diff --git a/.github/workflows/deploy.yml b/.github/workflows/deploy.yml
index 8e33da8a..916777b1 100644
--- a/.github/workflows/deploy.yml
+++ b/.github/workflows/deploy.yml
@@ -429,6 +429,7 @@ jobs:
           CHROMIUM_REQUIRE=1 BASE_URL=http://localhost:13581 node test-channels-ws-race-1498-e2e.js 2>&1 | tee -a e2e-output.txt
           CHROMIUM_REQUIRE=1 BASE_URL=http://localhost:13581 node test-issue-1487-byop-modal-layout-e2e.js 2>&1 | tee -a e2e-output.txt
           CHROMIUM_REQUIRE=1 BASE_URL=http://localhost:13581 node test-issue-1630-reach-mobile-e2e.js 2>&1 | tee -a e2e-output.txt
+          CHROMIUM_REQUIRE=1 BASE_URL=http://localhost:13581 node test-issue-1640-compare-discovery-e2e.js 2>&1 | tee -a e2e-output.txt
 
       # #1616: slide-over focus-restore flake-gate. Runs the slide-over
       # E2E 20 consecutive times against the SAME backend instance so
diff --git a/test-issue-1640-compare-discovery-e2e.js b/test-issue-1640-compare-discovery-e2e.js
new file mode 100644
index 00000000..c67b301a
--- /dev/null
+++ b/test-issue-1640-compare-discovery-e2e.js
@@ -0,0 +1,171 @@
+/**
+ * E2E test (#1640): Observer comparison must be a first-class IA citizen.
+ *
+ * Asserts THREE new entry points to `#/compare`, beyond the pre-existing
+ * 🔍 button on the observers page header:
+ *
+ *   (A) Observers page header — a labeled button reading "Compare observers"
+ *       (text + icon, NOT a bare emoji).
+ *   (B) Observer-detail page — a "Compare with…" affordance that opens
+ *       #/compare?a=<this>&b=<picked> pre-populated.
+ *   (D) Multi-select on observers table — checkbox-per-row, enabling a
+ *       "Compare selected" button once exactly two observers are checked.
+ *
+ * Also asserts:
+ *   - The compare page renders breadcrumb links back to BOTH observer
+ *     detail pages.
+ *   - The legacy deep-link `#/compare?a=...&b=...` continues to work.
+ *
+ * Usage: BASE_URL=http://localhost:13581 node test-issue-1640-compare-discovery-e2e.js
+ */
+const { chromium } = require('playwright');
+
+const BASE = process.env.BASE_URL || 'http://localhost:3000';
+
+let passed = 0, failed = 0;
+async function step(name, fn) {
+  try { await fn(); passed++; console.log('  \u2705 ' + name); }
+  catch (e) { failed++; console.error('  \u274c ' + name + ': ' + e.message); }
+}
+function assert(c, m) { if (!c) throw new Error(m || 'assertion failed'); }
+
+async function pickTwoObserverIds(page) {
+  await page.goto(BASE + '/#/observers', { waitUntil: 'domcontentloaded' });
+  await page.waitForSelector('#obsTable tbody tr', { timeout: 15000 });
+  const ids = await page.$$eval('#obsTable tbody tr[data-value]', rows =>
+    rows.slice(0, 2).map(r => decodeURIComponent(
+      (r.getAttribute('data-value') || '').replace('#/observers/', '')
+    ))
+  );
+  assert(ids.length === 2, 'need at least 2 observers in fixture, got ' + ids.length);
+  return ids;
+}
+
+async function run() {
+  const browser = await chromium.launch({
+    headless: true,
+    executablePath: process.env.CHROMIUM_PATH || undefined,
+    args: ['--no-sandbox', '--disable-gpu', '--disable-dev-shm-usage']
+  });
+  const ctx = await browser.newContext({ viewport: { width: 1400, height: 900 } });
+  const page = await ctx.newPage();
+  page.setDefaultTimeout(15000);
+  page.on('pageerror', e => console.error('  pageerror:', e.message));
+
+  console.log('\nRunning #1640 compare-discovery E2E tests against ' + BASE + '\n');
+
+  const [idA, idB] = await pickTwoObserverIds(page);
