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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
  • Hard extension reload when runtime-critical 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) Hard extension reload

Some file changes require reloading the extension runtime itself. Typical hard reload triggers:
  • manifest.json
  • _locales/**.json
  • Service worker/background runtime-critical changes
  • Content script output changes (the Extension.js browser runner enforces extension reload for consistency)
Extension.js applies browser-specific hard reload mechanisms internally (Chromium and Firefox flows differ under the hood) but unifies behavior at the CLI level. Each dev reload is announced with a single context label built by the dev server (naming what changed and which surface it affects). The same label appears 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.

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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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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