# ACP: Session Config Options

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | cap-protocol |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | ACP: Session Config Options |
| Source URL | https://agentclientprotocol.com/protocol/session-config-options |
| Source publisher / author | Agent Client Protocol project |
| Source type | standard |
| Source date (published) | Unknown — not stated |
| Date researched | 2026-10-25 |
| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
| Research method | direct page fetch |
| Confidence | High |
| Status | Active |

## Summary

The ACP mechanism by which an agent advertises per-session selectors (model, mode, reasoning level) and a client changes them. It is the closest existing standard to 'list an agent's models and pick one'.

## Key findings

- Each config option has `id`, `name`, optional `description`, optional `category`, `type` (`select` by default, `boolean` only if the client advertised support), `currentValue`, and `options` (flat values or groups, not mixed).
- Reserved categories: `mode`, `model`, `model_config`, `thought_level`; names starting with `_` are custom; clients MUST handle missing or unknown categories gracefully.
- The agent MAY return `configOptions` during session setup; the client sets one with `session/set_config_option` (`sessionId`, `configId`, `value`) and the agent MUST answer with the complete list of options and values.
- The agent announces changes with a `session/update` of `sessionUpdate: config_option_update` carrying the full `configOptions` array.
- Agents MUST always provide a default for every option; a client that meets an unknown `type` SHOULD ignore that option and the agent keeps its default.
- Order is significant: higher-priority options first; clients use the order to break ties within a category.
- If an agent provides `configOptions`, clients SHOULD use them instead of `modes`, which will be removed.

## Pros / strengths

Normative (MUST/SHOULD) and small; gives a live, per-agent, per-session model list instead of a static one, with a defined default and ordering.

## Cons / limitations / caveats

Only exists for agents that speak ACP; the page does not say how large option lists should be capped; the `model_config` category is separate RFD territory.

## Relevance to tai

tai's catalog reads a manifest list or a `--cap-models` probe, both static. For `acp_stdio` agents the live truth is the `model`-category config option from session setup. Recommended as the first discovery source for that binding, with ACP's default-on-unknown rule as the error-resistance model.

## Changelog

| Date | Change |
|---|---|
| 2026-10-25 | Initial dossier created. |