fix(model_metadata): consult DEFAULT_CONTEXT_LENGTHS before 256K fallback on custom endpoints

Problem: get_model_context_length() had an early return at the end of the
custom-endpoint probe branch (step 3) that returned DEFAULT_FALLBACK_CONTEXT
(256K) without ever consulting the hardcoded DEFAULT_CONTEXT_LENGTHS catalog
(step 8). Models served through a custom/proxied gateway (e.g. corporate
Anthropic proxy) that didn't expose Ollama or local-server endpoints would
hit this path and get capped at 256K, even when the model name clearly
matched a known entry in the catalog (e.g. claude-opus-4-8 → 1M).

Changes:
- agent/model_metadata.py: Before returning DEFAULT_FALLBACK_CONTEXT at the
  end of the custom-endpoint branch, consult DEFAULT_CONTEXT_LENGTHS using
  the same longest-key-first fuzzy matching as step 8. Only fall through
  to 256K if no catalog entry matches.
- tests/agent/test_model_metadata.py: Updated existing test and added new
  test covering the custom-endpoint → catalog fallback behavior.

Fixes #38865

tests/agent/test_model_metadata.py

@@ -18,6 +18,7 @@ from unittest.mock import patch, MagicMock
 from agent.model_metadata import (
     CONTEXT_PROBE_TIERS,
     DEFAULT_CONTEXT_LENGTHS,
+    DEFAULT_FALLBACK_CONTEXT,
     _strip_provider_prefix,
     estimate_tokens_rough,
     estimate_messages_tokens_rough,
@@ -773,17 +774,24 @@ class TestGetModelContextLength:
 
     @patch("agent.model_metadata.fetch_model_metadata")
     @patch("agent.model_metadata.fetch_endpoint_model_metadata")
-    def test_custom_endpoint_without_metadata_skips_name_based_default(self, mock_endpoint_fetch, mock_fetch):
+    def test_custom_endpoint_without_metadata_falls_back_to_catalog(self, mock_endpoint_fetch, mock_fetch):
+        """Custom endpoint with no metadata should fall back to the hardcoded
+        catalog (not 256K) when the model name matches a known entry.
+
+        Previously this returned CONTEXT_PROBE_TIERS[0] (256K) because the
+        custom-endpoint branch short-circuited before the catalog lookup.
+        See #38865.
+        """
         mock_fetch.return_value = {}
         mock_endpoint_fetch.return_value = {}
 
+        # GLM-5-TEE matches the "glm" entry in DEFAULT_CONTEXT_LENGTHS
         result = get_model_context_length(
             "zai-org/GLM-5-TEE",
             base_url="https://llm.chutes.ai/v1",
             api_key="test-key",
         )
-
-        assert result == CONTEXT_PROBE_TIERS[0]
+        assert result == 202752  # "glm" entry in DEFAULT_CONTEXT_LENGTHS
 
     @patch("agent.model_metadata.fetch_model_metadata")
     @patch("agent.model_metadata.fetch_endpoint_model_metadata")
@@ -858,6 +866,64 @@ class TestGetModelContextLength:
 
         assert result == 200000
 
+    @patch("agent.model_metadata.fetch_model_metadata")
+    def test_custom_endpoint_falls_back_to_hardcoded_catalog(self, mock_fetch):
+        """Custom/proxied endpoint that fails all probes should still resolve
+        via DEFAULT_CONTEXT_LENGTHS instead of returning 256K.
+
+        Regression test for #38865: a corporate Anthropic proxy (custom
+        base_url) caused the custom-endpoint branch to short-circuit before
+        the catalog lookup, capping context at 256K even for models like
+        claude-opus-4-8 that are in the hardcoded catalog with 1M.
+        """
+        mock_fetch.return_value = {}
+
+        # Patch all the probe functions that the custom-endpoint branch calls
+        # so they all fail (return None/empty), simulating a proxy that
+        # doesn't expose Ollama or local-server endpoints.
+        with (
+            patch(
+                "agent.model_metadata._resolve_endpoint_context_length",
+                return_value=None,
+            ),
+            patch(
+                "agent.model_metadata._query_ollama_api_show",
+                return_value=None,
+            ),
+            patch(
+                "agent.model_metadata._query_local_context_length",
+                return_value=None,
+            ),
+            patch(
+                "agent.model_metadata.is_local_endpoint",
+                return_value=False,
+            ),
+        ):
+            # A known model behind a custom proxy should resolve to its
+            # catalog value (1M), NOT the 256K fallback.
+            ctx = get_model_context_length(
+                "claude-opus-4-8",
+                base_url="https://my-gateway.example.com/v1/claude",
+            )
+            assert ctx == 1000000, f"Expected 1000000, got {ctx}"
+
+            # Another known model
+            ctx2 = get_model_context_length(
+                "claude-sonnet-4-6",
+                base_url="https://my-gateway.example.com/v1/claude",
+            )
+            assert ctx2 == 1000000, f"Expected 1000000, got {ctx2}"
+
+            # An unknown model on a custom endpoint should still fall back
+            # to 256K (no catalog match).
+            ctx3 = get_model_context_length(
+                "totally-unknown-model",
+                base_url="https://my-gateway.example.com/v1/claude",
+            )
+            assert ctx3 == DEFAULT_FALLBACK_CONTEXT, (
+                f"Expected {DEFAULT_FALLBACK_CONTEXT}, got {ctx3}"
+            )
+
 
 # =========================================================================
 # Bedrock context resolution — must run BEFORE custom-endpoint probe