diff --git a/agent/gemini_schema.py b/agent/gemini_schema.py
index 6822558837..134667bbe9 100644
--- a/agent/gemini_schema.py
+++ b/agent/gemini_schema.py
@@ -16,6 +16,12 @@ _GEMINI_SCHEMA_ALLOWED_KEYS = {
 }
 
 
+_GEMINI_STRUCTURAL_KEYS = {
+    "array": {"items", "minItems", "maxItems"},
+    "object": {"properties", "required", "minProperties", "maxProperties", "propertyOrdering"},
+}
+
+
 def _stringify_enum_value(item: Any) -> Any:
     """Gemini-safe string for a scalar enum entry, or None to drop it."""
     if isinstance(item, bool):
@@ -25,6 +31,30 @@ def _stringify_enum_value(item: Any) -> Any:
     return item if isinstance(item, str) else None
 
 
+def _normalize_gemini_type_array(type_array: list, cleaned: Dict[str, Any]) -> None:
+    """Keep union alternatives and their branch-local structural constraints."""
+    derived: Dict[str, Any] = {}
+    _normalize_type_array(type_array, derived)
+    if "anyOf" in derived:
+        constraints = {"anyOf": cleaned["anyOf"]} if "anyOf" in cleaned else {}
+        # Gemini requires items/properties on the typed branch itself, not
+        # its typeless parent. Keep required paired with those properties.
+        structural = {key: cleaned.pop(key) for keys in _GEMINI_STRUCTURAL_KEYS.values()
+                      for key in keys if key in cleaned}
+        cleaned["anyOf"] = [
+            sanitize_gemini_schema({**branch, **constraints, **{
+                key: value for key, value in structural.items()
+                if key in _GEMINI_STRUCTURAL_KEYS.get(branch["type"], ())
+            }}) for branch in derived["anyOf"]
+        ]
+    else:
+        cleaned["type"] = derived["type"]
+    if derived.get("nullable"):
+        # Derived from "null" in the array. Set AFTER the loop so it beats an input
+        # ``nullable: false`` regardless of which key the producer emitted first.
+        cleaned["nullable"] = True
+
+
 def sanitize_gemini_schema(schema: Any) -> Dict[str, Any]:
     """Gemini-compatible copy of a tool parameter schema: keeps only the documented subset
     (drops e.g. ``$schema`` / ``additionalProperties``) and recursively sanitizes nested
@@ -32,7 +62,6 @@ def sanitize_gemini_schema(schema: Any) -> Dict[str, Any]:
     if not isinstance(schema, dict):
         return {}
     cleaned: Dict[str, Any] = {}
-    type_array: Any = None
     for key, value in schema.items():
         if key not in _GEMINI_SCHEMA_ALLOWED_KEYS:
             continue
@@ -44,33 +73,21 @@ def sanitize_gemini_schema(schema: Any) -> Dict[str, Any]:
         elif key == "anyOf":
             if isinstance(value, list):
                 cleaned[key] = [sanitize_gemini_schema(item) for item in value if isinstance(item, dict)]
-        elif key == "type" and isinstance(value, list):
-            type_array = value  # normalized after the loop, so the derived ``nullable`` wins
         else:
             cleaned[key] = value
 
-    # JSON Schema allows an array ``type`` (``["string", "null"]``, ``["string", "integer"]``).
-    # Gemini's ``Schema`` accepts only a single string, and the enum check below would evaluate
-    # ``[...] in {...}`` -> TypeError: unhashable type: 'list', aborting translation of the WHOLE
-    # tool catalog. Reuse the sanitizer's normalization so a multi-type array becomes an ``anyOf``
-    # of single-type branches (no branch dropped) rather than keeping only the first.
-    if type_array is not None:
-        derived: Dict[str, Any] = {}
-        _normalize_type_array(type_array, derived)
-        if "anyOf" in derived:
-            cleaned["anyOf"] = derived["anyOf"]
-        else:
-            cleaned["type"] = derived["type"]
-        if derived.get("nullable"):
-            # Derived from "null" in the array. Set AFTER the loop so it beats an input
-            # ``nullable: false`` regardless of which key the producer emitted first.
-            cleaned["nullable"] = True
+    type_array = cleaned.get("type")
+    if isinstance(type_array, list):
+        cleaned.pop("type")
+        _normalize_gemini_type_array(type_array, cleaned)
 
