client}/login?redirect=false&error=${ErrorTypes.AUTH_FAILED}`);
+  redirectToAuthFailure(res, authFailureRedirectOptions);
 });
 
 /**
@@ -100,11 +122,7 @@ router.get('/openid', (req, res, next) => {
 
 router.get(
   '/openid/callback',
-  passport.authenticate('openid', {
-    failureRedirect: `${domains.client}/oauth/error`,
-    failureMessage: true,
-    session: false,
-  }),
+  authenticateOpenIDCallback,
   setBalanceConfig,
   checkDomainAllowed,
   oauthHandler,
diff --git a/api/server/routes/oauth.test.js b/api/server/routes/oauth.test.js
new file mode 100644
index 0000000000..20661a9084
--- /dev/null
+++ b/api/server/routes/oauth.test.js
@@ -0,0 +1,190 @@
+const express = require('express');
+const request = require('supertest');
+
+const originalDomainClient = process.env.DOMAIN_CLIENT;
+process.env.DOMAIN_CLIENT = 'http://client.test';
+
+const mockLogger = {
+  warn: jest.fn(),
+  error: jest.fn(),
+  info: jest.fn(),
+  debug: jest.fn(),
+};
+
+const mockOAuthHandler = jest.fn((_req, res) => res.status(204).end());
+const mockOpenIDCallbackMiddleware = jest.fn((_req, _res, next) => next());
+let mockOpenIDCallbackAuthenticatorOptions;
+const mockCreateOpenIDCallbackAuthenticator = jest.fn((options) => {
+  mockOpenIDCallbackAuthenticatorOptions = options;
+  return mockOpenIDCallbackMiddleware;
+});
+const mockBuildOAuthFailureLog = jest.fn(({ provider, req, err, info, defaultMessage }) => ({
+  provider,
+  code: err?.code ?? info?.code ?? info?.error ?? req.query?.error,
+  name: err?.name ?? info?.name,
+  message:
+    err?.message ??
+    info?.message ??
+    info?.error_description ??
+    req.query?.error_description ??
+    defaultMessage,
+  cause_code: err?.cause?.code ?? info?.cause?.code,
+  cause_name: err?.cause?.name ?? info?.cause?.name,
+  has_code: req.query?.code != null,
+  has_state: req.query?.state != null,
+  query_error: req.query?.error,
+  query_error_description: req.query?.error_description,
+  path: req.path,
+  forwarded_for: req.headers?.['x-forwarded-for'],
+  user_agent: req.headers?.['user-agent'],
+}));
+const mockGetOAuthFailureMessage = jest.fn(
+  (req) =>
+    req.session?.messages?.pop() ??
+    req.query?.error_description ??
+    req.query?.error ??
+    'OAuth authentication failed',
+);
+const mockRedirectToAuthFailure = jest.fn((res, { clientDomain, authFailedError }) =>
+  res.redirect(`${clientDomain}/login?redirect=false&error=${authFailedError}`),
+);
+const mockPassportAuthenticate = jest.fn(() => (_req, _res, next) => next());
+
+jest.mock('passport', () => ({
+  authenticate: (...args) => mockPassportAuthenticate(...args),
+}));
+
+jest.mock('openid-client', () => ({
+  randomState: jest.fn(() => 'random-state'),
+}));
+
+jest.mock('@librechat/data-schemas', () => ({
+  logger: mockLogger,
+}));
+
+jest.mock('librechat-data-provider', () => ({
+  ErrorTypes: {
+    AUTH_FAILED: 'auth_failed',
+  },
+}));
+
+jest.mock(
+  '@librechat/api',
+  () => ({
+    buildOAuthFailureLog: (...args) => mockBuildOAuthFailureLog(...args),
+    createOpenIDCallbackAuthenticator: (...args) => mockCreateOpenIDCallbackAuthenticator(...args),
+    createSetBalanceConfig: jest.fn(() => (_req, _res, next) => next()),
+    getOAuthFailureMessage: (...args) => mockGetOAuthFailureMessage(...args),
+    redirectToAuthFailure: (...args) => mockRedirectToAuthFailure(...args),
+  }),
+  { virtual: true },
+);
+
+jest.mock('~/server/middleware', () => ({
+  checkDomainAllowed: jest.fn((_req, _res, next) => next()),
+  loginLimiter: jest.fn((_req, _res, next) => next()),
+  logHeaders: jest.fn((_req, _res, next) => next()),
+}));
+
+jest.mock('~/server/controllers/auth/oauth', () => ({
+  createOAuthHandler: jest.fn(() => mockOAuthHandler),
+}));
+
+jest.mock('~/models', () => ({
+  findBalanceByUser: jest.fn(),
+  upsertBalanceFields: jest.fn(),
+}));
+
+jest.mock('~/server/services/Config', () => ({
