diff --git a/packages/core/package.json b/packages/core/package.json
index c441082bd4..8182660c7f 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -41,8 +41,9 @@
     "fix": "npx run-s fix:oxlint fix:fmt",
     "fix:oxlint": "OXLINT_TSGOLINT_DANGEROUSLY_SUPPRESS_PROGRAM_DIAGNOSTICS=true oxlint --type-aware --tsconfig tsconfig.lint.json --fix",
     "fix:fmt": "oxfmt \"{src,test,scripts,plugin/src}/**/**.ts\" \"{src,test}/**/**.tsx\"",
-    "lint": "npx run-s lint:oxlint lint:fmt",
+    "lint": "npx run-s lint:oxlint lint:fmt lint:side-effects",
     "lint:oxlint": "sh -c 'OUT=$(OXLINT_TSGOLINT_DANGEROUSLY_SUPPRESS_PROGRAM_DIAGNOSTICS=true oxlint --type-aware --tsconfig tsconfig.lint.json --deny-warnings 2>&1); echo \"$OUT\"; echo \"$OUT\" | grep -qE \"Found 0 warnings and 0 errors\"'",
+    "lint:side-effects": "node scripts/check-side-effects.js",
     "lint:fmt": "oxfmt --check \"{src,test,scripts,plugin/src}/**/**.ts\" \"{src,test}/**/**.tsx\"",
     "api-report:generate": "api-extractor run --local --verbose",
     "api-report:check": "api-extractor run --verbose"
diff --git a/packages/core/scripts/check-side-effects.js b/packages/core/scripts/check-side-effects.js
new file mode 100644
index 0000000000..9da99af602
--- /dev/null
+++ b/packages/core/scripts/check-side-effects.js
@@ -0,0 +1,101 @@
+const fs = require('fs');
+const path = require('path');
+const ts = require('typescript');
+
+// Guards the `"sideEffects": false` contract in package.json: no module that ships
+// in the bundle may run code at import time. A bundler is free to drop such a module
+// when its exports are unused, so an import-time side effect would silently vanish
+// from a consumer's build. This fails if one is introduced.
+//
+// Allowlist approach: only pure declarations are permitted at module top level;
+// every other statement is flagged, since anything else can execute at import time
+// (an `if`/`for`/`try`/block wrapping a call, a bare expression, an IIFE, a global
+// write). A side-effect import (`import 'x'` or `import {} from 'x'`, i.e. one that
+// binds nothing) is flagged too. Declarations are fine, including `const x = f()`:
+// they are module-local and ride with the module only when it is kept.
+//
+// Excluded: `tools/` (Node-only build tools, never bundled into an app) and `vendor/`
+// (audited third-party code reviewed when it is vendored in). The guard's job is to
+// stop first-party SDK code from gaining an import-time side effect.
+
+const ROOT = path.resolve(__dirname, '..');
+const SRC = path.join(ROOT, 'src', 'js');
+const EXCLUDE_DIRS = [path.join(SRC, 'tools'), path.join(SRC, 'vendor')];
+
+// Top-level statement kinds that are pure declarations (no import-time execution).
+const ALLOWED_KINDS = new Set([
+  ts.SyntaxKind.FunctionDeclaration,
+  ts.SyntaxKind.ClassDeclaration,
+  ts.SyntaxKind.InterfaceDeclaration,
+  ts.SyntaxKind.TypeAliasDeclaration,
+  ts.SyntaxKind.EnumDeclaration,
+  ts.SyntaxKind.ModuleDeclaration, // namespace / declare module
+  ts.SyntaxKind.VariableStatement, // const/let/var — incl. `const x = f()`
+  ts.SyntaxKind.ExportDeclaration, // export { ... } / export * from
+  ts.SyntaxKind.ExportAssignment, // export default ... / export =
+  ts.SyntaxKind.ImportEqualsDeclaration,
+  ts.SyntaxKind.EmptyStatement,
+]);
+
+// An import binds nothing (pure side-effect import) when it has no clause, or a clause
+// with neither a default name nor namespace nor any named binding. Type-only imports
+// are erased at compile time, so they never run.
+function isSideEffectImport(node) {
+  const clause = node.importClause;
+  if (clause && clause.isTypeOnly) {
+    return false;
+  }
+  if (!clause) {
+    return true;
+  }
+  if (clause.name) {
+    return false;
+  }
+  const bindings = clause.namedBindings;
+  if (!bindings) {
+    return true;
+  }
+  return ts.isNamedImports(bindings) && bindings.elements.length === 0;
+}
+
+function walk(dir, out) {
+  for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+    const p = path.join(dir, entry.name);
+    if (entry.isDirectory()) {
+      if (EXCLUDE_DIRS.some(d => p === d || p.startsWith(d + path.sep))) {
+        continue;
+      }
+      walk(p, out);
+    } else if (/\.tsx?$/.test(entry.name) && !/\.(test|spec)\.tsx?$/.test(entry.name)) {
+      out.push(p);
+    }
+  }
+  return out;
+}
+
+const findings = [];
+for (const file of walk(SRC, [])) {
+  const text = fs.readFileSync(file, 'utf8');
+  const sourceFile = ts.createSourceFile(file, text, ts.ScriptTarget.ES2018, true, ts.ScriptKind.TSX);
+  for (const stmt of sourceFile.statements) {
+    const lineOf = () => sourceFile.getLineAndCharacterOfPosition(stmt.getStart(sourceFile)).line + 1;
+    if (ts.isImportDeclaration(stmt)) {
+      if (isSideEffectImport(stmt)) {
+        findings.push({ file, line: lineOf(), kind: 'side-effect import', code: stmt.getText(sourceFile) });
+      }
+    } else if (!ALLOWED_KINDS.has(stmt.kind)) {
+      findings.push({ file, line: lineOf(), kind: 'top-level executed statement', code: stmt.getText(sourceFile).slice(0, 100) });
+    }
+  }
+}
+
+const rel = f => path.relative(ROOT, f);
+if (findings.length) {
+  console.error(`\n✖ import-time side effects found (${findings.length}) — these break the "sideEffects": false contract:\n`);
+  for (const f of findings) {
+    console.error(`  ${rel(f.file)}:${f.line}  [${f.kind}]  ${f.code.replace(/\n/g, ' ')}`);
+  }
+  console.error('\nMove the side effect into a function called by init()/wrap()/an integration factory.\n');
+  process.exit(1);
+}
+console.log('✓ no import-time side effects in src/js (tools/, vendor/ excluded) — "sideEffects": false is safe');