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Pass Metro's own @babel/runtime to Babel presets - #1957

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Oct 19, 2026
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Pass Metro's own @babel/runtime to Babel presets#1957
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Helpers that @babel/plugin-transform-runtime imports from @babel/runtime can fail to resolve, depending on where, or whether, a project installs it. Worse, presets don't know which version they'll get, so the plugin assumes the oldest and inlines newer helpers instead, eg _interopRequireWildcard into every module that uses import * as (react/react-native#57123).

This tells Babel presets to import helpers from Metro's own @babel/runtime, which always exists, at its installed version, so the plugin can target that version. It builds on the scheme resolvers from #1804, which Metro uses for its own metro: namespace.

With transformer.enableBabelRuntime: true (the default), the wiring is:

  1. The transform worker passes the Babel transformer metro:babel-runtime and the installed version of Metro's @babel/runtime. That copy's package.json joins the transform cache key. (This PR)
  2. metro-babel-transformer forwards both to presets as Babel caller data, named babelRuntimeModuleName and enableBabelRuntime. (This PR)
  3. @react-native/metro-babel-transformer, which React Native projects use instead, builds its own caller and needs to forward them too. (To do)
  4. @react-native/babel-preset configures @babel/plugin-transform-runtime with that moduleName and version. (Landed in Allow configuring Babel runtime via caller in @react-native/babel-preset (#57974) react-native#57974)
  5. Helpers are emitted as metro:babel-runtime/helpers/... imports, which Metro's metro: resolver maps to metro-runtime's @babel/runtime. (Landed in Metro 0.87.1)
  6. Metro adds that @babel/runtime to the file map, wherever it's installed. (Landed in Add Metro's own @babel/runtime to the file map as a dynamic root #2065, unreleased)

Caller data reaches a preset however it's configured, including from a project babel.config.js. In a React Native project, this is a no-op until step 3. Expo CLI's Babel transformer doesn't forward these yet either.

A string enableBabelRuntime (a version of the project's own @babel/runtime) keeps its meaning and passes neither value. It's now documented as deprecated.

Changelog

 - **[Feature]**: When `transformer.enableBabelRuntime` is `true`, pass Metro's own `@babel/runtime` (`metro:babel-runtime`) and its installed version to Babel presets via caller data
 - **[Deprecated]**: String values of `transformer.enableBabelRuntime` (use `true`)

Test plan

New integration test bundles and runs a fixture that does import * as values from './values', with a babel.config.js that reads the caller data as @react-native/babel-preset does. @babel/runtime only exports this helper from 7.14.0, so it's imported rather than inlined only if the version reaches the preset. Metro's @babel/runtime is outside the test's watch folders, so this also exercises #2065.

With enableBabelRuntime: false, the preset gets no caller data and the helper is inlined, as it is in React Native today:

__d(function (global, require, _$$_IMPORT_DEFAULT, _$$_IMPORT_ALL, module, exports, _dependencyMap) {
  "use strict";

  Object.defineProperty(exports, "__esModule", {
    value: true
  });
  exports.answer = void 0;
  var values = _interopRequireWildcard(require(_dependencyMap[0], "./values"));
  function _interopRequireWildcard(e, t) { /* 616 characters, in every module that uses `import * as` */ }
  const answer = exports.answer = values.answer;
},0,[1],"index.js");

With enableBabelRuntime: true, it's imported from Metro's own copy, which the test checks is metro-runtime's @babel/runtime (module 1, node_modules/@babel/runtime/helpers/interopRequireWildcard.js):

__d(function (global, require, _$$_IMPORT_DEFAULT, _$$_IMPORT_ALL, module, exports, _dependencyMap) {
  "use strict";

  var _interopRequireWildcard = require(_dependencyMap[0], "metro:babel-runtime/helpers/interopRequireWildcard").default;
  Object.defineProperty(exports, "__esModule", {
    value: true
  });
  exports.answer = void 0;
  var values = _interopRequireWildcard(require(_dependencyMap[1], "./values"));
  const answer = exports.answer = values.answer;
},0,[1,3],"index.js");

End to end with the RN wiring

This compares the iOS release bundle of React Native's private/helloworld, which has a babel.config.js naming @react-native/babel-preset as in the template. Current defaults are Metro main with React Native main, against this PR with the RN wiring from step 3 above. Metro's @babel/runtime is 7.29.7.