+  getAppConfig: jest.fn(),
+}));
+
+const oauthRouter = require('./oauth');
+
+afterAll(() => {
+  if (originalDomainClient === undefined) {
+    delete process.env.DOMAIN_CLIENT;
+    return;
+  }
+  process.env.DOMAIN_CLIENT = originalDomainClient;
+});
+
+function createApp(sessionMessages) {
+  const app = express();
+  app.use((req, _res, next) => {
+    if (sessionMessages) {
+      req.session = { messages: [...sessionMessages] };
+    }
+    next();
+  });
+  app.use('/oauth', oauthRouter);
+  app.use((err, _req, res, _next) => {
+    res.status(500).json({ message: err.message });
+  });
+  return app;
+}
+
+describe('OAuth route failure logging', () => {
+  beforeEach(() => {
+    mockLogger.warn.mockClear();
+    mockLogger.error.mockClear();
+    mockLogger.info.mockClear();
+    mockLogger.debug.mockClear();
+    mockOAuthHandler.mockClear();
+    mockOpenIDCallbackMiddleware.mockClear();
+    mockBuildOAuthFailureLog.mockClear();
+    mockGetOAuthFailureMessage.mockClear();
+    mockRedirectToAuthFailure.mockClear();
+    mockPassportAuthenticate.mockClear();
+    mockPassportAuthenticate.mockImplementation(() => (_req, _res, next) => next());
+    mockOpenIDCallbackMiddleware.mockImplementation((_req, _res, next) => next());
+  });
+
+  it('wires the package OpenID callback middleware into the route', async () => {
+    const app = createApp();
+
+    await request(app)
+      .get('/oauth/openid/callback?code=secret-code&state=secret-state')
+      .expect(204);
+
+    expect(mockOpenIDCallbackAuthenticatorOptions).toEqual({
+      passport: expect.objectContaining({ authenticate: expect.any(Function) }),
+      logger: mockLogger,
+      clientDomain: 'http://client.test',
+      authFailedError: 'auth_failed',
+    });
+    expect(mockOpenIDCallbackMiddleware).toHaveBeenCalledWith(
+      expect.any(Object),
+      expect.any(Object),
+      expect.any(Function),
+    );
+    expect(mockOAuthHandler).toHaveBeenCalled();
+  });
+
+  it('logs structured fallback errors without using Unknown OAuth error', async () => {
+    const app = createApp();
+
+    const response = await request(app)
+      .get('/oauth/error?error=access_denied&error_description=Denied%20by%20provider')
+      .set('x-forwarded-for', '203.0.113.10')
+      .expect(302);
+
+    expect(response.headers.location).toBe(
+      'http://client.test/login?redirect=false&error=auth_failed',
+    );
+    expect(mockLogger.warn).toHaveBeenCalledWith(
+      '[OAuth] Authentication failed',
+      expect.objectContaining({
+        provider: 'unknown',
+        code: 'access_denied',
+        message: 'Denied by provider',
+        query_error: 'access_denied',
+        query_error_description: 'Denied by provider',
+        has_code: false,
+        has_state: false,
+        forwarded_for: '203.0.113.10',
+      }),
+    );
+    expect(JSON.stringify(mockLogger.warn.mock.calls[0])).not.toContain('Unknown OAuth error');
+  });
+});
diff --git a/packages/api/src/oauth/callback.spec.ts b/packages/api/src/oauth/callback.spec.ts
new file mode 100644
index 0000000000..981b21defc
--- /dev/null
+++ b/packages/api/src/oauth/callback.spec.ts
@@ -0,0 +1,274 @@
+import type { NextFunction, Response } from 'express';
+import {
+  createOpenIDCallbackAuthenticator,
+  logOpenIDCallbackFailure,
+  redirectToAuthFailure,
+  type OpenIDCallbackRequest,
+  type OpenIDCallbackAuthenticatorOptions,
+} from './callback';
+
+type CallbackFn = (err: unknown, user: unknown, info: unknown) => void;
+type TestRequest = OpenIDCallbackRequest;
+
+const logger = {
+  warn: jest.fn(),
+  error: jest.fn(),
+};
+
+function createRequest(overrides: Partial<TestRequest> = {}): TestRequest {
+  return {
+    headers: {},
+    method: 'GET',
+    path: '/openid/callback',
+    originalUrl: '/openid/callback',
+    query: {},
+    ...overrides,
+  } as TestRequest;
+}
+
