diff --git a/agent/gemini_schema.py b/agent/gemini_schema.py
index 904c99d31b8..1f179fb2af4 100644
--- a/agent/gemini_schema.py
+++ b/agent/gemini_schema.py
@@ -72,6 +72,18 @@ def sanitize_gemini_schema(schema: Any) -> Dict[str, Any]:
                 if isinstance(item, dict)
             ]
             continue
+        if key == "enum":
+            # Gemini's Schema.enum requires TYPE_STRING values — it rejects
+            # integer/float/bool enum entries even when the schema's ``type``
+            # is ``integer`` or ``number``.  Stringify every value so tools
+            # like discord_server's auto_archive_duration=[60, 1440, ...]
+            # don't trigger "Invalid value at enum[0] (TYPE_STRING)" 400s.
+            if isinstance(value, list):
+                cleaned[key] = [
+                    str(v) if not isinstance(v, str) else v
+                    for v in value
+                ]
+            continue
         cleaned[key] = value
     return cleaned
 
diff --git a/tests/agent/test_gemini_schema.py b/tests/agent/test_gemini_schema.py
new file mode 100644
index 00000000000..94f4538b72f
--- /dev/null
+++ b/tests/agent/test_gemini_schema.py
@@ -0,0 +1,66 @@
+"""Tests for agent.gemini_schema.sanitize_gemini_schema."""
+
+from agent.gemini_schema import (
+    sanitize_gemini_schema,
+    sanitize_gemini_tool_parameters,
+)
+
+
+def test_integer_enum_is_stringified():
+    """Gemini's Schema.enum only accepts TYPE_STRING values.
+
+    Tools like discord_server's auto_archive_duration declare integer enums
+    ([60, 1440, 4320, 10080]). Those would raise Gemini HTTP 400
+    'Invalid value at enum[0] (TYPE_STRING)' if passed through unchanged.
+    """
+    schema = {
+        "type": "integer",
+        "enum": [60, 1440, 4320, 10080],
+        "description": "Thread archive duration in minutes.",
+    }
+    cleaned = sanitize_gemini_schema(schema)
+    assert cleaned["enum"] == ["60", "1440", "4320", "10080"]
+    assert cleaned["type"] == "integer"
+
+
+def test_string_enum_is_preserved():
+    schema = {"type": "string", "enum": ["alpha", "beta", "gamma"]}
+    cleaned = sanitize_gemini_schema(schema)
+    assert cleaned["enum"] == ["alpha", "beta", "gamma"]
+
+
+def test_mixed_enum_is_stringified():
+    schema = {"enum": ["a", 1, 2.5, True]}
+    cleaned = sanitize_gemini_schema(schema)
+    assert cleaned["enum"] == ["a", "1", "2.5", "True"]
+
+
+def test_nested_enum_inside_properties_is_stringified():
+    schema = {
+        "type": "object",
+        "properties": {
+            "auto_archive_duration": {
+                "type": "integer",
+                "enum": [60, 1440],
+            },
+            "name": {"type": "string"},
+        },
+    }
+    cleaned = sanitize_gemini_tool_parameters(schema)
+    assert cleaned["properties"]["auto_archive_duration"]["enum"] == ["60", "1440"]
+    assert "enum" not in cleaned["properties"]["name"]
+
+
+def test_enum_in_items_is_stringified():
+    schema = {
+        "type": "array",
+        "items": {"type": "integer", "enum": [1, 2, 3]},
+    }
+    cleaned = sanitize_gemini_schema(schema)
+    assert cleaned["items"]["enum"] == ["1", "2", "3"]
+
+
+def test_non_list_enum_is_dropped():
+    schema = {"enum": "not-a-list"}
+    cleaned = sanitize_gemini_schema(schema)
+    assert "enum" not in cleaned