     # Gemini requires every ``enum`` entry to be a string even for
     # integer/number/boolean types; the declared type stays intact and Gemini
     # still emits typed tool arguments at runtime. dict.fromkeys = ordered dedupe.
     enum_val = cleaned.get("enum")
-    if isinstance(enum_val, list) and cleaned.get("type") in {"integer", "number", "boolean"}:
+    if isinstance(enum_val, list) and (
+        isinstance(type_array, list) or cleaned.get("type") in {"integer", "number", "boolean"}
+    ):
         if stringified := list(dict.fromkeys(v for v in map(_stringify_enum_value, enum_val) if v is not None)):
             cleaned["enum"] = stringified
         else:
diff --git a/evals/gemini_type_array_probe.py b/evals/gemini_type_array_probe.py
new file mode 100644
index 0000000000..1873dbab32
--- /dev/null
+++ b/evals/gemini_type_array_probe.py
@@ -0,0 +1,120 @@
+"""Offline A/B native adapter + Google SDK schema validation (no API calls).
+
+Run with httpx and google-genai==1.47.0 installed:
+    python evals/gemini_type_array_probe.py --base origin/main
+"""
+from __future__ import annotations
+
+import argparse
+import copy
+import importlib
+import json
+import os
+from pathlib import Path
+import subprocess
+import sys
+import tempfile
+import threading
+import types
+from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
+
+
+class WireCapture(BaseHTTPRequestHandler):
+    requests = []
+
+    def do_POST(self):
+        size = int(self.headers["Content-Length"])
+        self.requests.append(json.loads(self.rfile.read(size)))
+        # Intentionally stop at the wire boundary; never fake a model response.
+        self.send_response(418)
+        self.end_headers()
+        self.wfile.write(b"Local schema capture only")
+
+    def log_message(self, *_):
+        pass
+
+
+def wire_translate(adapter, tools):
+    server = ThreadingHTTPServer(("127.0.0.1", 0), WireCapture)
+    thread = threading.Thread(target=server.serve_forever, daemon=True)
+    thread.start()
+    try:
+        with adapter.GeminiNativeClient(
+            api_key="local-probe-not-a-secret",
+            base_url=f"http://127.0.0.1:{server.server_port}/v1beta",
+        ) as client:
+            try:
+                client.chat.completions.create(
+                    model="gemini-2.5-flash", tools=tools,
+                    messages=[{"role": "user", "content": "Schema validation probe"}],
+                )
+            except adapter.GeminiAPIError as exc:
+                assert exc.status_code == 418
+        return WireCapture.requests.pop()["tools"]
+    finally:
+        server.shutdown()
+        server.server_close()
+        thread.join()
+
+
+def probe(label, translate):
+    from google.genai.types import FunctionDeclaration
+
+    cases = {
+        "nullable": {"type": ["number", "null"]},
+        "nullable_enum": {"type": ["integer", "null"], "enum": [1, 2]},
+        "union_enum": {"type": ["string", "integer"], "enum": ["one", 2]},
+        "nested_items": {"type": "array", "items": {"type": ["string", "null"]}},
+        "nested_anyof": {"anyOf": [{"type": ["string", "integer"]}, {"type": "boolean"}]},
+        "constrained_union": {"type": ["string", "integer"], "anyOf": [{"enum": ["one"]}, {"enum": ["two"]}]},
+        "structural_union": {"type": ["array", "object", "string"], "items": {"type": "integer"}, "properties": {"name": {"type": "string"}}, "required": ["name"]},
+        "scalar_control": {"type": "number"},
+    }
+    results = {}
+    for name, node in cases.items():
+        parameters = {"type": "object", "properties": {"value": node}}
+        original = copy.deepcopy(parameters)
+        try:
+            wire = translate([{"type": "function", "function": {"name": "probe", "parameters": parameters}}])
+            declaration = wire[0]["functionDeclarations"][0]
+            # Validate the actual adapter declaration, not a reconstructed schema.
+            FunctionDeclaration.model_validate(json.loads(json.dumps(declaration)))
+            results[name] = "accepted"
+        except (TypeError, ValueError) as exc:
+            results[name] = f"{type(exc).__name__}: {str(exc).splitlines()[0]}"
+        assert parameters == original, name
+    print(json.dumps({"label": label, "results": results}, indent=2))
+    return results
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--base", default="origin/main")
+    args = parser.parse_args()
+    root = Path(__file__).resolve().parents[1]
+    sys.path.insert(0, str(root))
+    for name in list(sys.modules):
+        if name == "agent" or name.startswith(("agent.", "tools", "hermes")):
+            del sys.modules[name]
+    with tempfile.TemporaryDirectory(prefix="gemini-schema-home-") as home:
+        os.environ["HERMES_HOME"] = home
+        adapter = importlib.import_module("agent.gemini_native_adapter")
+        baseline = types.ModuleType("baseline_gemini_schema")
+        source = subprocess.check_output(
+            ["git", "show", f"{args.base}:agent/gemini_schema.py"], cwd=root, text=True,
+        )
+        exec(compile(source, f"{args.base}:agent/gemini_schema.py", "exec"), baseline.__dict__)
+        fixed = adapter.sanitize_gemini_tool_parameters
+        adapter.sanitize_gemini_tool_parameters = baseline.sanitize_gemini_tool_parameters