+function createResponse(): Response {
+  return {
+    redirect: jest.fn(),
+  } as unknown as Response;
+}
+
+function createNext(): jest.MockedFunction<NextFunction> {
+  return jest.fn() as jest.MockedFunction<NextFunction>;
+}
+
+function createAuthenticator(
+  callbackHandler: (
+    callback: CallbackFn,
+    req: TestRequest,
+    res: Response,
+    next: NextFunction,
+  ) => void,
+) {
+  const passport = {
+    authenticate: jest.fn((_strategy: 'openid', _options, callback: CallbackFn) => {
+      return (req: TestRequest, res: Response, next: NextFunction) => {
+        callbackHandler(callback, req, res, next);
+      };
+    }),
+  };
+  const options: OpenIDCallbackAuthenticatorOptions = {
+    passport,
+    logger,
+    clientDomain: 'http://client.test',
+    authFailedError: 'auth_failed',
+  };
+
+  return {
+    middleware: createOpenIDCallbackAuthenticator(options),
+    passport,
+  };
+}
+
+describe('OpenID OAuth callback helpers', () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it('redirects failed auth attempts to the login failure URL', () => {
+    const res = createResponse();
+
+    redirectToAuthFailure(res, {
+      clientDomain: 'http://client.test',
+      authFailedError: 'auth_failed',
+    });
+
+    expect(res.redirect).toHaveBeenCalledWith(
+      'http://client.test/login?redirect=false&error=auth_failed',
+    );
+  });
+
+  it('logs OpenID callback failures with structured OAuth context', () => {
+    const req = createRequest({
+      query: {
+        code: 'secret-code',
+        state: 'secret-state',
+      },
+    });
+    const error = Object.assign(new Error('invalid response encountered'), {
+      code: 'OAUTH_INVALID_RESPONSE',
+      name: 'ClientError',
+    });
+
+    logOpenIDCallbackFailure({
+      logger,
+      req,
+      err: error,
+      info: { message: 'provider info' },
+    });
+
+    expect(logger.warn).toHaveBeenCalledWith(
+      '[OpenID OAuth] Callback authentication failed',
+      expect.objectContaining({
+        provider: 'openid',
+        code: 'OAUTH_INVALID_RESPONSE',
+        name: 'ClientError',
+        message: 'invalid response encountered',
+        has_code: true,
+        has_state: true,
+      }),
+    );
+  });
+
+  it('continues the successful callback path after logging in without a session', () => {
+    const user = { id: 'user-1' };
+    const req = createRequest();
+    const res = createResponse();
+    const next = createNext();
+    const logIn = jest.fn((loginUser, _options, done) => {
+      req.user = loginUser;
+      done();
+    });
+    req.logIn = logIn;
+    const { middleware, passport } = createAuthenticator((callback) =>
+      callback(null, user, { message: 'ok' }),
+    );
+
+    middleware(req, res, next);
+
+    expect(passport.authenticate).toHaveBeenCalledWith(
+      'openid',
+      { failureMessage: true, session: false },
+      expect.any(Function),
+    );
+    expect(logIn).toHaveBeenCalledWith(user, { session: false }, expect.any(Function));
+    expect(req.user).toBe(user);
+    expect(next).toHaveBeenCalledWith();
+    expect(logger.warn).not.toHaveBeenCalled();
+    expect(logger.error).not.toHaveBeenCalled();
+  });
+
+  it('sets req.user and continues when req.logIn is unavailable', () => {
+    const user = { id: 'user-1' };
+    const req = createRequest();
+    const res = createResponse();
+    const next = createNext();
+    const { middleware } = createAuthenticator((callback) => callback(null, user, undefined));
+
+    middleware(req, res, next);
+
+    expect(req.user).toBe(user);
+    expect(next).toHaveBeenCalledWith();
+  });
+
+  it('logs OpenID protocol failures and redirects without escalating', () => {
+    const req = createRequest({
+      query: {
+        code: 'secret-code',
+        state: 'secret-state',
+      },
+    });
+    const res = createResponse();
