diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index fbc59fc7..02c51124 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -75,7 +75,7 @@ jobs:
         uses: actions/setup-node@v4
         with: { node-version: '20' }
       - if: ${{ !cancelled() }}
-        run: node --test agent/hook.test.mjs
+        run: node --test agent/hook.test.mjs npm/zerograph/test/launcher.test.js
 
       # A default Git for Windows install puts `git` on PATH and not `sh` — the Unix tools are an
       # opt-in — so `ask --rerank` looks for one. Route 1: Git's shell directories
@@ -281,8 +281,8 @@ jobs:
           npx zerograph --repo $R --no-dense trace write refund
           if ($LASTEXITCODE -ne 3) { throw "a verdict through the shim should exit 3, not $LASTEXITCODE" }
 
-          # An upgrade under a live server: the launcher runs a per-version copy, so the package's
-          # own binary can be overwritten while a detached `serve` answers, and `stop` ends it.
+          # An upgrade under a live server: the launcher runs a copy kept under LOCALAPPDATA, so the
+          # package's own binary can be overwritten while a detached `serve` answers, and `stop` ends it.
           if ($IsWindows) {
             npx zerograph --repo $R --no-dense serve --detach --idle 600
             if ($LASTEXITCODE -ne 0) { throw "serve --detach through the launcher failed: $LASTEXITCODE" }
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 75b27c88..f785b365 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -97,7 +97,9 @@ jobs:
   # can only be configured on a package that exists, so its first version was published by
   # hand (docs/bench/runbook.md). scripts/npm-pack.sh packs the same tarball.
   npm:
-    needs: build
+    # After `verify`, so a version whose attestation failed is never published: npm will not take
+    # the same version twice, and the docs promise every binary it ships verifies.
+    needs: [build, verify]
     # Only a tag has a release to download from and a version to publish under.
     if: startsWith(github.ref, 'refs/tags/')
     runs-on: ubuntu-latest
diff --git a/npm/zerograph/bin/zerograph.js b/npm/zerograph/bin/zerograph.js
index 04ae2c74..22cdfd55 100644
--- a/npm/zerograph/bin/zerograph.js
+++ b/npm/zerograph/bin/zerograph.js
@@ -33,32 +33,60 @@ function binaryPath() {
 // failed with EBUSY. The binary runs from a copy under LOCALAPPDATA instead, and the package's own
 // file is never held open. Any failure here runs the package's file as before.
 //
-// One slot per install, not per version: a project's devDependency and a global install, or two
-// packs of one version, each keep their own copy rather than evicting each other's on every run.
-// npm writes every file with one fixed mtime, so an install is told apart by its size and the time
-// it created the file.
+// One slot per version and size, so a project's devDependency and a global install at different
+// versions keep their own copies instead of evicting each other on every run, and a reinstall of
+// the same version reuses its copy rather than adding a 95 MB one. The size is not proof of the
+// bytes (the linker pads a PE file to 512 bytes, so a rebuild often keeps it), so an install the
+// slot has not seen is hashed once against the copy; `seen` remembers the installs that matched.
+// npm writes every file with one fixed mtime, so an install is told apart by when it created it.
 const SLOT_TTL_MS = 30 * 24 * 3600 * 1000;
 const TOUCH_MS = 24 * 3600 * 1000;
 
