test: add tests for compression config_context_length passthrough

- Test that auxiliary.compression.context_length from config is forwarded
  to get_model_context_length (positive case)
- Test that invalid/non-integer config values are silently ignored
- Fix _make_agent() to set config=None (cherry-picked code reads self.config)

tests/run_agent/test_compression_feasibility.py

@@ -38,6 +38,7 @@ def _make_agent(
     agent.status_callback = None
     agent.tool_progress_callback = None
     agent._compression_warning = None
+    agent.config = None
 
     compressor = MagicMock(spec=ContextCompressor)
     compressor.context_length = main_context
@@ -130,6 +131,64 @@ def test_feasibility_check_passes_live_main_runtime():
     )
 
 
+@patch("agent.model_metadata.get_model_context_length", return_value=1_000_000)
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_feasibility_check_passes_config_context_length(mock_get_client, mock_ctx_len):
+    """auxiliary.compression.context_length from config is forwarded to
+    get_model_context_length so custom endpoints that lack /models still
+    report the correct context window (fixes #8499)."""
+    agent = _make_agent(main_context=200_000, threshold_percent=0.85)
+    agent.config = {
+        "auxiliary": {
+            "compression": {
+                "context_length": 1_000_000,
+            },
+        },
+    }
+    mock_client = MagicMock()
+    mock_client.base_url = "http://custom-endpoint:8080/v1"
+    mock_client.api_key = "sk-custom"
+    mock_get_client.return_value = (mock_client, "custom/big-model")
+
+    agent._emit_status = lambda msg: None
+    agent._check_compression_model_feasibility()
+
+    mock_ctx_len.assert_called_once_with(
+        "custom/big-model",
+        base_url="http://custom-endpoint:8080/v1",
+        api_key="sk-custom",
+        config_context_length=1_000_000,
+    )
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=128_000)
+@patch("agent.auxiliary_client.get_text_auxiliary_client")
+def test_feasibility_check_ignores_invalid_context_length(mock_get_client, mock_ctx_len):
+    """Non-integer context_length in config is silently ignored."""
+    agent = _make_agent(main_context=200_000, threshold_percent=0.50)
+    agent.config = {
+        "auxiliary": {
+            "compression": {
+                "context_length": "not-a-number",
+            },
+        },
+    }
+    mock_client = MagicMock()
+    mock_client.base_url = "http://custom:8080/v1"
+    mock_client.api_key = "sk-test"
+    mock_get_client.return_value = (mock_client, "custom/model")
+
+    agent._emit_status = lambda msg: None
+    agent._check_compression_model_feasibility()
+
+    mock_ctx_len.assert_called_once_with(
+        "custom/model",
+        base_url="http://custom:8080/v1",
+        api_key="sk-test",
+        config_context_length=None,
+    )
+
+
 @patch("agent.auxiliary_client.get_text_auxiliary_client")
 def test_warns_when_no_auxiliary_provider(mock_get_client):
     """Warning emitted when no auxiliary provider is configured."""