test(integration): Bedrock Mantle context overflow surfaces as 400 prompt is too long (Pylon #7732)

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/integration/contracts.json

@@ -223,6 +223,9 @@
     "tests/integration/providers/test_xiaomi_mimo_wire.py::test_xiaomi_mimo_tool_call_is_forwarded_and_returned": [
       "other.provider_wire.xiaomi_mimo.tool_call_survives_translation"
     ],
+    "tests/integration/providers/test_bedrock_mantle_wire.py::test_bedrock_mantle_context_overflow_returns_400_saying_prompt_is_too_long": [
+      "other.provider_wire.bedrock_mantle.context_overflow_is_reported_as_prompt_too_long"
+    ],
     "tests/integration/providers/test_fal_ai_video_wire.py::test_fal_h3_video_create_uses_canonical_body_and_status_path": [
       "other.provider_wire.fal_ai.video_queue_create_status_and_content_download"
     ],

tests/integration/providers/test_bedrock_mantle_wire.py

@@ -0,0 +1,53 @@
+import json
+from typing import Final
+
+import pytest
+from integration._support.client import Gateway
+from integration._support.wire import Reply, Request, wire_server
+from pydantic import JsonValue, TypeAdapter
+
+_BACKEND: Final = "openai.gpt-5.6-sol"
+_API_KEY: Final = "synthetic-mantle-bearer"
+_PROMPT: Final = "synthetic long conversation control"
+_PROMPT_TOKENS: Final = 1055489
+_MODEL_MAXIMUM: Final = 1050000
+_RESPONSES_PATH: Final = "/openai/v1/responses"
+_JSON_OBJECT: Final = TypeAdapter(dict[str, JsonValue])
+_OVERFLOW_BODY: Final = json.dumps(
+    {
+        "error": {
+            "code": "validation_error",
+            "message": f"prompt tokens ({_PROMPT_TOKENS}) exceed model maximum ({_MODEL_MAXIMUM}) for {_BACKEND}",
+            "type": "invalid_request_error",
+        }
+    }
+).encode()
+
+
+def _overflow_peer(request: Request) -> Reply:
+    assert request.method == "POST"
+    assert request.target == _RESPONSES_PATH
+    assert request.headers["authorization"] == f"Bearer {_API_KEY}"
+    body: Final = _JSON_OBJECT.validate_json(request.body)
+    assert body["model"] == _BACKEND
+    assert _PROMPT in json.dumps(body["input"]), body
+    return Reply(status=400, body=_OVERFLOW_BODY)
+
+
+@pytest.mark.covers("other.provider_wire.bedrock_mantle.context_overflow_is_reported_as_prompt_too_long")
+def test_bedrock_mantle_context_overflow_returns_400_saying_prompt_is_too_long(gateway: Gateway) -> None:
+    with wire_server(_overflow_peer) as wire, gateway.scenario() as scenario:
+        model: Final = scenario.model(model=f"bedrock_mantle/{_BACKEND}", api_base=wire.url, api_key=_API_KEY)
+        response: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {"model": model, "messages": [{"role": "user", "content": _PROMPT}]},
+        )
+        assert response.status_code == 400, response.text
+        error: Final = _JSON_OBJECT.validate_json(response.content)["error"]
+        assert isinstance(error, dict), response.text
+        assert error["code"] == "400", response.text
+        message: Final = error["message"]
+        assert isinstance(message, str), response.text
+        assert f"prompt is too long: {_PROMPT_TOKENS} tokens > {_MODEL_MAXIMUM} maximum" in message, response.text
+        assert [(request.method, request.target) for request in wire.drain()] == [("POST", _RESPONSES_PATH)]