diff --git a/src/web-server/routes/account-routes.ts b/src/web-server/routes/account-routes.ts
index 4a6520db..f00973cf 100644
--- a/src/web-server/routes/account-routes.ts
+++ b/src/web-server/routes/account-routes.ts
@@ -44,6 +44,10 @@ import {
   mutateConfig,
 } from '../../config/config-loader-facade';
 import type { AccountTier } from '../../cliproxy/accounts/types';
+import {
+  isManagedQuotaProvider,
+  MANAGED_QUOTA_PROVIDERS,
+} from '../../cliproxy/quota/quota-manager';
 
 /** Valid account tier values for tier-lock validation */
 const VALID_ACCOUNT_TIERS: ReadonlySet<string> = new Set<AccountTier>([
@@ -669,6 +673,18 @@ router.post('/tier-lock', (req: Request, res: Response): void => {
       return;
     }
 
+    // Fix #8: tier_lock is only enforced by quota-manager for managed-quota providers.
+    // Locking a non-managed provider persists a silently-unenforced entry.
+    // Reject with 400 to avoid misleading the caller.
+    if (!isManagedQuotaProvider(provider)) {
+      res.status(400).json({
+        error:
+          `Provider "${provider}" does not support tier-lock. ` +
+          `Only managed-quota providers support it: ${[...MANAGED_QUOTA_PROVIDERS].join(', ')}.`,
+      });
+      return;
+    }
+
     // tier must be explicitly present in body (key exists), and must be string or null
     if (!('tier' in req.body)) {
       res.status(400).json({ error: 'Missing required field: tier (string | null)' });
diff --git a/tests/unit/web-server/tier-lock-routes.test.ts b/tests/unit/web-server/tier-lock-routes.test.ts
index 55736614..0aaa875a 100644
--- a/tests/unit/web-server/tier-lock-routes.test.ts
+++ b/tests/unit/web-server/tier-lock-routes.test.ts
@@ -164,6 +164,41 @@ describe('POST /api/accounts/tier-lock', () => {
     expect(body.error).toMatch(/tier/i);
   });
 
+  it('rejects a non-managed provider with 400 (fix #8)', async () => {
+    // Providers like 'kiro' or unknown CLIProxy providers accept isCLIProxyProvider()
+    // but quota-manager does not enforce tier_lock for them. Persisting a lock entry
+    // would silently have no effect, misleading the caller. Must 400.
+    // Note: 'kiro' passes isCLIProxyProvider but is NOT in MANAGED_QUOTA_PROVIDERS.
+    // We test with a provider that is valid (passes CLIProxy check) but not managed.
+    // In practice this means any provider added to CLIProxy that is not in
+    // MANAGED_QUOTA_PROVIDERS = ['agy', 'claude', 'codex', 'gemini', 'ghcp'].
+    // We use a string that is a known CLIProxy provider but not managed.
+    // Since the set of CLIProxy providers is dynamic we test the error message content.
+    const res = await postJson(baseUrl, '/api/accounts/tier-lock', {
+      provider: 'kiro',
+      tier: 'pro',
+    });
+    // If kiro is a CLIProxy provider but not managed: expect 400
+    // If kiro is not a CLIProxy provider at all: also 400 (from isCLIProxyProvider check)
+    expect(res.status).toBe(400);
+    const body = (await res.json()) as { error: string };
+    expect(body.error).toMatch(/provider/i);
+  });
+
+  it('accepts all five managed-quota providers (agy, claude, codex, gemini, ghcp)', async () => {
+    const managedProviders = ['agy', 'claude', 'codex', 'gemini', 'ghcp'];
+    for (const provider of managedProviders) {
+      const res = await postJson(baseUrl, '/api/accounts/tier-lock', {
+        provider,
+        tier: 'pro',
+      });
+      // All managed providers should succeed (200)
+      expect(res.status).toBe(200);
+      const body = (await res.json()) as { provider: string; tier_lock: string | null };
+      expect(body.provider).toBe(provider);
+    }
+  });
+
   it('persists tier_lock as per-provider map in the config', async () => {
     await postJson(baseUrl, '/api/accounts/tier-lock', { provider: 'agy', tier: 'pro' });