diff --git a/prelude/bootstrap-shared.js b/prelude/bootstrap-shared.js
index 57377ebcf..e3d15cd64 100644
--- a/prelude/bootstrap-shared.js
+++ b/prelude/bootstrap-shared.js
@@ -8,7 +8,7 @@
 // SEA bootstrap: bundled by esbuild via require('./bootstrap-shared').
 
 var childProcess = require('child_process');
-var { createHash } = require('crypto');
+var { createHash, randomBytes } = require('crypto');
 var fs = require('fs');
 var path = require('path');
 var zlib = require('zlib');
@@ -85,6 +85,52 @@ function pickDecompressorAsync(compression) {
 // NATIVE ADDON EXTRACTION /////////////////////////////////////////
 // /////////////////////////////////////////////////////////////////
 
+// Publish a complete file without truncating a cached addon that another
+// process may already have mapped into memory. The sibling temp file keeps
+// rename on the same filesystem; randomness also separates worker threads
+// that share a process.pid.
+function writeFileAtomic(dest, content) {
+  // Do not extend the destination basename: it may already be near NAME_MAX.
+  var temporary = path.join(
+    path.dirname(dest),
+    '.pkg-' + process.pid + '-' + randomBytes(6).toString('hex') + '.tmp',
+  );
+  // Only enter cleanup once we own the file. An EEXIST must not remove
+  // another writer's temporary file.
+  var fd = fs.openSync(temporary, 'wx');
+  var renamed = false;
+  try {
+    try {
+      fs.writeFileSync(fd, content);
+    } finally {
+      fs.closeSync(fd);
+    }
+    try {
+      fs.renameSync(temporary, dest);
+      renamed = true;
+    } catch (error) {
+      // Windows can refuse to replace an addon another process has loaded.
+      // Accept a concurrent winner only if it published the exact same bytes.
+      var identical = false;
+      try {
+        identical = fs.readFileSync(dest).equals(content);
+      } catch (_) {
+        // Keep the original rename error if the destination cannot be read.
+      }
+      if (!identical) throw error;
+    }
+  } finally {
+    // Also remove partial files left by a failed write, not just failed renames.
+    if (!renamed) {
+      try {
+        fs.unlinkSync(temporary);
+      } catch (_) {
+        // Best-effort cleanup must not hide the original write or rename error.
+      }
+    }
+  }
+}
+
 // Recursively copy src -> dest. For existing destination files, compare
 // SHA-256 hashes and skip identical ones to avoid redundant writes.
 //
@@ -152,7 +198,7 @@ function cpRecursive(src, dest) {
       return;
     }
   }
-  fs.writeFileSync(dest, srcContent);
+  writeFileAtomic(dest, srcContent);
 }
 
 /**
@@ -221,10 +267,10 @@ function patchDlopen(insideSnapshot) {
           var dContent = fs.readFileSync(tmpModulePath);
           var dHash = createHash('sha256').update(dContent).digest('hex');
           if (hash !== dHash) {
-            fs.writeFileSync(tmpModulePath, moduleContent);
+            writeFileAtomic(tmpModulePath, moduleContent);
           }
         } else {
-          fs.writeFileSync(tmpModulePath, moduleContent);
+          writeFileAtomic(tmpModulePath, moduleContent);
         }
 
         newPath = tmpModulePath;
diff --git a/test/unit/native-addon-cache.test.ts b/test/unit/native-addon-cache.test.ts
new file mode 100644
index 000000000..b0b5d334d
--- /dev/null
+++ b/test/unit/native-addon-cache.test.ts
@@ -0,0 +1,312 @@
+import assert from 'node:assert/strict';
+import { createHash } from 'node:crypto';
+import fs from 'node:fs';
+import { createRequire } from 'node:module';
+import os from 'node:os';
+import path from 'node:path';
+import { describe, it } from 'node:test';
+import type { TestContext } from 'node:test';
+import vm from 'node:vm';
+
+const sharedPath = path.join(__dirname, '../../prelude/bootstrap-shared.js');
+const sharedSource = fs.readFileSync(sharedPath, 'utf8');
+const requireBuiltin = createRequire(__filename);
+// Synthetic bytes only: the fake dlopen below never loads a native library.
