Feature: Dynamic model selection via ACP configOptions + Discord slash command

[chaodu-agent]

Summary

Add a /model Discord slash command that dynamically lists available models from the ACP agent backend (e.g. Kiro CLI) and lets users switch models via an interactive Discord Select Menu — all driven by the ACP configOptions protocol.

Motivation

Currently, OpenAB binds to a single agent backend configured in config.toml. There is no way for users to discover or switch models at runtime from Discord. The ACP spec already defines session/configOptions with category: "model" for exactly this purpose — OpenAB just needs to bridge it to Discord UI.

Proposed Flow

User types /model in Discord
        ↓
OpenAB reads configOptions from the current ACP session
  (category: "model" → list of available models)
        ↓
OpenAB renders a Discord Select Menu with the model options
        ↓
User picks a model (e.g. "opus-4")
        ↓
OpenAB sends session/set_config_option to the ACP agent
  { configId: "model", value: "opus-4" }
        ↓
Agent confirms → OpenAB updates the Discord message
  "✅ Switched to Opus 4"

Implementation Details

1. Discord Slash Command Registration

Register /model as a guild command on bot startup (EventHandler::ready):

guild_id.set_commands(&ctx.http, vec![
    CreateCommand::new("model")
        .description("Select the AI model for this session")
]).await?;

2. ACP configOptions Parsing

After session/create, parse the configOptions array from the response and cache options with category: "model".

3. Discord Select Menu

Convert ACP configOptions into a Discord StringSelectMenu:

• Each ConfigOptionValue → one select menu option
• currentValue → default selection
• custom_id → acp_config_{configOption.id}

4. Interaction Handler

On interaction_create for component interactions with custom_id starting with acp_config_:

1. interaction.defer_update()
2. Send session/set_config_option via ACP JSON-RPC
3. Update the Discord message with confirmation

5. Agent-Initiated Updates

Handle config_option_update notifications from the agent (e.g. model fallback due to rate limits) and optionally notify the Discord thread.

Extensibility

The same mechanism can be reused for other ACP config categories:

Slash Command	ACP Category	Purpose
/model	model	Switch AI model
/mode	mode	Switch session mode (ask/code)
/thinking	thought_level	Set reasoning level

References

• ACP Session Config Options spec
• ACP Slash Commands spec
• Discord Select Menu docs

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