Current defaults This PR + RN wiring Change
Minified JS 906,504 B 807,389 B -99,115 B (-10.9%)
Minified JS, gzip -9 217,794 B 204,595 B -13,199 B (-6.1%)
Hermes bytecode (hermesc -O) 1,261,863 B 1,173,832 B -88,031 B (-7.0%)
Modules 518 521 +3
Inlined _interopRequireWildcard 199 0
Inlined _callSuper / _isNativeReflectConstruct 63 / 65 0 / 0
Inlined _superPropGet 25 0

Every module that uses import * as or extends a class carries its own copy of these helpers today. For example, React Native's UnimplementedView with current defaults (the three helper bodies, 1,091 bytes unminified, elided):

__d(function (global, require, _$$_IMPORT_DEFAULT, _$$_IMPORT_ALL, module, exports, _dependencyMap) {
  'use strict';

  var _interopRequireDefault = require(_dependencyMap[0]);
  Object.defineProperty(exports, "__esModule", {
    value: true
  });
  exports.default = undefined;
  var _classCallCheck2 = _interopRequireDefault(require(_dependencyMap[1]));
  var _createClass2 = _interopRequireDefault(require(_dependencyMap[2]));
  var _possibleConstructorReturn2 = _interopRequireDefault(require(_dependencyMap[3]));
  var _getPrototypeOf2 = _interopRequireDefault(require(_dependencyMap[4]));
  var _inherits2 = _interopRequireDefault(require(_dependencyMap[5]));
  var _StyleSheet = _interopRequireDefault(require(_dependencyMap[6]));
  var React = _interopRequireWildcard(require(_dependencyMap[7]));
  var _jsxRuntime = require(_dependencyMap[8]);
  function _interopRequireWildcard(e, t) { /* ... */ }
  function _callSuper(t, o, e) { /* ... */ }
  function _isNativeReflectConstruct() { /* ... */ }
  var UnimplementedView = /*#__PURE__*/function (_React$Component) {
    function UnimplementedView() {
      (0, _classCallCheck2.default)(this, UnimplementedView);
      return _callSuper(this, UnimplementedView, arguments);
    }
    (0, _inherits2.default)(UnimplementedView, _React$Component);
    return (0, _createClass2.default)(UnimplementedView, [{
      key: "render",
      value: function render() {
        var View = require(_dependencyMap[9]).default;
        return /*#__PURE__*/(0, _jsxRuntime.jsx)(View, {
          style: [styles.unimplementedView, this.props.style],
          children: this.props.children
        });
      }
    }]);
  }(React.Component);
  var styles = _StyleSheet.default.create({
    unimplementedView: {}
  });
  var _default = exports.default = UnimplementedView;
},404,[1,25,26,58,60,63,7,3,220,70]);

With this PR and the RN wiring, it imports them from Metro's @babel/runtime instead:

__d(function (global, require, _$$_IMPORT_DEFAULT, _$$_IMPORT_ALL, module, exports, _dependencyMap) {
  'use strict';

  Object.defineProperty(exports, "__esModule", {
    value: true
  });
  exports.default = undefined;
  var _classCallCheck2 = require(_dependencyMap[0]).default(require(_dependencyMap[1]));
  var _createClass2 = require(_dependencyMap[0]).default(require(_dependencyMap[2]));
  var _callSuper2 = require(_dependencyMap[0]).default(require(_dependencyMap[3]));
  var _inherits2 = require(_dependencyMap[0]).default(require(_dependencyMap[4]));
  var _StyleSheet = require(_dependencyMap[0]).default(require(_dependencyMap[5]));
  var React = require(_dependencyMap[6]).default(require(_dependencyMap[7]));
  var _jsxRuntime = require(_dependencyMap[8]);
  var UnimplementedView = /*#__PURE__*/function (_React$Component) {
    function UnimplementedView() {
      (0, _classCallCheck2.default)(this, UnimplementedView);
      return (0, _callSuper2.default)(this, UnimplementedView, arguments);
    }
    (0, _inherits2.default)(UnimplementedView, _React$Component);
    return (0, _createClass2.default)(UnimplementedView, [{
      key: "render",
      value: function render() {
        var View = require(_dependencyMap[9]).default;
        return /*#__PURE__*/(0, _jsxRuntime.jsx)(View, {
          style: [styles.unimplementedView, this.props.style],
          children: this.props.children
        });
      }
    }]);
  }(React.Component);
  var styles = _StyleSheet.default.create({
    unimplementedView: {}
  });
  var _default = exports.default = UnimplementedView;
},407,[1,27,28,59,67,9,3,5,223,74]);

Not run on a device.

