diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index 2b91b9e4d8e6..5f2d7e806aed 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -1337,6 +1337,31 @@ class _AnthropicCompletionsAdapter:
             if not _forbids_sampling_params(model):
                 anthropic_kwargs["temperature"] = temperature
 
+        # Pass through caller-supplied extra_body so providers behind
+        # Anthropic-compatible gateways receive their per-vendor request
+        # fields (thinking control, metadata, portal tags, ...). The dict
+        # form is the documented Anthropic SDK passthrough for non-standard
+        # request body keys; merge on top of whatever build_anthropic_kwargs
+        # already produced (e.g. fast-mode ``speed``) so call-time settings
+        # survive. Two exclusions:
+        #   - ``reasoning``: the OpenAI-shaped config dict is TRANSLATED into
+        #     the native ``thinking`` field above (build_anthropic_kwargs);
+        #     forwarding the raw field alongside would double-specify
+        #     reasoning and 400 on strict gateways.
+        #   - ``_``-prefixed keys: private Hermes plumbing (_reasoning_config
+        #     et al.), never wire fields.
+        caller_extra_body = kwargs.get("extra_body")
+        if caller_extra_body and isinstance(caller_extra_body, dict):
+            passthrough = {
+                k: v for k, v in caller_extra_body.items()
+                if k != "reasoning" and not str(k).startswith("_")
+            }
+            if passthrough:
+                existing = anthropic_kwargs.get("extra_body") or {}
+                if not isinstance(existing, dict):
+                    existing = {}
+                anthropic_kwargs["extra_body"] = {**existing, **passthrough}
+
         response = create_anthropic_message(self._client, anthropic_kwargs)
         _transport = get_transport("anthropic_messages")
         _nr = _transport.normalize_response(