diff --git a/homeassistant/components/google_generative_ai_conversation/conversation.py b/homeassistant/components/google_generative_ai_conversation/conversation.py
index 4a93f6ca569..8dec62ad26b 100644
--- a/homeassistant/components/google_generative_ai_conversation/conversation.py
+++ b/homeassistant/components/google_generative_ai_conversation/conversation.py
@@ -73,6 +73,14 @@ SUPPORTED_SCHEMA_KEYS = {
 
 def _format_schema(schema: dict[str, Any]) -> dict[str, Any]:
     """Format the schema to protobuf."""
+    if (subschemas := schema.get("anyOf")) or (subschemas := schema.get("allOf")):
+        for subschema in subschemas:  # Gemini API does not support anyOf and allOf keys
+            if "type" in subschema:  # Fallback to first subschema with 'type' field
+                return _format_schema(subschema)
+        return _format_schema(
+            subschemas[0]
+        )  # Or, if not found, to any of the subschemas
+
     result = {}
     for key, val in schema.items():
         if key not in SUPPORTED_SCHEMA_KEYS:
@@ -81,7 +89,9 @@ def _format_schema(schema: dict[str, Any]) -> dict[str, Any]:
             key = "type_"
             val = val.upper()
         elif key == "format":
-            if schema.get("type") == "string" and val != "enum":
+            if (schema.get("type") == "string" and val != "enum") or (
+                schema.get("type") not in ("number", "integer", "string")
+            ):
                 continue
             key = "format_"
         elif key == "items":
@@ -89,6 +99,12 @@ def _format_schema(schema: dict[str, Any]) -> dict[str, Any]:
         elif key == "properties":
             val = {k: _format_schema(v) for k, v in val.items()}
         result[key] = val
+
+    if result.get("type_") == "OBJECT" and not result.get("properties"):
+        # An object with undefined properties is not supported by Gemini API.
+        # Fallback to JSON string. This will probably fail for most tools that want it,
+        # but we don't have a better fallback strategy so far.
+        result["properties"] = {"json": {"type_": "STRING"}}
     return result
 
 
diff --git a/tests/components/google_generative_ai_conversation/snapshots/test_conversation.ambr b/tests/components/google_generative_ai_conversation/snapshots/test_conversation.ambr
index 66caf4c7218..abd3658e869 100644
--- a/tests/components/google_generative_ai_conversation/snapshots/test_conversation.ambr
+++ b/tests/components/google_generative_ai_conversation/snapshots/test_conversation.ambr
@@ -442,6 +442,24 @@
     description: "Test function"
     parameters {
       type_: OBJECT
+      properties {
+        key: "param3"
+        value {
+          type_: OBJECT
+          properties {
+            key: "json"
+            value {
+              type_: STRING
+            }
+          }
+        }
+      }
+      properties {
+        key: "param2"
+        value {
+          type_: NUMBER
+        }
+      }
       properties {
         key: "param1"
         value {
diff --git a/tests/components/google_generative_ai_conversation/test_conversation.py b/tests/components/google_generative_ai_conversation/test_conversation.py
index dc713f09454..afeb6d01faa 100644
--- a/tests/components/google_generative_ai_conversation/test_conversation.py
+++ b/tests/components/google_generative_ai_conversation/test_conversation.py
@@ -185,7 +185,9 @@ async def test_function_call(
         {
             vol.Optional("param1", description="Test parameters"): [
                 vol.All(str, vol.Lower)
-            ]
+            ],
+            vol.Optional("param2"): vol.Any(float, int),
+            vol.Optional("param3"): dict,
         }
     )