@meta-cla meta-cla Bot added the CLA Signed This label is managed by the Facebook bot. Authors need to sign the CLA before a PR can be reviewed. label Sep 21, 2026
@robhogan
robhogan force-pushed the pr1957 branch 2 times, most recently from d9dcf8f to cb7a1fe Compare September 21, 2026 14:34
@robhogan robhogan changed the title Pass metro:babel-runtime to Babel presets as the runtime module name via caller Pass Metro's own @babel/runtime and its version to Babel presets via caller Sep 21, 2026
@robhogan
robhogan force-pushed the pr1957 branch 2 times, most recently from 17e21c7 to fea257f Compare September 21, 2026 15:10
@robhogan robhogan mentioned this pull request Oct 6, 2026
Open
@robhogan robhogan changed the title Pass Metro's own @babel/runtime and its version to Babel presets via caller Pass Metro's own @babel/runtime to Babel presets Oct 16, 2026
@robhogan
robhogan changed the base branch from main to pr2065 October 16, 2026 10:35
@robhogan
robhogan changed the base branch from pr2065 to main October 16, 2026 10:52
@robhogan
robhogan force-pushed the pr1957 branch 5 times, most recently from 4ceb264 to ef1e685 Compare October 16, 2026 16:03
@robhogan
Pass Metro's own @babel/runtime to Babel presets
82837f1 
Helpers that `@babel/plugin-transform-runtime` imports from `@babel/runtime` can fail to resolve, depending on where, or whether, a project installs it. Worse, presets don't know which version they'll get, so the plugin assumes the oldest and inlines newer helpers instead, eg `_interopRequireWildcard` into every module that uses `import * as` (react/react-native#57123).

This tells Babel presets to import helpers from Metro's own `@babel/runtime`, which always exists, at its installed version, so the plugin can target that version. It builds on the scheme resolvers from #1804, which Metro uses for its own `metro:` namespace.

With `transformer.enableBabelRuntime: true` (the default), the wiring is:

1. The transform worker passes the Babel transformer `metro:babel-runtime` and the installed version of Metro's `@babel/runtime`. That copy's `package.json` joins the transform cache key. (This PR)
2. `metro-babel-transformer` forwards both to presets as Babel caller data, named `babelRuntimeModuleName` and `enableBabelRuntime`. (This PR)
3. `@react-native/metro-babel-transformer`, which React Native projects use instead, builds its own caller and needs to forward them too. (To do)
4. `@react-native/babel-preset` configures `@babel/plugin-transform-runtime` with that `moduleName` and `version`. (Landed in react/react-native#57974)
5. Helpers are emitted as `metro:babel-runtime/helpers/...` imports, which Metro's `metro:` resolver maps to metro-runtime's `@babel/runtime`. (Landed in Metro 0.87.1)
6. Metro adds that `@babel/runtime` to the file map, wherever it's installed. (Landed in #2065, unreleased)

Caller data reaches a preset however it's configured, including from a project `babel.config.js`. In a React Native project, this is a no-op until step 3. Expo CLI's Babel transformer doesn't forward these yet either.

A string `enableBabelRuntime` (a version of the project's own `@babel/runtime`) keeps its meaning and passes neither value. It's now documented as deprecated.

## Changelog

```
 - **[Feature]**: When `transformer.enableBabelRuntime` is `true`, pass Metro's own `@babel/runtime` (`metro:babel-runtime`) and its installed version to Babel presets via caller data
 - **[Deprecated]**: String values of `transformer.enableBabelRuntime` (use `true`)
```

## Test plan

New integration test bundles and runs a fixture that does `import * as values from './values'`, with a `babel.config.js` that reads the caller data as `@react-native/babel-preset` does. `@babel/runtime` only exports this helper from 7.14.0, so it's imported rather than inlined only if the version reaches the preset. Metro's `@babel/runtime` is outside the test's watch folders, so this also exercises #2065.

With `enableBabelRuntime: false`, the preset gets no caller data and the helper is inlined, as it is in React Native today:

```js
__d(function (global, require, _$$_IMPORT_DEFAULT, _$$_IMPORT_ALL, module, exports, _dependencyMap) {
  "use strict";

  Object.defineProperty(exports, "__esModule", {
    value: true
  });
  exports.answer = void 0;
  var values = _interopRequireWildcard(require(_dependencyMap[0], "./values"));
  function _interopRequireWildcard(e, t) { /* 616 characters, in every module that uses `import * as` */ }
  const answer = exports.answer = values.answer;
},0,[1],"index.js");
```

With `enableBabelRuntime: true`, it's imported from Metro's own copy, which the test checks is metro-runtime's `@babel/runtime` (module `1`, `node_modules/@babel/runtime/helpers/interopRequireWildcard.js`):

```js
__d(function (global, require, _$$_IMPORT_DEFAULT, _$$_IMPORT_ALL, module, exports, _dependencyMap) {
  "use strict";

  var _interopRequireWildcard = require(_dependencyMap[0], "metro:babel-runtime/helpers/interopRequireWildcard").default;
  Object.defineProperty(exports, "__esModule", {
    value: true
  });
  exports.answer = void 0;
  var values = _interopRequireWildcard(require(_dependencyMap[1], "./values"));
  const answer = exports.answer = values.answer;
},0,[1,3],"index.js");
```

