diff --git a/helpers/litellm_transport.py b/helpers/litellm_transport.py
index 9e7a290f9..a66ed63c6 100644
--- a/helpers/litellm_transport.py
+++ b/helpers/litellm_transport.py
@@ -640,13 +640,11 @@ class ResponsesTransport:
         response_builtin_tools: Any = None,
     ) -> list[Any]:
         merged: list[Any] = []
-        if isinstance(tools, list):
-            merged.extend(tools)
-        elif tools:
-            merged.append(tools)
-
-        for source in (response_function_tools, response_builtin_tools):
-            for tool in _as_list(source):
+        for source in (tools, response_function_tools, response_builtin_tools):
+            source_tools = (
+                source if isinstance(source, list) else [source] if source else []
+            )
+            for tool in source_tools:
                 normalized = cls.normalize_response_tool(tool)
                 if normalized:
                     merged.append(normalized)
@@ -658,7 +656,12 @@ class ResponsesTransport:
             tool = {"type": tool}
         if not isinstance(tool, dict):
             return None
-        return dict(tool)
+        normalized = dict(tool)
+        if normalized.get("type") == "function":
+            normalized["parameters"] = _normalize_function_parameters(
+                normalized.get("parameters")
+            )
+        return normalized
 
     @staticmethod
     def prepare_prompt_caching(
@@ -800,7 +803,9 @@ class ResponsesTransport:
                     "type": "function",
                     "name": function.get("name", ""),
                     "description": function.get("description", ""),
-                    "parameters": function.get("parameters", {}),
+                    "parameters": _normalize_function_parameters(
+                        function.get("parameters")
+                    ),
                 }
                 if "strict" in function:
                     response_tool["strict"] = function["strict"]
@@ -1206,6 +1211,23 @@ def _response_tool_type(tool: Any) -> str:
     return ""
 
 
+def _normalize_function_parameters(parameters: Any) -> dict[str, Any]:
+    if not isinstance(parameters, dict):
+        return _permissive_function_parameters()
+
+    normalized = dict(parameters)
+    normalized.setdefault("type", "object")
+    if normalized.get("type") == "object" and not isinstance(
+        normalized.get("properties"), dict
+    ):
+        normalized["properties"] = {}
+    return normalized or _permissive_function_parameters()
+
+
+def _permissive_function_parameters() -> dict[str, Any]:
+    return {"type": "object", "properties": {}, "additionalProperties": True}
+
+
 def apply_chat_prompt_cache_markers(
     messages: list[dict[str, Any]],
     *,
diff --git a/helpers/litellm_transport.py.dox.md b/helpers/litellm_transport.py.dox.md
index 938af1432..a3b7bb73f 100644
--- a/helpers/litellm_transport.py.dox.md
+++ b/helpers/litellm_transport.py.dox.md
@@ -25,6 +25,7 @@
 - Keep provider selection and provider-specific defaults outside this helper; callers pass a resolved LiteLLM model name and kwargs.
 - Strip Agent Zero internal kwargs before sending requests to LiteLLM.
 - Do not send orphan tool controls when no tools are present; strict OpenAI-compatible servers can reject empty `tools` arrays.
+- Normalize function tool parameter schemas with an explicit object `properties` field before Responses requests so OpenAI-compatible chat backends reached through LiteLLM can validate them.
 - Prefer Responses API when configured, but fallback to Chat Completions when the provider does not support Responses.
 - Fall back to Chat Completions when a Responses request is rejected before any output by an endpoint-specific or shape-specific Bad Request indicating the provider cannot parse Responses payloads.
 - Preserve provider-state metadata when Responses API calls succeed, and fall back to local replay when provider state is unsupported.
diff --git a/helpers/responses_tools.py b/helpers/responses_tools.py
index e0d0e6fc3..453e7d731 100644
--- a/helpers/responses_tools.py
+++ b/helpers/responses_tools.py
@@ -217,13 +217,17 @@ def _schema_from_any(schema: Any) -> dict[str, Any]:
     if isinstance(schema, dict):
         normalized = dict(schema)
         normalized.setdefault("type", "object")
+        if normalized.get("type") == "object" and not isinstance(
+            normalized.get("properties"), dict
+        ):
+            normalized["properties"] = {}
         normalized.setdefault("additionalProperties", True)
         return normalized
     return _permissive_schema()
 
 
 def _permissive_schema() -> dict[str, Any]:
-    return {"type": "object", "additionalProperties": True}
+    return {"type": "object", "properties": {}, "additionalProperties": True}
 
 
 def _balanced_json_object(text: str) -> str:
diff --git a/helpers/responses_tools.py.dox.md b/helpers/responses_tools.py.dox.md
index 094f4b522..e7ec0e760 100644
--- a/helpers/responses_tools.py.dox.md
+++ b/helpers/responses_tools.py.dox.md
@@ -14,6 +14,7 @@
 
