Windows on ARM64: Claude managed agents fail every session/new with -32603 (win-x64 Node provisioned on ARM64 host; error.data dropped from logs)

[Quintusnz]

Summary

On Windows 11 ARM64, every Claude-backed managed agent fails every room mention with:

Agent reported error (code -32603): Internal error

while OpenAI-backed managed agents in the same room work normally. Two separate defects combine here:

1. Buzz provisions a win-x64 Node runtime on ARM64 hosts, which breaks architecture-sensitive native-binary resolution inside Node-based ACP adapters (@agentclientprotocol/claude-agent-acp).
2. buzz-acp discards the JSON-RPC error.data payload when logging agent errors, which hid the adapter's fully actionable error message, making this look like an opaque Claude failure.

Environment

• Windows 11 Home ARM64 (10.0.26200) — Snapdragon-class machine
• Buzz desktop 0.5.3 (x64 build, running under Windows x64 emulation)
• @agentclientprotocol/claude-agent-acp 0.64.0, installed into the global npm prefix using the system arm64 Node v24.8.0 / npm
• Claude Code CLI 2.1.220, claude.ai subscription auth (claude auth status healthy)
• Buzz-managed Node runtime: %APPDATA%\Buzz\runtimes\node\v24.18.0\win-x64\node.exe (PE header: x64)

Root cause chain

1. buzz-desktop.exe and buzz-acp.exe are x64 binaries. On an ARM64 host they run under emulation, and managed_node_paths.rs selects the Node platform via std::env::consts::ARCH — a compile-time constant that is "x86_64" inside an x64 build regardless of the real machine. So Buzz provisions win-x64 Node. (The ("windows", "aarch64") => ("win-arm64", ...) arm added on main can never be taken by an x64-built binary.)

2. The npm shim claude-agent-acp.cmd resolves node from PATH; under Buzz that is the bundled win-x64 Node, so the adapter runs with process.arch === "x64".

3. On session/new, the adapter's claudeCliPath() resolves the SDK CLI as @anthropic-ai/claude-agent-sdk-win32-x64/claude.exe. Because the adapter was installed by an arm64 npm, only @anthropic-ai/claude-agent-sdk-win32-arm64 is vendored. Resolution fails and the adapter throws:

   Claude native binary not found for win32-x64. Reinstall @anthropic-ai/claude-agent-sdk without --omit=optional, or set CLAUDE_CODE_EXECUTABLE.

4. The ACP TypeScript SDK wraps this plain exception as { code: -32603, message: "Internal error", data: { details: "<the message above>" } }.

5. agent_error_from_json in crates/buzz-acp/src/acp.rs keeps only code and message whenever message is a string — error.data is dropped, so the actionable text in step 3 never appears in any Buzz log. Operators see only a generic agent_returned … error=Agent reported error (code -32603): Internal error.

Because the failure is thrown before the CLI is ever spawned, no amount of model-id, parallelism, or permission-mode configuration changes the outcome — which cost us several hours of ruling those out.

Reproduction (standalone, no Buzz needed)

Pipe ACP over stdio into the adapter, once under each Node:

initialize        {"protocolVersion":2, "clientInfo":{"name":"buzz-acp","version":"0.5.3"}, ...}
session/new       {"cwd":"<absolute existing dir>", "mcpServers":[], "_meta":{"sessionTitle":"repro"}}

Adapter launched under	session/new result
Buzz-bundled win-x64 Node v24.18.0	-32603 Internal error, data.details = "Claude native binary not found for win32-x64…"
System arm64 Node v24.8.0	success
win-x64 Node + CLAUDE_CODE_EXECUTABLE → the vendored win32-arm64 claude.exe	success

Workaround (confirmed working)

Add to the Claude agents' env_vars in managed-agents.json:

"env_vars": {
  "CLAUDE_CODE_EXECUTABLE": "C:\\Users\\<user>\\AppData\\Roaming\\npm\\node_modules\\@agentclientprotocol\\claude-agent-acp\\node_modules\\@anthropic-ai\\claude-agent-sdk-win32-arm64\\claude.exe"
}

The x64-emulated adapter process spawns the ARM64 CLI without issue.

Suggested fixes

1. Runtime arch detection for the managed Node runtime — decide win-x64 vs win-arm64 from the OS architecture at runtime (e.g. IsWow64Process2, or Rust: checking PROCESSOR_ARCHITEW6432/RuntimeInformation-equivalent), not std::env::consts::ARCH; or ship a native ARM64 build of Buzz.
2. Stop dropping error.data — agent_error_from_json should append data (or at least data.details) to the logged message when present. The adapter had put the exact remedy on the wire; Buzz logged "Internal error".

Possibly related in spirit but distinct from #2698 / #2692 (those are model-picker issues; this fails before any model is reached).

Also affected

The bundled Claude installer flow has the same arch blindness: install-claude.log on this machine shows Buzz fetching claude-2.1.220-win32-x64.exe, which then crashed with 0xC0000005 on the ARM64 host.

[iroiro147]

Proposed fix in #4121: resolves Windows ARM64/x64-emulation Node provisioning via IsWow64Process2 runtime detection, and preserves JSON-RPC error.data fields in agent_error_from_json (fixes both compounding defects). 2090 desktop lib + 662 buzz-acp lib tests pass; clippy clean.