fix: follow-up polish for salvaged PR #85512

- Log (debug) instead of silently swallowing capability-lookup failures in
  anthropic_prompt_cache_policy — a swallowed failure would otherwise
  downgrade an explicit prompt_caching: true to (False, False) with zero
  trace. Matches the sibling MoA branch's logger.debug style.
- Use load_config_readonly() for the None-fallback in
  get_custom_provider_model_capability: the helper only reads, and the
  fallback fires on the blank-stub paths (agent init before
  _custom_providers is assigned, MoA/auxiliary destination planning), so
  skip the ~135us defensive deepcopy per call.
- Add route-isolation regression tests at both levels (config helper +
  agent policy): a prompt_caching declaration for one provider route must
  never apply to another route with the same model name. Mutation-checked:
  both tests fail when the URL match is disabled.

agent/agent_runtime_helpers.py

@@ -2248,8 +2248,11 @@ def anthropic_prompt_cache_policy(
                 capability="prompt_caching",
                 custom_providers=getattr(agent, "_custom_providers", None),
             )
-        except Exception:
-            pass
+        except Exception as _cap_exc:
+            logger.debug(
+                "custom-provider prompt_caching capability lookup failed: %s",
+                _cap_exc,
+            )
     if custom_prompt_caching is not None:
         return custom_prompt_caching, custom_prompt_caching
 

hermes_cli/config.py

@@ -1799,6 +1799,12 @@ def get_custom_provider_model_capability(
         return None
     if custom_providers is None:
         try:
+            if config is None:
+                # Read-only path: this helper never mutates the entries it
+                # scans, and get_compatible_custom_providers shallow-copies
+                # each entry before normalizing, so the no-deepcopy cache is
+                # safe here (~135us saved per call on the blank-stub paths).
+                config = load_config_readonly()
             custom_providers = get_compatible_custom_providers(config)
         except Exception:
             return None

tests/hermes_cli/test_custom_provider_context_length.py

@@ -98,6 +98,27 @@ class TestGetCustomProviderModelCapability:
             custom,
         ) is None
 
+    def test_capability_is_route_isolated(self):
+        """A declaration for one route must not apply to another route.
+
+        Guards normalize_route_base_url matching: if the URL comparison ever
+        regresses to a model-only (or hostname-only) shortcut, this pins the
+        failure.
+        """
+        custom = [
+            {
+                "base_url": "https://other.example.invalid/anthropic",
+                "models": {"fable": {"prompt_caching": True}},
+            }
+        ]
+
+        assert get_custom_provider_model_capability(
+            "fable",
+            "https://example.invalid/anthropic",
+            "prompt_caching",
+            custom,
+        ) is None
+
 
 
 class TestGetModelContextLengthHonorsOverride:

tests/run_agent/test_anthropic_prompt_cache_policy.py

@@ -212,6 +212,25 @@ class TestThirdPartyAnthropicGateway:
 
         assert agent._anthropic_prompt_cache_policy() == (False, False)
 
+    def test_capability_on_other_route_does_not_apply(self):
+        """prompt_caching declared for a DIFFERENT base_url must not enable
+        caching for this agent's route — route isolation at the policy level."""
+        agent = _make_agent(
+            provider="custom:anthropic-proxy",
+            base_url="https://gateway.example.com/anthropic",
+            api_mode="anthropic_messages",
+            model="fable",
+        )
+        agent._custom_providers = [
+            {
+                "name": "other-proxy",
+                "base_url": "https://other.example.com/anthropic",
+                "models": {"fable": {"prompt_caching": True}},
+            }
+        ]
+
+        assert agent._anthropic_prompt_cache_policy() == (False, False)
+
 
 class TestMiniMaxAnthropicWire:
     """MiniMax's own model family on its Anthropic-compatible endpoint.