diff --git a/agent/error_classifier.py b/agent/error_classifier.py
index 59b244b7b9..b49ff9b7b3 100644
--- a/agent/error_classifier.py
+++ b/agent/error_classifier.py
@@ -92,6 +92,11 @@ _BILLING_PATTERNS = (
     "billing hard limit", "exceeded your current quota", "account is deactivated", "plan does not include",
     "out of extra usage", "out of funds", "run out of funds", "balance_depleted",
     "model_not_supported_on_free_tier", "not available on the free tier",
+    # LiteLLM proxies word a hard cap as "hard billing limit" (structured twin:
+    # ``terminal_quota_exhausted`` in _BILLING_ERROR_CODES). "terminal billing
+    # limit" free text is NOT matched: substring rules can't negate the
+    # "non-terminal billing limit" wording, and the structured code covers it.
+    "hard billing limit",
 )
 
 # Not proof of exhaustion: Anthropic returns the same "out of extra usage" body
@@ -107,7 +112,7 @@ _XAI_SPENDING_LIMIT_ERROR_CODE = "personal-team-blocked:spending-limit"
 _BILLING_ERROR_CODES = frozenset({
     "insufficient_quota", "billing_not_active", "payment_required", "insufficient_credits",
     "no_usable_credits", "balance_depleted", "model_not_supported_on_free_tier",
-    "member_spend_cap_exceeded", _XAI_SPENDING_LIMIT_ERROR_CODE,
+    "member_spend_cap_exceeded", "terminal_quota_exhausted", _XAI_SPENDING_LIMIT_ERROR_CODE,
 })
 
 # Transient rate limiting. Bedrock "Throttling error: Too many tokens" also
@@ -725,6 +730,11 @@ def _status_404(c: _Ctx) -> Verdict:
 
 
 def _status_429(c: _Ctx) -> Verdict:
+    # A structured billing code is decisive: LiteLLM stamps
+    # ``terminal_quota_exhausted`` (a hard cap, not throttling) on 429s, and
+    # this handler always returns, so _by_error_code never sees the code.
+    if c.code in _BILLING_ERROR_CODES:
+        return _V_BILLING
     # Z.AI/Zhipu reuse 429 for server-wide overload: back off on the same
     # key instead of burning the pool (#14038).
     if any(p in c.msg for p in _OVERLOADED_PATTERNS):
diff --git a/tests/agent/test_error_classifier.py b/tests/agent/test_error_classifier.py
index 13cb0ab873..995a91fb40 100644
--- a/tests/agent/test_error_classifier.py
+++ b/tests/agent/test_error_classifier.py
@@ -539,6 +539,30 @@ class TestClassifyApiError:
         assert result.reason == FailoverReason.rate_limit
         assert result.should_rotate_credential is True
 
+    def test_429_with_structured_terminal_quota_code_is_billing(self):
+        """LiteLLM stamps ``terminal_quota_exhausted`` on a hard-cap 429. The
+        429 handler always returns a verdict, so the structured billing code
+        must be honored inside it — otherwise the exhausted key is retried
+        (upstream this respawned duplicate subagents; ported from
+        code-yeongyu/oh-my-openagent#6677)."""
+        e = MockAPIError(
+            "request failed", status_code=429,
+            body={"error": {"code": "terminal_quota_exhausted", "message": "request failed"}},
+        )
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.billing
+        assert result.retryable is False
+        assert result.should_fallback is True
+
+    def test_429_hard_billing_limit_text_is_billing(self):
+        """The free-text twin: "hard billing limit" is exhaustion wording, not
+        throttling, even though it contains no reset signal to disambiguate."""
+        result = classify_api_error(
+            MockAPIError("hard billing limit reached for this key", status_code=429)
+        )
+        assert result.reason == FailoverReason.billing
+        assert result.retryable is False
+
     # ── 5xx that are actually request-validation errors ──
     # Some OpenAI-compatible gateways (e.g. codex.nekos.me) return
     # request-validation failures with a 5xx status. These are