Overview

Atomic is an open-source AI coding agent runtime with a Rust-powered TUI (terminal UI), built by a small San Francisco startup. I contributed a feature that visually distinguishes which subagent stage is active when the agent pauses for human input — a frequently requested UX improvement.

The Problem

When Atomic runs multi-stage workflows, the user can be prompted for input by the main thread or by any number of named subagents. Before this change, all input prompts looked identical — there was no way to know which stage was asking, making multi-agent workflows confusing to navigate.

What I Built

Three distinct UI cases, each requiring a different injection strategy: Case 1 — Editor input (typing mode) A [stage: name] label is injected post-render into the editor's top border rule, replacing ─ fill characters. Done after CustomEditor.render() completes so the internal border detection and cursor placement (❯) remain unaffected. Case 2 — ask_user_question widget Label injected into the widget's top rule via a dedicated embedStageLabelInWidgetTopRule() helper, which runs independently of the existing ctrl+x hint injection pass. Case 3 — AWAITING INPUT banner (simple prompts) Label appended to the top border of the awaiting input card via a new makeBorderTopStyled() helper that accepts a pre-styled ANSI string. stageName was threaded through PromptCardIdentity and all call sites. Label styling: [stage: in muted text, stage name in bold, truncated with … at the 40-column floor. No background tint — dropped after reading the maintainer's design notes (warning colors conflict with bash mode and disappear under NO_COLOR).

Code Review

The PR went through Greptile automated review, which surfaced two issues I fixed:

• P1 — Off-by-2 in the truncation budget for long stage names, which would overflow the terminal border at exactly the worst time. Fixed the column arithmetic.
• P2 — Test border regex was too permissive after my changes. Tightened to /^╭[^╮]*╮$/ plus an explicit 100-column width assertion to catch future layout regressions.

Result

867/867 test files pass (1 pre-existing failure on main unchanged). PR open, awaiting maintainer review.

Tech Stack

• TypeScript — TUI rendering logic
• Vitest — Test suite
• ANSI escape codes — Terminal styling
• Atomic TUI architecture — packages/workflows/src/tui/