diff --git a/agent/error_classifier.py b/agent/error_classifier.py
index 7fa38bbcf70..0afcf66d445 100644
--- a/agent/error_classifier.py
+++ b/agent/error_classifier.py
@@ -240,6 +240,24 @@ _MODEL_NOT_FOUND_PATTERNS = [
     "unsupported model",
 ]
 
+# Request-validation patterns — the request is malformed and will fail
+# identically on every retry. Some OpenAI-compatible gateways (notably
+# codex.nekos.me) return these as 5xx instead of the standard 4xx, which
+# makes the generic "5xx → retryable server_error" rule misfire: the retry
+# loop hammers the same deterministic rejection 3+ times, then the
+# transport-recovery path resets the counter and does it again, producing
+# a request flood. When a 5xx body carries one of these unambiguous
+# request-validation signals, classify as a non-retryable format_error so
+# the loop fails fast and falls back instead of looping.
+_REQUEST_VALIDATION_PATTERNS = [
+    "unknown parameter",
+    "unsupported parameter",
+    "unrecognized request argument",
+    "invalid_request_error",
+    "unknown_parameter",
+    "unsupported_parameter",
+]
+
 # OpenRouter aggregator policy-block patterns.
 #
 # When a user's OpenRouter account privacy setting (or a per-request
@@ -745,6 +763,23 @@ def _classify_by_status(
         )
 
     if status_code in {500, 502}:
+        # Some OpenAI-compatible gateways return request-validation errors
+        # with a 5xx status (codex.nekos.me returns 502 for unknown/
+        # unsupported parameters). These are deterministic — every retry
+        # gets the identical rejection — so the generic "5xx → retryable
+        # server_error" rule turns one bad request into a retry flood.
+        # Detect the unambiguous request-validation signals (in either the
+        # message text or the structured error code) and fail fast.
+        if (
+            any(p in error_msg for p in _REQUEST_VALIDATION_PATTERNS)
+            or error_code.lower() in {"invalid_request_error", "unknown_parameter",
+                                      "unsupported_parameter"}
+        ):
+            return result_fn(
+                FailoverReason.format_error,
+                retryable=False,
+                should_fallback=True,
+            )
         return result_fn(FailoverReason.server_error, retryable=True)
 
     if status_code in {503, 529}:
diff --git a/tests/agent/test_error_classifier.py b/tests/agent/test_error_classifier.py
index eef3650347c..397d2673552 100644
--- a/tests/agent/test_error_classifier.py
+++ b/tests/agent/test_error_classifier.py
@@ -293,6 +293,64 @@ class TestClassifyApiError:
         result = classify_api_error(e)
         assert result.reason == FailoverReason.overloaded
 
+    # ── 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
+    # deterministic, so they must NOT be retried — otherwise the retry
+    # loop hammers the identical bad request into a flood.
+
+    def test_502_with_unknown_parameter_is_non_retryable(self):
+        e = MockAPIError(
+            "Unknown parameter: 'input[617]._empty_recovery_synthetic'",
+            status_code=502,
+            body={
+                "error": {
+                    "type": "invalid_request_error",
+                    "message": (
+                        "[ObjectParam] [input[617]._empty_recovery_synthetic] "
+                        "[unknown_parameter] Unknown parameter: "
+                        "'input[617]._empty_recovery_synthetic'."
+                    ),
+                }
+            },
+        )
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.format_error
+        assert result.retryable is False
+        assert result.should_fallback is True
+
+    def test_502_with_unsupported_parameter_is_non_retryable(self):
+        e = MockAPIError(
+            "Unsupported parameter: logprobs",
+            status_code=502,
+            body={
+                "error": {
+                    "type": "invalid_request_error",
+                    "message": "Unsupported parameter: logprobs",
+                }
+            },
+        )
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.format_error
+        assert result.retryable is False
+
+    def test_500_with_invalid_request_error_type_is_non_retryable(self):
+        e = MockAPIError(
+            "bad request",
+            status_code=500,
+            body={"error": {"type": "invalid_request_error", "message": "bad request"}},
+        )
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.format_error
+        assert result.retryable is False
+
+    def test_502_plain_bad_gateway_still_retryable(self):
+        """A genuine 502 with no request-validation signal stays retryable."""
+        e = MockAPIError("Bad Gateway", status_code=502)
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.server_error
+        assert result.retryable is True
+
     # ── Model not found ──
 
     def test_404_model_not_found(self):