+    const next = createNext();
+    const error = Object.assign(new Error('invalid response encountered'), {
+      code: 'OAUTH_INVALID_RESPONSE',
+      name: 'ClientError',
+    });
+    const { middleware } = createAuthenticator((callback) => callback(error, false, undefined));
+
+    middleware(req, res, next);
+
+    expect(logger.warn).toHaveBeenCalledWith(
+      '[OpenID OAuth] Callback authentication failed',
+      expect.objectContaining({
+        provider: 'openid',
+        code: 'OAUTH_INVALID_RESPONSE',
+        name: 'ClientError',
+        message: 'invalid response encountered',
+        has_code: true,
+        has_state: true,
+      }),
+    );
+    expect(res.redirect).toHaveBeenCalledWith(
+      'http://client.test/login?redirect=false&error=auth_failed',
+    );
+    expect(next).not.toHaveBeenCalled();
+  });
+
+  it('logs unexpected OpenID errors with context before escalating', () => {
+    const req = createRequest({
+      query: {
+        code: 'secret-code',
+        state: 'secret-state',
+      },
+    });
+    const res = createResponse();
+    const next = createNext();
+    const error = Object.assign(new Error('database exploded'), {
+      name: 'DatabaseError',
+    });
+    const { middleware } = createAuthenticator((callback) => callback(error, false, undefined));
+
+    middleware(req, res, next);
+
+    expect(logger.error).toHaveBeenCalledWith(
+      '[OpenID OAuth] Callback authentication error',
+      expect.objectContaining({
+        provider: 'openid',
+        name: 'DatabaseError',
+        message: 'database exploded',
+        has_code: true,
+        has_state: true,
+      }),
+    );
+    expect(next).toHaveBeenCalledWith(error);
+    expect(res.redirect).not.toHaveBeenCalled();
+  });
+
+  it('logs Passport info failures and redirects without escalating', () => {
+    const req = createRequest();
+    const res = createResponse();
+    const next = createNext();
+    const { middleware } = createAuthenticator((callback) =>
+      callback(null, false, {
+        code: 'DOMAIN_DENIED',
+        message: 'Email domain not allowed',
+      }),
+    );
+
+    middleware(req, res, next);
+
+    expect(logger.warn).toHaveBeenCalledWith(
+      '[OpenID OAuth] Callback authentication failed',
+      expect.objectContaining({
+        provider: 'openid',
+        code: 'DOMAIN_DENIED',
+        message: 'Email domain not allowed',
+      }),
+    );
+    expect(res.redirect).toHaveBeenCalledWith(
+      'http://client.test/login?redirect=false&error=auth_failed',
+    );
+    expect(next).not.toHaveBeenCalled();
+  });
+
+  it('logs login errors and sends them to the error handler', () => {
+    const user = { id: 'user-1' };
+    const req = createRequest();
+    const res = createResponse();
+    const next = createNext();
+    const error = Object.assign(new Error('login failed'), {
+      name: 'LoginError',
+    });
+    req.logIn = jest.fn((_loginUser, _options, done) => done(error));
+    const { middleware } = createAuthenticator((callback) =>
+      callback(null, user, { message: 'provider info' }),
+    );
+
+    middleware(req, res, next);
+
+    expect(logger.error).toHaveBeenCalledWith(
+      '[OpenID OAuth] Callback authentication error',
+      expect.objectContaining({
+        provider: 'openid',
+        name: 'LoginError',
+        message: 'login failed',
+      }),
+    );
+    expect(next).toHaveBeenCalledWith(error);
+  });
+});
diff --git a/packages/api/src/oauth/callback.ts b/packages/api/src/oauth/callback.ts
new file mode 100644
index 0000000000..4570c91e6a
--- /dev/null
+++ b/packages/api/src/oauth/callback.ts
@@ -0,0 +1,140 @@
+import type { NextFunction, Response } from 'express';
+import {
+  buildOAuthFailureLog,
+  isOAuthProtocolFailure,
+  type OAuthFailureLog,
+  type OAuthFailureRequest,
+} from './failure';
+
+type LoginFunction = (
