diff --git a/.env.example b/.env.example
index 0dbeda45..fd61a1af 100644
--- a/.env.example
+++ b/.env.example
@@ -29,7 +29,7 @@ API_RATE_LIMIT=200
 # Increase for environments with many concurrent browser sessions behind shared NAT.
 API_POLLING_RATE_LIMIT=300
 
-# ─── SSO / LDAP Configuration (Admiral) ───────────────────────────
+# ─── SSO / LDAP Configuration ────────────────────────────────────
 
 # LDAP / Active Directory
 SSO_LDAP_ENABLED=false
@@ -59,6 +59,17 @@ SSO_OIDC_OKTA_ISSUER_URL=
 SSO_OIDC_OKTA_CLIENT_ID=
 SSO_OIDC_OKTA_CLIENT_SECRET=
 
+# Custom OIDC (Keycloak, Authentik, Authelia, Zitadel, etc.)
+SSO_OIDC_CUSTOM_ENABLED=false
+SSO_OIDC_CUSTOM_DISPLAY_NAME=Custom OIDC
+SSO_OIDC_CUSTOM_ISSUER_URL=
+SSO_OIDC_CUSTOM_CLIENT_ID=
+SSO_OIDC_CUSTOM_CLIENT_SECRET=
+SSO_OIDC_CUSTOM_SCOPES=openid email profile
+SSO_OIDC_CUSTOM_ID_CLAIM=
+SSO_OIDC_CUSTOM_USERNAME_CLAIM=
+SSO_OIDC_CUSTOM_EMAIL_CLAIM=
+
 # Role mapping (shared across OIDC providers)
 SSO_OIDC_ADMIN_CLAIM=groups
 SSO_OIDC_ADMIN_CLAIM_VALUE=sencho-admins
diff --git a/backend/src/__tests__/sso.test.ts b/backend/src/__tests__/sso.test.ts
index 94a8eb92..400712d5 100644
--- a/backend/src/__tests__/sso.test.ts
+++ b/backend/src/__tests__/sso.test.ts
@@ -2,6 +2,7 @@ import { describe, it, expect, beforeAll, afterAll, vi } from 'vitest';
 import { setupTestDb, cleanupTestDb, TEST_JWT_SECRET } from './helpers/setupTestDb';
 import supertest from 'supertest';
 import jwt from 'jsonwebtoken';
+import crypto from 'crypto';
 import type { Express } from 'express';
 
 let tmpDir: string;
@@ -505,4 +506,120 @@ describe('SSO Claim Mapping', () => {
     // Claim present but no match
     expect(resolve({ roles: 'user-role' }, { oidcAdminClaim: 'roles', oidcAdminClaimValue: 'admin-role', oidcDefaultRole: 'viewer' })).toBe('viewer');
   });
+
+  it('resolveRoleFromOidc handles edge cases', async () => {
+    const { SSOService } = await import('../services/SSOService');
+    const sso = SSOService.getInstance();
+    const resolve = (sso as unknown as {
+      resolveRoleFromOidc: (userInfo: Record<string, unknown>, config: { oidcAdminClaim?: string; oidcAdminClaimValue?: string; oidcDefaultRole?: string }) => string;
+    }).resolveRoleFromOidc.bind(sso);
+
+    // Empty oidcAdminClaimValue falls back to default 'sencho-admins' via || operator,
+    // so a matching claim still resolves to admin
+    expect(resolve({ groups: ['sencho-admins'] }, { oidcAdminClaim: 'groups', oidcAdminClaimValue: '', oidcDefaultRole: 'viewer' })).toBe('admin');
+
+    // Claim exists but is an empty array (no match possible)
+    expect(resolve({ groups: [] }, { oidcAdminClaim: 'groups', oidcAdminClaimValue: 'sencho-admins', oidcDefaultRole: 'viewer' })).toBe('viewer');
+
+    // Claim exists but is an empty string (no match)
+    expect(resolve({ groups: '' }, { oidcAdminClaim: 'groups', oidcAdminClaimValue: 'sencho-admins', oidcDefaultRole: 'viewer' })).toBe('viewer');
+
+    // Claim value is a number (should be coerced to string for comparison)
+    expect(resolve({ role_id: 42 }, { oidcAdminClaim: 'role_id', oidcAdminClaimValue: '42', oidcDefaultRole: 'viewer' })).toBe('admin');
+  });
+});
+
+describe('SSO Config - Role Enforcement', () => {
+  let viewerToken: string;
+
+  beforeAll(async () => {
+    // Create a viewer user in the DB so the auth middleware can resolve it
+    const { DatabaseService } = await import('../services/DatabaseService');
+    const db = DatabaseService.getInstance();
+    if (!db.getUserByUsername('sso_test_viewer')) {
+      db.addUser({ username: 'sso_test_viewer', password_hash: '$2b$10$fake', role: 'viewer' });
+    }
+    viewerToken = jwt.sign({ username: 'sso_test_viewer', role: 'viewer' }, TEST_JWT_SECRET, { expiresIn: '1h' });
+  });
+
+  it('GET /api/sso/config returns 403 for viewer role', async () => {
+    const res = await supertest(app)
+      .get('/api/sso/config')
+      .set('Authorization', `Bearer ${viewerToken}`);
+    expect(res.status).toBe(403);
+    expect(res.body.code).toBe('ADMIN_REQUIRED');
+  });
+
+  it('PUT /api/sso/config/:provider returns 403 for viewer role', async () => {
+    const res = await supertest(app)
+      .put('/api/sso/config/ldap')
+      .set('Authorization', `Bearer ${viewerToken}`)
+      .send({ enabled: false });
+    expect(res.status).toBe(403);
+    expect(res.body.code).toBe('ADMIN_REQUIRED');
+  });
+
+  it('DELETE /api/sso/config/:provider returns 403 for viewer role', async () => {
+    const res = await supertest(app).delete('/api/sso/config/ldap').set('Authorization', `Bearer ${viewerToken}`);
+    expect(res.status).toBe(403);
+    expect(res.body.code).toBe('ADMIN_REQUIRED');
+  });
+});
+
+describe('SSO Config - API Token Denied', () => {
+  let apiRawToken: string;
+
+  beforeAll(async () => {
+    const { DatabaseService } = await import('../services/DatabaseService');
+    const db = DatabaseService.getInstance();
+    apiRawToken = jwt.sign({ scope: 'api_token', jti: crypto.randomUUID() }, TEST_JWT_SECRET, { expiresIn: '1h' });
+    const tokenHash = crypto.createHash('sha256').update(apiRawToken).digest('hex');
+    const admin = db.getUserByUsername('testadmin');
+    db.addApiToken({ token_hash: tokenHash, name: `sso-scope-${Date.now()}`, scope: 'full-admin', user_id: admin!.id, created_at: Date.now(), expires_at: null });
+  });
+
+  it('GET /api/sso/config returns 403 SCOPE_DENIED for API token', async () => {
+    const res = await supertest(app)
+      .get('/api/sso/config')
+      .set('Authorization', `Bearer ${apiRawToken}`);
+    expect(res.status).toBe(403);
+    expect(res.body.code).toBe('SCOPE_DENIED');
+  });
+
+  it('PUT /api/sso/config/:provider returns 403 SCOPE_DENIED for API token', async () => {
+    const res = await supertest(app)
+      .put('/api/sso/config/ldap')
+      .set('Authorization', `Bearer ${apiRawToken}`)
+      .send({ enabled: false });
+    expect(res.status).toBe(403);
+    expect(res.body.code).toBe('SCOPE_DENIED');
+  });
+});
+
+describe('SSO Test Connection - Custom OIDC', () => {
+  it('POST /api/sso/config/oidc_custom/test returns failure when not configured', async () => {
+    const res = await supertest(app)
+      .post('/api/sso/config/oidc_custom/test')
+      .set('Authorization', `Bearer ${adminToken}`);
+    expect(res.status).toBe(200);
+    expect(res.body.success).toBe(false);
+    expect(res.body.error).toBeDefined();
+  });
+});
+
+describe('SSO OIDC Callback - Additional Error Handling', () => {
+  it('handles error param without error_description', async () => {
+    const res = await supertest(app)
+      .get('/api/auth/sso/oidc/oidc_google/callback?error=server_error');
+    expect(res.status).toBe(302);
+    expect(res.headers.location).toContain('sso_error');
+    expect(res.headers.location).toContain('server_error');
+  });
+
+  it('handles error with description for oidc_custom provider', async () => {
+    const res = await supertest(app)
+      .get('/api/auth/sso/oidc/oidc_custom/callback?error=consent_required&error_description=User+must+consent');
+    expect(res.status).toBe(302);
+    expect(res.headers.location).toContain('User');
+  });
 });
diff --git a/backend/src/index.ts b/backend/src/index.ts
index baa3eb47..6d04bffd 100644
--- a/backend/src/index.ts
+++ b/backend/src/index.ts
@@ -858,6 +858,20 @@ const ssoRateLimiter = rateLimit({
   message: { error: 'Too many SSO attempts. Please try again later.' },
 });
 
+/** Derive the OAuth callback base URL from SSO_CALLBACK_URL or the request Host header, with injection validation. */
+function getSSOBaseUrl(req: Request, res: Response): string | null {
+  const host = req.get('host') || '';
+  if (!process.env.SSO_CALLBACK_URL && /[\s<>\r\n]/.test(host)) {
+    console.error('[SSO] Rejected suspicious Host header');
+    res.redirect('/?sso_error=Invalid+request');
+    return null;
+  }
+  if (!process.env.SSO_CALLBACK_URL && isDebugEnabled()) {
+    console.debug('[SSO:debug] SSO_CALLBACK_URL not set; using Host header for callback URL:', host);
+  }
+  return process.env.SSO_CALLBACK_URL || `${req.protocol}://${host}`;
+}
+
 // List enabled SSO providers (for login page)
 app.get('/api/auth/sso/providers', (_req: Request, res: Response): void => {
   try {
@@ -929,7 +943,8 @@ app.get('/api/auth/sso/oidc/:provider/authorize', ssoRateLimiter, async (req: Re
       return;
     }
 
-    const baseUrl = process.env.SSO_CALLBACK_URL || `${req.protocol}://${req.get('host')}`;
+    const baseUrl = getSSOBaseUrl(req, res);
+    if (!baseUrl) return;
     const callbackUrl = `${baseUrl}/api/auth/sso/oidc/${provider}/callback`;
 
     const { url, state, codeVerifier } = await SSOService.getInstance().getOIDCAuthorizationUrl(provider, callbackUrl);
@@ -995,7 +1010,8 @@ app.get('/api/auth/sso/oidc/:provider/callback', ssoRateLimiter, async (req: Req
       return;
     }
 
-    const baseUrl = process.env.SSO_CALLBACK_URL || `${req.protocol}://${req.get('host')}`;
+    const baseUrl = getSSOBaseUrl(req, res);
+    if (!baseUrl) return;
     const callbackUrl = `${baseUrl}/api/auth/sso/oidc/${provider}/callback`;
 
     const result = await SSOService.getInstance().handleOIDCCallback(
diff --git a/backend/src/services/SSOService.ts b/backend/src/services/SSOService.ts
index 451301d2..e4b46995 100644
--- a/backend/src/services/SSOService.ts
+++ b/backend/src/services/SSOService.ts
@@ -91,6 +91,15 @@ export class SSOService {
         this.seedOidcFromEnv('oidc_github', 'SSO_OIDC_GITHUB');
         this.seedOidcFromEnv('oidc_okta', 'SSO_OIDC_OKTA');
         this.seedOidcFromEnv('oidc_custom', 'SSO_OIDC_CUSTOM');
+
+        // Warn if OIDC providers are enabled but SSO_CALLBACK_URL is not set
+        if (!process.env.SSO_CALLBACK_URL) {
+            const enabled = DatabaseService.getInstance().getEnabledSSOConfigs();
+            const hasOidc = enabled.some(c => c.provider !== 'ldap');
+            if (hasOidc) {
+                console.warn('[SSO] SSO_CALLBACK_URL is not set. OAuth callback URLs will be derived from the request Host header. Set SSO_CALLBACK_URL to your external URL for deployments behind a reverse proxy.');
+            }
+        }
     }
 
     private seedLdapFromEnv(): void {
@@ -430,6 +439,18 @@ export class SSOService {
             const usernameClaimName = config.oidcUsernameClaim || 'preferred_username';
             const emailClaimName = config.oidcEmailClaim || 'email';
 
+            if (isDebugEnabled()) {
+                if (config.oidcIdClaim && !(config.oidcIdClaim in userInfo)) {
+                    console.debug(`[SSO:debug] Custom ID claim '${config.oidcIdClaim}' not found in token; falling back to 'sub'`);
+                }
+                if (config.oidcUsernameClaim && !(config.oidcUsernameClaim in userInfo)) {
+                    console.debug(`[SSO:debug] Custom username claim '${config.oidcUsernameClaim}' not found in token; falling back to standard claims`);
+                }
+                if (config.oidcEmailClaim && !(config.oidcEmailClaim in userInfo)) {
+                    console.debug(`[SSO:debug] Custom email claim '${config.oidcEmailClaim}' not found in token; falling back to 'email'`);
+                }
+            }
+
             const sub = String(userInfo[idClaimName] ?? userInfo.sub ?? userInfo.id ?? '');
             if (!sub) {
                 return { success: false, error: 'Could not determine user identity from provider' };
@@ -559,6 +580,10 @@ export class SSOService {
 
