diff --git a/‎.github/scripts/docs_preview.js‎
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diff --git a/‎.github/scripts/docs_preview.test.js‎
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+// Docs preview gate and PR comment. .github/workflows/docs-preview.yml wires
+// these up: `authorize` decides whether a run may build and deploy a preview
+// (and for which commit), `comment` reports the outcome on the pull request.
+// The security model is described in the workflow's header.
+'use strict';
+
+const MARKER = '<!-- docs-preview -->';
+const BOT_LOGIN = 'github-actions[bot]';
+
+// Sets the job outputs `authorized`, `pr_number`, `head_sha` and
+// `slash_attempt` for a pull_request_target or /preview-docs issue_comment run.
+async function authorize({ github, context, core }) {
+  const { owner, repo } = context.repo;
+
+  async function permissionFor(username) {
+    const { data } = await github.rest.repos.getCollaboratorPermissionLevel({ owner, repo, username });
+    return { level: data.permission, role: data.role_name };
+  }
+
+  let authorized = false;
+  let prNumber = '';
+  let headSha = '';
+  let slashAttempt = false;
+
+  if (context.eventName === 'pull_request_target') {
+    // Gate on the *sender* (whoever caused this run — on synchronize that
+    // is the pusher), not the PR author, so a non-admin pushing to an
+    // admin-opened branch does not get an automatic build.
+    const actor = context.payload.sender.login;
+    prNumber = String(context.payload.pull_request.number);
+    headSha = context.payload.pull_request.head.sha;
+    const perm = await permissionFor(actor);
+    authorized = perm.level === 'admin';
+    core.info(`pull_request_target by ${actor} (level=${perm.level}, role=${perm.role}) → authorized=${authorized}`);
+  } else {
+    // issue_comment: the job-level `if:` already guarantees this is a PR
+    // comment starting with /preview-docs.
+    slashAttempt = true;
+    const actor = context.payload.comment.user.login;
+    prNumber = String(context.payload.issue.number);
+    const perm = await permissionFor(actor);
+    authorized = perm.level === 'admin' || perm.role === 'maintain';
+    if (authorized) {
+      const { data: pr } = await github.rest.pulls.get({ owner, repo, pull_number: Number(prNumber) });
+      if (pr.state !== 'open') {
+        authorized = false;
+        core.info(`PR #${prNumber} is ${pr.state}; refusing to preview.`);
+      } else {
+        headSha = pr.head.sha;
+      }
+    }
+    core.info(`/preview-docs by ${actor} (level=${perm.level}, role=${perm.role}) → authorized=${authorized}`);
+  }
+
+  core.setOutput('authorized', String(authorized));
+  core.setOutput('pr_number', prNumber);
+  core.setOutput('head_sha', headSha);
+  core.setOutput('slash_attempt', String(slashAttempt));
+}
+
+// Posts or updates the preview comment on the PR. Reads the outcome of the
+// earlier jobs from the step's env: AUTHORIZED, PR_NUMBER, HEAD_SHA,
+// DEPLOY_RESULT, DEPLOYMENT_URL, ALIAS_URL, RUN_URL.
+async function comment({ github, context }) {
+  const { owner, repo } = context.repo;
+  const env = process.env;
+  const issue_number = Number(env.PR_NUMBER);
+
+  async function upsert(body) {
+    const comments = await github.paginate(github.rest.issues.listComments, { owner, repo, issue_number, per_page: 100 });
+    const existing = comments.find((c) => c.user?.login === BOT_LOGIN && c.body?.includes(MARKER));
+    if (existing) {
+      await github.rest.issues.updateComment({ owner, repo, comment_id: existing.id, body });
+    } else {
+      await github.rest.issues.createComment({ owner, repo, issue_number, body });
+    }
+  }
+
+  if (env.AUTHORIZED !== 'true') {
+    await github.rest.issues.createComment({
+      owner, repo, issue_number,
+      body: `@${context.actor} — only repository admins or maintainers can run \`/preview-docs\` (and the PR must be open).`,
+    });
+    return;
+  }
+
+  if (env.DEPLOY_RESULT !== 'success') {
+    await upsert(
+      `${MARKER}\n### 📚 Documentation preview\n\n` +
+      `❌ Preview build **failed** for \`${env.HEAD_SHA.slice(0, 7)}\` — [workflow logs](${env.RUN_URL}).`
+    );
+    return;
+  }
+
+  const previewUrl = env.ALIAS_URL || env.DEPLOYMENT_URL;
+  const ts = new Date().toISOString().replace('T', ' ').replace(/\.\d+Z$/, ' UTC');
+  await upsert(
+    `${MARKER}\n### 📚 Documentation preview\n\n` +
+    `| | |\n|---|---|\n` +
+    `| **Preview** | ${previewUrl} |\n` +
+    `| **Deployment** | ${env.DEPLOYMENT_URL} |\n` +
+    `| **Commit** | \`${env.HEAD_SHA.slice(0, 7)}\` |\n` +
+    `| **Triggered by** | @${context.actor} |\n` +
+    `| **Updated** | ${ts} |\n`
+  );
+}
+
+module.exports = { authorize, comment };
@@ -0,0 +1,212 @@
+// Scenario tests for docs_preview.js: who gets a preview build (`authorize`)
+// and what the pull request shows afterwards (`comment`).
+//
+//   node --test .github/scripts/docs_preview.test.js
+//
+// No dependencies; the GitHub client is a small fake defined at the bottom.
+// CI runs it in the checks job (.github/workflows/shared.yml).
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { authorize, comment } = require('./docs_preview.js');
+
+const REPO = { owner: 'modelcontextprotocol', repo: 'python-sdk' };
+const HEAD = 'e4dfda7baa127ab00ebcd1d5324560cbe3cdfe42';
+const MARKER = '<!-- docs-preview -->';
+
+// `permission` / `role_name` as the collaborators API reports them.
+const PEOPLE = {
+  admin: { permission: 'admin', role_name: 'admin' },
+  maintainer: { permission: 'write', role_name: 'maintain' },
+  writer: { permission: 'write', role_name: 'write' },
+  outsider: { permission: 'read', role_name: 'read' },
+};
+
+// ── authorize ──────────────────────────────────────────────────────────────
+// `expect` is the full set of job outputs the step writes.
+
+const authorizeScenarios = [
+  {
+    name: 'admin pushes to (or opens) a PR → automatic preview of that head',
+    event: pushed(7, 'admin'),
+    expect: { authorized: 'true', pr_number: '7', head_sha: HEAD, slash_attempt: 'false' },
+  },
+  {
+    name: 'someone with write but not admin pushes → no automatic preview',
+    event: pushed(7, 'writer'),
+    expect: { authorized: 'false', pr_number: '7', head_sha: HEAD, slash_attempt: 'false' },
+  },
+  {
+    name: 'maintainer comments /preview-docs on an open PR → preview of its current head',
+    event: slash(7, 'maintainer'),
+    expect: { authorized: 'true', pr_number: '7', head_sha: HEAD, slash_attempt: 'true' },
+  },
+  {
+    name: 'admin comments /preview-docs → authorized as well',
+    event: slash(7, 'admin'),
+    expect: { authorized: 'true', pr_number: '7', head_sha: HEAD, slash_attempt: 'true' },
+  },
+  {
+    name: 'writer without the maintain role comments /preview-docs → refused, recorded as an attempt',
+    event: slash(7, 'writer'),
+    expect: { authorized: 'false', pr_number: '7', head_sha: '', slash_attempt: 'true' },
+  },
+  {
+    name: 'outsider comments /preview-docs → refused, recorded as an attempt',
+    event: slash(7, 'outsider'),
+    expect: { authorized: 'false', pr_number: '7', head_sha: '', slash_attempt: 'true' },
+  },
+  {
+    name: '/preview-docs on a closed PR → refused even for a maintainer',
+    pr: { state: 'closed' },
+    event: slash(7, 'maintainer'),
+    expect: { authorized: 'false', pr_number: '7', head_sha: '', slash_attempt: 'true' },
+  },
+];
+
+for (const s of authorizeScenarios) {
+  test(`authorize: ${s.name}`, async () => {
+    const world = makeWorld({ pr: { number: 7, ...s.pr } });
+    assert.deepEqual(await runAuthorize(world, s.event), s.expect);
+    assert.equal(world.writes.length, 0);
+  });
+}
+
+test('authorize: a failing permission lookup fails the step instead of deciding either way', async () => {
+  const world = makeWorld({ pr: { number: 7 } });
+  world.failPermissionLookup = true;
+  await assert.rejects(runAuthorize(world, pushed(7, 'admin')), /boom/);
+});
+
+// ── comment ────────────────────────────────────────────────────────────────
+
+const DEPLOYED = {
+  AUTHORIZED: 'true',
+  PR_NUMBER: '7',
+  HEAD_SHA: HEAD,
+  DEPLOY_RESULT: 'success',
+  DEPLOYMENT_URL: 'https://1a2b3c.mcp-python-sdk-docs.pages.dev',
+  ALIAS_URL: 'https://pr-7.mcp-python-sdk-docs.pages.dev',
+  RUN_URL: 'https://github.com/modelcontextprotocol/python-sdk/actions/runs/1',
+};
+
+test('comment: a refused /preview-docs gets a plain reply to the commenter, not a preview comment', async () => {
+  const world = makeWorld({ pr: { number: 7 } });
+  await runComment(world, { ...DEPLOYED, AUTHORIZED: 'false', HEAD_SHA: '', DEPLOY_RESULT: 'skipped' }, 'outsider');
+  assert.equal(world.comments.length, 1);
+  assert.match(world.comments[0].body, /^@outsider — only repository admins or maintainers can run `\/preview-docs`/);
+  assert.ok(!world.comments[0].body.includes(MARKER));
+});
+
+test('comment: first successful deploy posts one preview comment linking the alias URL and the commit', async () => {
+  const world = makeWorld({ pr: { number: 7 } });
+  await runComment(world, DEPLOYED, 'admin');
+  assert.equal(world.comments.length, 1);
+  const body = world.comments[0].body;
+  assert.ok(body.startsWith(`${MARKER}\n### 📚 Documentation preview`));
+  assert.match(body, /\| \*\*Preview\*\* \| https:\/\/pr-7\.mcp-python-sdk-docs\.pages\.dev \|/);
+  assert.match(body, /\| \*\*Deployment\*\* \| https:\/\/1a2b3c\.mcp-python-sdk-docs\.pages\.dev \|/);
+  assert.match(body, /\| \*\*Commit\*\* \| `e4dfda7` \|/);
+  assert.match(body, /\| \*\*Triggered by\*\* \| @admin \|/);
+  assert.deepEqual(world.writes, ['comment on #7']);
+});
+
+test('comment: a later deploy edits the existing preview comment instead of adding another', async () => {
+  const world = makeWorld({ pr: { number: 7 }, comments: [{ user: 'someone', body: 'LGTM' }, { user: 'github-actions[bot]', body: `${MARKER}\nold table` }] });
+  await runComment(world, { ...DEPLOYED, HEAD_SHA: 'f'.repeat(40) }, 'admin');
+  assert.equal(world.comments.length, 2);
+  assert.match(world.comments[1].body, /\| \*\*Commit\*\* \| `fffffff` \|/);
+  assert.deepEqual(world.writes, ['edit comment 101']);
+});
+
+test("comment: someone else's comment that happens to contain the marker is left alone", async () => {
+  const world = makeWorld({ pr: { number: 7 }, comments: [{ user: 'someone', body: `quoting ${MARKER} here` }] });
+  await runComment(world, DEPLOYED, 'admin');
+  assert.equal(world.comments.length, 2);
+  assert.equal(world.comments[0].body, `quoting ${MARKER} here`);
+  assert.deepEqual(world.writes, ['comment on #7']);
+});
+
+test('comment: with no alias URL the preview link falls back to the deployment URL', async () => {
+  const world = makeWorld({ pr: { number: 7 } });
+  await runComment(world, { ...DEPLOYED, ALIAS_URL: '' }, 'admin');
+  assert.match(world.comments[0].body, /\| \*\*Preview\*\* \| https:\/\/1a2b3c\.mcp-python-sdk-docs\.pages\.dev \|/);
+});
+
+test('comment: a build or deploy that did not succeed is reported with the short SHA and a link to the run', async () => {
+  const world = makeWorld({ pr: { number: 7 }, comments: [{ user: 'github-actions[bot]', body: `${MARKER}\nold table` }] });
+  await runComment(world, { ...DEPLOYED, DEPLOY_RESULT: 'skipped', DEPLOYMENT_URL: '', ALIAS_URL: '' }, 'admin');
+  assert.equal(world.comments.length, 1);
+  assert.equal(
+    world.comments[0].body,
+    `${MARKER}\n### 📚 Documentation preview\n\n❌ Preview build **failed** for \`e4dfda7\` — [workflow logs](${DEPLOYED.RUN_URL}).`
+  );
+});
+
+// ── Harness ────────────────────────────────────────────────────────────────
+
+function pushed(number, sender) {
+  return { eventName: 'pull_request_target', actor: sender, payload: { action: 'synchronize', pull_request: { number, head: { sha: HEAD } }, sender: { login: sender } } };
+}
+function slash(number, commenter) {
+  return { eventName: 'issue_comment', actor: commenter, payload: { action: 'created', issue: { number, pull_request: {} }, comment: { body: '/preview-docs', user: { login: commenter } } } };
+}
+
+async function runAuthorize(world, event) {
+  const outputs = {};
+  const core = { info: () => {}, setOutput: (k, v) => { outputs[k] = v; } };
+  await authorize({ github: world.github, context: { repo: REPO, ...event }, core });
+  return outputs;
+}
+
+async function runComment(world, env, actor) {
+  const saved = {};
+  for (const [k, v] of Object.entries(env)) { saved[k] = process.env[k]; process.env[k] = v; }
+  try {
+    await comment({ github: world.github, context: { repo: REPO, actor }, core: {} });
+  } finally {
+    for (const [k, v] of Object.entries(saved)) { if (v === undefined) delete process.env[k]; else process.env[k] = v; }
+  }
+}
+
+// ── A tiny in-memory GitHub ────────────────────────────────────────────────
+
+function makeWorld({ pr, comments = [] }) {
+  const world = { pr: { state: 'open', ...pr }, comments: [], writes: [], failPermissionLookup: false, nextCommentId: 100 };
+  for (const c of comments) world.comments.push({ id: world.nextCommentId++, ...c });
+
+  const err = (status, message = 'fake error') => Object.assign(new Error(message), { status });
+  const write = (what) => world.writes.push(what);
+  const checkPr = (n) => { if (n !== world.pr.number) throw err(404); };
+
+  const rest = {
+    repos: {
+      getCollaboratorPermissionLevel: async ({ username }) => {
+        if (world.failPermissionLookup) throw err(500, 'boom');
+        const person = PEOPLE[username];
+        if (!person) throw err(404, 'not a user');
+        return { data: { ...person, user: { login: username } } };
+      },
+    },
+    pulls: {
+      get: async ({ pull_number }) => { checkPr(pull_number); return { data: { number: pull_number, state: world.pr.state, head: { sha: HEAD } } }; },
+    },
+    issues: {
+      listComments: async ({ issue_number }) => { checkPr(issue_number); return { data: world.comments.map((c) => ({ id: c.id, body: c.body, user: { login: c.user } })) }; },
+      createComment: async ({ issue_number, body }) => {
+        checkPr(issue_number);
+        write(`comment on #${issue_number}`);
+        world.comments.push({ id: world.nextCommentId++, user: 'github-actions[bot]', body });
+      },
+      updateComment: async ({ comment_id, body }) => {
+        write(`edit comment ${comment_id}`);
+        const c = world.comments.find((x) => x.id === comment_id);
+        if (!c) throw err(404);
+        c.body = body;
+      },
+    },
+  };
+  world.github = { rest, paginate: async (fn, args) => (await fn(args)).data };
+  return world;
+}