+  user: unknown,
+  options: { session: false },
+  done: (err?: unknown) => void,
+) => void;
+
+export type OpenIDCallbackRequest = OAuthFailureRequest & {
+  logIn?: LoginFunction;
+  user?: unknown;
+};
+
+type OpenIDCallback = (err: unknown, user: unknown, info: unknown) => void;
+
+type PassportMiddleware = (
+  req: OpenIDCallbackRequest,
+  res: Response,
+  next: NextFunction,
+) => unknown;
+
+type PassportLike = {
+  authenticate: (
+    strategy: 'openid',
+    options: {
+      failureMessage: true;
+      session: false;
+    },
+    callback: OpenIDCallback,
+  ) => PassportMiddleware;
+};
+
+type OAuthCallbackLogLevel = 'warn' | 'error';
+
+type OAuthCallbackLogger = Record<
+  OAuthCallbackLogLevel,
+  (message: string, details: OAuthFailureLog) => void
+>;
+
+export type AuthFailureRedirectOptions = {
+  clientDomain?: string;
+  authFailedError: string;
+};
+
+export type LogOpenIDCallbackFailureOptions = {
+  logger: OAuthCallbackLogger;
+  req: OAuthFailureRequest;
+  err?: unknown;
+  info?: unknown;
+  level?: OAuthCallbackLogLevel;
+};
+
+export type OpenIDCallbackAuthenticatorOptions = AuthFailureRedirectOptions & {
+  logger: OAuthCallbackLogger;
+  passport: PassportLike;
+};
+
+export function redirectToAuthFailure(
+  res: Response,
+  { clientDomain, authFailedError }: AuthFailureRedirectOptions,
+): void {
+  res.redirect(`${clientDomain}/login?redirect=false&error=${authFailedError}`);
+}
+
+export function logOpenIDCallbackFailure({
+  logger,
+  req,
+  err,
+  info,
+  level = 'warn',
+}: LogOpenIDCallbackFailureOptions): void {
+  logger[level](
+    level === 'error'
+      ? '[OpenID OAuth] Callback authentication error'
+      : '[OpenID OAuth] Callback authentication failed',
+    buildOAuthFailureLog({
+      provider: 'openid',
+      req,
+      err,
+      info,
+      defaultMessage: 'OpenID authentication failed',
+    }),
+  );
+}
+
+export function createOpenIDCallbackAuthenticator({
+  passport,
+  logger,
+  clientDomain,
+  authFailedError,
+}: OpenIDCallbackAuthenticatorOptions): (
+  req: OpenIDCallbackRequest,
+  res: Response,
+  next: NextFunction,
+) => unknown {
+  return (req: OpenIDCallbackRequest, res: Response, next: NextFunction): unknown => {
+    return passport.authenticate(
+      'openid',
+      {
+        failureMessage: true,
+        session: false,
+      },
+      (err: unknown, user: unknown, info: unknown) => {
+        if (err) {
+          if (isOAuthProtocolFailure(err, info)) {
+            logOpenIDCallbackFailure({ logger, req, err, info });
+            return redirectToAuthFailure(res, { clientDomain, authFailedError });
+          }
+
+          logOpenIDCallbackFailure({ logger, req, err, info, level: 'error' });
+          return next(err);
+        }
+
+        if (!user) {
+          logOpenIDCallbackFailure({ logger, req, err, info });
+          return redirectToAuthFailure(res, { clientDomain, authFailedError });
+        }
+
+        if (typeof req.logIn !== 'function') {
+          req.user = user;
+          return next();
+        }
+
+        return req.logIn(user, { session: false }, (loginErr?: unknown) => {
+          if (loginErr) {
+            logOpenIDCallbackFailure({ logger, req, err: loginErr, info, level: 'error' });
+            return next(loginErr);
+          }
+          return next();
+        });
+      },
+    )(req, res, next);
+  };
+}
diff --git a/packages/api/src/oauth/failure.spec.ts b/packages/api/src/oauth/failure.spec.ts
new file mode 100644
index 0000000000..c7178030bd
--- /dev/null
+++ b/packages/api/src/oauth/failure.spec.ts
@@ -0,0 +1,139 @@
+import { buildOAuthFailureLog, getOAuthFailureMessage, isOAuthProtocolFailure } from './failure';
+import type { OAuthFailureRequest } from './failure';
+
+function createRequest(overrides: Partial<OAuthFailureRequest> = {}): OAuthFailureRequest {
+  return {
+    headers: {},
