Responses Lite sends an empty description for the default functions namespace, rejected by Azure OpenAI

What version of Codex CLI is running?

codex-cli 0.147.0

What subscription do you have?

API/custom model provider using Azure OpenAI through LiteLLM; not tied to a ChatGPT subscription.

Which model were you using?

gpt-5.6-sol

What platform is your computer?

Windows 11, arm64

What terminal emulator and version are you using (if applicable)?

PowerShell

Codex doctor report

{
  "status": "not available for the affected Codex build"
}

What issue are you seeing?

When Responses Lite is enabled for a model and the provider supports namespace
tools, Codex groups normal function/custom tools into an additional_tools
item under the default functions namespace.

Codex serializes this namespace with an empty description:

{
  "type": "namespace",
  "name": "functions",
  "description": "",
  "tools": [...]
}

Azure OpenAI's Responses API rejects the request before inference:

HTTP 400
Invalid 'input[0].tools[0].description': empty string

Removing the field also fails:

Missing required parameter: 'input[0].tools[0].description'.

Changing only the description to a non-empty value, such as
"Built-in function tools", makes the same request succeed with HTTP 200.

This issue is conditional: it only occurs when the selected model has
use_responses_lite=true. Normal Responses requests do not reproduce it.

What steps can reproduce the bug?

1. Configure a Responses-compatible custom provider backed by Azure OpenAI.
2. Select a model whose metadata has `use_responses_lite=true`.
3. Ensure namespace tools are supported and start a normal Codex turn containing
   function tools.
4. Inspect the generated Responses request.
5. Observe that the first `additional_tools` item contains a `functions`
   namespace whose `description` is `""`.
6. Send the request to Azure OpenAI and observe HTTP 400.

Minimal relevant request shape:

```json
{
  "model": "affected-model",
  "input": [
    {
      "type": "additional_tools",
      "role": "developer",
      "tools": [
        {
          "type": "namespace",
          "name": "functions",
          "description": "",
          "tools": [
            {
              "type": "function",
              "name": "lookup_order",
              "description": "Look up an order",
              "parameters": {
                "type": "object",
                "properties": {}
              }
            }
          ]
        }
      ]
    },
    {
      "type": "message",
      "role": "user",
      "content": [
        {
          "type": "input_text",
          "text": "Reply only: ok."
        }
      ]
    }
  ]
}

### What is the expected behavior?

```markdown
Codex should not emit an empty required namespace description.

Possible fixes include:

- use a non-empty default such as `"Built-in function tools"` for the
  `functions` namespace;
- normalize empty descriptions for non-OpenAI/Azure providers; or
- disable Responses Lite namespace serialization when the configured provider
  cannot accept that wire format.

Existing non-empty namespace descriptions should remain unchanged.

Additional information

Current source explicitly creates and preserves this empty value:

• codex-rs/tools/src/responses_api.rs:
  default_namespace_description("functions") returns String::new().
• codex-rs/tools/src/tool_spec_tests.rs:
  responses_lite_preserves_empty_functions_namespace_description asserts that
  the serialized description equals "".
• codex-rs/core/tests/suite/responses_lite.rs also asserts the empty
  description in the generated additional_tools request.

Relevant source:

https://github.com/openai/codex/blob/main/codex-rs/tools/src/responses_api.rs

https://github.com/openai/codex/blob/main/codex-rs/tools/src/tool_spec.rs

https://github.com/openai/codex/blob/main/codex-rs/tools/src/tool_spec_tests.rs

The failure was validated through LiteLLM 1.95.0 against Azure OpenAI.
LiteLLM forwards the nested namespace description unchanged.

Transport does not appear to be the root cause: the invalid payload is produced
by Responses Lite serialization and can affect HTTP or WebSocket transports.

Start in codex-rs/tools/src/responses_api.rs at default_namespace_description("functions"), then read the related assertions in codex-rs/tools/src/tool_spec_tests.rs and codex-rs/core/tests/suite/responses_lite.rs. Update the behavior so the default functions namespace has a non-empty description while existing non-empty descriptions remain unchanged, and run the affected Responses Lite tests.