### End to end with the RN wiring

This compares the iOS release bundle of React Native's `private/helloworld`, which has a `babel.config.js` naming `@react-native/babel-preset` as in the template. Current defaults are Metro `main` with React Native `main`, against this PR with the RN wiring from step 3 above. Metro's `@babel/runtime` is 7.29.7.

| | Current defaults | This PR + RN wiring | Change |
|---|---|---|---|
| Minified JS | 906,504 B | 807,389 B | -99,115 B (-10.9%) |
| Minified JS, gzip -9 | 217,794 B | 204,595 B | -13,199 B (-6.1%) |
| Hermes bytecode (`hermesc -O`) | 1,261,863 B | 1,173,832 B | -88,031 B (-7.0%) |
| Modules | 518 | 521 | +3 |
| Inlined `_interopRequireWildcard` | 199 | 0 | |
| Inlined `_callSuper` / `_isNativeReflectConstruct` | 63 / 65 | 0 / 0 | |
| Inlined `_superPropGet` | 25 | 0 | |

Every module that uses `import * as` or extends a class carries its own copy of these helpers today. For example, React Native's `UnimplementedView` with current defaults (the three helper bodies, 1,091 bytes unminified, elided):

```js
__d(function (global, require, _$$_IMPORT_DEFAULT, _$$_IMPORT_ALL, module, exports, _dependencyMap) {
  'use strict';

  var _interopRequireDefault = require(_dependencyMap[0]);
  Object.defineProperty(exports, "__esModule", {
    value: true
  });
  exports.default = undefined;
  var _classCallCheck2 = _interopRequireDefault(require(_dependencyMap[1]));
  var _createClass2 = _interopRequireDefault(require(_dependencyMap[2]));
  var _possibleConstructorReturn2 = _interopRequireDefault(require(_dependencyMap[3]));
  var _getPrototypeOf2 = _interopRequireDefault(require(_dependencyMap[4]));
  var _inherits2 = _interopRequireDefault(require(_dependencyMap[5]));
  var _StyleSheet = _interopRequireDefault(require(_dependencyMap[6]));
  var React = _interopRequireWildcard(require(_dependencyMap[7]));
  var _jsxRuntime = require(_dependencyMap[8]);
  function _interopRequireWildcard(e, t) { /* ... */ }
  function _callSuper(t, o, e) { /* ... */ }
  function _isNativeReflectConstruct() { /* ... */ }
  var UnimplementedView = /*#__PURE__*/function (_React$Component) {
    function UnimplementedView() {
      (0, _classCallCheck2.default)(this, UnimplementedView);
      return _callSuper(this, UnimplementedView, arguments);
    }
    (0, _inherits2.default)(UnimplementedView, _React$Component);
    return (0, _createClass2.default)(UnimplementedView, [{
      key: "render",
      value: function render() {
        var View = require(_dependencyMap[9]).default;
        return /*#__PURE__*/(0, _jsxRuntime.jsx)(View, {
          style: [styles.unimplementedView, this.props.style],
          children: this.props.children
        });
      }
    }]);
  }(React.Component);
  var styles = _StyleSheet.default.create({
    unimplementedView: {}
  });
  var _default = exports.default = UnimplementedView;
},404,[1,25,26,58,60,63,7,3,220,70]);
```

With this PR and the RN wiring, it imports them from Metro's `@babel/runtime` instead:

```js
__d(function (global, require, _$$_IMPORT_DEFAULT, _$$_IMPORT_ALL, module, exports, _dependencyMap) {
  'use strict';

  Object.defineProperty(exports, "__esModule", {
    value: true
  });
  exports.default = undefined;
  var _classCallCheck2 = require(_dependencyMap[0]).default(require(_dependencyMap[1]));
  var _createClass2 = require(_dependencyMap[0]).default(require(_dependencyMap[2]));
  var _callSuper2 = require(_dependencyMap[0]).default(require(_dependencyMap[3]));
  var _inherits2 = require(_dependencyMap[0]).default(require(_dependencyMap[4]));
  var _StyleSheet = require(_dependencyMap[0]).default(require(_dependencyMap[5]));
  var React = require(_dependencyMap[6]).default(require(_dependencyMap[7]));
  var _jsxRuntime = require(_dependencyMap[8]);
  var UnimplementedView = /*#__PURE__*/function (_React$Component) {
    function UnimplementedView() {
      (0, _classCallCheck2.default)(this, UnimplementedView);
      return (0, _callSuper2.default)(this, UnimplementedView, arguments);
    }
    (0, _inherits2.default)(UnimplementedView, _React$Component);
    return (0, _createClass2.default)(UnimplementedView, [{
      key: "render",
      value: function render() {
        var View = require(_dependencyMap[9]).default;
        return /*#__PURE__*/(0, _jsxRuntime.jsx)(View, {
          style: [styles.unimplementedView, this.props.style],
          children: this.props.children
        });
      }
    }]);
  }(React.Component);
  var styles = _StyleSheet.default.create({
    unimplementedView: {}
  });
  var _default = exports.default = UnimplementedView;
},407,[1,27,28,59,67,9,3,5,223,74]);
```