+    method: 'GET',
+    path: '/openid/callback',
+    originalUrl: '/openid/callback',
+    query: {},
+    ...overrides,
+  };
+}
+
+describe('OAuth failure logging helpers', () => {
+  it('prefers session failure messages and removes the consumed message', () => {
+    const req = createRequest({
+      session: {
+        messages: ['first', 'latest'],
+      },
+    });
+
+    expect(getOAuthFailureMessage(req)).toBe('latest');
+    expect(req.session?.messages).toEqual(['first']);
+  });
+
+  it('falls back to provider query error details without returning Unknown OAuth error', () => {
+    const req = createRequest({
+      query: {
+        error: 'access_denied',
+        error_description: 'Denied by provider',
+      },
+    });
+
+    expect(getOAuthFailureMessage(req)).toBe('Denied by provider');
+  });
+
+  it('logs OpenID protocol failure metadata without raw code or state values', () => {
+    const req = createRequest({
+      headers: {
+        host: 'chat.example.com',
+        'x-forwarded-for': '203.0.113.10',
+        'x-forwarded-proto': 'https',
+        'user-agent': 'test-agent',
+      },
+      id: 'request-id',
+      originalUrl: '/openid/callback?code=secret-code&state=secret-state',
+      query: {
+        code: 'secret-code',
+        state: 'secret-state',
+      },
+    });
+    const error = Object.assign(new Error('invalid response encountered'), {
+      code: 'OAUTH_INVALID_RESPONSE',
+      name: 'ClientError',
+      cause: {
+        code: 'OAUTH_INVALID_RESPONSE',
+        name: 'OperationProcessingError',
+        message: 'invalid response encountered',
+      },
+    });
+
+    const log = buildOAuthFailureLog({
+      provider: 'openid',
+      req,
+      err: error,
+      defaultMessage: 'OpenID authentication failed',
+    });
+
+    expect(log).toEqual({
+      provider: 'openid',
+      code: 'OAUTH_INVALID_RESPONSE',
+      name: 'ClientError',
+      message: 'invalid response encountered',
+      cause_code: 'OAUTH_INVALID_RESPONSE',
+      cause_name: 'OperationProcessingError',
+      cause_message: 'invalid response encountered',
+      has_code: true,
+      has_state: true,
+      method: 'GET',
+      path: '/openid/callback',
+      request_id: 'request-id',
+      host: 'chat.example.com',
+      forwarded_proto: 'https',
+      forwarded_for: '203.0.113.10',
+      user_agent: 'test-agent',
+    });
+    expect(JSON.stringify(log)).not.toContain('secret-code');
+    expect(JSON.stringify(log)).not.toContain('secret-state');
+  });
+
+  it('captures provider response error fields from Passport info', () => {
+    const log = buildOAuthFailureLog({
+      provider: 'openid',
+      req: createRequest(),
+      info: {
+        error: 'access_denied',
+        error_description: 'User denied consent',
+      },
+    });
+
+    expect(log).toEqual({
+      provider: 'openid',
+      code: 'access_denied',
+      message: 'User denied consent',
+      has_code: false,
+      has_state: false,
+      method: 'GET',
+      path: '/openid/callback',
+    });
+  });
+
+  it('truncates very long messages', () => {
+    const longMessage = 'x'.repeat(320);
+
+    const log = buildOAuthFailureLog({
+      provider: 'openid',
+      req: createRequest(),
+      info: {
+        message: longMessage,
+      },
+    });
+
+    expect(log.message).toHaveLength(315);
+    expect(log.message?.endsWith('... [truncated]')).toBe(true);
+  });
+
+  it.each([
+    [{ code: 'OAUTH_INVALID_RESPONSE' }, true],
+    [{ name: 'AuthorizationResponseError' }, true],
+    [
+      { cause: { name: 'OperationProcessingError', message: 'invalid response encountered' } },
+      true,
+    ],
+    [{ name: 'DatabaseError', message: 'database exploded' }, false],
+  ])('classifies OAuth protocol failure %j as %s', (error, expected) => {
+    expect(isOAuthProtocolFailure(error)).toBe(expected);
+  });
+});
diff --git a/packages/api/src/oauth/failure.ts b/packages/api/src/oauth/failure.ts
new file mode 100644
index 0000000000..a899ac0ffa
--- /dev/null
+++ b/packages/api/src/oauth/failure.ts
@@ -0,0 +1,258 @@
+import type { Request } from 'express';
+
+const MAX_LOG_VALUE_LENGTH = 300;
+
+type LogValue = string | boolean;
+
+type FailureLike = {
+  code?: unknown;
+  error?: unknown;
+  name?: unknown;
+  message?: unknown;
+  error_description?: unknown;
+  cause?: unknown;
+};
+
+export type OAuthFailureRequest = Pick<
+  Request,
+  'headers' | 'method' | 'path' | 'originalUrl' | 'query'
+> & {
+  id?: string;
+  requestId?: string;
+  session?: {
+    messages?: unknown[];
+  };
+};
+
+export type OAuthFailureLog = {
+  provider: string;
+  code?: string;
+  name?: string;
+  message?: string;
+  cause_code?: string;
+  cause_name?: string;
+  cause_message?: string;
+  has_code: boolean;
+  has_state: boolean;
+  query_error?: string;
+  query_error_description?: string;
+  method?: string;
+  path?: string;
+  request_id?: string;
+  host?: string;
+  forwarded_host?: string;
+  forwarded_proto?: string;
+  forwarded_for?: string;
+  real_ip?: string;
+  user_agent?: string;
+};
+
+export type BuildOAuthFailureLogParams = {
+  provider: string;
+  req: OAuthFailureRequest;
+  err?: unknown;
+  info?: unknown;
+  defaultMessage?: string;
+};
+
+function normalizeLogValue(value: unknown): string | undefined {
+  if (value == null) {
+    return undefined;
+  }
+
+  if (Array.isArray(value)) {
+    for (const entry of value) {
+      const normalized = normalizeLogValue(entry);
+      if (normalized) {
+        return normalized;
+      }
+    }
+    return undefined;
+  }
+
+  if (typeof value === 'string') {
+    const trimmed = value.replace(/\s+/g, ' ').trim();
+    if (!trimmed) {
+      return undefined;
+    }
+    if (trimmed.length <= MAX_LOG_VALUE_LENGTH) {
+      return trimmed;
+    }
+    return `${trimmed.slice(0, MAX_LOG_VALUE_LENGTH)}... [truncated]`;
+  }
+
+  if (typeof value === 'number' || typeof value === 'boolean') {
+    return String(value);
+  }
+
+  return undefined;
+}
+
+function compactLogObject(
+  log: Partial<OAuthFailureLog> & Pick<OAuthFailureLog, 'provider' | 'has_code' | 'has_state'>,
+): OAuthFailureLog {
+  const compacted: Partial<OAuthFailureLog> = {};
+  const keys = Object.keys(log) as Array<keyof OAuthFailureLog>;
+  for (const key of keys) {
+    const value = log[key];
+    if (value !== undefined) {
+      Object.assign(compacted, { [key]: value as LogValue });
+    }
+  }
+  return compacted as OAuthFailureLog;
+}
+
+function getField(source: unknown, field: keyof FailureLike): unknown {
+  if (!source) {
+    return undefined;
+  }
+  if (typeof source === 'string') {
+    return field === 'message' ? source : undefined;
+  }
+  if (typeof source === 'object') {
+    return (source as FailureLike)[field];
+  }
+  return undefined;
+}
+
+function firstLogValue(...values: unknown[]): string | undefined {
+  for (const value of values) {
+    const normalized = normalizeLogValue(value);
+    if (normalized) {
+      return normalized;
+    }
+  }
+  return undefined;
+}
+
+function getCause(source: unknown): unknown {
+  const cause = getField(source, 'cause');
+  return cause && typeof cause === 'object' ? cause : undefined;
+}
+
+function getHeader(req: OAuthFailureRequest, headerName: string): string | undefined {
+  return normalizeLogValue(req.headers?.[headerName]);
+}
+
+function getQueryValue(req: OAuthFailureRequest, queryName: string): string | undefined {
+  return normalizeLogValue(req.query?.[queryName]);
+}
+
+function hasQueryValue(req: OAuthFailureRequest, queryName: string): boolean {
+  return getQueryValue(req, queryName) !== undefined;
+}
+
+function getRequestPath(req: OAuthFailureRequest): string | undefined {
+  return firstLogValue(req.path, req.originalUrl?.split('?')[0]);
