diff --git a/agent/error_classifier.py b/agent/error_classifier.py
index 7550fd65d3..7b88e2f2de 100644
--- a/agent/error_classifier.py
+++ b/agent/error_classifier.py
@@ -1257,12 +1257,18 @@ def _build_error_msg(error: Exception, body: Any) -> str:
 
 
 def _body_message_candidates(body: dict) -> Iterator[Any]:
-    """Body message fields in priority order (OpenAI, flat, litellm/Bedrock proxy shapes)."""
+    """Body message fields in priority order (OpenAI, flat, litellm/Bedrock proxy, FastAPI shapes)."""
     yield _error_obj(body).get("message")
     yield body.get("message")
     yield body.get("errorMessage")
     args = body.get("errorArgs")
     yield args.get("reason") if isinstance(args, dict) else None
+    # FastAPI/Starlette relays and the Codex gateway answer {"detail": "..."} (or a nested
+    # OpenAI-ish object); without it a descriptive rejection reads as a bare 400 and the
+    # large-session heuristic sends it into compression (#81558). A list here is pydantic's
+    # validation shape, read by _oversized_message_content_rejection.
+    detail = body.get("detail")
+    yield detail.get("message") if isinstance(detail, dict) else detail if isinstance(detail, str) else None
 
 
 def _from_cause_chain(error: Exception, pick: Callable[[Any], Any], default: Any) -> Any:
diff --git a/contributors/emails/nagornyy.o@gmail.com b/contributors/emails/nagornyy.o@gmail.com
new file mode 100644
index 0000000000..fa64384f79
--- /dev/null
+++ b/contributors/emails/nagornyy.o@gmail.com
@@ -0,0 +1 @@
+i-Hun
diff --git a/tests/agent/test_error_classifier.py b/tests/agent/test_error_classifier.py
index 047ae48188..2e7ed82670 100644
--- a/tests/agent/test_error_classifier.py
+++ b/tests/agent/test_error_classifier.py
@@ -1181,6 +1181,28 @@ class TestClassifyApiError:
             for r in caplog.records
         ), "Expected a distinct warning identifying the malformed-body 400"
 
+    def test_400_top_level_detail_body_is_not_a_bare_400_on_large_session(self):
+        """FastAPI-style ``{"detail": "..."}`` bodies (Codex gateway, Starlette relays) →
+        the descriptive text is read, so the large-session heuristic does not route a
+        model entitlement/retirement rejection into compression (#81558, #106475).
+        ``str(error)`` is the SDK's ``Error code: 400 - {...}`` form, exactly as on the wire.
+        Salvaged from #100783 (@i-Hun)."""
+        detail = "The 'gpt-5.5-codex' model is not supported when using Codex with a ChatGPT account."
+        large = dict(provider="openai-codex", model="gpt-5.5-codex",
+                     approx_tokens=109_962, context_length=272_000, num_messages=223)
+        for body in ({"detail": detail}, {"detail": {"message": detail}}):
+            e = MockAPIError(f"Error code: 400 - {body!r}", status_code=400, body=body)
+            result = classify_api_error(e, **large)  # type: ignore[arg-type]
+            assert result.reason is not FailoverReason.context_overflow, body
+            assert result.should_compress is False
+            assert result.should_fallback is True
+            assert result.message == detail
+        # Control: the genuinely bare body the heuristic exists for still compresses.
+        bare = classify_api_error(
+            MockAPIError("Error code: 400 - {'error': {'message': 'Error'}}", status_code=400,
+                         body={"error": {"message": "Error"}}), **large)  # type: ignore[arg-type]
+        assert bare.reason is FailoverReason.context_overflow
+
 
     # ── Peer closed + large session ──