Stream Bash tool output in real-time during command execution

Problem

When Claude Code executes a Bash command (e.g., a build or evaluation script), the user sees only a spinner with elapsed time until the command completes. All stdout/stderr output is buffered and returned at once. During long-running commands (30s–5min+), there is no way to tell whether the process is making progress or frozen.

This is especially problematic when:

• Running test suites or evaluation scripts that report per-item progress.
• Building large projects where compiler output would indicate status.
• Executing multi-step scripts where each step produces output.

Current behavior

⠋ Running Bash command… (45s)

The user sees only this until the entire command finishes. Scripts that use Write-Host, Write-Progress, echo, or similar progress output are invisible during execution.

Expected behavior

Stream command output incrementally so the user can see real-time progress:

⠋ Running Bash command… (45s)
  [  5/20] seed 0004: 145832  (mean: 152340.2, 3.1s/seed)
  ##########---------- 50%  ETA: 47s

Motivation

• Eliminates ambiguity between "still processing" and "frozen" — a significant source of user anxiety and unnecessary session restarts.
• Enables scripts to communicate progress (progress bars, per-item results) as designed.
• Related: #14280 (VS Code-specific), #12996 (blank screen on Windows).

Environment

• Claude Code CLI v2.1.47 (standalone binary, Windows 11).
• Commands run via the Bash tool in terminal sessions.