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Bug Fix: "Cannot call a class constructor without |new|"

The Problem

When loading the plugin, OpenCode would fail with:

TypeError: Cannot call a class constructor without |new|
    at BraintrustClient (/Users/ankur/.config/opencode/plugin/braintrust.js:12672:14)

Root Cause

OpenCode's plugin loader (packages/opencode/src/plugin/index.ts line 67-72) iterates through all exports of a plugin module and attempts to call each one as a plugin function:

for (const [_name, fn] of Object.entries<PluginInstance>(mod)) {
  if (seen.has(fn)) continue
  seen.add(fn)
  const init = await fn(input)  // Line 70 - calls every export as a function!
  hooks.push(init)
}

The original plugin was exporting:

  • BraintrustPlugin (function) ✅
  • default (same function) ✅
  • BraintrustClient (class) ❌

When OpenCode tried to call BraintrustClient(input) instead of new BraintrustClient(), it failed because classes must be called with new.

The Fix

Changed the export in src/index.ts from:

// BAD - exports the class
export { BraintrustClient } from "./client"

To:

// GOOD - only exports the type
export type { BraintrustClient } from "./client"

Now the plugin only exports:

  • BraintrustPlugin (function)
  • default (function)

Both can be safely called by OpenCode's loader.

Lessons Learned

  1. OpenCode plugins should only export functions that match the Plugin type signature
  2. Export types, not runtime values for classes and utilities
  3. Use export type { ... } for TypeScript types that shouldn't appear in the runtime exports
  4. Test plugin loading behavior by checking Object.keys() of the imported module

Verification

# Check exports (should only show functions)
node -e "import('./dist/index.js').then(m => console.log(Object.keys(m)))"
# Output: [ 'BraintrustPlugin', 'default' ]

# Test with OpenCode
BRAINTRUST_API_KEY=test-key opencode
# Should show "Login failed: 401" instead of class constructor error