Native provider controls

Use native controls when the portable request cannot express a provider feature precisely enough. This is an intentional trade: exact provider behavior in exchange for portability.

Availability: Rust: top-level x-* key · CLI: not exposed · Proxy/clients: provider_config["x-*"]

The built-in namespaces are:

Namespace	Target
`x-openai`	OpenAI Responses request fields
`x-chatgpt`	Supported ChatGPT subscription Responses fields
`x-anthropic`	Anthropic Messages fields and llmshim-managed Anthropic header controls
`x-gemini`	Gemini request fields and `thinkingConfig`

There is no x-xai namespace. llmshim's unified reasoning control already emits xAI's supported native reasoning: {effort} shape, including its model-family clamps.

OpenAI

x-openai fields are copied into the OpenAI Responses request body. This gives access to the full native reasoning object, including effort, mode, summary, and context when accepted by the selected model.

{
  "model": "openai/gpt-6-sol",
  "messages": [{"role": "user", "content": "Analyze this carefully."}],
  "x-openai": {
    "reasoning": {
      "effort": "high",
      "mode": "pro",
      "summary": "auto"
    }
  }
}

The object is native OpenAI configuration. llmshim does not clamp it, so the selected model must accept the values.

ChatGPT subscription

Use x-chatgpt in an engine request, or provider_config["x-chatgpt"] through the proxy. Supported fields are input, instructions, include, tools, tool_choice, reasoning, previous_response_id, and truncation. The routed model, stream: true, and store: false are enforced after native overrides; unsupported fields, including all token limits and metadata, are removed. reasoning.encrypted_content is always included in the upstream request. Returned chat history uses llmshim's existing normalized messages; it does not preserve opaque encrypted reasoning items. x-openai does not apply to this provider.

Anthropic

Most x-anthropic fields become Anthropic Messages body fields. Two special keys control headers instead:

disable_1m_context disables llmshim's automatic 1M-context beta header;
extra_betas appends and de-duplicates caller-supplied anthropic-beta tokens.

{
  "model": "anthropic/claude-sonnet-5-5",
  "messages": [{"role": "user", "content": "Analyze this carefully."}],
  "x-anthropic": {
    "thinking": {"type": "adaptive"},
    "output_config": {"effort": "high"},
    "disable_1m_context": true,
    "extra_betas": ["extended-cache-ttl-2025-04-11"]
  }
}

thinking and output_config are sent in the request body. The two control keys are consumed by llmshim while building Anthropic headers and are not sent as body fields.

The Rust contract also accepts top-level thinking and output_config, but the namespace keeps provider-native fields visibly grouped.

Gemini

x-gemini.thinkingConfig replaces the unified Gemini thinking configuration. It accepts Gemini's native thinking configuration:

{
  "model": "gemini/gemini-3.8-flash",
  "messages": [{"role": "user", "content": "Analyze this carefully."}],
  "x-gemini": {
    "thinkingConfig": {
      "thinkingLevel": "high",
      "includeThoughts": true
    }
  }
}

Other keys inside x-gemini are copied to the Gemini request body. As with all native controls, the provider validates their shapes and values.

Where the namespace goes

In the Rust serde_json::Value, use the namespace as a top-level key, as in the examples above.

The proxy reserves provider_config for fields that are merged into that core request. Wrap the same namespace inside it:

{
  "model": "openai/gpt-6-sol",
  "messages": [{"role": "user", "content": "Analyze this carefully."}],
  "provider_config": {
    "x-openai": {
      "reasoning": {
        "effort": "high",
        "mode": "pro",
        "summary": "auto"
      }
    }
  }
}

Language clients send the same object through their provider_config field. The CLI chat workflow does not expose native request configuration.

Precedence and portability

An explicit native reasoning object takes precedence over reasoning_effort/reasoning_mode. llmshim does not translate the object into another provider's dialect. Anthropic header controls are applied as headers; the other native fields are copied to their documented native destination.

If you change the model address to another provider, remove or replace the old provider namespace. Keeping x-openai in an Anthropic request does not recreate the OpenAI behavior. See Portable core, native edges for the underlying contract.

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