Not run on a device.
@robhogan
robhogan marked this pull request as ready for review October 17, 2026 10:05
@robhogan
robhogan requested a balanced review from Copilot October 17, 2026 10:05
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Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

@robhogan
robhogan requested review from huntie and vzaidman October 17, 2026 10:05
@facebook-github-tools facebook-github-tools Bot added the Shared with Meta Applied via automation to indicate that an Issue or Pull Request has been shared with the team. label Oct 17, 2026
huntie
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Woop, a long time coming! 🙌🏻

@robhogan
robhogan merged commit 64e949e into main Oct 19, 2026
16 checks passed
@robhogan robhogan mentioned this pull request Oct 19, 2026
Closed
meta-codesync Bot pushed a commit to react/react-native that referenced this pull request Oct 21, 2026
@robhogan @meta-codesync
Pass Metro's own @babel/runtime through @react-native/metro-babel-tra…
89db5ce 
…nsformer (#59205)

Summary:
This is the RN side of react/metro#1957

Helpers that `babel/plugin-transform-runtime` imports from `babel/runtime` can fail to resolve, depending on where, or whether, a project installs it. Worse, `react-native/babel-preset` doesn't know which version it'll get, so the plugin assumes the oldest and inlines newer helpers instead, eg `_interopRequireWildcard` into every module that uses `import * as` (#57123).

Metro now offers its own `babel/runtime`, which always exists, at a version it knows. This forwards it from `react-native/metro-babel-transformer`, which React Native projects use and which currently drops it. With `transformer.enableBabelRuntime: true` (the default), the wiring is:

1. Metro's transform worker passes the Babel transformer `babelRuntimeModuleName: 'metro:babel-runtime'` and the installed version of Metro's `babel/runtime`. (Landed in react/metro#1957, unreleased)
2. `react-native/metro-babel-transformer` forwards both to the preset. (This PR)
3. `react-native/babel-preset` configures `babel/plugin-transform-runtime` with that `moduleName` and `version`. (Landed in #57974)
4. Helpers are emitted as `metro:babel-runtime/helpers/...` imports, which Metro's `metro:` resolver maps to its own `babel/runtime`. (Landed in Metro 0.87.1)
5. Metro adds that `babel/runtime` to the file map, wherever it's installed. (Landed in react/metro#2065, unreleased)

How this PR forwards them depends on the project's Babel config:

- With a project Babel config naming the preset, as in the template, they reach the preset as Babel caller data.
- Without one, the version replaces `enableBabelRuntime: true` in the preset's options, because preset options take precedence over caller data. `babelRuntimeModuleName` already reaches the preset there.

Until the next Metro release, Metro doesn't pass either value, so this is a no-op, but harmless. The Flow suppression for the new fields can go once React Native depends on that release.

## Changelog:

[GENERAL] [ADDED] - `react-native/metro-babel-transformer` passes Metro's own `babel/runtime` and its version to `react-native/babel-preset`, when Metro provides them

Pull Request resolved: #59205

Test Plan:
New tests transform `import * as foo from './foo'` with and without a project `babel.config.js` naming the preset. When Metro passes `metro:babel-runtime` and a version, the helper is imported from `metro:babel-runtime/helpers/interopRequireWildcard`, and otherwise it's inlined as before. The import cases fail without this change.

End to end, with Metro from react/metro#1957, the iOS release bundle of `private/helloworld` no longer inlines helpers: 199 copies of `_interopRequireWildcard` and 63 of `_callSuper` become three shared modules. Against current defaults:

| | Current defaults | With this PR | Change |
|---|---|---|---|
| Minified JS | 906,504 B | 807,389 B | -10.9% |
| Minified JS, gzip -9 | 217,794 B | 204,595 B | -6.1% |
| Hermes bytecode | 1,261,863 B | 1,173,832 B | -7.0% |

Sample output is in react/metro#1957. Not run on a device.