             // Update email if changed
             if (params.email && params.email !== existing.email) {
+                if (debug) console.debug('[SSO:debug] Email changed on re-login', {
+                    userId: existing.id, username: existing.username,
+                    oldEmail: existing.email || '(none)', newEmail: params.email,
+                });
                 updates.email = params.email;
             }
 
@@ -568,8 +593,11 @@ export class SSOService {
                     const seatLimits = LicenseService.getInstance().getSeatLimits();
                     if (seatLimits.maxAdmins === null || db.getAdminCount() < seatLimits.maxAdmins) {
                         updates.role = params.role;
+                    } else if (debug) {
+                        console.debug('[SSO:debug] Admin seat limit reached; keeping current role for existing user', {
+                            userId: existing.id, username: existing.username,
+                        });
                     }
-                    // If admin seats full, keep current role rather than upgrading
                 } else {
                     // Always allow demotion (e.g., removed from admin group)
                     updates.role = params.role;
diff --git a/docs/features/sso.mdx b/docs/features/sso.mdx
index bd34505a..c6e75e4e 100644
--- a/docs/features/sso.mdx
+++ b/docs/features/sso.mdx
@@ -192,7 +192,7 @@ Environment variables are useful for initial deployment or infrastructure-as-cod
 | Variable | Default | Description |
 |----------|---------|-------------|
 | `SSO_OIDC_CUSTOM_ENABLED` | `false` | Enable the custom OIDC provider |
-| `SSO_OIDC_CUSTOM_DISPLAY_NAME` | `oidc_custom` | Label shown on the login button |
+| `SSO_OIDC_CUSTOM_DISPLAY_NAME` | `Custom OIDC` | Label shown on the login button |
 | `SSO_OIDC_CUSTOM_ISSUER_URL` | - | OIDC issuer URL (the base of the discovery endpoint) |
 | `SSO_OIDC_CUSTOM_CLIENT_ID` | - | OAuth client ID from your identity provider |
 | `SSO_OIDC_CUSTOM_CLIENT_SECRET` | - | OAuth client secret (encrypted at rest) |
@@ -298,7 +298,7 @@ Fix: set the Issuer URL to exactly match the `issuer` field returned by your pro
 
 If users are created with incorrect usernames or missing emails:
 
-- Enable debug mode (`DEBUG=true` environment variable) to see the raw claims Sencho receives from the provider
+- Enable **Developer Mode** (Settings > Developer Mode toggle) to see the raw claims Sencho receives from the provider in the server logs
 - Check your provider's documentation for which claims it includes in the ID token and userinfo response
 - Verify that the scopes you configured include the necessary permissions (some providers require explicit `profile` or `email` scopes)
 - Set the appropriate claim names in the Custom OIDC claim mapping fields
diff --git a/docs/images/sso/sso-settings-custom-oidc.png b/docs/images/sso/sso-settings-custom-oidc.png
index b53e9b55..b3b41988 100644
Binary files a/docs/images/sso/sso-settings-custom-oidc.png and b/docs/images/sso/sso-settings-custom-oidc.png differ
diff --git a/docs/images/sso/sso-settings-ldap.png b/docs/images/sso/sso-settings-ldap.png
index c83b24a4..eefdba0d 100644
Binary files a/docs/images/sso/sso-settings-ldap.png and b/docs/images/sso/sso-settings-ldap.png differ
diff --git a/docs/images/sso/sso-settings-oidc.png b/docs/images/sso/sso-settings-oidc.png
index ea05a0fd..1817a7b2 100644
Binary files a/docs/images/sso/sso-settings-oidc.png and b/docs/images/sso/sso-settings-oidc.png differ
diff --git a/docs/images/sso/sso-settings.png b/docs/images/sso/sso-settings.png
index f649d422..35a37a10 100644
Binary files a/docs/images/sso/sso-settings.png and b/docs/images/sso/sso-settings.png differ
diff --git a/frontend/src/components/SSOSection.tsx b/frontend/src/components/SSOSection.tsx
index 91004a01..62a16397 100644
--- a/frontend/src/components/SSOSection.tsx
+++ b/frontend/src/components/SSOSection.tsx
@@ -386,7 +386,7 @@ export function SSOSection() {
         try {
             const res = await apiFetch('/sso/config');
             if (res.ok) setConfigs(await res.json());
-        } catch { /* ignore - AdmiralGate will handle non-pro */ }
+        } catch { /* ignore fetch errors */ }
     };
 
     // eslint-disable-next-line react-hooks/set-state-in-effect