MCP schema conversion collapses array items.oneOf(object) to string[]

Summary

Codex appears to lose MCP tool input schema fidelity when an array's items schema is expressed via oneOf.

In practice, this causes object-array parameters from MCP tools to be exposed to the model as string[] instead of an array of structured objects.

Expected behavior

Given a schema like:

{
  "type": "array",
  "items": {
    "oneOf": [
      {
        "type": "object",
        "properties": {
          "id": { "type": "string" }
        },
        "required": ["id"]
      }
    ]
  }
}

Codex should preserve the nested union/object shape, or at least normalize it to an equivalent supported union form.

Actual behavior

Codex parses the items schema as string, which effectively turns the overall parameter type into string[].

Minimal repro

I added a failing regression test on my fork here:

https://github.com/attila-kun/codex/commit/86258a13e12873823d072d977d1aabdcc73eb328

The test is:

• json_schema::tests::parse_tool_input_schema_preserves_array_item_one_of_object_shape

Run:

cd codex-rs
cargo test -p codex-tools parse_tool_input_schema_preserves_array_item_one_of_object_shape

Failing output

running 1 test
test json_schema::tests::parse_tool_input_schema_preserves_array_item_one_of_object_shape ... FAILED

Diff < left / right > :
 JsonSchema {
     schema_type: Some(
         Single(
             Array,
         ),
     ),
     ...
     items: Some(
         JsonSchema {
<            schema_type: Some(
<                Single(
<                    String,
<                ),
<            ),
>            schema_type: None,
             ...
<            any_of: None,
>            any_of: Some(
>                [
>                    JsonSchema {
>                        schema_type: Some(
>                            Single(
>                                Object,
>                            ),
>                        ),
>                        ...
>                    },
>                ],
>            ),
         },
     ),
     ...
 }