diff --git a/model_tools.py b/model_tools.py
index 31e3115dcf..5c0559f00d 100644
--- a/model_tools.py
+++ b/model_tools.py
@@ -415,24 +415,27 @@ def coerce_tool_args(tool_name: str, args: Dict[str, Any]) -> Dict[str, Any]:
         if not prop_schema:
             continue
         expected = prop_schema.get("type")
-        if not expected:
+        if not expected and not _schema_allows_null(prop_schema):
             continue
-        coerced = _coerce_value(value, expected)
+        coerced = _coerce_value(value, expected, schema=prop_schema)
         if coerced is not value:
             args[key] = coerced
 
     return args
 
 
-def _coerce_value(value: str, expected_type):
+def _coerce_value(value: str, expected_type, schema: dict | None = None):
     """Attempt to coerce a string *value* to *expected_type*.
 
     Returns the original string when coercion is not applicable or fails.
     """
+    if _schema_allows_null(schema) and value.strip().lower() == "null":
+        return None
+
     if isinstance(expected_type, list):
         # Union type — try each in order, return first successful coercion
         for t in expected_type:
-            result = _coerce_value(value, t)
+            result = _coerce_value(value, t, schema=schema)
             if result is not value:
                 return result
         return value
@@ -445,9 +448,35 @@ def _coerce_value(value: str, expected_type):
         return _coerce_json(value, list)
     if expected_type == "object":
         return _coerce_json(value, dict)
+    if expected_type == "null" and value.strip().lower() == "null":
+        return None
     return value
 
 
+def _schema_allows_null(schema: dict | None) -> bool:
+    """Return True when a JSON Schema fragment explicitly permits null."""
+    if not isinstance(schema, dict):
+        return False
+
+    schema_type = schema.get("type")
+    if schema_type == "null":
+        return True
+    if isinstance(schema_type, list) and "null" in schema_type:
+        return True
+    if schema.get("nullable") is True:
+        return True
+
+    for union_key in ("anyOf", "oneOf"):
+        variants = schema.get(union_key)
+        if not isinstance(variants, list):
+            continue
+        for variant in variants:
+            if isinstance(variant, dict) and variant.get("type") == "null":
+                return True
+
+    return False
+
+
 def _coerce_json(value: str, expected_python_type: type):
     """Parse *value* as JSON when the schema expects an array or object.
 
diff --git a/tests/run_agent/test_tool_arg_coercion.py b/tests/run_agent/test_tool_arg_coercion.py
index bc84b2bf60..8a14da9ea2 100644
--- a/tests/run_agent/test_tool_arg_coercion.py
+++ b/tests/run_agent/test_tool_arg_coercion.py
@@ -67,7 +67,7 @@ class TestCoerceNumber:
     def test_inf_stays_string_for_integer_only(self):
         """Infinity should not be converted to int."""
         result = _coerce_number("inf")
-        assert result == float("inf")
+        assert result == "inf"
 
     def test_negative_float(self):
         assert _coerce_number("-2.5") == -2.5
@@ -255,6 +255,35 @@ class TestCoerceToolArgs:
             result = coerce_tool_args("test_tool", args)
             assert result["config"] == {"max": 50}
 
+    def test_coerces_string_null_for_nullable_object_arg(self):
+        """Models often emit literal "null" for optional MCP object args."""
+        schema = self._mock_schema({
+            "setting": {
+                "type": "object",
+                "additionalProperties": True,
+                "nullable": True,
+                "default": None,
+            },
+        })
+        with patch("model_tools.registry.get_schema", return_value=schema):
+            args = {"setting": "null"}
+            result = coerce_tool_args("test_tool", args)
+            assert result["setting"] is None
+
+    def test_coerces_string_null_for_nullable_array_arg(self):
+        schema = self._mock_schema({
+            "stages": {
+                "type": "array",
+                "items": {"type": "object"},
+                "nullable": True,
+                "default": None,
+            },
+        })
+        with patch("model_tools.registry.get_schema", return_value=schema):
+            args = {"stages": "null"}
+            result = coerce_tool_args("test_tool", args)
+            assert result["stages"] is None
+
     def test_invalid_json_array_preserved_as_string(self):
         """If the string isn't valid JSON, pass it through — let the tool decide."""
         schema = self._mock_schema({"items": {"type": "array"}})
diff --git a/tests/tools/test_mcp_tool.py b/tests/tools/test_mcp_tool.py
index 5fddf8ec09..fd19eefa47 100644
--- a/tests/tools/test_mcp_tool.py
+++ b/tests/tools/test_mcp_tool.py
@@ -285,6 +285,7 @@ class TestSchemaConversion:
 
         assert schema["properties"]["workdir"] == {
             "type": "string",
+            "nullable": True,
             "default": None,
             "description": "Optional working directory",
         }
@@ -314,6 +315,7 @@ class TestSchemaConversion:
         assert schema["properties"]["filters"]["items"] == {
             "type": "object",
             "properties": {"field": {"type": "string"}},
+            "nullable": True,
         }
 
     def test_convert_mcp_schema_survives_missing_inputschema_attribute(self):
diff --git a/tools/mcp_tool.py b/tools/mcp_tool.py
index d3868f7c20..217318127a 100644
--- a/tools/mcp_tool.py
+++ b/tools/mcp_tool.py
@@ -2441,6 +2441,7 @@ def _normalize_mcp_input_schema(schema: dict | None) -> dict:
             ]
             if len(non_null) == 1 and len(non_null) != len(variants):
                 replacement = dict(non_null[0]) if isinstance(non_null[0], dict) else {}
+                replacement.setdefault("nullable", True)
                 for meta_key in ("title", "description", "default", "examples"):
                     if meta_key in stripped and meta_key not in replacement:
                         replacement[meta_key] = stripped[meta_key]