If a module worker’s dependency graph imports the worker’s own entry URL, Safari can evaluate that entry script a second time and create a second module instance. That is what WebKit Bugzilla issue 324459 reports for Safari 26.5 on macOS 26.5.1. The issue was filed on September 17, 2026, is marked NEW, and has no resolution in the accessible record. It is one reporter’s account, and Safari 27 is not confirmed as fixed or affected.
What the bug report says
A page starts a worker with new Worker(url, { type: "module" }). Something in the worker’s graph then imports the worker’s entry URL. The reporter says this can happen three ways:
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- a static import cycle back to the entry,
- a dynamic
import()of the entry, - a code-split lazy chunk that imports from the worker entry.
In the minimal reproduction, the expected result is {"evals":1,"same":true}. The reported Safari result is {"evals":2,"same":false}. The entry body ran twice, and the namespace you get from the import is not the one the running worker already has. The report cites Chrome 153 and Firefox 156 as returning the existing module. Those are the reporter’s tested versions, not a guarantee for any other release.
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The report names Safari 26.5 on macOS 26.5.1. It also says another developer hit the problem in Safari 26.3.
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What is not affected, according to the reporter
- The same import pattern in a document, rather than a worker, behaves correctly in Safari.
- A worker entry and a chunk that both import a separate shared module are not affected.
- Importing the entry by absolute URL, or by dynamic import, is described as affected like the relative static case.
Treat this as what one person observed, not a full compatibility matrix.
Why it violates the expected behavior
The WHATWG HTML Standard (a living document; the page read for this article was last updated 2026-10-04) says: “the module map is used to ensure that imported module scripts are only fetched, parsed, and evaluated once per Document or worker”. The same module URL should therefore evaluate once.
The reporter’s explanation is that Safari behaves as if the worker’s top-level script is missing from that module map. That is a diagnosis of the observed behavior, not a root cause confirmed by WebKit. The map is keyed by URL, so genuinely different URLs, such as a cache-busted entry.js?v=2, are not necessarily the same module. Only the same URL should be deduplicated.
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The reporter’s production case was a lazily loaded ProRes decoder that registered itself in a module registry. The code that checked the registry was looking at a different copy. The reporter attributes this to a Vite 6.4 / Rollup 4 build in which a lazy chunk imported the worker’s entry chunk.
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A second evaluation can silently duplicate anything held at module scope:
- registries and caches,
- singletons and initialization flags,
- WebAssembly instances,
- top-level side effects such as message handlers.
This is one concrete report. It does not show that all Vite or Rollup builds, or all Safari apps, are affected.
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- Quickly import your W-2, 1099, 1098, and last year's personal tax return, even from TurboTax and Quicken Software
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How to check your own project
- Build the project and open the emitted worker entry file and its chunks.
- Search the output for imports that reference the worker entry’s filename, as the reporter suggests. Any lazy or shared chunk importing the entry is the risky shape.
- Reproduce the generated graph in Safari. Serve it over local HTTP, because the supplied reproduction needs it: module workers do not load from a
file://address. - Add a temporary counter or log at the top of the entry and check whether it fires twice, or compare the namespace identity of the imported entry with the running one.
- Run the same test in Chrome and Firefox to confirm the behavior is Safari-specific.
Workarounds that follow from the report
The report does not give an official fix, so these are inferences from its stated boundaries, not tested recommendations. The only boundary it states is that a worker entry and chunk both importing a separate shared module is not affected. So the safest approach is to keep stateful code out of the entry file. Put registries, caches and singletons in a separate shared module, and have both the entry and any lazy chunks import that module. Avoid chunk layouts where a lazy chunk imports back from the entry.
Safari 27 status
A WebKit contributor, Alexey Proskuryakov, replied on September 17, 2026: “Thank you for the report! Could you please try Safari 27?” WebKit’s September 2, 2026 post says Safari 27 adds top-level-await spec compliance through a ground-up module-loader rewrite. That post does not claim bug 324459 is fixed, and the accessible issue record has no result from the Safari 27 test. Until that is reported, test your own build on Safari 27 before assuming either way.
Limits of the evidence
The main source is an open issue and the reporter’s account. The Chrome and Firefox results are the reporter’s. Version numbers and the {"evals":2,"same":false} output come from a single reproduction, not a survey of how many sites are affected.
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