+}
+
+function popSessionFailureMessage(req: OAuthFailureRequest): unknown {
+  const messages = req.session?.messages;
+  if (!Array.isArray(messages) || messages.length === 0) {
+    return undefined;
+  }
+  return messages.pop();
+}
+
+export function getOAuthFailureMessage(
+  req: OAuthFailureRequest,
+  defaultMessage = 'OAuth authentication failed',
+): string {
+  return (
+    firstLogValue(
+      popSessionFailureMessage(req),
+      getQueryValue(req, 'error_description'),
+      getQueryValue(req, 'error'),
+      defaultMessage,
+    ) ?? defaultMessage
+  );
+}
+
+export function buildOAuthFailureLog({
+  provider,
+  req,
+  err,
+  info,
+  defaultMessage,
+}: BuildOAuthFailureLogParams): OAuthFailureLog {
+  const errCause = getCause(err);
+  const infoCause = getCause(info);
+  return compactLogObject({
+    provider,
+    code: firstLogValue(
+      getField(err, 'code'),
+      getField(err, 'error'),
+      getField(errCause, 'code'),
+      getField(errCause, 'error'),
+      getField(info, 'code'),
+      getField(info, 'error'),
+      getField(infoCause, 'code'),
+      getField(infoCause, 'error'),
+      getQueryValue(req, 'error'),
+    ),
+    name: firstLogValue(getField(err, 'name'), getField(info, 'name')),
+    message: firstLogValue(
+      getField(err, 'message'),
+      getField(err, 'error_description'),
+      getField(info, 'message'),
+      getField(info, 'error_description'),
+      getQueryValue(req, 'error_description'),
+      getQueryValue(req, 'error'),
+      defaultMessage,
+    ),
+    cause_code: firstLogValue(getField(errCause, 'code'), getField(infoCause, 'code')),
+    cause_name: firstLogValue(getField(errCause, 'name'), getField(infoCause, 'name')),
+    cause_message: firstLogValue(getField(errCause, 'message'), getField(infoCause, 'message')),
+    has_code: hasQueryValue(req, 'code'),
+    has_state: hasQueryValue(req, 'state'),
+    query_error: getQueryValue(req, 'error'),
+    query_error_description: getQueryValue(req, 'error_description'),
+    method: normalizeLogValue(req.method),
+    path: getRequestPath(req),
+    request_id: firstLogValue(req.requestId, req.id, getHeader(req, 'x-request-id')),
+    host: getHeader(req, 'host'),
+    forwarded_host: getHeader(req, 'x-forwarded-host'),
+    forwarded_proto: getHeader(req, 'x-forwarded-proto'),
+    forwarded_for: getHeader(req, 'x-forwarded-for'),
+    real_ip: getHeader(req, 'x-real-ip'),
+    user_agent: getHeader(req, 'user-agent'),
+  });
+}
+
+export function isOAuthProtocolFailure(err?: unknown, info?: unknown): boolean {
+  const errCause = getCause(err);
+  const infoCause = getCause(info);
+  const code = firstLogValue(
+    getField(err, 'code'),
+    getField(err, 'error'),
+    getField(errCause, 'code'),
+    getField(errCause, 'error'),
+    getField(info, 'code'),
+    getField(info, 'error'),
+    getField(infoCause, 'code'),
+    getField(infoCause, 'error'),
+  );
+
+  if (code?.startsWith('OAUTH_')) {
+    return true;
+  }
+
+  const name = firstLogValue(
+    getField(err, 'name'),
+    getField(errCause, 'name'),
+    getField(info, 'name'),
+    getField(infoCause, 'name'),
+  );
+  if (name === 'AuthorizationResponseError') {
+    return true;
+  }
+
+  const message = firstLogValue(
+    getField(err, 'message'),
+    getField(errCause, 'message'),
+    getField(info, 'message'),
+    getField(infoCause, 'message'),
+  );
+
+  return name === 'OperationProcessingError' && /invalid response/i.test(message ?? '');
+}
diff --git a/packages/api/src/oauth/index.ts b/packages/api/src/oauth/index.ts
index f5a7a41334..4c3fea9ea4 100644
--- a/packages/api/src/oauth/index.ts
+++ b/packages/api/src/oauth/index.ts
@@ -1,3 +1,5 @@
 export * from './csrf';
+export * from './callback';
+export * from './failure';
 export * from './tokens';
 export * from './validation';