-function runnable(path) {
-  if (process.platform !== "win32" || !process.env.LOCALAPPDATA) return path;
+function sha256(file) {
+  const hash = require("node:crypto").createHash("sha256");
+  const fd = fs.openSync(file, "r");
+  try {
+    const buf = Buffer.alloc(1 << 20);
+    let n;
+    while ((n = fs.readSync(fd, buf, 0, buf.length, null)) > 0) hash.update(buf.subarray(0, n));
+  } finally {
+    fs.closeSync(fd);
+  }
+  return hash.digest("hex");
+}
+
+function runnable(path, { platform = process.platform, localAppData = process.env.LOCALAPPDATA, now = Date.now() } = {}) {
+  if (platform !== "win32" || !localAppData) return path;
   try {
     const { version } = require("../package.json");
-    const root = join(process.env.LOCALAPPDATA, "zerograph", "bin");
+    const root = join(localAppData, "zerograph", "bin");
     const src = fs.statSync(path);
-    const slot = `${version}-${src.size}-${Math.trunc(src.ctimeMs)}`;
+    const slot = `${version}-${src.size}`;
+    const install = `${src.size}:${Math.trunc(src.ctimeMs)}`;
     const dir = join(root, slot);
     const copy = join(dir, "zerograph.exe");
     const used = join(dir, "used");
-    const now = Date.now();
-    if (fs.existsSync(copy)) {
-      // Marked at most once a day, so a slot in use is never the one the sweep below removes.
+    const seen = join(dir, "seen");
+    const digest = join(dir, "sha256");
+    const read = (f) => { try { return fs.readFileSync(f, "utf8"); } catch { return ""; } };
+    const mark = () => {
+      // At most once a day, so a slot in use is never the one the sweep below removes.
       try {
-        if (now - fs.statSync(used).mtimeMs > TOUCH_MS) fs.utimesSync(used, new Date(), new Date());
+        if (now - fs.statSync(used).mtimeMs <= TOUCH_MS) return;
       } catch {
         fs.writeFileSync(used, "");
       }
-      return copy;
+      fs.utimesSync(used, new Date(now), new Date(now));
+    };
+    let hash;
+    if (fs.existsSync(copy)) {
+      if (read(seen).split("\n").includes(install)) { mark(); return copy; }
+      hash = sha256(path);
+      if (hash === read(digest).trim()) {
+        fs.appendFileSync(seen, `${install}\n`);
+        mark();
+        return copy;
+      }
     }
     fs.mkdirSync(dir, { recursive: true });
     // Left by a launcher killed mid-copy; one another launcher is still writing stays locked.
@@ -70,11 +98,14 @@ function runnable(path) {
       fs.copyFileSync(path, tmp);
       fs.renameSync(tmp, copy);
     } catch (e) {
-      // A full disk, or another launcher that renamed the same bytes into place first.
+      // A full disk, an older build of these bytes still running, or another launcher that renamed
+      // the same bytes into place first: only the last leaves a copy that is ours to run.
       fs.rmSync(tmp, { force: true });
-      if (!fs.existsSync(copy)) throw e;
+      if (!fs.existsSync(copy) || (hash ?? sha256(path)) !== sha256(copy)) return path;
     }
-    fs.writeFileSync(used, "");
+    fs.writeFileSync(digest, hash ?? sha256(path));
+    fs.writeFileSync(seen, `${install}\n`);
+    mark();
     // Slots nothing has launched for a month; one whose binary still runs fails the delete and
     // stays.
     for (const other of fs.readdirSync(root)) {
@@ -92,16 +123,27 @@ function runnable(path) {
   }
 }
 
-const found = binaryPath();
-if (found.error) {
-  console.error(found.error);
-  process.exit(2);
+// A sweep in another launcher can remove an old slot between `runnable` and the spawn.
+function spawnBinary(exe, pkg, args, spawn = spawnSync) {
+  const run = spawn(exe, args, { stdio: "inherit" });
+  if (run.error && run.error.code === "ENOENT" && exe !== pkg) return spawn(pkg, args, { stdio: "inherit" });
+  return run;
 }
 
