npx vitest run hangs when running full test suites via Copilot CLI terminal

[JackUkleja]

Describe the bug

Summary

Running npx vitest run for a full test suite hangs indefinitely when executed through the GitHub Copilot CLI terminal. The same command works fine in a regular PowerShell/cmd terminal.

Environment

• Tool: GitHub Copilot CLI (VS Code)
• OS: Windows
• Node: v22.14.0
• npm: 10.9.2
• Vitest: v4.1.2

Reproduction Steps

1. Open a Copilot CLI terminal session in VS Code
2. Run npx vitest run
3. Vitest starts, queues all test files, runs a few, then hangs with no further output
4. Typically nothing is seen piped back to the session window (the above has been deduced)

What Works

• Running a single test file works fine: npx vitest run some.test.ts
• Running batches of up to ~10 files works fine
• Running with JSON output to file works and completes successfully:

  npx vitest run --reporter=json --outputFile=test-results.json
  

Root Cause

According to copilot which tried to diagnose this:
Vitest's default live reporter continuously rewrites terminal output using ANSI escape sequences (cursor movement, line clearing) for all queued files simultaneously. This high-volume ANSI output appears to overflow or deadlock the Copilot CLI's stdout pipe, causing the process to hang.

Redirecting output to a file bypasses the pipe entirely and the suite completes normally.

Workaround

npx vitest run --reporter=json --outputFile=test-results.json

Suggested Fix

Affected version

vs code 1.120.0

Steps to reproduce the behavior

1. Open a Copilot CLI terminal session in VS Code
2. Run npx vitest run
3. Vitest starts, queues all test files, runs a few, then hangs with no further output
4. Typically nothing is seen piped back to the session window (the above has been deduced)

Expected behavior

See output of vitest

Additional context

No response