[Windows] ACP local connectors spawn literal .exe names — miss .cmd shims, unhelpful ENOENT, mojibake in ACP log

[equalsthreerussian-collab]

Summary

On Windows, Devin Desktop's local ACP connectors spawn agent binaries by literal executable name. Two of the three built-in external agents fail out of the box:

• OpenCode: spawn opencode.exe ENOENT — a standard npm install of opencode only places opencode (shell script) and opencode.cmd shims on PATH (%APPDATA%\npm). The real opencode.exe is not exposed as a top-level binary, so a literal-name lookup never finds it.
• Google Antigravity: spawn agy_acp_server.exe ENOENT — the ACP kernel is a separate artifact distributed via the ACP registry (dl.google.com/agy-extensions/releases/windows/agy-acp-server-...zip). Neither the Antigravity IDE nor the agy.exe CLI installs it, so the connector can never succeed after a normal Antigravity install.

Codex works because its connector runs npx -y @agentclientprotocol/codex-acp@... — npx resolves .cmd shims itself, so its PATH lookup succeeds.

Actual behavior

Devin ACP.log shows the connector flagging the binary missing, then failing spawn:

[warning] [antigravity-acp] Binary "agy_acp_server.exe" not found in PATH; spawn will fail and the agent will be disabled
[info]    [antigravity-acp] Spawning process: agy_acp_server.exe
[error]   [antigravity-acp] [error] spawn agy_acp_server.exe ENOENT
[error]   Agent "antigravity-acp" failed to activate: Command "agy_acp_server.exe" not found. Ensure the agent binary is installed and on your PATH.
[error]   [antigravity-acp] Activation failed: Command "agy_acp_server.exe" not found. Ensure the agent binary is installed and on your PATH. — ��������, �ᯮ��塞�� �ணࠬ��� ��� ������ 䠩���.

The agent then shows Failed in Settings → Agents with no actionable guidance. Three compounding issues:

1. PATHEXT resolution missing: the warning is correct that the literal .exe name isn't on PATH, but a opencode.cmd shim is — the lookup ignores the PATHEXT-style resolution that cmd.exe/shell spawns get for free.
2. Unhelpful error for separate ACP artifacts: for Antigravity, "ensure the agent binary is installed and on your PATH" gives no hint that agy_acp_server is a distinct download not shipped with the IDE/CLI.
3. Mojibake in the log: the localized Windows error appended to the failure (— ��������, �ᯮ��塞��...) is CP866/CP1251 text decoded as UTF-8. On non-English locales this makes the failure look like an encoding bug rather than a missing binary — it sent me debugging the wrong thing.

Expected behavior

• Binary resolution on Windows should try PATHEXT extensions (agent.exe, agent.cmd, agent.bat) or spawn via a shell, matching how these CLIs are actually installed.
• For agents whose ACP binary is a separate artifact, the error should name the artifact and where to get it (e.g. "install agy_acp_server from the ACP registry").
• Log output should decode child-process text using the OEM/ANSI codepage (or sanitize non-UTF-8 bytes) so localized Windows errors don't render as mojibake.

Environment

• Windows 11, Russian system locale (CP1251 ANSI / CP866 OEM)
• Devin Desktop (Agents panel / ACP connectors), session logs dated 2026-09-18
• opencode 1.18.31 via npm; Antigravity agy.exe 1.x

Workarounds verified locally

• Copied the real opencode.exe into %APPDATA%\npm → ACP handshake succeeds (protocolVersion 1, agent OpenCode 1.18.31).
• Downloaded agy_acp_server_1.1.1-windows-x86_64.zip via the official ACP registry entry into %LOCALAPPDATA%\agy\bin → connector initializes and advertises oauth-personal / gemini-api-key auth methods.