+
+  // ── Entry point A: labeled "Compare observers" on observers page ──
+  await step('(A) Observers page header has labeled "Compare observers" button', async () => {
+    await page.goto(BASE + '/#/observers', { waitUntil: 'domcontentloaded' });
+    await page.waitForSelector('#obsTable tbody tr', { timeout: 15000 });
+    const btn = await page.$('[data-action="compare-observers"]');
+    assert(btn, 'expected element with data-action="compare-observers" in observers page header');
+    const text = (await btn.textContent() || '').trim();
+    assert(/compare/i.test(text),
+      'compare button must have visible text mentioning "Compare", got "' + text + '"');
+  });
+
+  await step('(A) Clicking "Compare observers" navigates to #/compare', async () => {
+    await page.goto(BASE + '/#/observers', { waitUntil: 'domcontentloaded' });
+    await page.waitForSelector('[data-action="compare-observers"]', { timeout: 10000 });
+    await page.click('[data-action="compare-observers"]');
+    await page.waitForFunction(() => location.hash.startsWith('#/compare'), null, { timeout: 5000 });
+    assert(/^#\/compare/.test(await page.evaluate(() => location.hash)),
+      'expected hash to become #/compare');
+  });
+
+  // ── Entry point B: observer-detail "Compare with…" picker ──
+  await step('(B) Observer detail page exposes a "Compare with…" affordance', async () => {
+    await page.goto(BASE + '/#/observers/' + idA, { waitUntil: 'domcontentloaded' });
+    await page.waitForSelector('#obsTitle', { timeout: 15000 });
+    // Give detail page a moment to finish rendering.
+    await page.waitForSelector('[data-action="compare-with-picker"]', { timeout: 10000 });
+    const picker = await page.$('[data-action="compare-with-picker"]');
+    assert(picker, 'expected [data-action="compare-with-picker"] (select) on observer-detail');
+    const options = await picker.$$('option');
+    assert(options.length >= 2, 'compare-with picker should be populated with other observers');
+  });
+
+  await step('(B) Picking another observer + Compare navigates to #/compare?a=<idA>&b=<other>', async () => {
+    await page.goto(BASE + '/#/observers/' + idA, { waitUntil: 'domcontentloaded' });
+    await page.waitForSelector('[data-action="compare-with-picker"]', { timeout: 10000 });
+    await page.selectOption('[data-action="compare-with-picker"]', idB);
+    await page.click('[data-action="compare-with-go"]');
+    await page.waitForFunction((idA) =>
+      location.hash.indexOf('#/compare') === 0 &&
+      location.hash.indexOf('a=' + idA) >= 0, idA, { timeout: 5000 });
+    const h = await page.evaluate(() => location.hash);
+    assert(h.indexOf('b=' + idB) >= 0, 'expected deep-link to carry b=<picked>, got: ' + h);
+  });
+
+  // ── Entry point D: multi-select on observers table ──
+  await step('(D) Observers table renders one checkbox per row + "Compare selected" button', async () => {
+    await page.goto(BASE + '/#/observers', { waitUntil: 'domcontentloaded' });
+    await page.waitForSelector('#obsTable tbody tr', { timeout: 15000 });
+    const boxes = await page.$$('#obsTable tbody input[type="checkbox"][data-compare-select]');
+    assert(boxes.length >= 2,
+      'expected per-row checkboxes ([data-compare-select]); got ' + boxes.length);
+    const btn = await page.$('[data-action="compare-selected"]');
+    assert(btn, 'expected [data-action="compare-selected"] button');
+    const disabled = await btn.evaluate(el => el.disabled || el.getAttribute('aria-disabled') === 'true');