Reviewed By: christophpurrer

Differential Revision: D126404231

Pulled By: javache

fbshipit-source-id: b85e157205abc5de437f7b68aae887d5c9010c95
robhogan added a commit that referenced this pull request Oct 21, 2026
@robhogan
Publish 0.87.2
417b7e7 
Summary:
Publish Metro 0.87.2 from `main`.

Changelog: [Internal]

Test plan:
```
node scripts/updateVersion.js 0.87.2
sl diff --stat
```

Only the 17 `packages/*/package.json` manifests change, and every `0.87.1` reference is replaced.

Draft release notes (from the 95 commits in v0.87.1...main - internal-only changes omitted):

```
 - **[Feature]**: Add a built-in `metro:runtime/*` specifier, resolvable from any importer, eg for `transformer.asyncRequireModulePath` (#2083 by @robhogan)
 - **[Feature]**: When `transformer.enableBabelRuntime` is `true`, pass Metro's own `@babel/runtime` (`metro:babel-runtime`) and its installed version to Babel presets via caller data (#1957 by @robhogan)
 - **[Fix]**: Resolver: Fix stale resolutions after adding a nearer `package.json` or retargeting a symlinked directory, which previously needed a restart (#1948 by @robhogan)
 - **[Fix]**: Resolver: Packages and directories named with an asset extension (eg `lodash.zip`) resolve as modules rather than failing as assets (#2055 by @robhogan)
 - **[Fix]**: Resolver: Fall through to parent conditions when a nested `"exports"` or `"imports"` condition has no matching target (#2074 by @maximilliangrand)
 - **[Fix]**: Resolver: An exact `"exports"` or `"imports"` subpath with a `null` or unmatched target no longer resolves through a less specific subpath pattern (#1987 by @kwy404)
 - **[Fix]**: Use indexed watch folder paths (`[metro-watchFolders]/N/`) for assets outside `projectRoot` (#1892 by @robhogan)
 - **[Fix]**: Fix `/[metro-project]/` and `/[metro-watchFolders]/` bundle serving on Windows (#2015 by @motiz88)
 - **[Fix]**: Prevent HMR crash on `[metro-watchFolders]` or `[metro-project]` URLs (#2017 by @motiz88)
 - **[Fix]**: Remove erroneous `[metro-project]` and `[metro-watchFolders]` resolution of entry points from the API and CLI (#2030 by @motiz88)
 - **[Fix]**: Fix backslashes in async bundle paths on Windows (`includeAsyncPaths`) (#1938 by @robhogan)
 - **[Fix]**: Fix stale modules on the client when a file is deleted and recreated during a build, or changes during a build that fails (#2007 by @YevheniiKotyrlo)
 - **[Fix]**: Fix files created or modified while Metro starts up being missed until restart (#2054 by @cpruijsen)
 - **[Fix]**: Fix watch mode losing files when a directory replaces a file or symlink, eg when unlinking a package (#2009 by @robhogan)
 - **[Fix]**: Fix the fallback (Linux/Windows) watcher hanging on stop, and missing changes in a replaced directory, after a watch error (#1967 by @robhogan)
 - **[Fix]**: Fix the fallback watcher missing changes when a directory changes faster than Windows can report (#1967 by @robhogan)
 - **[Fix]**: Close the watch handles of a deleted directory and every directory under it in `FallbackWatcher`, which kept the process busy on Windows (#2004 by @YevheniiKotyrlo)
 - **[Fix]**: `metro-file-map`: Fix removing a non-active duplicate mock changing which mock is active (#1756 by @durvesh1992)
 - **[Fix]**: Propagate transformer initialisation errors from `Bundler` (#1854 by @Boulea7)
