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fix(chrome): enable WebMCP on managed Chrome 157 launches - #2516

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sx4im:fix-2513-managed-chrome-157-webmcp-flag
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sx4im wants to merge 1 commit into
vercel-labs:mainfrom
sx4im:fix-2513-managed-chrome-157-webmcp-flag

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@sx4im

@sx4im sx4im commented Nov 10, 2026

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Managed Chrome 157 launches still pass WebMCPTesting, which Chromium 157 removed, so WebMCP never gets enabled. The discovery test then sees a null WebMCP field and the fixture fails registering tools on an undefined document.modelContext.

This enables the WebMCP feature directly in the WebMCP launch branch. The legacy WebMCPTesting flag stays so older managed Chrome builds keep working. The launch-args unit tests now assert the exact WebMCP token is present by default and absent with --no-webmcp.

Verified: the new unit-test assertions fail against the old flag list and pass against the new one (checked with a standalone run of the same token logic), and cargo fmt is clean. I could not run the cargo test suite locally: the build was killed twice by the sandbox running out of memory, and a runtime restart wiped the build cache. CI is the gate for the unit tests and the Chrome 157 discovery test.

Fixes #2513

Chromium 157 removed the WebMCPTesting feature alias, so the managed
launch no longer exposed WebMCP: the discovery test saw a null WebMCP
field and the fixture's registerTool call failed on an undefined
document.modelContext. Enable the WebMCP feature directly in the WebMCP
launch branch. The legacy WebMCPTesting flag stays so older managed
Chrome builds keep working.

The launch-args unit tests now assert the exact WebMCP token is present
by default and absent with --no-webmcp.

Fixes vercel-labs#2513
@vercel

vercel Bot commented Nov 10, 2026

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@sx4im is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

@Osraka

Osraka commented Nov 10, 2026

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I ran a focused Ubuntu comparison for #2513 on upstream e7519957b9 with Chrome for Testing 157.0.8103.38. The existing discovery/invocation/cancellation E2E test failed at the expected null WebMCP assertion with the original flags, then passed after adding the explicit WebMCP feature while retaining the legacy flag. The existing opt-out E2E test passed both before and after.

Linux run and logs. This verifies the flag change in isolation, not this PR's exact head, the whole E2E suite, or older Chrome releases.

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Managed Chrome 157 launch still uses the removed WebMCPTesting flag

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