cli: opencode api silently truncates piped stdout at 256 KiB boundary (rc=0) #60395

Summary

opencode api get silently truncates large response bodies when stdout is a pipe: the output ends mid-JSON-string at 256 KiB / 320 KiB boundaries, with exit code 0 and no error on stderr. Redirecting to a file returns the full body. Consumers piping the CLI (scripts, subprocess capture) get corrupt JSON with no signal that anything went wrong.

Environment

• opencode version: 2.1.2
• OS: Linux 6.8.0-147-generic (linux x64)
• Terminal: TERM=xterm-256color, COLORTERM=truecolor
• Shell: /bin/bash
• Install/channel: latest (standalone binary, ~/.opencode/bin/opencode)
• Active plugins: None found in config

Reproduction

1. Find any GET endpoint whose response body exceeds 256 KiB. We used the session message list of a large session: opencode api get "/api/session/{sessionID}/message?limit=200" (body: 1,472,857 bytes).
2. Redirect to a file:

opencode api get "/api/session/{id}/message?limit=200" > out.json
# → 1,472,857 bytes, stable across 5 consecutive runs

3. Pipe the output (any pipe consumer):

opencode api get "/api/session/{id}/message?limit=200" | cat > out2.json
# → 262,144 bytes (exactly 256 KiB), body ends mid-string

4. Pipe from a program (Python):

import subprocess
r = subprocess.run(
    ["opencode", "api", "get", "/api/session/{id}/message?limit=200"],
    capture_output=True, text=True)
print(len(r.stdout), r.returncode, r.stderr)
# → 259721 rc=0 stderr=''   (first run)
# → 422484 rc=0 stderr=''   (second run)
# both truncate mid-JSON; json.loads() fails with "Unterminated string"

Expected Behavior

The full response body on stdout, identical to the file-redirect case, regardless of whether stdout is a TTY, file, or pipe.

Actual Behavior

When stdout is a pipe, the written body stops at a multiple of 64 KiB (observed: 262,144 = 256 KiB, and 259,721 / 422,484 ≈ 253.6 / 412.6 KiB via subprocess), mid-string in the middle of the JSON document. Exit code is 0 and stderr is empty, so the truncation is completely silent. File redirection is always complete and stable.

Additional Context

• Truncation boundaries are exact 64 KiB multiples, consistent with the final buffered chunk(s) being dropped on process exit when stdout is a pipe (buffered write not flushed, or non-blocking write loss on EAGAIN at shutdown).
• Reproduced consistently on 2026-10-30 with 2.1.2 (multiple runs, two different capture methods); file redirect never truncated in ~10 runs.
• Impact: any scripted consumption of opencode api output (automation, SDK-adjacent tooling, CI) can silently receive corrupt JSON for large responses. A size check against the Content-Length/body, or simply flushing before exit, should fix it; a warning when stdout is a pipe and the body exceeds some threshold would at least make it detectable.