 - **[Fix]**: Release transform workers and file watchers when Metro fails to initialise (#1963 by @robhogan)
 - **[Fix]**: Ignore hyphenated and namespaced SVG attributes (eg `stroke-width`) when reading SVG asset dimensions (#1972 by @GabrielDumbrava)
 - **[Fix]**: Fix generated positions when iterating the mappings of indexed source maps, eg in `composeSourceMaps` (#1976 by @kwy404)
 - **[Fix]**: `composeSourceMaps` keeps every mapping at a generated column that the last map maps more than once (#1981 by @robhogan)
 - **[Fix]**: Source maps no longer lose lines after a gap of more than about two million unmapped generated lines (#1983 by @robhogan)
 - **[Fix]**: `metro-symbolicate` no longer fails on stack frames with no source position, such as `[native code]:0:0` in crash reporter output (#1891 by @robhogan)
 - **[Fix]**: Fix `metro-symbolicate` parsing of stack frames with Windows absolute paths (#1939 by @robhogan)
 - **[Performance]**: `metro-source-map`: Faster `Consumer` decoding and position lookups, and `composeSourceMaps` encodes its output directly - 3.2x faster composition of a bundle map with its Hermes map, using 15% less memory (#1978, #1979, #1980, #1983 by @robhogan)
 - **[Performance]**: Resolver: Ask for the package of a directory once for all extension candidates, and check `hierarchicalLookup` candidates from their own file map nodes. First-pass resolution is 35% faster (172ms -> 113ms on a mid-size RN app), net of a ~3% first-build cost from dropping `PackageCache`'s closest-package memo, which also cuts the resolver's retained heap by 37% (#1945, #1946, #1948 by @robhogan)
 - **[Performance]**: 33% faster non-Watchman file system crawl on startup (#1677 by @kitten)
 - **[Deprecated]**: `server.enhanceMiddleware` logs a deprecation warning when set to anything other than its default. Use `server.unstable_middleware` or `server.unstable_priorityMiddleware` (#2031 by @vzaidman)
 - **[Deprecated]**: `runServer`'s `unstable_extraMiddleware` option. Use `unstable_priorityMiddleware`, which runs the same middleware in the same place (#2031 by @vzaidman)
 - **[Deprecated]**: String values of `transformer.enableBabelRuntime` (use `true`) (#1957 by @robhogan)
 - **[Types]**: Add TypeScript definitions for `metro-runtime` modules, and generate (rather than hand-maintain) types for `metro-file-map` workers (#1996 by @robhogan)

> NOTE: Experimental features are not covered by semver and can change at any time.

 - **[Experimental]**: Add `server.unstable_middleware` and `server.unstable_priorityMiddleware` config options, and `runServer` options of the same names, to add middleware after and before Metro's own. `mergeConfig` combines them from both configs: a later config's `unstable_middleware` runs last and its `unstable_priorityMiddleware` runs first (#2031 by @vzaidman)
 - **[Experimental]**: Add `server.unstable_onServerCreated` config option and `runServer` option of the same name, called with Metro's server instance before the HTTP server listens (#2031 by @vzaidman)
 - **[Experimental]**: Fix mismatched asset paths in (release) builds under `unstable_serverRoot`, make them always relative to `projectRoot` (#1917 by @robhogan)

 **Full Changelog**: v0.87.1...v0.87.2
```
robhogan added a commit that referenced this pull request Oct 21, 2026
@robhogan
Publish 0.87.2
d9abde7 
Summary:
Publish Metro 0.87.2 from `main`.

