`cargo build 2>&1 | tail -20` exits with tail's 0 even when the build
failed — bash without pipefail reports the last pipeline command's
status, and `cmd || echo failed` swallows the status the same way. The
model reads exit_code: 0 as a strong success signal and can conclude a
build passed while the visible output says it failed (community report,
Windows Rust builds; not platform-specific).
Two-part fix, mirroring OpenCode's prompt-side approach plus a
result-side backstop they don't have:
- Tool description now forbids piping builds/tests through
tail/head/cat (output is already auto-truncated + spilled to a file)
and warns that pipes/|| fallbacks mask exit codes.
- New annotate_masked_success() in tools/terminal_hints.py: when
exit_code == 0, the command shape can mask an upstream status
(top-level pipe into a passthrough consumer, or || echo/true), AND
the output carries strong tool-specific failure shapes (rustc,
cargo, pytest, gcc, npm, make, ninja), attach an advisory 'hint'
telling the model to treat the run as failed and re-run bare.
exit_code itself is never modified. Search/content heads
(grep/rg/echo/printf/...) are excluded to avoid false positives on
pipelines whose output legitimately contains error text.
E2E-verified through the real terminal tool path: hint fires on masked
cargo-style failures, silent on bare commands, clean pipes, and
grep/printf pipelines. 42 targeted tests pass.
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