Windows: postinstall fails to copy binary from optionalDependency — stub remains 479 bytes

Description

After installing or updating opencode-ai via npm on Windows, the opencode command fails with:

Error al ejecutar el programa 'opencode.exe': El ejecutable especificado no es una aplicación válida para esta plataforma de sistema operativo.

The root cause: the postinstall.mjs script fails to copy the real platform-specific binary over the stub. The bin/opencode.exe remains a 479-byte stub instead of the ~174MB real binary.

Environment

• OS: Windows 10/11 x64
• Node.js: v22.16.0
• npm: global install
• opencode-ai version: 1.18.11 (also reproducible on 1.18.15)

Steps to Reproduce

1. npm install -g opencode-ai
2. opencode --version
3. Error: binary not valid for platform

Diagnosis

The package structure:

opencode-ai/
  bin/opencode.exe          ← 479 bytes (STUB - broken)
  postinstall.mjs           ← should copy real binary
  node_modules/
    opencode-windows-x64/
      bin/opencode.exe      ← 174MB (REAL binary)

The postinstall.mjs script:

1. Detects platform (windows) and arch (x64)
2. Checks AVX2 support
3. Should copy opencode-windows-x64/bin/opencode.exe to bin/opencode.exe
4. Fails silently — stub remains unchanged

Running the postinstall manually works:

node "C:\Users\...\npm\node_modules\opencode-ai\postinstall.mjs"
# After this, bin/opencode.exe is 174MB and works

Additional Context

• The real binary IS downloaded as an optionalDependency (opencode-windows-x64)
• The postinstall.mjs uses fs.linkSync with fallback to fs.copyFileSync
• No error is thrown — the script exits cleanly but the stub is not replaced
• This happens consistently across multiple install/update cycles
• Windows Defender may be blocking the binary copy operation

Suggested Fix

The postinstall.mjs should:

1. Verify the target binary was actually replaced (check file size or hash)
2. Throw an explicit error if the copy/link failed
3. Or: skip the stub entirely and have npm use the optionalDependency binary directly

Workaround

Run the postinstall manually after each install/update:

node "$(npm root -g)/opencode-ai/postinstall.mjs"