v5.1.0

The engine closed its entire open bug range (Extension.js 61-73) in the
4.0.14 canary line. This release re-aligns the MCP with the fixed engine,
finishes the MCP-side half of those bugs, and continues the
report-failure-not-false-success program that 5.0.0 started. 5.0.0 was never
published to npm; installing 5.1.0 picks up both.

Fixed

• Sessions now genuinely survive the MCP process. detached: true alone
  never did it: the child held pipes to the MCP, so when the MCP exited the
  next compile log line killed the dev server with EPIPE. Launch tools
  (extension_dev/start/preview) now stream the child's output to a
  session log file (returned as logPath) instead of pipes. A detached
  session outlives the MCP and a fresh MCP process rediscovers it through
  ready.json and can stop it. Pinned by a detach-contract test.
• extension_preview no longer reports launched for a process that died
  in seconds. It health-checks the child like dev/start (the MCP half of
  engine bug 72), and all three launch tools read the engine's new
  browser_exited stamp, so a browser that dies after launch (for example a
  rejected add-on) returns status:"browser-exited" instead of success.
• extension_doctor names a dead browser. A browser_exited ready
  contract now produces a runtime-errors failure that says the browser died,
  with the matching remedy, instead of the generic "fix the build error"
  wording that pointed at a build that was fine.
• extension_create verifies the scaffold. A resolved create over a
  partial tree (an interrupted template download) returned nextSteps
  pointing at a project that could not compile. It now checks the manifest
  exists and returns status:"incomplete" when it does not.
• extension_manifest_validate is per-target honest. chromium:/
  firefox: prefixed keys resolve per target, edge joins the default
  matrix, manifest_version must be 2 or 3, a default_locale without its
  _locales catalog blocks, and a missing 128px store icon warns
  (extension_inspect reports has128Icon).
• Stale state stops being served as live. extension_logs stamps
  stale:true when the producing session is dead or from a different run;
  extension_wait returns runtimeErrors alongside ready instead of a bare
  green over a crashing worker; extension_build reports
  productionDivergence when the production manifest lost permissions or
  resources relative to source.
• extension_open's asTab fallback fires on the user-gesture wall, and
  extension_storage set without a key answers in MCP vocabulary rather
  than CLI flags.

Added

• Structured bundler warnings on extension_build. The engine now
  persists its build summary to dist/extension-js/<browser>/ build-summary.json (the transport half of engine bug 73), and the tool
  returns it as buildWarnings (with buildWarningsTruncated naming the
  true count when the engine capped the list). Older engines simply omit the
  field; nothing is scraped from stdout.
• Popup-faithful headless rendering. A popup rendered as a tab is now
  sized like the real popup: the document's content size is measured over
  CDP, clamped to Chrome's 25x25-800x600 popup bounds, and the window is
  resized to it (reported as renderedAsTab.popupBounds). If the browser
  does not verifiably honor the resize, the tool keeps saying "no popup
  sizing" instead of implying fidelity. Note headless-new is one such
  browser: it accepts Browser.setWindowBounds and changes nothing, so
  headless sessions get the honest fallback, not a resized window. The
  measurement also leaves body's authored width alone; only the root takes
  the temporary fit-content override, so a popup that sizes itself through
  body { width } measures at its real width.
• CI typechecks the tests. pnpm typecheck covers src/ and the test
  tsconfig, wired into the CI matrix, so type drift between tools and their
  tests cannot accumulate silently again.

Changed

• Tool prose caught up with the fixed engine. extension_eval and
  extension_dom_inspect now advertise the surface contexts
  (popup/options/sidebar/devtools) and override pages
  (newtab/history/bookmarks) the engine's relay serves, needing no tab
  id. The "content eval is known-broken" guard is version-honest: on
  Extension.js >= 4.0.14 a null is the expression's real result, and the note
  says so instead of condemning a repaired path. Firefox hints name every
  working route.

A pass focused on a single question: when something has gone wrong, does the
tool say so? Five tools were reporting success over a failure. All five now
verify before they claim anything.

Breaking

• extension_build refuses a broken build. It runs the
  extension_manifest_validate checks as a preflight and returns
  status:"blocked" on build-blocking errors instead of shelling out to a build
  it knows is broken. Pass skipValidation: true for the old behavior. It also
  returns success:false with status:"incomplete" when the bundler exits 0 but
  a declared entrypoint never reached dist/, because the browser refuses to
  load that artifact. Non-blocking findings ride along as manifestWarnings.
• Browser resolution defaults to chrome, not chromium. A dead session
  used to fall through to a blind default, so every call after a dev server
  exited silently retargeted a browser the caller never ran. A dead session now
  resolves to its own browser with source:"stale".
• extension_open renames tab to target. The value is a CDP target id,
  not a chrome.tabs id, and the old name invited callers to pass it straight
  into tools that need a numeric tab id.

Fixed

• extension_doctor no longer reports healthy:true over a crashing
  extension. Its runtime-error check read the wrong field, so every error row
  in logs.ndjson collapsed to an empty string and was skipped. It now reads the
  engine's messageParts payload, with an errorName/stack fallback, and
  collapses a throw that repeats on every event.
• extension_dev and extension_start no longer report status:"started"
  for a server that already exited. Both health-check the child process and
  return status:"exited" with the exit code, signal, and the child's own output
  as evidence.
• extension_open no longer reports success for a navigation that failed.
  Navigating to a chrome-extension:// origin is cross process and swaps the
  render frame, so the pre-navigation session reported a stale error URL on
  success and success on failure. It now confirms against a fresh target list.
  This affected the url navigation path shipped in 4.9.0, not only the new
  surface rendering.
• extension_open targets the right extension. A dev session also loads
  Extension.js's own manager extension, and taking the first extension target
  navigated against the wrong origin. The id is now derived from the dist path
  the session actually loaded.

Added

• Headless surface rendering. extension_open accepts asTab for
  popup/options/sidebar, rendering the surface document in a real tab so it
  can be inspected where no window exists to host a popup. It is applied
  automatically when a headless session refuses to open the surface, with a note
  saying what was substituted.
• Tab targeting by url. extension_eval and extension_dom_inspect take a
  url and otherwise default to the active tab, and extension_dom_inspect
  gains listTabs for discovery. The engine gained this in 4.0.13; the tool
  descriptions had been telling callers a numeric tab id was required.
• Friendlier arguments. timeoutMs, lines, tabId, href and
  browserName fold onto their canonical names, withConsole accepts true,
  and the input validator understands union types.
• extension_create matches your package manager. Hints and the engine
  warning now use bun, pnpm or yarn when that is what the scaffold used, and the
  warning reads the pin the scaffold actually wrote.