Windows: worker system prompt via argv exceeds 32,767-char limit → bare 'spawn ENAMETOOLONG' for every lane (0/N batch)

[HenryLach]

Summary

On Windows, spawnAgent (extensions/taskplane/agent-host.ts:455) passes the composed worker system prompt as a single argv element (--system-prompt <text>). The base worker template (templates/agents/task-worker.md) is 28,497 bytes in 0.30.6; once a project's .pi/agents/task-worker.md overlay pushes the composed command line past Windows' 32,767-char CreateProcess limit, every lane fails at spawn with a bare spawn ENAMETOOLONG in the engine-worker stderr. The engine then marks the head task failed and skips its lane dependents — a 0/N batch with zero worker execution and no actionable message.

Reproduction (second occurrence)

• 2026-09-08, batch 20260908T234433 (Penster, taskplane 0.30.6, Windows 11, node 24): 6 tasks / 3 lanes; all three lane heads Runtime V2 execution error: spawn ENAMETOOLONG; 3 failed + 3 skipped in 2m16s. Overlay was 4,433 bytes → composed argv ≈ 33.2K.
• 2026-07-17, batch 20260717T235502 TP-101: same signature (first occurrence).

Diagnosis took the operator's supervisor a lane-log read plus a byte count; the engine log gave nothing beyond the errno.

Two asks, one issue

1. Immediate — preflight length check on win32 before spawn

In spawnAgent, on process.platform === "win32", compute the composed command-line length (execPath + cli + all args incl. --system-prompt, with Windows quoting) and, if > 32,767, fail BEFORE spawn with an actionable error, e.g.:

worker command line is 33,226 chars; Windows limit is 32,767.
System prompt = base template (28,497) + .pi/agents/task-worker.md (4,433).
Trim the overlay by ≥ 459 chars or upgrade to a build that passes the prompt via file.

Surface it as the task's failure reason (so the supervisor alert names it), not only engine stderr.

2. Real fix — do not pass the system prompt via argv

Write the composed prompt to a temp file under the batch runtime dir (or pipe via stdin) and pass --system-prompt-file <path> (or equivalent) to pi. Removes the platform limit entirely and lets the base template grow without silently eating every project's overlay headroom (today a Windows project has ~3.9K chars of overlay ceiling).

Notes

• Reviewer (11.1K base) and merger (7.1K base) compositions are far from the limit; only the worker template is close.
• Related Windows-arg finding filed as Intra-lane dependency promotes the dependent to the next wave, gating it on unrelated lanes #639 (intra-lane dependency wave promotion) — unrelated cause, same batch family.