diff --git a/run_agent.py b/run_agent.py
index afee105e8..d8ca03da0 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -5555,7 +5555,13 @@ class AIAgent:
                     # are programming bugs, not transient failures.
                     _RETRYABLE_STATUS_CODES = {413, 429, 529}
                     is_local_validation_error = isinstance(api_error, (ValueError, TypeError))
-                    is_client_status_error = isinstance(status_code, int) and 400 <= status_code < 500 and status_code not in _RETRYABLE_STATUS_CODES
+                    # Detect generic 400s from Anthropic OAuth (transient server-side failures).
+                    # Real invalid_request_error responses include a descriptive message;
+                    # transient ones contain only "Error" or are empty. (ref: issue #1608)
+                    _err_body = getattr(api_error, "body", None) or {}
+                    _err_message = (_err_body.get("error", {}).get("message", "") if isinstance(_err_body, dict) else "")
+                    _is_generic_400 = (status_code == 400 and _err_message.strip().lower() in ("error", ""))
+                    is_client_status_error = isinstance(status_code, int) and 400 <= status_code < 500 and status_code not in _RETRYABLE_STATUS_CODES and not _is_generic_400
                     is_client_error = (is_local_validation_error or is_client_status_error or any(phrase in error_msg for phrase in [
                         'error code: 401', 'error code: 403',
                         'error code: 404', 'error code: 422',
diff --git a/tests/test_anthropic_adapter.py b/tests/test_anthropic_adapter.py
index e36f0b2c9..7203de7e0 100644
--- a/tests/test_anthropic_adapter.py
+++ b/tests/test_anthropic_adapter.py
@@ -144,9 +144,11 @@ class TestIsClaudeCodeTokenValid:
 
 
 class TestResolveAnthropicToken:
-    def test_prefers_oauth_token_over_api_key(self, monkeypatch):
+    def test_prefers_oauth_token_over_api_key(self, monkeypatch, tmp_path):
         monkeypatch.setenv("ANTHROPIC_API_KEY", "sk-ant-api03-mykey")
         monkeypatch.setenv("ANTHROPIC_TOKEN", "sk-ant-oat01-mytoken")
+        monkeypatch.delenv("CLAUDE_CODE_OAUTH_TOKEN", raising=False)
+        monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
         assert resolve_anthropic_token() == "sk-ant-oat01-mytoken"
 
     def test_reports_claude_json_primary_key_source(self, monkeypatch, tmp_path):
@@ -174,9 +176,11 @@ class TestResolveAnthropicToken:
         monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
         assert resolve_anthropic_token() == "sk-ant-api03-mykey"
 
-    def test_falls_back_to_token(self, monkeypatch):
+    def test_falls_back_to_token(self, monkeypatch, tmp_path):
         monkeypatch.delenv("ANTHROPIC_API_KEY", raising=False)
         monkeypatch.setenv("ANTHROPIC_TOKEN", "sk-ant-oat01-mytoken")
+        monkeypatch.delenv("CLAUDE_CODE_OAUTH_TOKEN", raising=False)
+        monkeypatch.setattr("agent.anthropic_adapter.Path.home", lambda: tmp_path)
         assert resolve_anthropic_token() == "sk-ant-oat01-mytoken"
 
     def test_returns_none_with_no_creds(self, monkeypatch, tmp_path):