Windows: claude-code / claude_agent_sdk providers can't spawn the CLI — system prompt passed as argv exceeds the 32,767-char command-line limit (os error 206)

[edewveall-dev]

Summary

On Windows, routing chat through either Claude CLI provider (claude-code:<model> or claude_agent_sdk) fails on every turn because the harness system prompt is passed as a single command-line argument. The resulting command line exceeds Windows' 32,767-character CreateProcess limit, so the spawn fails before the CLI ever runs.

Problem

Environment: Windows 11 Home build 26200, OpenHuman beta v0.61.8 (official installer, desktop), Claude Code CLI v2.1.156 at C:\Users\<user>\.local\bin\claude.exe (the CLI works fine when invoked directly from a shell, both interactive and claude -p).

Steps to reproduce:

1. On Windows, install the Claude Code CLI and sign in.
2. Set chat_provider = "claude-code:claude-sonnet-5" in config.toml (or connect the Claude Code provider in the LLM settings UI).
3. Send any chat message.
4. Every turn fails; the UI shows the generic "Something went wrong" toast.

Expected: the turn is routed through the local Claude Code CLI.

Actual: the daemon log shows the spawn failing with the underlying OS error:

WRN [journal] run failed error=model error: model error: openhuman provider chat failed: failed to spawn `claude`: The filename or extension is too long. (os error 206)

With chat_provider = "claude_agent_sdk" the same failure occurs, but the log drops the OS error (the anyhow source isn't printed, which made this much harder to diagnose):

WRN [journal] run failed error=model error: model error: openhuman provider chat failed: failed to spawn claude binary 'C:\Users\<user>\.local\bin\claude.exe'

os error 206 (ERROR_FILENAME_EXCED_RANGE) is what CreateProcess returns when the command line exceeds 32,767 UTF-16 characters.

Solution (suspected cause + proposed fix)

• src/openhuman/inference/provider/claude_code/driver.rs builds argv with --append-system-prompt <system prompt> — the full tinyagents harness system prompt (log shows tools=78 on a plain chat turn) goes into one argv entry. Windows argv is a flat command-line string capped at 32,767 chars; macOS/Linux allow ~1–2 MB, which is why this only breaks on Windows.
• src/openhuman/inference/provider/claude_agent_sdk/subprocess.rs passes the entire composed message via -p <full_message> argv — same wall.
• Diagnostic confirmation: substituting the configured binary with C:\Windows\System32\cmd.exe fails identically, proving the args (not the binary) are the problem.

Proposed fix: deliver the system prompt out-of-band on Windows — the claude-code driver already streams messages over stdin as stream-json, so the system prompt could ride the same channel, or be written to a temp file referenced via the CLI's settings mechanisms — keeping argv short. Independent quick win: print the full error chain ({:#}) when spawn fails in claude_agent_sdk/subprocess.rs so the OS error isn't swallowed.

Acceptance criteria

• Repro gone — a chat turn routed through claude-code:<model> completes on Windows 11 with a harness system prompt larger than 32 KB.
• Regression safety — coverage for argv length on Windows spawn paths (or system prompt delivered via stdin/file on all platforms).
• Spawn errors logged with source — failed to spawn log lines include the underlying io::Error / OS error code.

Related

• Desktop shortcut doesn't restore app when minimized — only system tray works #4809 / fix(tauri/windows): restore minimized window on desktop shortcut/taskbar click #4818 (earlier Windows desktop-shortcut restore issue reported from this machine — same environment)