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Documentation Index

Fetch the complete documentation index at: https://agentclientprotocol.com/llms.txt

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Elevator pitch

Add a new model_config category to session configuration options, so that agents can expose model-related parameters (context size, speed/quality trade-offs, etc) and clients can group them alongside the main model selector in the UI.

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Status quo

The category field on SessionConfigOption currently supports three values: mode, model, and thought_level. This works well when the model is a single selector, but some agents expose many model configurations — context window size, speed tier, and similar settings that logically belong next to the model picker.

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What we propose to do about it

Add a model_config variant to SessionConfigOptionCategory. Agents tag any model-related parameters with "category": "model_config", and clients render them near the primary model selector — for example as secondary controls within a model-picker popover or panel.

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Relationship to thought_level

Once model_config exists, thought_level is semantically a special case of a model configuration parameter. We keep thought_level as-is for backward compatibility — existing clients already handle it — but new model-related options should use model_config.

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Shiny future

Agents expose rich, parameterized model configurations over ACP.

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Implementation details and plan

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JSON format

An agent declares model-config options in configOptions: 
{
  "configOptions": [
    {
      "id": "model",
      "name": "Model",
      "category": "model",
      "type": "select",
      "currentValue": "sonnet-4.5",
      "options": [
        { "value": "sonnet-4.5", "name": "Sonnet 4.5" },
        { "value": "opus-4.6", "name": "Opus 4.6" }
      ]
    },
    {
      "id": "context_size",
      "name": "Context Size",
      "category": "model_config",
      "type": "select",
      "currentValue": "200k",
      "options": [
        { "value": "200k", "name": "200K" },
        { "value": "1m", "name": "1M" }
      ]
    },
    {
      "id": "fast_mode",
      "name": "Fast Mode",
      "category": "model_config",
      "type": "boolean",
      "currentValue": false
    }
  ]
}

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Client behavior

  • Clients SHOULD render model_config options near the model selector (e.g., in the same popover or panel).
  • Clients that do not recognize the category MUST handle it gracefully per the existing spec — the option still renders, just without special placement.
  • No new client capability negotiation is needed.

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Should thought_level move under model_config?

Not now. Existing clients already handle thought_level, so changing its semantics would be a breaking change. New model-related parameters should use model_config; thought_level remains for backward compatibility.

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Revision history

  • 2026-04-08: Initial proposal