+    assert(disabled, '"Compare selected" must be disabled when 0 rows are selected');
+  });
+
+  await step('(D) Selecting exactly two rows enables "Compare selected" and navigates correctly', async () => {
+    await page.goto(BASE + '/#/observers', { waitUntil: 'domcontentloaded' });
+    await page.waitForSelector('#obsTable tbody tr', { timeout: 15000 });
+    const boxes = await page.$$('#obsTable tbody input[type="checkbox"][data-compare-select]');
+    await boxes[0].check();
+    await boxes[1].check();
+    const btn = await page.$('[data-action="compare-selected"]');
+    const stillDisabled = await btn.evaluate(el => el.disabled || el.getAttribute('aria-disabled') === 'true');
+    assert(!stillDisabled, '"Compare selected" must be enabled when exactly 2 rows are checked');
+    await btn.click();
+    await page.waitForFunction(() => location.hash.indexOf('#/compare?') === 0, null, { timeout: 5000 });
+    const h = await page.evaluate(() => location.hash);
+    assert(/a=[^&]+&b=[^&]+/.test(h),
+      'expected hash to carry both ?a=&b= deep-link params, got: ' + h);
+  });
+
+  await step('(D) Selecting a third row disables "Compare selected" again', async () => {
+    await page.goto(BASE + '/#/observers', { waitUntil: 'domcontentloaded' });
+    await page.waitForSelector('#obsTable tbody tr', { timeout: 15000 });
+    const boxes = await page.$$('#obsTable tbody input[type="checkbox"][data-compare-select]');
+    if (boxes.length < 3) return; // fixture might only have 2; skip silently
+    await boxes[0].check();
+    await boxes[1].check();
+    await boxes[2].check();
+    const btn = await page.$('[data-action="compare-selected"]');
+    const disabled = await btn.evaluate(el => el.disabled || el.getAttribute('aria-disabled') === 'true');
+    assert(disabled, '"Compare selected" must re-disable when count !== 2');
+  });
+
+  // ── Compare page breadcrumbs to both observer detail pages ──
+  await step('Compare page renders breadcrumb links back to both observer detail pages', async () => {
+    await page.goto(BASE + '/#/compare?a=' + idA + '&b=' + idB, { waitUntil: 'domcontentloaded' });
+    await page.waitForSelector('.compare-page', { timeout: 15000 });
+    await page.waitForSelector('[data-role="compare-breadcrumbs"]', { timeout: 10000 });
+    const linkA = await page.$('[data-role="compare-breadcrumbs"] a[href="#/observers/' + idA + '"]');
+    const linkB = await page.$('[data-role="compare-breadcrumbs"] a[href="#/observers/' + idB + '"]');
+    assert(linkA, 'expected breadcrumb anchor → #/observers/<idA>');
+    assert(linkB, 'expected breadcrumb anchor → #/observers/<idB>');
+  });
+
+  // ── Legacy deep-link regression guard ──
+  await step('Legacy deep-link #/compare?a=...&b=... still pre-populates both selects', async () => {
+    await page.goto(BASE + '/#/compare?a=' + idA + '&b=' + idB, { waitUntil: 'domcontentloaded' });
+    await page.waitForSelector('#compareObsA', { timeout: 15000 });
+    const valA = await page.$eval('#compareObsA', el => el.value);
+    const valB = await page.$eval('#compareObsB', el => el.value);
+    assert(valA === idA, 'compareObsA should be pre-selected to a=, got ' + valA);
+    assert(valB === idB, 'compareObsB should be pre-selected to b=, got ' + valB);
+  });
+
+  await browser.close();
+  console.log('\n' + passed + ' passed, ' + failed + ' failed');
+  if (failed) process.exit(1);
+}
+
+run().catch(e => { console.error(e); process.exit(1); });