diff --git a/coworker/providers/gemini_provider.py b/coworker/providers/gemini_provider.py
index 683d21b2..93ef234a 100644
--- a/coworker/providers/gemini_provider.py
+++ b/coworker/providers/gemini_provider.py
@@ -257,7 +257,11 @@ def convert_messages(
 
 
 def _sanitize_schema(schema: Any) -> Any:
-    """Strip JSON Schema keys Gemini's OpenAPI subset rejects (recursively)."""
+    """Strip JSON Schema keys Gemini's OpenAPI subset rejects (recursively), and coerce
+    list-valued `type` (JSON Schema union, e.g. ["string", "number"] — common in vendor
+    MCP tool schemas) into shapes the API accepts: null joins as `nullable`, a single
+    remaining type stays `type`, several become `anyOf` (owner-hit 2026-07-23: monday's
+    compareValue union 400'd every Gemini turn in sessions with MCP tools)."""
     if not isinstance(schema, dict):
         return schema
     cleaned: dict[str, Any] = {}
@@ -270,6 +274,14 @@ def _sanitize_schema(schema: Any) -> Any:
             cleaned[key] = _sanitize_schema(value)
         elif key == "anyOf" and isinstance(value, list):
             cleaned[key] = [_sanitize_schema(sub) for sub in value]
+        elif key == "type" and isinstance(value, list):
+            types = [t for t in value if t != "null"]
+            if len(value) != len(types):
+                cleaned["nullable"] = True
+            if len(types) == 1:
+                cleaned["type"] = types[0]
+            elif types:
+                cleaned["anyOf"] = [{"type": t} for t in types]
         else:
             cleaned[key] = value
     return cleaned
diff --git a/tests/test_gemini_provider.py b/tests/test_gemini_provider.py
index f356042f..5064a7a8 100644
--- a/tests/test_gemini_provider.py
+++ b/tests/test_gemini_provider.py
@@ -634,3 +634,66 @@ def test_stream_yields_reasoning_deltas_for_thought_parts():
     assert [c.reasoning_delta for c in out if c.reasoning_delta] == ["mull ", "it over"]
     final = out[-1].turn
     assert final.text == "done" and final.reasoning == "mull it over"
+
+
+def test_sanitize_coerces_union_types():
+    """Vendor MCP schemas use JSON-Schema union types (owner-hit 2026-07-23: monday's
+    compareValue `type: ['string','number']` 400'd every Gemini turn). Nullable unions
+    become `nullable`, multi-type unions become anyOf."""
+    cleaned = _sanitize_schema(
+        {
+            "type": "object",
+            "properties": {
+                "compareValue": {
+                    "anyOf": [
+                        {"type": "string"},
+                        {"type": "array", "items": {"type": ["string", "number"]}},
+                    ]
+                },
+                "maybe": {"type": ["string", "null"]},
+                "nothing": {"type": ["null"]},
+            },
+        }
+    )
+    items = cleaned["properties"]["compareValue"]["anyOf"][1]["items"]
+    assert items == {"anyOf": [{"type": "string"}, {"type": "number"}]}
+    assert cleaned["properties"]["maybe"] == {"type": "string", "nullable": True}
+    assert cleaned["properties"]["nothing"] == {"nullable": True}
+
+
+def test_union_type_schema_validates_as_sdk_config():
+    """The exact failing shape must pass the SDK's GenerateContentConfig validation."""
+    types_mod = pytest.importorskip("google.genai.types")
+    tools = convert_tools(
+        [
+            {
+                "type": "function",
+                "function": {
+                    "name": "mcp__monday__search",
+                    "parameters": {
+                        "type": "object",
+                        "properties": {
+                            "filters": {
+                                "type": "array",
+                                "items": {
+                                    "type": "object",
+                                    "properties": {
+                                        "compareValue": {
+                                            "anyOf": [
+                                                {"type": "string"},
+                                                {"type": "number"},
+                                                {"type": "array", "items": {"type": "string"}},
+                                                {"type": "array", "items": {"type": ["string", "number"]}},
+                                            ]
+                                        }
+                                    },
+                                },
+                            }
+                        },
+                    },
+                },
+            }
+        ]
+    )
+    config = types_mod.GenerateContentConfig.model_validate({"tools": tools})
+    assert config.tools