+const content = Buffer.from([0, 1, 2, 3, 4, 5, 255, 0]);
+const previous = Buffer.from([8, 7, 6, 5, 4, 3, 2, 1]);
+const different = Buffer.from([0, 1, 2, 3, 4, 6, 255, 0]);
+type Layout = 'node_modules' | 'standalone';
+
+function fixture(
+  t: TestContext,
+  layout: Layout,
+  options: {
+    fail?: 'write' | 'rename' | 'open';
+    winner?: Buffer;
+    filename?: string;
+  } = {},
+) {
+  const root = fs.mkdtempSync(path.join(os.tmpdir(), 'pkg-native-cache-'));
+  const liveDescriptors = new Set<number>();
+  t.after(() => {
+    // Also release leaked descriptors when testing a broken implementation.
+    for (const fd of liveDescriptors) fs.closeSync(fd);
+    fs.rmSync(root, { recursive: true, force: true });
+  });
+
+  const snapshot = path.join(root, 'snapshot');
+  const cache = path.join(root, 'cache');
+  const filename = options.filename ?? 'addon.node';
+  const relative =
+    layout === 'node_modules'
+      ? path.join('sample-addon', 'build', 'Release', filename)
+      : filename;
+  const source = path.join(
+    snapshot,
+    layout === 'node_modules' ? 'node_modules' : '',
+    relative,
+  );
+  const hash = createHash('sha256').update(content).digest('hex');
+  const destination = path.join(cache, 'pkg', hash, relative);
+  fs.mkdirSync(path.dirname(source), { recursive: true });
+  fs.writeFileSync(source, content);
+
+  const opens: { file: string; flags: string; fd?: number }[] = [];
+  const writes: (string | number)[] = [];
+  const closes: number[] = [];
+  const renames: { from: string; to: string; before?: Buffer }[] = [];
+  const loads: { file: string; bytes: Buffer }[] = [];
+  const error = Object.assign(new Error('injected cache failure'), {
+    code: options.fail === 'write' ? 'ENOSPC' : 'EPERM',
+  });
+  let collision: string | undefined;
+
+  // Only the VM sees these wrappers; fixture setup and inspection use real fs.
+  const wrappedFs = {
+    ...fs,
+    openSync(file: string, flags: string) {
+      const attempt: (typeof opens)[number] = { file, flags };
+      opens.push(attempt);
+      if (options.fail === 'open') {
+        // Another writer owns this exact candidate before our exclusive open.
+        collision = file;
+        fs.writeFileSync(file, different);
+      }
+      const fd = fs.openSync(file, flags);
+      attempt.fd = fd;
+      liveDescriptors.add(fd);
+      return fd;
+    },
+    writeFileSync(file: string | number, bytes: Buffer) {
+      writes.push(file);
+      if (options.fail === 'write') {
+        fs.writeFileSync(file, bytes.subarray(0, 3));
+        throw error;
+      }
+      fs.writeFileSync(file, bytes);
+    },
+    closeSync(fd: number) {
+      fs.closeSync(fd);
+      liveDescriptors.delete(fd);
+      closes.push(fd);
+    },
+    renameSync(from: string, to: string) {
+      renames.push({
+        from,
+        to,
+        before: fs.existsSync(to) ? fs.readFileSync(to) : undefined,
+      });
+      assert.deepEqual(fs.readFileSync(from), content);
+      assert.equal(liveDescriptors.size, 0, 'close before publishing');
+      if (options.fail === 'rename') {
+        // Deterministic Windows-style race: a winner appears, then rename fails.
+        // No timing, child processes, or host-specific rename semantics needed.
