diff --git a/CHANGELOG.md b/CHANGELOG.md
index 21a1848..9639d36 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -43,7 +43,7 @@ Measured, minified, across esbuild / rollup / webpack:
 
 | What you import | esbuild | rollup | webpack |
 | --- | ---: | ---: | ---: |
-| `flattenErrors` | 287 | 292 | 291 |
+| `flattenErrors` | 394 | 367 | 394 |
 | one decorator | 1,837 | 1,823 | 1,818 |
 | `validateSync` | 3,722 | 3,554 | 3,823 |
 | `toPlainSync` | 7,744 | 7,769 | 7,832 |
@@ -60,6 +60,22 @@ names the rules that must not appear — and one case asserts that everything IS
 everything is used, so a "shaken" result cannot come from a bundle that failed to build. Strip
 the annotations and it fails with `"must be a latitude" should have been shaken out`.
 
+The same pass shook out something that was supposed to stay. Cereale installs `Symbol.metadata`
+when the runtime lacks it, and `sideEffects` named the module holding that install — but not the
+barrel that re-exports it. A side-effect-free barrel is droppable as a whole, so all three
+bundlers pruned the `export * from './metadata.js'` edge before metadata.js's own marking was
+ever consulted: `import { configure } from 'cereale'` came out at 145 bytes through esbuild, 143
+through webpack and 144 through rollup, with `Symbol.metadata` in none of them. That matters
+because `tsc`'s decorator emit reads the well-known symbol directly — `typeof Symbol ===
+"function" && Symbol.metadata ? Object.create(null) : void 0` — so without the install a
+decorated class gets `metadata: undefined`, which is to say no rules at all.
+
+`index.js` and `index.ts` are now listed too. It costs about 100 bytes, and only on imports that
+reach nothing else; every row of the table above except the first was byte-identical before and
+after, across all three bundlers. Two more cases in `treeshake.test.ts` pin it, one on the source
+and one on `dist/esm`, because those are separate paths in the manifest and a typo in either is
+invisible from the other side.
+
 ### Frameworks
 
 [FRAMEWORKS.md](FRAMEWORKS.md) — a setup recipe for each, every one run before it was written,
diff --git a/README.md b/README.md
index 15abc69..1e3d79e 100644
--- a/README.md
+++ b/README.md
@@ -506,7 +506,7 @@ source and the published bundle:
 
 | What you import | esbuild | rollup | webpack |
 | --- | ---: | ---: | ---: |
-| `flattenErrors` | 287 | 292 | 291 |
+| `flattenErrors` | 394 | 367 | 394 |
 | one decorator | 1,837 | 1,823 | 1,818 |
 | `validateSync` | 3,722 | 3,554 | 3,738 |
 | `toPlainSync` | 7,744 | 7,769 | 7,771 |
@@ -518,6 +518,13 @@ The serializer and the deserializer drop independently: read JSON and you do not
 writing it. The validator is kept by both, because `validate` defaults to `true` and the entry
 points reference it whatever a given call site passes.
 
+The floor is about a hundred bytes: cereale installs `Symbol.metadata` if the runtime lacks it,
+and that install has to survive tree-shaking or a `tsc`-compiled consumer decorates its classes
+with no metadata at all. It is why `sideEffects` names `index.js` as well as `metadata.js` —
+marking only the latter leaves the barrel itself droppable, so the edge to it is pruned before
+its own marking is ever read. That cost lands only on the first row; every import that touches a
+model was already carrying it.
+
 This did not come for free. Thirty of the rules are declared as top-level calls —
 `export const IsString = rule(…)` — and rollup can prove such a call side-effect-free by reading
 the factory, but esbuild and webpack will not. Without a `/*#__PURE__*/` annotation on each of
diff --git a/package.json b/package.json
index 2392cb4..11edee9 100644
--- a/package.json
+++ b/package.json
@@ -23,8 +23,10 @@
     }
   },
   "sideEffects": [
+    "./dist/esm/index.js",
     "./dist/esm/metadata.js",
     "./dist/cereale.min.js",
+    "./src/index.ts",
     "./src/metadata.ts"
   ],
   "files": [
diff --git a/src/metadata.ts b/src/metadata.ts
index 50c261a..04138f8 100644
--- a/src/metadata.ts
+++ b/src/metadata.ts
@@ -15,8 +15,22 @@ import type { ClassConstructor } from './interfaces.js';
  */
 const METADATA_KEY: symbol = (Symbol as { metadata?: symbol }).metadata ?? Symbol.for('Symbol.metadata');
 
