# Check stdin rather than stdout for interactive terminals (GitLab MR #3383)

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | shell-multiplexer-auto-attach |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | Check stdin rather than stdout for interactive terminals |
| Source URL | https://gitlab.com/gitlab-org/gitlab-development-kit/-/merge_requests/3383/pipelines |
| Source publisher / author | GitLab (gitlab-development-kit project) |
| Source type | repository |
| Source date (published) | Unknown — not stated |
| Date researched | 2026-10-09 |
| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
| Research method | web search synthesis |
| Confidence | Medium |
| Status | Active |

## Summary

A real-world merge request fixing an interactivity check that tested `stdout.isatty()` when it should have tested `stdin.isatty()` — the two can legitimately disagree (e.g. output piped to a pager while input stays a real terminal, or vice versa).

## Key findings

- `stdin.isatty()` and `stdout.isatty()` are independent facts and can differ in either direction in real, common shell usage (piping, redirection).
- Which one is the "right" one to check depends on what the check is actually trying to answer (can I read user input? vs. can I write formatted/colored output?) — conflating them is a real, shipped bug class.

## Pros / strengths

- A fixed, real production bug (not a hypothetical) — concrete evidence this mismatch actually bites people, not just a theoretical edge case.

## Cons / limitations / caveats

- Specific to one project's bug fix; doesn't generalize a universal rule beyond "check the stream that matches what you actually need, and don't assume stdin/stdout agree."

## Relevance to tai

A direct implementation warning for the proposal's `.bashrc` detection logic: if it ever needs to distinguish "can prompt a human" from "output looks like a terminal," check `stdin` specifically (`[ -t 0 ]`), not `stdout`, and never assume the two will match.

## Changelog

| Date | Change |
|---|---|
| 2026-10-09 | Initial dossier created. |