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#968·sandcastle

OpenCode agent provider fails with spawn E2BIG for prompts over Linux argv limit

Author: zero-two-rafaeltabCreated Aug 29, 2026Updated Sep 1, 2026

Description

The OpenCode agent provider fails with spawn E2BIG when a resolved prompt exceeds Linux's per-argument process limit. Sandcastle currently shell-escapes the full prompt into the positional opencode run command, so the process fails to spawn before OpenCode starts.

Reproduction

Minimal Docker-sandbox reproduction:

https://github.com/zero-two-rafaeltab/sandcastle-opencode-e2big-repro/tree/43d385175449ecfba54dc7683b7619a085e020c0

bash
git clone https://github.com/zero-two-rafaeltab/sandcastle-opencode-e2big-repro.git
cd sandcastle-opencode-e2big-repro
git checkout 43d385175449ecfba54dc7683b7619a085e020c0
make reproduce

The reproduction uses a synthetic 150,000-byte ASCII inline prompt and exits with:

ExecError: exec failed: spawn E2BIG

No credentials are required and no model request is made.

Versions

Verified on 2026-08-29:

  • @ai-hero/sandcastle 0.12.0 — the version in this repository and the current latest npm release
  • opencode-ai 1.18.25 — the current latest npm release
  • OpenCode model configuration: openai/gpt-5.6-luna, variant low
  • Linux with the Docker sandbox provider

Root cause

For the same 150,000-byte prompt, the built-in providers produce:

Agent provider Command bytes Stdin bytes
OpenCode 150,106 0
Codex 78 150,000
Claude Code 115 150,000
Pi 47 150,000

OpenCode places the prompt in the command; the other providers transport it via stdin.

On Linux, a single process argument reaches the boundary at 128 KiB:

131071 ok
131072 E2BIG

This is a process argument limit rather than an agent context-window error. Shell escaping may make the assembled command larger still.

Expected behavior

The OpenCode agent provider should transport prompts that are valid for the agent without failing at the operating-system argument boundary. A clearer preflight error would improve diagnostics but would not resolve the transport limitation.

Related issues

  • #387 diagnosed the general Linux MAX_ARG_STRLEN failure.
  • #393 moved another provider's prompt transport to stdin to fix #387.
  • #703 introduced the current OpenCode provider behavior.
  • #928 discusses prompt transport and shell-command expansion for Claude Code.

Source: mattpocock/sandcastle

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