Agent detection disables colors in user code via shared tinyrainbow singleton

[christopher-buss]

Describe the bug

Vitest 4.1.2 calls disableDefaultColors() from tinyrainbow 3.1.0 when std-env's isAgent is true (e.g. running inside Claude Code, Cursor, etc.):

// vitest/dist/chunks/cac.DRKYQDPl.js
import { isAgent } from 'std-env';
import { disableDefaultColors } from 'tinyrainbow';

if (isAgent) disableDefaultColors();

disableDefaultColors() mutates tinyrainbow's shared default export singleton via Object.assign(), turning all color functions into identity functions. This affects not just Vitest's own output, but also user code under test that imports tinyrainbow — because both Vitest and user code resolve to the same singleton instance.

This means any test that asserts on ANSI-colored output (e.g. testing a CLI formatter that uses tinyrainbow) silently breaks when run inside an AI agent environment.

Reproduction

1. Create a module that uses tinyrainbow:

import color from "tinyrainbow";
export const warn = (msg: string) => `${color.yellow("⚠")} ${msg}`;

2. Write a test asserting colored output:

it("should produce yellow warning symbol", () => {
  expect(warn("oops")).toContain("\x1b[33m⚠\x1b[39m");
});

3. Run with CLAUDECODE=1 (or any env that makes std-env detect an agent):

CLAUDECODE=1 vitest run

The test fails because color.yellow becomes an identity function.

Expected behavior: Vitest's agent detection should only affect Vitest's own reporter output, not mutate shared state that user code depends on.

Possible solutions:

• Use createColors() for Vitest's internal rendering instead of mutating the default singleton
• Scope the disable to Vitest's reporter layer rather than the global default

System Info

  System:
    OS: Windows 11 10.0.26100
    CPU: (32) x64 AMD Ryzen 9 9950X3D 16-Core Processor
    Memory: 37.30 GB / 93.64 GB
  Binaries:
    Node: 25.8.2 - C:\Users\Christopher\.local\share\mise\installs\node\25.8.2\node.EXE
    npm: 11.11.1 - C:\Users\Christopher\.local\share\mise\installs\node\25.8.2\npm.CMD
    pnpm: 10.33.0 - C:\Users\Christopher\.local\share\mise\installs\npm-corepack\0.34.6\pnpm.CMD
    bun: 1.3.11 - C:\Users\Christopher\.local\share\mise\installs\bun\1.3.11\bin\bun.EXE
    Deno: 2.7.10 - C:\Users\Christopher\.local\share\mise\installs\deno\2.7.10\bin\deno.EXE
  Browsers:
    Edge: Chromium (140.0.3485.54)
    Firefox: 149.0 - C:\Program Files\Mozilla Firefox\firefox.exe
  npmPackages:
    @vitest/coverage-v8: catalog:test => 4.1.2
    @vitest/eslint-plugin: catalog:lint => 1.6.13
    vitest: catalog:test => 4.1.2

Used Package Manager

pnpm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.

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Generated with Claude Code
Verified manually by me!

[github-actions]

Hello @christopher-buss. Your content has been labeled maybe automated because it appears to have been fully generated by AI with no human involvement. It will be closed automatically in 3 days unless a real person responds.

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[christopher-buss]

I have personally reviewed and standby this content - I asked Claude Code to add this issue for me! Came up because my snapshot tests suddenly started failing using an agent, but would pass when I ran myself in powershell. Sorry should've added the CC attribution in the PR. Apologise for missing the GH template. Will update.