 - Build local function tools from enabled `agent.system.tool.*.md` prompt files.
 - Local prompt-derived function names prefer explicit `"tool_name"` examples, then the first prompt heading, and only fall back to the prompt filename when the prompt declares no callable name.
+- Function parameter schemas are object schemas with an explicit `properties` object so OpenAI-compatible servers that validate chat-style tool payloads accept permissive tools.
 - Preserve original Agent Zero tool names through the native Responses name map.
 - Keep MCP tool schemas merged after local prompt-derived tools.
 - Connector remote tools are advertised only when `_a0_connector` runtime metadata says the matching connected CLI capability is currently available.
diff --git a/tests/test_responses_tools.py b/tests/test_responses_tools.py
index f6aa5155b..fc187d1f7 100644
--- a/tests/test_responses_tools.py
+++ b/tests/test_responses_tools.py
@@ -101,3 +101,48 @@ def test_responses_function_tools_use_prompt_declared_names(monkeypatch, tmp_pat
     assert name_map["call_subordinate"] == "call_subordinate"
     assert name_map["behaviour_adjustment"] == "behaviour_adjustment"
     assert name_map["filename_only"] == "filename_only"
+    assert all(isinstance(tool["parameters"].get("properties"), dict) for tool in tools)
+
+
+def test_responses_function_tools_add_empty_properties_to_mcp_schemas(
+    monkeypatch,
+    tmp_path,
+):
+    prompt_root = tmp_path / "prompts"
+    prompt_root.mkdir()
+
+    monkeypatch.setattr(
+        responses_tools.subagents,
+        "get_paths",
+        lambda *args, **kwargs: [str(prompt_root)],
+    )
+    monkeypatch.setattr(
+        responses_tools,
+        "_mcp_tools",
+        lambda agent: [
+            (
+                "remote_noop",
+                {
+                    "description": "Remote noop",
+                    "input_schema": {"type": "object"},
+                },
+            )
+        ],
+    )
+
+    tools, _name_map = responses_tools.build_responses_function_tools(
+        FakeAgent(prompt_root)
+    )
+
+    assert tools == [
+        {
+            "type": "function",
+            "name": "remote_noop",
+            "description": "Remote noop",
+            "parameters": {
+                "type": "object",
+                "properties": {},
+                "additionalProperties": True,
+            },
+        }
+    ]
diff --git a/tests/test_stream_tool_early_stop.py b/tests/test_stream_tool_early_stop.py
index a1109948f..2b73c9503 100644
--- a/tests/test_stream_tool_early_stop.py
+++ b/tests/test_stream_tool_early_stop.py
@@ -780,7 +780,7 @@ def test_responses_request_translates_messages_and_params():
             "type": "function",
             "name": "lookup",
             "description": "Search",
-            "parameters": {"type": "object"},
+            "parameters": {"type": "object", "properties": {}},
             "strict": True,
         }
     ]
@@ -839,6 +839,61 @@ def test_responses_request_normalizes_reasoning_and_orphan_tool_choice():
     assert "reasoning" not in request
 
 
+def test_responses_request_normalizes_function_tool_parameter_shapes():
+    request = litellm_transport.ResponsesTransport.from_chat(
+        [],
+        {
+            "functions": [
+                {
+                    "name": "legacy_noop",
+                    "description": "Legacy function",
+                    "parameters": {},
+                }
+            ],
+        },
+    )
+
+    assert request["tools"] == [
+        {
+            "type": "function",
+            "name": "legacy_noop",
+            "description": "Legacy function",
+            "parameters": {
+                "type": "object",
+                "properties": {},
+            },
+        }
+    ]
+
+    request = litellm_transport.ResponsesTransport.from_chat(
+        [],
+        {
+            "a0_responses_function_tools": [
+                {
+                    "type": "function",
+                    "name": "native_noop",
+                    "description": "Native function",
+                    "parameters": {"type": "object"},
+                }
+            ],
+            "responses_builtin_tools": [{"type": "web_search"}],
+        },
+    )
+
+    assert request["tools"] == [
+        {
+            "type": "function",
+            "name": "native_noop",
+            "description": "Native function",
+            "parameters": {
+                "type": "object",
+                "properties": {},
+            },
+        },
+        {"type": "web_search"},
+    ]
+
+
 def test_chat_completions_kwargs_omit_empty_tools():
     kwargs = litellm_transport.ChatCompletionsTransport.prepare_kwargs(
         {