diff --git a/agent/anthropic_adapter.py b/agent/anthropic_adapter.py
index 7cdac560b1..8c468e8686 100644
--- a/agent/anthropic_adapter.py
+++ b/agent/anthropic_adapter.py
@@ -1225,6 +1225,14 @@ def _normalize_tool_input_schema(schema: Any) -> Dict[str, Any]:
     ``keep_nullable_hint=False`` because the Anthropic validator does not
     recognize the OpenAPI-style ``nullable: true`` extension and strict
     schema-to-grammar converters may reject unknown keywords.
+
+    Top-level ``oneOf``/``allOf``/``anyOf`` are also stripped here: the
+    Anthropic API rejects union keywords at the schema root with a generic
+    HTTP 400. Several upstream and plugin tools ship schemas with one of
+    these keywords at the top level (commonly for Pydantic discriminated
+    unions). If we land here with those keywords still present after
+    nullable-union stripping, drop them and fall back to a plain object
+    schema so the tool still validates at the Anthropic boundary.
     """
     if not schema:
         return {"type": "object", "properties": {}}
@@ -1234,6 +1242,12 @@ def _normalize_tool_input_schema(schema: Any) -> Dict[str, Any]:
     normalized = strip_nullable_unions(schema, keep_nullable_hint=False)
     if not isinstance(normalized, dict):
         return {"type": "object", "properties": {}}
+    # Strip top-level union keywords that Anthropic's validator rejects.
+    banned = {"oneOf", "allOf", "anyOf"}
+    if banned & normalized.keys():
+        normalized = {k: v for k, v in normalized.items() if k not in banned}
+        if "type" not in normalized:
+            normalized["type"] = "object"
     if normalized.get("type") == "object" and not isinstance(normalized.get("properties"), dict):
         normalized = {**normalized, "properties": {}}
     return normalized