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Keep development fast by using the lightest update strategy for each file change. Need fast feedback after each file change? Extension.js chooses the safest and fastest update path automatically:
  • HMR (hot module replacement) when module updates are safe
  • Classified reloads (full, service worker, content scripts, or notify-only page) when runtime assets change
  • Restart required errors/warnings when entrypoint structure changes

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Reload prerequisites (devtools setup dialog)

Before evaluating reload behavior, confirm the same setup checks shown in the Extension.js devtools Confirm setup dialog: If you skip either step, reload behavior can appear inconsistent even when the build pipeline is healthy.

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Reload tiers

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1) Hot module replacement (fastest path)

Extension.js uses HMR when code can update without restarting the extension’s background processes and event listeners. Common examples:
  • Scripts attached to extension HTML pages (accept updates through injected HMR wrappers)
  • Background script modules in non-service-worker flows
  • Scripts registered via the userScripts API
  • CSS updates in content-script/runtime wrappers where supported

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2) Classified reloads

When a change needs more than HMR, Extension.js classifies it into one of four reload kinds: The decision comes from the compiler’s chunk graph, not from filenames, and follows a fixed order:
  1. Forced full. A change to manifest.json or anything under _locales/ always classifies as full, no matter what else changed.
  2. Chunk membership. Extension.js looks up which chunks contain each changed source. A source in a background/ chunk means service-worker. A source in a content_scripts/ chunk means content-scripts.
  3. Changed static asset. Some changed files exist in the output directory but belong to no chunk: an icon, a web-accessible resource, a DNR ruleset. These force full, so the browser re-reads them from disk.
  4. Name heuristics. Only for sources the chunk graph does not know: paths matching background or service worker patterns classify as service-worker.
  5. Content fallback. If the manifest declares content scripts, remaining unknown changes re-inject every content-script entry.
  6. Notify-only page. Everything else is a page instruction: livereload owns the refresh, and the reload announcement still fires.
A source that lives in both a service-worker chunk and a content-script chunk fans out to both paths. One save triggers the SW restart and carries the stale content-script entries for re-injection, in a single classified instruction. Each dev reload is announced with a single context label built by the dev server. The format is context (fileA, fileB +2 more), for example service_worker + content_script (shared/api.ts). The same label appears verbatim in CLI stdout, in the page devtools console, and in the devtools pill, so you can correlate a reload across all three without guessing. If the browser serves a stale cached service worker after a reload, the dev server detects the version mismatch and resynchronizes the extension automatically. You should not need to manually remove and re-add the extension to recover.
Why do content script filenames include hashes in dev? Chrome aggressively caches chrome-extension:// resources. A stable filename like content-0.js can serve stale code even after a full extension reload. Extension.js appends a short build hash in development (for example, content-0.abcd1234.js). Each rebuild produces a fresh URL that bypasses the cache. Production builds use clean names. Set hashContentScripts: false in extension.config.* to opt out and keep stable dev filenames.

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3) Restart required (dev server)

When extension entrypoint references change (not just module contents), Extension.js reports restart-required diagnostics. This prevents you from continuing with a stale dependency graph. Typical restart-required scenarios:
  • Manifest script entrypoint list changes
  • HTML entrypoint script/style references change
  • pages/ / scripts/ file set changes in watch mode (especially removals)

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Behavior matrix

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How the dev runtime gets injected

The reload plugin instruments your build in five injection steps, then prunes:
  1. Strip the dev-server runtime from content-script chunks, which cannot host it.
  2. Set up the reload strategy on the background and content-script entries.
  3. Inject the service-worker scripts-replay shim, so /scripts/* injections re-run on edit.
  4. Inject the bridge producer, so the background forwards console output to the control channel.
  5. Inject the bridge relay, so content-script console output reaches the same channel.
Afterward, a pruning step manages hot/. Hot-update chunks are fetched from disk on the extension origin, so stale generations would accumulate in what ships. Extension.js keeps the current generation plus one previous generation for in-flight fetches, and deletes the rest after every compile. The whole pipeline is development-only. It is a no-op in production builds and when you pass --no-reload.

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Reloading scripts and HTML outside the manifest

pages/ and scripts/ follow the same reload strategy as manifest-declared assets.
  • Existing module updates can use hot-update paths.
  • Entrypoint set changes (add/remove or reference graph changes) may require restart.
Example:
Extension.js recognizes the /pages and /scripts folders for hot-reloading and treats each entry as a separate page or script it can reload independently.
Scripts you inject programmatically with chrome.scripting.executeScript from /scripts/* are replayed on edit. When you change a file under /scripts/, Extension.js re-runs the same injection on the same tab and tears down the previous mount, so dynamically injected scripts update live, the way declarative content_scripts do, instead of leaving stale DOM until you manually re-trigger the call. This is a dev-only convenience.

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Best practices

  • Keep entrypoint references stable during active dev sessions to maximize HMR.
  • Batch manifest.json and locale edits to avoid repeated hard extension reloads.
  • Use pages/ and scripts/ for off-manifest assets, then restart when file sets or entry wiring changes.
  • Treat restart required diagnostics as intentional safety checks, not transient warnings.

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Next steps