fix: classify provider code-only errors into structured failover reasons

A provider error body that carries only a structured code and no HTTP
status (Gemini UNAVAILABLE / DEADLINE_EXCEEDED / INTERNAL, Anthropic
API_ERROR, OpenAI SERVER_ERROR) fell through every classifier stage to
FailoverReason.unknown, so the retry loop treated a provider overload
or timeout as a generic retryable failure and never reached the
overload/timeout recovery hints. Add a per-provider code table consulted
after the shared _ERROR_CODE_VERDICTS map; codes are scoped to their
provider family so a same-named code from another backend stays unknown.

Fixes #70414
Salvages #70425 (@ooiuuii) onto the rule-table classifier layout.

agent/error_classifier.py

@@ -596,6 +596,19 @@ _ERROR_CODE_VERDICTS: Dict[str, Verdict] = {
     "invalid_encrypted_content": _V_INVALID_ENCRYPTED,
 }
 
+# Provider-native status codes that arrive as a bare ``{"error": {"code": …}}`` body
+# (no HTTP status, no prose): gRPC canonical names from Gemini, Anthropic error
+# types, OpenAI's ``server_error``. Scoped per provider so a coincidentally named
+# code from another backend stays ``unknown`` (#70414). Provider aliases collapse
+# to the family key before lookup.
+_PROVIDER_CODE_FAMILIES = {"openai-codex": "openai", "google": "gemini", "google-gemini": "gemini",
+                           "google-ai-studio": "gemini", "vertex": "gemini", "google-vertex": "gemini"}
+_PROVIDER_CODE_VERDICTS: Dict[str, Dict[str, Verdict]] = {
+    "openai": {"server_error": _V_SERVER_ERROR},
+    "gemini": {"unavailable": _V_OVERLOADED, "deadline_exceeded": _V_TIMEOUT, "internal": _V_SERVER_ERROR},
+    "anthropic": {"api_error": _V_SERVER_ERROR, "rate_limit_error": _V_RATE_LIMIT},
+}
+
 # Generic ``invalid_request_error`` is deliberately NOT a 400 validation
 # signal — OpenAI stamps it on genuine overflow 400s too.
 _400_VALIDATION_CODES = {"unknown_parameter", "unsupported_parameter"}
@@ -770,7 +783,11 @@ def _by_error_code(c: _Ctx) -> Optional[Verdict]:
     # HTTP 200: retrying cannot succeed, a configured fallback still may.
     if c.code == PROVIDER_STREAM_NON_JSON_ERROR_CODE and "request validation failed:" in c.msg:
         return _V_FORMAT_ERROR
-    return _ERROR_CODE_VERDICTS.get(c.code)
+    verdict = _ERROR_CODE_VERDICTS.get(c.code)
+    if verdict is None:
+        family = _PROVIDER_CODE_FAMILIES.get(c.provider_slug, c.provider_slug)
+        verdict = _PROVIDER_CODE_VERDICTS.get(family, {}).get(c.code)
+    return verdict
 
 
 def _by_message(c: _Ctx) -> Optional[Verdict]:

tests/agent/test_error_classifier.py

@@ -1393,6 +1393,29 @@ class TestSSLCertVerificationFailFast:
 
 # ── Test: RateLimitError without status_code (Copilot/GitHub Models) ──────────
 
+class TestProviderCodeOnlyErrors:
+    """Bare ``{"error": {"code": …}}`` bodies with no HTTP status map to the
+    provider's structured reason instead of ``unknown`` (#70414)."""
+
+    @pytest.mark.parametrize("provider, code, reason", [
+        ("gemini", "UNAVAILABLE", FailoverReason.overloaded),
+        ("google", "DEADLINE_EXCEEDED", FailoverReason.timeout),
+        ("vertex", "INTERNAL", FailoverReason.server_error),
+        ("anthropic", "API_ERROR", FailoverReason.server_error),
+        ("openai-codex", "SERVER_ERROR", FailoverReason.server_error),
+    ])
+    def test_provider_native_code_maps_to_structured_reason(self, provider, code, reason):
+        e = MockAPIError(code, body={"error": {"code": code}})
+        result = classify_api_error(e, provider=provider)
+        assert result.reason == reason
+        assert result.retryable is True
+        assert result.should_rotate_credential is False
+
+    def test_code_meaning_does_not_leak_across_providers(self):
+        e = MockAPIError("UNAVAILABLE", body={"error": {"code": "UNAVAILABLE"}})
+        assert classify_api_error(e, provider="openai").reason == FailoverReason.unknown
+
+
 class TestRateLimitErrorWithoutStatusCode:
     """Regression tests for the Copilot/GitHub Models edge case where the
     OpenAI SDK raises RateLimitError but does not populate .status_code."""