diff --git a/strix/agents/factory.py b/strix/agents/factory.py
index c6e48fd9..5de2cbcb 100644
--- a/strix/agents/factory.py
+++ b/strix/agents/factory.py
@@ -287,13 +287,15 @@ def _with_strictness(tool: FunctionTool, strict_schemas: bool) -> FunctionTool:
 
 def _merged_variants(variants: list[dict[str, Any]]) -> dict[str, Any] | None:
     merged: dict[str, Any] = {}
+    types: list[str] = []
     for variant in variants:
-        if not isinstance(variant.get("type"), str):
+        if not isinstance(variant.get("type"), str) or variant["type"] in types:
             return None
         if merged.keys() & (variant.keys() - {"type"}) or variant.keys() & {"enum", "const"}:
             return None
         merged |= variant
-    merged["type"] = [variant["type"] for variant in variants]
+        types.append(variant["type"])
+    merged["type"] = types
     return merged
 
 
diff --git a/tests/test_agent_tool_registration.py b/tests/test_agent_tool_registration.py
index b9f6474a..069f8c57 100644
--- a/tests/test_agent_tool_registration.py
+++ b/tests/test_agent_tool_registration.py
@@ -184,6 +184,7 @@ def test_nullable_parameter_keeps_its_variant_schema() -> None:
         {"anyOf": [{"type": "string", "const": "a"}, {"type": "null"}]},
         {"anyOf": [{"type": "string", "format": "uri"}, {"type": "array", "format": "x"}]},
         {"anyOf": [{"$ref": "#/$defs/Mode"}, {"type": "null"}]},
+        {"anyOf": [{"type": "string", "minLength": 2}, {"type": "string"}]},
     ],
 )
 def test_unmergeable_unions_are_left_untouched(spec: dict[str, Any]) -> None: