diff --git a/.github/scripts/npm-preflight.mjs b/.github/scripts/npm-preflight.mjs
new file mode 100644
index 0000000..7ecc460
--- /dev/null
+++ b/.github/scripts/npm-preflight.mjs
@@ -0,0 +1,106 @@
+// Verifies that NPM_TOKEN can actually publish every publishable workspace
+// package, so a broken/expired/wrong-account token fails the PR instead of
+// only surfacing as an E403 during the master publish.
+//
+// Checks, for the token's identity:
+//   - the token is valid (npm whoami succeeds)
+//   - for each already-published package: identity has read-write access
+//   - for packages not yet on npm: a valid token is enough (first publish)
+
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { execFileSync } from "node:child_process";
+
+const REGISTRY = "https://registry.npmjs.org";
+const token = process.env.NPM_TOKEN;
+
+if (!token) {
+  console.log(
+    "npm_token secret is not available (e.g. a fork PR). Skipping npm publish preflight.",
+  );
+  process.exit(0);
+}
+
+// Point npm at the registry with the token, without touching the repo tree.
+fs.writeFileSync(
+  path.join(os.homedir(), ".npmrc"),
+  `//registry.npmjs.org/:_authToken=${token}\n`,
+);
+
+const npm = (args) =>
+  execFileSync("npm", [...args, "--registry", REGISTRY], {
+    encoding: "utf8",
+  }).trim();
+
+let whoami;
+try {
+  whoami = npm(["whoami"]);
+} catch {
+  console.error(
+    "::error::npm_token is invalid or expired (npm whoami failed). Publish on master will 403.",
+  );
+  process.exit(1);
+}
+console.log(`npm identity: ${whoami}`);
+
+const publishable = fs
+  .readdirSync("packages")
+  .map((dir) => path.join("packages", dir, "package.json"))
+  .filter((file) => fs.existsSync(file))
+  .map((file) => JSON.parse(fs.readFileSync(file, "utf8")))
+  .filter((pkg) => pkg && !pkg.private && pkg.name)
+  .map((pkg) => pkg.name);
+
+const existsOnNpm = async (name) => {
+  const res = await fetch(`${REGISTRY}/${encodeURIComponent(name)}`, {
+    method: "HEAD",
+  });
+  return res.status === 200;
+};
+
+// Per-package collaborator check. Unlike `npm access list packages` (which only
+// works for org accounts and 403s for a personal account), this works whether
+// the packages are owned by a user or an org.
+let failed = false;
+for (const name of publishable) {
+  let collaborators;
+  try {
+    collaborators = JSON.parse(
+      npm(["access", "list", "collaborators", name, "--json"]),
+    );
+  } catch {
+    // Couldn't read collaborators: fine if the package doesn't exist yet
+    // (first publish), otherwise the token genuinely can't see/access it.
+    if (await existsOnNpm(name)) {
+      console.log(
+        `FAIL  ${name}: '${whoami}' cannot read collaborators (token lacks access)`,
+      );
+      failed = true;
+    } else {
+      console.log(`ok    ${name}: not yet on npm, valid token can first-publish`);
+    }
+    continue;
+  }
+
+  const perm = collaborators[whoami];
+  if (perm && perm.includes("write")) {
+    console.log(`ok    ${name}: ${whoami} has ${perm}`);
+  } else if (perm) {
+    console.log(`FAIL  ${name}: ${whoami} has ${perm} (need read-write)`);
+    failed = true;
+  } else {
+    console.log(`FAIL  ${name}: ${whoami} is not a collaborator`);
+    failed = true;
+  }
+}
+
+if (failed) {
+  console.error(
+    `\n::error::npm token '${whoami}' cannot publish one or more packages. ` +
+      `Fix the token account/ownership before merging to master.`,
+  );
+  process.exit(1);
+}
+
+console.log("\nAll publishable packages are writable by this token.");
diff --git a/.github/workflows/checks.yml b/.github/workflows/checks.yml
index e97b22b..e7b43b8 100644
--- a/.github/workflows/checks.yml
+++ b/.github/workflows/checks.yml
@@ -51,3 +51,18 @@ jobs:
           cache-key: ci-types
       - name: types
         run: pnpm check-types
+
+  npm-publish-preflight:
+    name: npm-publish-preflight
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout
+        uses: actions/checkout@v4
+      - name: node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+      - name: verify npm publish credentials
+        env:
+          NPM_TOKEN: ${{ secrets.npm_token }}
+        run: node .github/scripts/npm-preflight.mjs