[aidan-sims]

Owner: Aidan Sims. Ticket: FDE-2585. Builds on #465 (builtin_tools, merged), which touched the same file.

run_opencode passed the whole prompt as a single argv element. Linux caps one argument at 128 KiB (MAX_ARG_STRLEN), so a longer prompt failed exec with E2BIG before the CLI started. opencode-ai 1.17.18 reads stdin when it isn't a TTY, so the prompt, including its output-format suffix, now goes there.

• The prompt is no longer appended to argv. The CLI is spawned with stdin=PIPE, and _stream_cli feeds the prompt in a task that runs while stdout is read. This avoids a deadlock when the CLI prints before it has read all of its input.
• If the CLI exits before reading stdin (BrokenPipe/ConnectionReset), the write error is ignored and the CLI's stderr explains the exit. Any other write error fails the turn, because the CLI may have received only part of the prompt.
• Behaviour change: on main, opencode wrapped a positional prompt that contained a space in quotes and escaped its inner quotes. On stdin the model gets the prompt verbatim. This applies to every opencode run, including runs with no builtin_tools selection, so pass@k on opencode agents may shift slightly compared with earlier runs.

Validation: uv run pytest tst/unit/agents/opencode gives 119 passed, 7 skipped (Linux-only); its two builtin_tools tests now read the prompt from stdin. New tests cover first and resumed turns, a 1 MiB prompt, a real exec of a 1 MiB prompt (E2BIG on main), a CLI that exits without reading stdin, a real CLI that writes 300 KB to stdout and stderr before reading its prompt, and a write error that fails the turn. The last two were checked against mutants of the fix and fail on them. ruff check passes on the touched files.

Not in this PR:

• A run of the opencode agent image (AC4). The real pinned CLI was run instead, against a local fake model: a 512 KiB prompt fails with E2BIG on main on Linux, and arrives byte for byte on stdin.
• Publishing new hub agent versions (done separately by Env Pod after release).
• Reformatting the three existing touched files, which already fail ruff format --check on main.

🤖 Generated with Claude Code

Confidence Score: 5/5

The PR appears safe to merge; no actionable issues were found.

Summary

OpenCode prompts now travel through stdin instead of command arguments, avoiding the per-argument size limit.

• OpenCode turns send the full prompt through stdin.

Diagram

sequenceDiagram
  participant Bridge
  participant CLI as OpenCode CLI
  Bridge->>CLI: Start with piped stdin, stdout, and stderr
  par Feed prompt
    Bridge->>CLI: Write prompt and close stdin
  and Read events
    CLI-->>Bridge: Stream stdout events
  and Read errors
    CLI-->>Bridge: Stream stderr
  end
  Bridge->>CLI: Wait for exit
  Bridge->>Bridge: Build turn result

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