-const run = spawnSync(runnable(found.path), process.argv.slice(2), { stdio: "inherit" });
-if (run.error) {
-  console.error(`zerograph: ${run.error.message}`);
-  process.exit(2);
+function main() {
+  const found = binaryPath();
+  if (found.error) {
+    console.error(found.error);
+    process.exit(2);
+  }
+  const run = spawnBinary(runnable(found.path), found.path, process.argv.slice(2));
+  if (run.error) {
+    console.error(`zerograph: ${run.error.message}`);
+    process.exit(2);
+  }
+  // A signal death has no status; report it the way a shell would.
+  process.exit(run.status === null ? 128 : run.status);
 }
-// A signal death has no status; report it the way a shell would.
-process.exit(run.status === null ? 128 : run.status);
+
+if (require.main === module) main();
+module.exports = { runnable, spawnBinary };
diff --git a/npm/zerograph/test/launcher.test.js b/npm/zerograph/test/launcher.test.js
new file mode 100644
index 00000000..5750d685
--- /dev/null
+++ b/npm/zerograph/test/launcher.test.js
@@ -0,0 +1,108 @@
+"use strict";
+
+// The Windows copy logic runs on every platform here: `runnable` takes the platform and
+// LOCALAPPDATA as arguments, so the slot rules are pinned without a Windows box.
+const test = require("node:test");
+const assert = require("node:assert");
+const fs = require("node:fs");
+const os = require("node:os");
+const { join } = require("node:path");
+const { runnable, spawnBinary } = require("../bin/zerograph.js");
+const { version } = require("../package.json");
+
+const DAY = 24 * 3600 * 1000;
+
+function sandbox() {
+  const base = fs.mkdtempSync(join(os.tmpdir(), "zerograph-launcher-"));
+  const pkg = join(base, "pkg.exe");
+  fs.writeFileSync(pkg, "x".repeat(1000));
+  const app = join(base, "app");
+  const root = join(app, "zerograph", "bin");
+  return { pkg, app, root, win: (now = Date.now()) => ({ platform: "win32", localAppData: app, now }) };
+}
+
+function slot(root, name, lastUsed) {
+  const dir = join(root, name);
+  fs.mkdirSync(dir, { recursive: true });
+  fs.writeFileSync(join(dir, "zerograph.exe"), "old");
+  fs.writeFileSync(join(dir, "used"), "");
+  fs.utimesSync(join(dir, "used"), new Date(lastUsed), new Date(lastUsed));
+  return dir;
+}
+
+test("off Windows the package's own binary runs", () => {
+  const s = sandbox();
+  assert.strictEqual(runnable(s.pkg, { platform: "linux", localAppData: s.app }), s.pkg);
+  assert.strictEqual(fs.existsSync(s.root), false);
+});
+
+test("the binary runs from a copy, and a second launch reuses it", () => {
+  const s = sandbox();
+  const first = runnable(s.pkg, s.win());
+  assert.notStrictEqual(first, s.pkg);
+  assert.strictEqual(fs.readFileSync(first, "utf8"), fs.readFileSync(s.pkg, "utf8"));
+  assert.strictEqual(runnable(s.pkg, s.win()), first);
+});
+
+test("a reinstall of the same version reuses its copy instead of adding one", () => {
+  const s = sandbox();
+  const first = runnable(s.pkg, s.win());
+  fs.rmSync(s.pkg);
+  fs.writeFileSync(s.pkg, "x".repeat(1000));
+  assert.strictEqual(runnable(s.pkg, s.win()), first);
+  assert.deepStrictEqual(fs.readdirSync(s.root), [`${version}-1000`]);
+});
+
+test("another version's copy used within the month stays", () => {
+  const s = sandbox();
+  const now = Date.now();
+  const other = slot(s.root, "0.6.0-999", now - 5 * DAY);
+  runnable(s.pkg, s.win(now));
+  assert.ok(fs.existsSync(join(other, "zerograph.exe")));
+});
+
+test("a copy nothing launched for a month is swept", () => {
+  const s = sandbox();
+  const now = Date.now();
+  const stale = slot(s.root, "0.6.0-999", now - 31 * DAY);
+  runnable(s.pkg, s.win(now));
+  assert.strictEqual(fs.existsSync(stale), false);
+});
+
+test("launching an old copy marks it, so the next sweep keeps it", () => {
+  const s = sandbox();
+  const now = Date.now();
+  const mine = runnable(s.pkg, s.win(now - 29 * DAY));
+  assert.strictEqual(runnable(s.pkg, s.win(now)), mine);
+  const used = fs.statSync(join(mine, "..", "used")).mtimeMs;
+  assert.ok(now - used < DAY, "the launch did not mark the slot");
+});
+
+test("a rebuild of the same version and size runs the new bytes, not the old copy", () => {
+  const s = sandbox();
+  runnable(s.pkg, s.win());
+  fs.rmSync(s.pkg);
+  fs.writeFileSync(s.pkg, "y".repeat(1000));
+  const copy = runnable(s.pkg, s.win());
+  assert.strictEqual(fs.readFileSync(copy, "utf8"), "y".repeat(1000));
+});
+
+test("a copy swept before the spawn falls back to the package's binary", () => {
+  const calls = [];
+  const spawn = (exe) => {
+    calls.push(exe);
+    return exe === "copy.exe" ? { error: Object.assign(new Error("gone"), { code: "ENOENT" }) } : { status: 0 };
+  };
+  assert.strictEqual(spawnBinary("copy.exe", "pkg.exe", [], spawn).status, 0);
+  assert.deepStrictEqual(calls, ["copy.exe", "pkg.exe"]);
+});
+
+test("a binary that fails for another reason is not run twice", () => {
+  const calls = [];
+  const spawn = (exe) => {
+    calls.push(exe);
+    return { error: Object.assign(new Error("denied"), { code: "EACCES" }) };
+  };
+  assert.strictEqual(spawnBinary("copy.exe", "pkg.exe", [], spawn).error.code, "EACCES");
+  assert.deepStrictEqual(calls, ["copy.exe"]);
+});