+        before = probe(args.base, lambda tools: wire_translate(adapter, tools))
+        adapter.sanitize_gemini_tool_parameters = fixed
+        after = probe("worktree", lambda tools: wire_translate(adapter, tools))
+        assert before["scalar_control"] == "accepted"
+        assert all(result != "accepted" for name, result in before.items() if name != "scalar_control")
+        assert all(result == "accepted" for result in after.values())
+        print(f"PASS: {len(before) - 1} failing base cases repaired; scalar control and inputs preserved.")
+        print("LOCAL SDK VALIDATION ONLY: no Google API call or Windows Desktop exercise.")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/agent/test_gemini_type_arrays.py b/tests/agent/test_gemini_type_arrays.py
new file mode 100644
index 0000000000..614bb78bef
--- /dev/null
+++ b/tests/agent/test_gemini_type_arrays.py
@@ -0,0 +1,55 @@
+"""Native tool translation must preserve union alternatives at every depth."""
+import copy
+
+import pytest
+
+from agent.gemini_native_adapter import _translate_tools_to_gemini
+
+
+@pytest.mark.parametrize("types", [["string", "null"], ["integer", "null"], ["string", "integer"], ["string", "integer", "boolean", "null"], ["array", "object", "string"], ["null"], [], "string"])
+@pytest.mark.parametrize("location", ["properties", "items", "anyOf"])
+@pytest.mark.parametrize("nullable_first", [False, True])
+@pytest.mark.parametrize("constrained", [False, True])
+def test_native_type_arrays_preserve_alternatives(types, location, nullable_first, constrained):
+    node = {"nullable": False, "type": types} if nullable_first else {"type": types, "nullable": False}
+    node["description"] = "Keep this guidance"
+    if "array" in types:
+        node.update(items={"type": "integer"}, minItems=1)
+    if "object" in types:
+        node.update(properties={"name": {"type": "string"}}, required=["name", "undefined"])
+    if constrained:
+        node["anyOf"] = [{"enum": ["one"]}, {"enum": ["two"]}]
+    wrapper = {"properties": {"value": node}} if location == "properties" else {location: node if location == "items" else [node]}
+    params = {"type": "object", "properties": {"nested": wrapper}}
+    original = copy.deepcopy(params)
+    tools = [{"type": "function", "function": {"name": "probe", "parameters": params}}]
+    out = _translate_tools_to_gemini(tools)[0]["functionDeclarations"][0]["parameters"]["properties"]["nested"][location]
+    out = out["value"] if location == "properties" else out[0] if location == "anyOf" else out
+    expected = {t for t in types if t != "null"} if isinstance(types, list) else {types}
+    branches = out["anyOf"] if "type" not in out else [out]
+    actual = {branch["type"] for branch in branches}
+    for branch in branches:
+        if branch["type"] == "array":
+            assert branch["items"] == node["items"]
+            assert branch["minItems"] == node["minItems"]
+        if branch["type"] == "object" and "object" in types:
+            assert branch["properties"] == node["properties"]
+            assert branch["required"] == ["name"]
+        if "array" in types and branch["type"] != "array":
+            assert "items" not in branch
+    if constrained:
+        assert all(branch["anyOf"] == node["anyOf"] for branch in branches)
+    assert actual == (expected or {"null" if "null" in types else "object"})
+    assert not isinstance(out.get("type"), list)
+    if isinstance(types, list) and "null" in types and expected:
+        assert out["nullable"] is True
+    assert out["description"] == node["description"]
+    assert params == original
+
+
+@pytest.mark.parametrize("types,enum", [(["integer", "null"], [1, 2]), (["string", "integer"], ["one", 2]), (["boolean", "string"], [True, "other"])])
+def test_native_type_array_enums_remain_wire_strings(types, enum):
+    params = {"type": "object", "properties": {"value": {"type": types, "enum": enum}}}
+    tools = [{"type": "function", "function": {"name": "probe", "parameters": params}}]
+    node = _translate_tools_to_gemini(tools)[0]["functionDeclarations"][0]["parameters"]["properties"]["value"]
+    assert node["enum"] == [str(v).lower() if isinstance(v, bool) else str(v) for v in enum]
diff --git a/website/docs/guides/google-gemini.md b/website/docs/guides/google-gemini.md
index 001c3cb710..d047916b7d 100644
--- a/website/docs/guides/google-gemini.md
+++ b/website/docs/guides/google-gemini.md
@@ -76,6 +76,11 @@ Hermes detects this endpoint and creates its native Gemini adapter. Internally,
 - tool results → Gemini `functionResponse` parts
 - streaming responses → OpenAI-shaped stream chunks for the Hermes loop
 
+Tool parameter type arrays such as `"type": ["number", "null"]` are translated
+into Gemini's scalar type plus `nullable` form. Multi-type unions keep every
+alternative through `anyOf`, including nested properties and array items. This
+happens automatically; no MCP server or provider configuration change is needed.
+
 :::note Gemini 3 thought signatures
 For Gemini 3 tool use, Hermes preserves the `thoughtSignature` values attached to function-call parts and replays them on the next tool turn. That covers the validation-critical path for multi-step agent workflows.