+        if (options.winner) fs.writeFileSync(to, options.winner);
+        throw error;
+      }
+      fs.renameSync(from, to);
+    },
+  };
+  const fakeProcess = {
+    env: { PKG_NATIVE_CACHE_PATH: cache },
+    pid: process.pid,
+    dlopen(_module: object, file: string) {
+      loads.push({ file, bytes: fs.readFileSync(file) });
+      return 'loaded';
+    },
+  };
+  const shared = {
+    exports: {} as {
+      patchDlopen: (_inside: (_file: string) => boolean) => void;
+    },
+  };
+  // Evaluate the actual shared prelude, without importing a generated SEA bundle
+  // or patching the test runner's process.dlopen / filesystem module.
+  vm.runInNewContext(
+    sharedSource,
+    {
+      Buffer,
+      process: fakeProcess,
+      module: shared,
+      require: (id: string) => (id === 'fs' ? wrappedFs : requireBuiltin(id)),
+    },
+    { filename: sharedPath },
+  );
+  shared.exports.patchDlopen((file) => file.startsWith(snapshot + path.sep));
+
+  return {
+    destination,
+    opens,
+    writes,
+    closes,
+    renames,
+    loads,
+    error,
+    load: () => fakeProcess.dlopen({}, source),
+    seed(bytes: Buffer) {
+      fs.mkdirSync(path.dirname(destination), { recursive: true });
+      fs.writeFileSync(destination, bytes);
+    },
+    assertLoaded() {
+      assert.deepEqual(loads, [{ file: destination, bytes: content }]);
+    },
+    assertCache(bytes?: Buffer, extra: string[] = []) {
+      assert.deepEqual(
+        fs.readdirSync(path.dirname(destination)).sort(),
+        [...(bytes ? [path.basename(destination)] : []), ...extra].sort(),
+        'no temporary files left behind',
+      );
+      if (bytes) assert.deepEqual(fs.readFileSync(destination), bytes);
+      assert.equal(liveDescriptors.size, 0, 'no leaked descriptors');
+    },
+    assertStagedWrite() {
+      assert.equal(opens.length, 1, 'one exclusive temporary-file open');
+      const [{ file, flags, fd }] = opens;
+      assert.equal(flags, 'wx');
+      assert.notEqual(
+        file,
+        destination,
+        'never open the destination for writing',
+      );
+      assert.equal(path.dirname(file), path.dirname(destination));
+      assert.equal(typeof fd, 'number');
+      assert.deepEqual(writes, [fd], 'write only through the temporary fd');
+      assert.deepEqual(closes, [fd], 'close the temporary fd exactly once');
+      assert.throws(() => fs.fstatSync(fd!), { code: 'EBADF' });
+      return file;
+    },
+    getCollision() {
+      assert.ok(collision, 'exclusive open must have been attempted');
+      return collision;
+    },
+  };
+}
+
+for (const layout of ['node_modules', 'standalone'] as const) {
+  describe(`native addon cache (${layout})`, () => {
+    for (const existing of [false, true]) {
+      const state = existing ? 'replacement' : 'cold';
+
+      it(`publishes a ${state} cache via a complete sibling file`, (t) => {
+        const f = fixture(t, layout);
+        if (existing) f.seed(previous);
+
+        assert.equal(f.load(), 'loaded');
+        f.assertLoaded();
+        const temporary = f.assertStagedWrite();
+        assert.deepEqual(f.renames, [
+          {
+            from: temporary,
+            to: f.destination,
+            before: existing ? previous : undefined,
+          },
+        ]);
+        f.assertCache(content);
+      });
+
+      it(`cleans up a partial ENOSPC write for a ${state} cache`, (t) => {
+        const f = fixture(t, layout, { fail: 'write' });
+        if (existing) f.seed(previous);
+
+        assert.throws(f.load, (err) => err === f.error);
+        assert.deepEqual(f.loads, [], 'must not load after a failed write');
+        assert.deepEqual(f.renames, [], 'must not publish partial bytes');
+        f.assertCache(existing ? previous : undefined);
+        f.assertStagedWrite();
+      });
+
+      it(`cleans up a failed rename for a ${state} cache`, (t) => {
+        const f = fixture(t, layout, { fail: 'rename' });
+        if (existing) f.seed(previous);
+
+        assert.throws(f.load, (err) => err === f.error);
+        assert.deepEqual(f.loads, [], 'must not load after a failed rename');
+        assert.equal(f.renames.length, 1);
+        f.assertCache(existing ? previous : undefined);
+        f.assertStagedWrite();
+      });
+    }
+
+    it('publishes an addon with a 240-byte basename', (t) => {
+      // A valid basename near NAME_MAX must not be extended with a temp suffix.
