[robobun]

Problem

• vm.runInContext(code, ctx) throws TypeError: Proxy is not allowed in the global prototype chain. when the context global has a Proxy anywhere in its prototype chain. jsdom 28+ puts one there (WindowProperties), so jsdom under node:vm and Vitest pool: 'vmThreads' fail on Bun. Node prints the result. Fixes node:vm rejects a Proxy in a DONT_CONTEXTIFY global prototype chain, breaking jsdom with Vitest vmThreads #42331.
• The main realm has the same problem. After Object.setPrototypeOf(globalThis, Object.create(new Proxy({}, {}))), vm.runInThisContext() and CommonJS require() throw the same error.
• The check is JavaScriptCore's, in ProgramExecutable::initializeGlobalProperties. It runs before every program links. Bun has no hook to skip it, so the fix needs [JSC] Let an embedder allow a Proxy in the prototype chain of its global object WebKit#626.

Fix

• [JSC] Let an embedder allow a Proxy in the prototype chain of its global object WebKit#626 adds JSGlobalObject::setAllowsProxyInPrototypeChain(). A global object that opts in skips the check. Bun::GlobalScope::finishCreation (src/jsc/bindings/BunGlobalScope.cpp) opts in, so every Bun global is covered: main, workers, shadow realms and node:vm contexts.
• Correct because every Bun global already clears IsImmutablePrototypeExoticObject from its structure (ZigGlobalObject.cpp:443, NodeVM.cpp:941), so user code can set its prototype. Global declaration instantiation in JSC reads only own properties of the global, so no Proxy trap runs while a program links. Name lookup at run time goes through the ordinary [[HasProperty]], [[Get]] and [[Set]] paths, which handle a Proxy. That matches V8.
• WEBKIT_VERSION points at the preview build of [JSC] Let an embedder allow a Proxy in the prototype chain of its global object WebKit#626. Repin it to the merged sha before this merges.
• Verified: test/js/node/vm/vm.test.ts, block "a Proxy in the prototype chain of the context global" (4 tests, all fail on Bun 1.4.2). Also the rest of test/js/node/vm, the 95 vendored test/js/node/test/parallel/test-vm-*.js, and jsdom 30 under vm.runInContext.

Background

• A ProgramExecutable is what vm.runInContext, vm.runInThisContext, new vm.Script().runIn* and the CommonJS loader evaluate. Modules and eval do not go through it, so they never hit the check.
• An immutable prototype exotic object is an object whose [[SetPrototypeOf]] refuses a new value. The spec makes Object.prototype one, and JSC makes global objects one by default. Bun clears that flag on its globals so jsdom can set __proto__ on the window.
• WebKit added the check in 2016 (bug 165205) for the web, where Window.prototype is mutable and var declaration walked the prototype chain at the time. JSC's declaration code is own-property only today.

Notes

Node behaviour the tests follow: in a DONT_CONTEXTIFY context a missing name throws ReferenceError. In a contextified context it reads as undefined (the sandbox interceptor). Node's has/get/set traps fire on a Proxy in the chain for unresolved names and sloppy assignments, and the Proxy target is never written (the receiver is the global). Bun now produces the same results.

Pre-existing, unrelated: var and function declarations in a DONT_CONTEXTIFY context do not create bindings in Bun, with or without a Proxy in the chain. The declaration test therefore uses a contextified context.

test/js/node/vm/script-leak.test.ts times out at 5 s in the container used here, on main as well as on this branch (16 s of parse time for 10000 scripts in a debug ASAN build).

Self-reviewed: the review found that the first version opted in only the node:vm global and missed the main realm, and that the PR body claimed the main global could not get a Proxy in its chain. Both fixed: the opt-in moved to Bun::GlobalScope, and a main realm test was added. The review also asked for the preview pin to be replaced by the merged sha, which is the repin step above.

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no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/vm/vm.test.ts

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Reviewing files that changed from the base of the PR and between 6a92015 and 776a00d.

📒 Files selected for processing (3)

• scripts/build/deps/webkit.ts
• src/jsc/bindings/BunGlobalScope.cpp
• test/js/node/vm/vm.test.ts

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[robobun]

^{Updated 11:07 AM PT - Sep 11th, 2026}

✅ @robobun, your commit 776a00d5645519a5b843f075a881f5a2f9dec7b0 passed in Build #114362! 🎉

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🧪   To try this PR locally:

bunx bun-pr 42347

That installs a local version of the PR into your bun-42347 executable, so you can run:

bun-42347 --bun

[claude]

🔴 WEBKIT_VERSION is pinned to autobuild-preview-pr-626-f1e23f97, an ephemeral preview-PR build tag; merging this makes every fresh checkout and CI lane depend on an artifact that disappears once oven-sh/WebKit#626 closes, breaking the build for everyone (base pins a merged SHA). Fix: before merge, repin to the merged upstream SHA and verify prebuilt WebKit artifacts exist for every platform × flavor, per scripts/build/deps/README.md and the Dependencies & vendoring rule. The PR body notes this as a repin step, but the pin as committed is still the merge-blocker.

Extended reasoning...

scripts/build/deps/webkit.ts:6 sets WEBKIT_VERSION = "autobuild-preview-pr-626-f1e23f97". WebKit is a prebuilt dep — resolveDep downloads a release artifact keyed on this string from oven-sh/WebKit. Preview-PR autobuild tags are deleted when the PR is merged or closed, so after that point dep_fetch 404s and bun bd / CI fails at the WebKit fetch step on any machine without a cached copy. The base branch pins a real merged SHA (cf1b36ec…) whose artifacts are permanent. The repo's dependency-pin convention explicitly forbids merging a pin to an ephemeral artifact; the author's own PR body says "Repin it to the merged sha before this merges", confirming the tag is temporary.

Verification: normal — acknowledged in diff: the PR body states "WEBKIT_VERSION points at the preview build of oven-sh/WebKit#626. Repin it to the merged sha before this merges", which is accurate but does not resolve the hazard; the code as it stands is not mergeable. scripts/build/deps/webkit.ts:6 changes the pin from a permanent merged SHA to an ephemeral preview-PR tag: ``` -export const…

[robobun]

Agreed. The preview tag is only there so CI can build against oven-sh/WebKit#626 before it merges. I will repin WEBKIT_VERSION to the merged sha as soon as that PR lands, and this PR stays a draft for merging until then. Leaving this thread open as the reminder.

[github-actions]

Superseded by #44833.

[robobun]

Confirmed. #44833 pins a WebKit that skips the Proxy check for every Bun global (ProgramExecutable.cpp, oven-sh/WebKit#781), so this change is no longer needed. The issue repro prints 2 on 1.4.4-canary.1.