v1 file-plugin loader never falls back to legacy named exports when a v2 default export is present

Start in packages/opencode/src/plugin/shared.ts at readV1Plugin, then trace applyPlugin and getLegacyPlugins in packages/opencode/src/plugin/index.ts. Run the relevant plugin loader tests and add coverage for a dual-shape module. Done means detect mode allows the named v1 hook to load while strict v1 handling still reports invalid defaults.

v1 file-plugin loader never falls back to legacy named exports when a v2 default export is present

Version: 1.18.31 (desktop AppImage; same core as CLI)
Area: packages/opencode/src/plugin/{index,shared}.ts — external file-plugin loading

What happens

A file plugin that carries both contracts — a named hook-map export (v1) and a default { id, setup } export (v2), like the official dcg-guard.js generated by dcg install --opencode — is silently skipped on the v1 runtime. Its tool.execute.before hook never fires. For a security guard this means protection is down with zero visible signal in the app (the failure is only an Effect log line).

Repro (live A/B, same hook function)

• ~/.config/opencode/plugins/dcg-guard.js (official, dual-shape): git reset --hard in a scratch repo executed (block missed).
• ~/.config/opencode/plugins/dcg-v1-shim.js containing only export const server = DcgGuard re-exported from the same file (same function reference, no default export): identical git reset --hard aborted with dcg's block message, worktree intact.

The only difference between the two cases is the module shape, so the loader — not the hook — is at fault.

Root cause

applyPlugin (plugin/index.ts) calls readV1Plugin(mod, spec, "server", "detect") first. With a default export of { id, setup } (has id, no server), readV1Plugin (plugin/shared.ts) does not bail out in detect mode — it falls through to:

if (kind === "server" && server === undefined) {
  throw new TypeError(`Plugin ${spec} must default export an object with server()`)
}

The throw rejects applyPlugin, the caller logs failed to load plugin and continues, and the legacy path (getLegacyPlugins, which would have picked up the named DcgGuard export) is never reached — it sits behind the if (plugin) branch that threw.

Notably the docs say a plugin "exports one or more plugin functions" and named-only modules (no default export) do load fine through the legacy path — so dual-shape modules are strictly worse than v1-only ones, which contradicts the documented contract.

Proposed patch

In detect mode, a non-matching default export should mean "not a v1 plugin", not an error — let the legacy path try:

--- a/packages/opencode/src/plugin/shared.ts
+++ b/packages/opencode/src/plugin/shared.ts
@@ readV1Plugin
   if (kind === "server" && server === undefined) {
+    if (mode === "detect") return
     throw new TypeError(`Plugin ${spec} must default export an object with server()`)
   }
   if (kind === "tui" && tui === undefined) {
+    if (mode === "detect") return
     throw new TypeError(`Plugin ${spec} must default export an object with tui()`)
   }

Strict mode (explicit v1 plugins) keeps throwing; only auto-detection falls back. Happy to PR this + a loader test if the approach looks right.