Agent defaults: "Couldn't save." discards the backend validation message, making the failure undiagnosable

Start in desktop/src/features/agents/ui/AgentDefaultsEditor.tsx around line 233 and inspect the error shape returned by the Tauri set_global_agent_config call. Reproduce the validation failure with a derived provider or model key in env_vars, then verify that the backend's validation message is shown instead of only "Couldn't save.".

Summary

AgentDefaultsEditor discards the backend's validation message and shows a bare "Couldn't save." The backend's reason is accurate and actionable — it just never reaches the user.

Result: a save that can never succeed, with no indication of why, anywhere. No app log, no relay log (even at DEBUG), no config file touched.

Reproduce

1. Put a derived provider/model key into the global agent config's env_vars — e.g. GOOSE_PROVIDER or GOOSE_MODEL (easy to do if you have edited global-agent-config.json directly, e.g. as a workaround for a provider issue).
2. Open Agents → Agent defaults, change the harness or model, press Save defaults.
3. Every save fails with Couldn't save. and no further detail.

Cause

The backend correctly rejects it — desktop/src-tauri/src/managed_agents/global_config/mod.rs:113:

if !derived.is_empty() {
    return Err(format!(
        "the following keys must be set via the structured provider/model fields, \
         not as env vars: {}",
        derived.join(", ")
    ));
}

against DERIVED_PROVIDER_MODEL_ENV_KEYS (GOOSE_MODEL, GOOSE_PROVIDER, BUZZ_AGENT_MODEL, BUZZ_AGENT_PROVIDER).

The rule itself is right — Buzz derives those vars from the structured fields, so allowing both would create two sources of truth. The problem is purely that the message is thrown away in desktop/src/features/agents/ui/AgentDefaultsEditor.tsx:233:

} catch (err) {
  setSaveState("error");
  setSaveError(typeof err === "string" ? err : "Couldn't save.");
}

set_global_agent_config returns Result<GlobalAgentConfigSaveResult, String>, so the reason is a string on the Rust side — but whatever reaches the catch fails the typeof err === "string" test (Tauri appears to wrap it), so the fallback always wins.

Suggested fix

Unwrap the Tauri error rather than testing it as a bare string, e.g.:

const message =
  typeof err === "string" ? err :
  (err as { message?: string })?.message ?? "Couldn't save.";
setSaveError(message);

Even surfacing String(err) would be a large improvement over the current dead end.

Impact

The validation is a guard rail users are quite likely to hit — anyone who has hand-edited global-agent-config.json to work around a provider problem will land on it. As shipped, the only way to discover the cause is to read the Rust source.

Environment

• Buzz Desktop 0.5.x, macOS arm64
• Relay: self-hosted, built from source

Related

Same family as #3636 and #4278 — a correct signal produced internally, then discarded before it reaches the operator.