buzz-acp model switch never matches claude-agent-acp config options: expects configId, adapter sends id #2794

Summary

resolve_model_switch_method() in crates/buzz-acp/src/acp.rs reads the configId key from each session/new configOptions entry and skips any entry that lacks it. The claude-agent-acp adapter (v0.61.0) keys those entries with id, not configId. Every model-category entry is therefore skipped, the desired model never matches, and the session silently falls back to the CLI default from the user's ~/.claude/settings.json.

Reproduction

Buzz Desktop 0.4.24, macOS arm64, claude-agent-acp 0.61.0, Claude Code CLI 2.1.219.

1. Set a Claude Code agent's model to Opus in Buzz Desktop (per-agent or global default).
2. Restart the agent. buzz-acp is launched with BUZZ_ACP_MODEL=opus[1m] as expected.
3. Ask the agent what model it is running. It reports whatever ~/.claude/settings.json pins (in my case claude-fable-5[1m]), not Opus.

Harness log shows the fallback:

WARN pool::model: desired model opus[1m] not found in agent's available models - proceeding with agent default

The pool::model tracing target is not written to the per-agent log files, so nothing user-visible records the miss.

Root cause

The adapter's actual session/new response (captured with a stdio JSON-RPC probe against the bundled adapter):

{
  "configOptions": [{
    "id": "model",
    "name": "Model",
    "category": "model",
    "type": "select",
    "currentValue": "claude-fable-5[1m]",
    "options": [
      { "value": "default" },
      { "value": "opus[1m]" },
      { "value": "claude-fable-5[1m]" },
      { "value": "sonnet" },
      { "value": "haiku" }
    ]
  }],
  "models": null
}

Note the key is id. But resolve_model_switch_method() does:

let config_id = match config_opt.get("configId").and_then(|v| v.as_str()) {
    Some(id) => id,
    None => continue,   // claude-agent-acp entries always take this branch
};

So opus[1m] is a valid option value sitting right there in the response, and the exact-match loop never sees it. The unstable models path cannot rescue it either because the adapter returns models: null.

The unit tests all use configId-keyed fixtures, which is why they pass despite the drift.

The set side is unaffected: the ACP SDK's SetSessionConfigOptionRequest schema takes configId as the request param, and the adapter's setSessionConfigOption handler looks entries up by params.configId against o.id. Only the read side in buzz-acp is wrong.

Relationship to existing issues and PRs

• [Bug] Claude Code agents ignore the model set in Buzz Desktop — model picker persists but nothing applies it at spawn #2692 reports the user-visible symptom. A comment there traces this exact WARN but attributes the miss to a cold-start model-discovery race. It is not a race: the option list is present and complete in the session/new response, the lookup just reads the wrong key.
• fix(desktop): apply the configured model to Claude Code agents via ANTHROPIC_MODEL #2695, fix(desktop): Claude model picks + agent avatars after restart #2701, fix(desktop): apply Claude models and run Windows installers natively #2696 inject ANTHROPIC_MODEL at spawn from the desktop side. That masks the symptom for spawn-time selection but leaves the ACP config-option switch path broken, including for any future mid-session switching (feat(agents): allow changing an agent's model mid-session (like /model in Claude Code) #2543).

Fix

Accept both spellings when extracting the config id:

config_opt.get("configId").or_else(|| config_opt.get("id")).and_then(|v| v.as_str())

PR incoming with this change plus a regression test using an id-keyed payload that mirrors the real adapter response.