Changelog: [Internal]

Test plan:
```
node scripts/updateVersion.js 0.87.2
sl diff --stat
```

Only the 17 `packages/*/package.json` manifests change, and every `0.87.1` reference is replaced.

Draft release notes (from the 95 commits in v0.87.1...main - internal-only changes omitted):

```
 - **[Feature]**: Add a built-in `metro:runtime/*` specifier, resolvable from any importer, eg for `transformer.asyncRequireModulePath` (#2083 by @robhogan)
 - **[Feature]**: When `transformer.enableBabelRuntime` is `true`, pass Metro's own `@babel/runtime` (`metro:babel-runtime`) and its installed version to Babel presets via caller data (#1957 by @robhogan)
 - **[Fix]**: Resolver: Fix stale resolutions after adding a nearer `package.json` or retargeting a symlinked directory, which previously needed a restart (#1948 by @robhogan)
 - **[Fix]**: Resolver: Packages and directories named with an asset extension (eg `lodash.zip`) resolve as modules rather than failing as assets (#2055 by @robhogan)
 - **[Fix]**: Resolver: Fall through to parent conditions when a nested `"exports"` or `"imports"` condition has no matching target (#2074 by @maximilliangrand)
 - **[Fix]**: Resolver: An exact `"exports"` or `"imports"` subpath with a `null` or unmatched target no longer resolves through a less specific subpath pattern (#1987 by @kwy404)
 - **[Fix]**: Use indexed watch folder paths (`[metro-watchFolders]/N/`) for assets outside `projectRoot` (#1892 by @robhogan)
 - **[Fix]**: Fix `/[metro-project]/` and `/[metro-watchFolders]/` bundle serving on Windows (#2015 by @motiz88)
 - **[Fix]**: Prevent HMR crash on `[metro-watchFolders]` or `[metro-project]` URLs (#2017 by @motiz88)
 - **[Fix]**: Remove erroneous `[metro-project]` and `[metro-watchFolders]` resolution of entry points from the API and CLI (#2030 by @motiz88)
 - **[Fix]**: Fix backslashes in async bundle paths on Windows (`includeAsyncPaths`) (#1938 by @robhogan)
 - **[Fix]**: Fix stale modules on the client when a file is deleted and recreated during a build, or changes during a build that fails (#2007 by @YevheniiKotyrlo)
 - **[Fix]**: Fix files created or modified while Metro starts up being missed until restart (#2054 by @cpruijsen)
 - **[Fix]**: Fix watch mode losing files when a directory replaces a file or symlink, eg when unlinking a package (#2009 by @robhogan)
 - **[Fix]**: Fix the fallback (Linux/Windows) watcher hanging on stop, and missing changes in a replaced directory, after a watch error (#1967 by @robhogan)
 - **[Fix]**: Fix the fallback watcher missing changes when a directory changes faster than Windows can report (#1967 by @robhogan)
 - **[Fix]**: Close the watch handles of a deleted directory and every directory under it in `FallbackWatcher`, which kept the process busy on Windows (#2004 by @YevheniiKotyrlo)
 - **[Fix]**: `metro-file-map`: Fix removing a non-active duplicate mock changing which mock is active (#1756 by @durvesh1992)
 - **[Fix]**: Propagate transformer initialisation errors from `Bundler` (#1854 by @Boulea7)
 - **[Fix]**: Release transform workers and file watchers when Metro fails to initialise (#1963 by @robhogan)
 - **[Fix]**: Ignore hyphenated and namespaced SVG attributes (eg `stroke-width`) when reading SVG asset dimensions (#1972 by @GabrielDumbrava)
 - **[Fix]**: Fix generated positions when iterating the mappings of indexed source maps, eg in `composeSourceMaps` (#1976 by @kwy404)
 - **[Fix]**: `composeSourceMaps` keeps every mapping at a generated column that the last map maps more than once (#1981 by @robhogan)
 - **[Fix]**: Source maps no longer lose lines after a gap of more than about two million unmapped generated lines (#1983 by @robhogan)
 - **[Fix]**: `metro-symbolicate` no longer fails on stack frames with no source position, such as `[native code]:0:0` in crash reporter output (#1891 by @robhogan)
 - **[Fix]**: Fix `metro-symbolicate` parsing of stack frames with Windows absolute paths (#1939 by @robhogan)
 - **[Performance]**: `metro-source-map`: Faster `Consumer` decoding and position lookups, and `composeSourceMaps` encodes its output directly - ~3.2x faster composition of a bundle map with its Hermes map, using ~15% less memory (#1978, #1979, #1980, #1983 by @robhogan)
 - **[Performance]**: Resolver: Ask for the package of a directory once for all extension candidates, and check `hierarchicalLookup` candidates from their own file map nodes. First-pass resolution is ~35% faster, net of a ~3% first-build cost from dropping `PackageCache`'s closest-package memo, which also cuts the resolver's retained heap by ~37% (#1945, #1946, #1948 by @robhogan)
 - **[Performance]**: ~33% faster non-Watchman file system crawl on startup (#1677 by @kitten)
 - **[Deprecated]**: `server.enhanceMiddleware` logs a deprecation warning when set to anything other than its default. Use `server.unstable_middleware` or `server.unstable_priorityMiddleware` (#2031 by @vzaidman)
 - **[Deprecated]**: `runServer`'s `unstable_extraMiddleware` option. Use `unstable_priorityMiddleware`, which runs the same middleware in the same place (#2031 by @vzaidman)
 - **[Deprecated]**: String values of `transformer.enableBabelRuntime` (use `true`) (#1957 by @robhogan)
 - **[Types]**: Add TypeScript definitions for `metro-runtime` modules, and generate (rather than hand-maintain) types for `metro-file-map` workers (#1996 by @robhogan)

> NOTE: Experimental features are not covered by semver and can change at any time.

 - **[Experimental]**: Add `server.unstable_middleware` and `server.unstable_priorityMiddleware` config options, and `runServer` options of the same names, to add middleware after and before Metro's own. `mergeConfig` combines them from both configs: a later config's `unstable_middleware` runs last and its `unstable_priorityMiddleware` runs first (#2031 by @vzaidman)
 - **[Experimental]**: Add `server.unstable_onServerCreated` config option and `runServer` option of the same name, called with Metro's server instance before the HTTP server listens (#2031 by @vzaidman)
 - **[Experimental]**: Fix mismatched asset paths in (release) builds under `unstable_serverRoot`, make them always relative to `projectRoot` (#1917 by @robhogan)

 **Full Changelog**: v0.87.1...v0.87.2
```
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@huntie huntie huntie approved these changes
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@robhogan @huntie