shell tool returns "(no output)" when a single line exceeds the byte limit

Description

When a command prints a single line larger than tool_output.max_bytes (default 50 KB) and the output ends with a newline, the shell tool hands the model a truncation banner followed by (no output) — the whole body is dropped. The same content without the trailing newline returns the expected 50 KB tail.

Cause: tail() in packages/opencode/src/tool/shell.ts:236-250 walks the lines backwards and byte-slices the last line when nothing else fits, guarded by out.length === 0. split("\n") appends an empty sentinel for the trailing newline; the sentinel costs 0 bytes but is unshifted into out first, so that guard is never true for output ending in a newline and the byte-slice fallback is unreachable. bytes === 0 is the check that was intended.

The existing truncates output exceeding byte limit test misses it because its generator writes the bytes without a trailing newline and only asserts the banner, not the body.

Not #40728 (that one is about the spill file on disk — the spill file is complete here), and not #36795 / #35511 (intermittent capture races — this is deterministic).

Disclosure: found and reproduced with AI assistance; I reviewed and verified every claim locally.

Plugins

No response

OpenCode version

dev @ cc4b456 (v1.18.18)

Steps to reproduce

1. Make a file that is one 80 KB line ending with a newline:

bun -e 'Bun.write("big.txt","a".repeat(80000)+"\n")'

2. Ask the agent to run cat big.txt.

Expected: the last 50 KB of the line. Actual, 191 characters in total:

...output truncated...

Full output saved to: /…/tool-output/tool_xxx

(no output)

Commands that hit this in practice: cat on a minified bundle or a single-line JSON/lockfile, curl, jq -c, base64, docker inspect.

Screenshot and/or share link

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Operating System

macOS 15

Terminal

No response