diff --git a/run_agent.py b/run_agent.py
index 080156051..e98a6e798 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -1268,6 +1268,19 @@ class AIAgent:
             try:
                 _config_context_length = int(_config_context_length)
             except (TypeError, ValueError):
+                logger.warning(
+                    "Invalid model.context_length in config.yaml: %r — "
+                    "must be a plain integer (e.g. 256000, not '256K'). "
+                    "Falling back to auto-detection.",
+                    _config_context_length,
+                )
+                import sys
+                print(
+                    f"\n⚠ Invalid model.context_length in config.yaml: {_config_context_length!r}\n"
+                    f"  Must be a plain integer (e.g. 256000, not '256K').\n"
+                    f"  Falling back to auto-detected context window.\n",
+                    file=sys.stderr,
+                )
                 _config_context_length = None
 
         # Store for reuse in switch_model (so config override persists across model switches)
@@ -1296,7 +1309,20 @@ class AIAgent:
                                 try:
                                     _config_context_length = int(_cp_ctx)
                                 except (TypeError, ValueError):
-                                    pass
+                                    logger.warning(
+                                        "Invalid context_length for model %r in "
+                                        "custom_providers: %r — must be a plain "
+                                        "integer (e.g. 256000, not '256K'). "
+                                        "Falling back to auto-detection.",
+                                        self.model, _cp_ctx,
+                                    )
+                                    import sys
+                                    print(
+                                        f"\n⚠ Invalid context_length for model {self.model!r} in custom_providers: {_cp_ctx!r}\n"
+                                        f"  Must be a plain integer (e.g. 256000, not '256K').\n"
+                                        f"  Falling back to auto-detected context window.\n",
+                                        file=sys.stderr,
+                                    )
                     break
         
         # Select context engine: config-driven (like memory providers).
diff --git a/tests/run_agent/test_invalid_context_length_warning.py b/tests/run_agent/test_invalid_context_length_warning.py
new file mode 100644
index 000000000..1ed72c951
--- /dev/null
+++ b/tests/run_agent/test_invalid_context_length_warning.py
@@ -0,0 +1,111 @@
+"""Tests that invalid context_length values in config produce visible warnings."""
+
+from unittest.mock import patch, MagicMock, call
+
+
+def _build_agent(model_cfg, custom_providers=None, model="anthropic/claude-opus-4.6"):
+    """Build an AIAgent with the given model config."""
+    cfg = {"model": model_cfg}
+    if custom_providers is not None:
+        cfg["custom_providers"] = custom_providers
+
+    with (
+        patch("hermes_cli.config.load_config", return_value=cfg),
+        patch("agent.model_metadata.get_model_context_length", return_value=128_000),
+        patch("run_agent.get_tool_definitions", return_value=[]),
+        patch("run_agent.check_toolset_requirements", return_value={}),
+        patch("run_agent.OpenAI"),
+    ):
+        from run_agent import AIAgent
+
+        agent = AIAgent(
+            model=model,
+            api_key="test-key-1234567890",
+            quiet_mode=True,
+            skip_context_files=True,
+            skip_memory=True,
+        )
+    return agent
+
+
+def test_valid_integer_context_length_no_warning():
+    """Plain integer context_length should work silently."""
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent({"default": "gpt5.4", "provider": "custom",
+                              "base_url": "http://localhost:4000/v1",
+                              "context_length": 256000})
+    assert agent._config_context_length == 256000
+    # No warning about invalid context_length
+    for c in mock_logger.warning.call_args_list:
+        assert "Invalid" not in str(c)
+
+
+def test_string_k_suffix_context_length_warns():
+    """context_length: '256K' should warn the user clearly."""
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent({"default": "gpt5.4", "provider": "custom",
+                              "base_url": "http://localhost:4000/v1",
+                              "context_length": "256K"})
+    assert agent._config_context_length is None
+    # Should have warned
+    warning_calls = [c for c in mock_logger.warning.call_args_list
+                     if "Invalid" in str(c) and "256K" in str(c)]
+    assert len(warning_calls) == 1
+    assert "plain integer" in str(warning_calls[0])
+
+
+def test_string_numeric_context_length_works():
+    """context_length: '256000' (string) should parse fine via int()."""
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent({"default": "gpt5.4", "provider": "custom",
+                              "base_url": "http://localhost:4000/v1",
+                              "context_length": "256000"})
+    assert agent._config_context_length == 256000
+    for c in mock_logger.warning.call_args_list:
+        assert "Invalid" not in str(c)
+
+
+def test_custom_providers_invalid_context_length_warns():
+    """Invalid context_length in custom_providers should warn."""
+    custom_providers = [
+        {
+            "name": "LiteLLM",
+            "base_url": "http://localhost:4000/v1",
+            "models": {
+                "gpt5.4": {"context_length": "256K"}
+            },
+        }
+    ]
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent(
+            {"default": "gpt5.4", "provider": "custom",
+             "base_url": "http://localhost:4000/v1"},
+            custom_providers=custom_providers,
+            model="gpt5.4",
+        )
+    warning_calls = [c for c in mock_logger.warning.call_args_list
+                     if "Invalid" in str(c) and "256K" in str(c)]
+    assert len(warning_calls) == 1
+    assert "custom_providers" in str(warning_calls[0])
+
+
+def test_custom_providers_valid_context_length():
+    """Valid integer in custom_providers should work silently."""
+    custom_providers = [
+        {
+            "name": "LiteLLM",
+            "base_url": "http://localhost:4000/v1",
+            "models": {
+                "gpt5.4": {"context_length": 256000}
+            },
+        }
+    ]
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent(
+            {"default": "gpt5.4", "provider": "custom",
+             "base_url": "http://localhost:4000/v1"},
+            custom_providers=custom_providers,
+            model="gpt5.4",
+        )
+    for c in mock_logger.warning.call_args_list:
+        assert "Invalid" not in str(c)