fix(plugin): background-agents fails to load on OpenCode Desktop due to class constructor and export issues

[sebascld]

📋 Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

📝 Bug Description

The background-agents.ts plugin fails to load entirely on OpenCode Desktop 1.2.27. Two separate errors occur in sequence, each preventing the plugin from initializing.

🔄 Steps to Reproduce

1. Install background-agents.ts in ~/.config/opencode/plugins/
2. Launch OpenCode Desktop 1.2.27
3. Observe sidecar logs

✅ Expected Behavior

The plugin loads successfully and registers the delegate, delegation_read, and delegation_list tools.

❌ Actual Behavior

Two sequential errors in the sidecar logs:

Error 1:
service=plugin error=Cannot call a class constructor TimeoutError without |new|

Cause: OpenCode's plugin bundler transpiles ES6 class syntax to functions. When
TimeoutError extends Error is transpiled, calls to new TimeoutError(...) inside
Promise.race() lose the new keyword, crashing the constructor.

Fix: Replace the class with an interface + factory function:

interface TimeoutError extends Error { readonly name: "TimeoutError"; readonly timeoutMs: number }
function makeTimeoutError(message: string, timeoutMs: number): TimeoutError { ... }
function isTimeoutError(e: unknown): e is TimeoutError { return e instanceof Error && (e as any).name === "TimeoutError" }

Error 2 (after fixing Error 1):
service=plugin error=undefined is not an object (evaluating 'client.app.log')

Cause: OpenCode calls every exported function/value from a plugin file as if it
were a plugin entry point. The file had export function logWarn(client, ...) and
export async function withTimeout(...) — OpenCode called logWarn(ctx) where
ctx is the PluginInput object, binding client = ctx. Since ctx has no .app
property, client.app.log throws.

Fix: Remove export from all utility functions — only BackgroundAgents and
export default should be exported.

Operating System

Windows

Agent / Client

OpenCode

Shell

Other

📋 Relevant Logs

2026-03-17T19:51:41Z service=plugin error=Cannot call a class constructor TimeoutError without |new| failed to load plugin
2026-03-17T20:02:35Z service=plugin error=undefined is not an object (evaluating 'client.app.log') failed to load plugin

💡 Additional Context

Both fixes are minimal and non-breaking. The root cause of Error 2 is an undocumented
behavior of OpenCode's plugin loader: it iterates all named exports and attempts to
invoke each one as a plugin. Plugin files should only export the plugin function itself.