fix(mcp): disable the chromium sandbox for the bundled browser on linux

[Yury Semikhatsky (yury-s)]

Summary

• An undefined channel with browserName: 'chromium' resolves to the same downloaded build as the chromium and chrome-for-testing channels, so treat it the same when defaulting chromiumSandbox on linux.

Fixes #42452

[Pavel Feldman (pavelfeldman)]

channel !== 'chrome-for-testing' <-- I would kick this out.

[Yury Semikhatsky (yury-s)]

It's in the same category. The downloaded builds ship chrome_sandbox, but it's extracted from a zip as -rwxr-xr-x user user and archives can't carry setuid root.

[github-actions]

Test results for "MCP"

1 failed
❌ [firefox] › mcp/cli-core.spec.ts:97 › fill @mcp-windows-latest-firefox

8307 passed, 1377 skipped

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Merge workflow run.

[github-actions]

Hi, I'm the Playwright bot and I took a first look at the CI failure here.

🟢 The one failure is a pre-existing flake — this PR is clear

The single red is [firefox] › mcp/cli-core.spec.ts:97 › fill on mcp-windows-latest-firefox. This PR only changes how chromiumSandbox defaults for downloaded Chromium builds on Linux — it can't affect a Firefox MCP test on Windows. And the same test failed on a non-PR push (SHA d529911d, 2026-07-28) with the identical error, so it predates your change.

Details

Pre-existing flake / infra

• [firefox] › mcp/cli-core.spec.ts:97 › fill — Windows-Firefox-only flake. Across the test-results DB this test is green almost everywhere: mcp-windows-latest-firefox 2 failed / 694 runs, and 0 failures on the ubuntu (701 runs) and macOS (707 runs) bots. Crucially, one of those two failures was run 30317255877 on SHA d529911d (a push, not this PR), with the exact same assertion:

  expect(received).toBe(expected)
  Expected: "- textbox [active] [ref=e2]: Hello, world!"
  Received: undefined
  

  The snapshot occasionally comes back without the filled textbox on Windows-Firefox. This PR touches only packages/playwright-core/src/tools/mcp/config.ts (chromiumSandbox defaulting for chromium/chrome-for-testing/undefined channels on Linux) and a Linux-gated test — nothing in the Firefox fill/snapshot path.

_{Triaged by the Playwright bot - agent run}