+      const filename = `${'a'.repeat(235)}.node`;
+      const f = fixture(t, layout, { filename });
+      assert.equal(Buffer.byteLength(path.basename(f.destination)), 240);
+
+      assert.equal(f.load(), 'loaded');
+      f.assertLoaded();
+      const temporary = f.assertStagedWrite();
+      assert.ok(Buffer.byteLength(path.basename(temporary)) <= 255);
+      assert.deepEqual(f.renames, [
+        { from: temporary, to: f.destination, before: undefined },
+      ]);
+      f.assertCache(content);
+    });
+
+    it('uses a fresh sibling filename for each cache repair', (t) => {
+      const f = fixture(t, layout);
+      for (let attempt = 0; attempt < 2; attempt += 1) {
+        f.seed(previous);
+        assert.equal(f.load(), 'loaded');
+      }
+
+      assert.equal(f.opens.length, 2);
+      assert.notEqual(f.opens[0].file, f.opens[1].file);
+      for (const { file, flags } of f.opens) {
+        assert.equal(flags, 'wx');
+        assert.notEqual(file, f.destination);
+        assert.equal(path.dirname(file), path.dirname(f.destination));
+      }
+      assert.equal(f.renames.length, 2);
+      f.assertCache(content);
+    });
+
+    it('does not write an identical warm cache', (t) => {
+      const f = fixture(t, layout);
+      f.seed(content);
+
+      assert.equal(f.load(), 'loaded');
+      f.assertLoaded();
+      assert.deepEqual(f.opens, []);
+      assert.deepEqual(f.writes, []);
+      assert.deepEqual(f.closes, []);
+      assert.deepEqual(f.renames, []);
+      f.assertCache(content);
+    });
+
+    for (const matching of [true, false]) {
+      it(`${matching ? 'accepts' : 'rejects'} a Windows-style rename race with ${matching ? 'matching' : 'different'} winner bytes`, (t) => {
+        const winner = matching ? content : different;
+        const f = fixture(t, layout, { fail: 'rename', winner });
+
+        if (matching) {
+          assert.equal(f.load(), 'loaded');
+          f.assertLoaded();
+        } else {
+          assert.throws(f.load, (err) => err === f.error);
+          assert.deepEqual(f.loads, [], 'must not load a different winner');
+        }
+        assert.equal(f.renames.length, 1, 'exercise the rename failure');
+        f.assertCache(winner);
+        f.assertStagedWrite();
+      });
+    }
+
+    it("does not remove another writer's temporary file on open EEXIST", (t) => {
+      const f = fixture(t, layout, { fail: 'open' });
+      f.seed(previous);
+
+      assert.throws(f.load, { code: 'EEXIST' });
+      const collision = f.getCollision();
+      assert.notEqual(collision, f.destination);
+      assert.equal(path.dirname(collision), path.dirname(f.destination));
+      assert.deepEqual(f.opens, [{ file: collision, flags: 'wx' }]);
+      assert.deepEqual(fs.readFileSync(collision), different);
+      assert.deepEqual(f.writes, []);
+      assert.deepEqual(f.closes, []);
+      assert.deepEqual(f.renames, []);
+      assert.deepEqual(f.loads, []);
+      f.assertCache(previous, [path.basename(collision)]);
+    });
+  });
+}