-// Also installed globally, because a consumer's own compiler emit may read `Symbol.metadata`
-// directly. package.json marks this module as having side effects so it survives bundling.
+// Also installed globally, because a consumer's own compiler emit reads `Symbol.metadata`
+// directly and does not share our fallback. tsc emits
+//
+//   const _metadata = typeof Symbol === "function" && Symbol.metadata ? Object.create(null) : void 0;
+//
+// so on a runtime without the well-known symbol the class is decorated with `metadata:
+// undefined` and ends up with no metadata at all. (esbuild's `__knownSymbol` has the same
+// `Symbol.for` fallback we do and needs nothing from us; tsc does.)
+//
+// `sideEffects` in package.json is what keeps this statement through bundling — and it has to
+// name `index.ts`/`index.js` as well as this module. Marking only this one is not enough: the
+// barrel is then itself side-effect-free, so a bundler drops the `export * from './metadata.js'`
+// edge before this module's own marking is ever consulted, and the install silently vanishes.
+// Measured on `import { configure } from 'cereale'`: absent from all three of esbuild, webpack
+// and rollup until the barrel was listed too. It costs ~100 bytes, and only for imports that
+// pull in nothing else — every entry point that touches a model was already byte-identical.
 ((Symbol as { metadata?: symbol }).metadata as symbol | undefined) ??= METADATA_KEY;
 
 export interface ValidationArguments {
diff --git a/src/treeshake.test.ts b/src/treeshake.test.ts
index a53026c..f3ef26f 100644
--- a/src/treeshake.test.ts
+++ b/src/treeshake.test.ts
@@ -117,6 +117,27 @@ describe('tree-shaking', () => {
     expectShaken(code, [MARKER.validator], [MARKER.serializer, MARKER.deserializer, MARKER.isString]);
   });
 
+  /**
+   * The `Symbol.metadata` install at the top of metadata.ts is a bare statement, not an export,
+   * so it survives only because `sideEffects` names that module — and naming it is one hop short
+   * of enough. The barrel that re-exports it is side-effect-free too, so a bundler prunes the
+   * `export * from './metadata.js'` edge before metadata.js's own marking is ever consulted.
+   *
+   * Nothing notices, in the usual way. `import { configure } from 'cereale'` came out at 145
+   * bytes through esbuild, 143 through webpack and 144 through rollup with `Symbol.metadata`
+   * absent from all three — and a tsc-compiled consumer on a runtime without the well-known
+   * symbol is then decorated with `metadata: undefined`, which is to say with no rules at all.
+   */
+  it('installs Symbol.metadata even when nothing model-shaped is imported', async () => {
+    const code = await bundle(`
+      import { configure } from CEREALE;
+      export const f = (o) => configure(o);
+    `);
+
+    expect(code, 'the Symbol.metadata install was pruned along with the barrel').toContain('Symbol.metadata');
+    expectShaken(code, [], [MARKER.isString, MARKER.serializer, MARKER.deserializer, MARKER.validator]);
+  });
+
   it('costs almost nothing to import only an error helper', async () => {
     const code = await bundle(`
       import { flattenErrors } from CEREALE;
@@ -160,6 +181,22 @@ describe('tree-shaking', () => {
       expect(code.length).toBeLessThan(3000);
     });
 
+    /**
+     * The source-level case above proves the `sideEffects` mechanism works; this one proves the
+     * two entries are spelled the way the published tree is laid out. `./src/index.ts` and
+     * `./dist/esm/index.js` are separate paths in the manifest, and a typo in either is invisible
+     * from the other side.
+     */
+    it.runIf(built)('installs Symbol.metadata from the published barrel too', async () => {
+      const code = await bundle(`
+        import { configure } from ${JSON.stringify(path.join(dist, 'esm/index.js'))};
+        export const f = (o) => configure(o);
+      `.replace('CEREALE', 'unused'));
+
+      expect(code, 'the Symbol.metadata install was pruned from dist/esm').toContain('Symbol.metadata');
+      expectShaken(code, [], [MARKER.isString, MARKER.serializer, MARKER.deserializer]);
+    });
+
     it.runIf(built)('keeps its purity annotations through minification', async () => {
       const flat = readFileSync(path.join(dist, 'cereale.min.js'), 'utf8');
       const perModule = readFileSync(path.join(dist, 'esm/decorators.js'), 'utf8');