feat: add Mantle override for AWS Bedrock Anthropic endpoint

Introduce `model_capabilities.mantle` flag that installs a provider-specific
override in fast-agent's `ModelDatabase._PROVIDER_MODEL_OVERRIDES` to strip
features the AWS Bedrock Mantle endpoint rejects (beta headers, extended
thinking, task budgets, web tools, prompt caching).

Without this override, fast-agent sends default beta headers and `thinking`
parameters for modern Claude models that Mantle rejects with a misleading
404 "model does not exist" error.

pallas/server.py

@@ -123,33 +123,85 @@ def _preflight_mcp_servers(agent_name: str, servers: dict[str, dict]) -> None:
 # ── Model registration ────────────────────────────────────────────────────────
 
 def _register_one_model(model_spec: str, capabilities: dict) -> None:
-    """Register a single model with fast-agent's ModelDatabase if unknown."""
+    """Register a single model with fast-agent's ModelDatabase.
+
+    Two cases:
+
+    1. **Unknown model** — if fast-agent has no built-in entry for this model,
+       register a minimal ``ModelParameters`` with the declared capabilities.
+
+    2. **Mantle-hosted model** (``capabilities.mantle: true``) — regardless of
+       whether the model has a built-in entry, install a provider-specific
+       override for ``(Provider.ANTHROPIC, model_name)`` in
+       ``_PROVIDER_MODEL_OVERRIDES`` that strips the features the AWS Bedrock
+       Mantle endpoint rejects:
+
+       - ``anthropic_required_betas`` (no ``anthropic-beta`` header)
+       - ``reasoning`` / ``reasoning_effort_spec`` (no extended-thinking request)
+       - ``anthropic_task_budget_supported``
+       - ``anthropic_web_fetch_version`` / ``anthropic_web_search_version``
+       - ``cache_ttl`` (prompt caching is not advertised as supported on
+         Mantle for every model; disable the cache planner by default)
+
+       Without this override fast-agent sends beta headers and ``thinking``
+       parameters that Mantle rejects with a misleading ``"model does not
+       exist"`` 404.
+    """
     from fast_agent.llm.model_database import ModelDatabase, ModelParameters
+    from fast_agent.llm.provider_types import Provider
 
     model_name = model_spec.split(".", 1)[-1] if "." in model_spec else model_spec
 
-    if ModelDatabase.get_model_params(model_name) is not None:
-        return
-
     is_vision = capabilities.get("vision", False)
     context_window = capabilities.get("context_window", 131072)
     max_output_tokens = capabilities.get("max_output_tokens", 16384)
+    is_mantle = capabilities.get("mantle", False)
 
-    if is_vision:
-        tokenizes = list(ModelDatabase.QWEN_MULTIMODAL)
-        logger.info("Registered model '%s' with vision capabilities", model_name)
+    existing = ModelDatabase.get_model_params(model_name)
+
+    if existing is None:
+        # Unknown model — register a fresh runtime entry.
+        if is_vision:
+            tokenizes = list(ModelDatabase.QWEN_MULTIMODAL)
+            logger.info("Registered model '%s' with vision capabilities", model_name)
+        else:
+            tokenizes = list(ModelDatabase.TEXT_ONLY)
+            logger.info("Registered model '%s' as text-only", model_name)
+
+        ModelDatabase.register_runtime_model_params(
+            model_name,
+            ModelParameters(
+                context_window=context_window,
+                max_output_tokens=max_output_tokens,
+                tokenizes=tokenizes,
+            ),
+        )
+        base_params = ModelDatabase.get_model_params(model_name)
     else:
-        tokenizes = list(ModelDatabase.TEXT_ONLY)
-        logger.info("Registered model '%s' as text-only", model_name)
+        base_params = existing
+
+    if is_mantle and base_params is not None:
+        # Clone the base params and strip Mantle-incompatible features.
+        override = base_params.model_copy(
+            update={
+                "context_window": context_window,
+                "max_output_tokens": max_output_tokens,
+                "anthropic_required_betas": None,
+                "reasoning": None,
+                "reasoning_effort_spec": None,
+                "anthropic_task_budget_supported": False,
+                "anthropic_web_fetch_version": None,
+                "anthropic_web_search_version": None,
+                "cache_ttl": None,
+            }
+        )
+        normalized = ModelDatabase.normalize_model_name(model_name)
+        ModelDatabase._PROVIDER_MODEL_OVERRIDES[(Provider.ANTHROPIC, normalized)] = override
+        logger.info(
+            "Registered Mantle override for anthropic/'%s' (strips beta headers, thinking, web tools, caching)",
+            model_name,
+        )
 
-    ModelDatabase.register_runtime_model_params(
-        model_name,
-        ModelParameters(
-            context_window=context_window,
-            max_output_tokens=max_output_tokens,
-            tokenizes=tokenizes,
-        ),
-    )
 
 
 def _register_unknown_models(deployment_config: dict) -> None: