diff --git a/public/observers.js b/public/observers.js
index 43b72d1b..d0dfdda4 100644
--- a/public/observers.js
+++ b/public/observers.js
@@ -320,6 +320,16 @@ window.preserveCompareSelection = function preserveCompareSelection(prevIds, tbo
     return timeAgo(o.last_packet_at);
   }
 
+  // #1789 — Firmware strings can be long, e.g.
+  // "v1.16.0-07a3ca9 Build: 2025-..."; truncate the "Build:" suffix for the
+  // display string, keeping the full value in the cell's title= attr.
+  function truncateBuildSuffix(s) {
+    if (!s) return '';
+    const idx = s.indexOf('Build:');
+    if (idx < 0) return s;
+    return s.slice(0, idx).replace(/[\s,;-]+$/, '');
+  }
+
   function uptimeStr(firstSeen) {
     if (!firstSeen) return '—';
     const ms = Date.now() - new Date(firstSeen).getTime();
@@ -374,7 +384,7 @@ window.preserveCompareSelection = function preserveCompareSelection(prevIds, tbo
         <caption class="sr-only">Observer status and statistics</caption>
         <thead><tr>
           <th scope="col" data-priority="1" data-sort-key="status" data-type="numeric">Status</th><th scope="col" data-priority="1" data-sort-key="name">Name</th><th scope="col" data-priority="3" data-sort-key="region">Region</th><th scope="col" data-priority="2" data-sort-key="last_seen" data-type="numeric">Last Status</th><th scope="col" data-priority="2" data-sort-key="last_packet_at" data-type="numeric">Last Packet</th>
-          <th scope="col" data-priority="3" data-sort-key="packet_health" data-type="numeric">Packet Health</th><th scope="col" data-priority="4" data-sort-key="packet_count" data-type="numeric">Total Packets</th><th scope="col" data-priority="3" data-sort-key="packets_hour" data-type="numeric">Packets/Hour</th><th scope="col" data-priority="4" data-sort-key="clock_offset" data-type="numeric">Clock Offset</th><th scope="col" data-priority="4" data-sort-key="uptime" data-type="numeric">Uptime</th>
+          <th scope="col" data-priority="3" data-sort-key="packet_health" data-type="numeric">Packet Health</th><th scope="col" data-priority="4" data-sort-key="packet_count" data-type="numeric">Total Packets</th><th scope="col" data-priority="3" data-sort-key="packets_hour" data-type="numeric">Packets/Hour</th><th scope="col" data-priority="4" data-sort-key="clock_offset" data-type="numeric">Clock Offset</th><th scope="col" data-priority="4" data-sort-key="uptime" data-type="numeric">Uptime</th><th scope="col" data-priority="4" data-sort-key="firmware">Firmware</th><th scope="col" data-priority="4" data-sort-key="client_version">Client</th>
           <th scope="col" data-priority="1" class="col-compare-select" style="width:32px"><span class="sr-only">Select for compare</span></th>
         </tr></thead>
         <tbody>${filtered.map(o => {
@@ -416,6 +426,8 @@ window.preserveCompareSelection = function preserveCompareSelection(prevIds, tbo
               return renderSkewBadge(sev, sk.offsetSec) + ' <span class="text-muted" title="Computed from ' + sk.samples + ' multi-observer packets. Positive = observer ahead of consensus.">(' + sk.samples + ')</span>';
             })()}</td>
             <td data-value="${_uptimeMs}">${uptimeStr(o.first_seen)}</td>
+            <td data-testid="obs-cell-firmware" data-value="${escapeHtml(String(o.firmware || ''))}" class="mono"${o.firmware ? ` title="${escapeHtml(String(o.firmware))}"` : ''}>${o.firmware ? escapeHtml(truncateBuildSuffix(String(o.firmware))) : '<span class="text-muted">—</span>'}</td>
+            <td data-testid="obs-cell-client-version" data-value="${escapeHtml(String(o.client_version || ''))}" class="mono"${o.client_version ? ` title="${escapeHtml(String(o.client_version))}"` : ''}>${o.client_version ? escapeHtml(String(o.client_version)) : '<span class="text-muted">—</span>'}</td>
             <td class="col-compare-select" onclick="event.stopPropagation()" style="text-align:center">
               <input type="checkbox" data-compare-select value="${escapeHtml(o.id)}"
                      aria-label="Select ${escapeHtml(o.name || o.id)} for comparison"
diff --git a/test-all.sh b/test-all.sh
index 9b7c1d55..57e56c78 100755
--- a/test-all.sh
+++ b/test-all.sh
@@ -28,6 +28,7 @@ node test-channel-issue-1087.js
 node test-issue-1409-no-encrypted-flood.js
 node test-analytics-channels-integration.js
 node test-observers-headings.js
+node test-issue-1789-observer-firmware-cols.js
 node test-issue-1648-m1-emoji-scan.js
 node test-issue-1648-m2-emoji-scan.js
 node test-issue-1648-m3-emoji-scan.js
diff --git a/test-issue-1789-observer-firmware-cols.js b/test-issue-1789-observer-firmware-cols.js
new file mode 100644
index 00000000..3190b2f8
--- /dev/null
+++ b/test-issue-1789-observer-firmware-cols.js
@@ -0,0 +1,112 @@
+/* test-issue-1789-observer-firmware-cols.js — Issue #1789 regression test.
+ *
+ * Asserts the observers table renders Firmware and Client (client_version)
+ * columns from data already on the wire from /api/observers. Both headers
+ * must carry data-priority="4" (matches Clock Offset/Uptime so
+ * TableResponsive hides them first on mobile) and data-sort-key, and both
+ * cells must carry a data-value (raw string for sorting) and class="mono".
+ *
+ * For the firmware cell, a long "... Build: ..." suffix on the displayed
+ * text must be truncated, with the full string preserved in a title=
+ * attribute.
+ *
+ * Static-source test (like test-observers-headings.js) — no server needed.
+ */
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const assert = require('assert');
+
+const src = fs.readFileSync(path.join(__dirname, 'public', 'observers.js'), 'utf8');
+
+let passed = 0, failed = 0;
+function test(name, fn) {
+  try { fn(); passed++; console.log(`  \u2713 ${name}`); }
+  catch (e) { failed++; console.log(`  \u2717 ${name}\n    ${e.message}`); }
+}
+
+function extractBlock(s, openRe, closeRe) {
+  const m = s.match(openRe);
+  if (!m) throw new Error('open marker not found');
+  const start = m.index + m[0].length;
+  const rest = s.slice(start);
+  const cm = rest.match(closeRe);
+  if (!cm) throw new Error('close marker not found');
+  return rest.slice(0, cm.index);
+}
+
+console.log('\u2500\u2500 Observers firmware/client columns (#1789) \u2500\u2500');
+
+test('thead contains Firmware header with data-priority=4 and data-sort-key=firmware', () => {
+  const thead = extractBlock(src, /<thead><tr>/, /<\/tr><\/thead>/);
+  assert.ok(
+    /<th[^>]*data-priority="4"[^>]*data-sort-key="firmware"[^>]*>\s*Firmware\s*<\/th>/.test(thead) ||
+    /<th[^>]*data-sort-key="firmware"[^>]*data-priority="4"[^>]*>\s*Firmware\s*<\/th>/.test(thead),
+    'Firmware <th> with data-priority="4" and data-sort-key="firmware" not found in thead'
+  );
+});
+
+test('thead contains Client header with data-priority=4 and data-sort-key=client_version', () => {
+  const thead = extractBlock(src, /<thead><tr>/, /<\/tr><\/thead>/);
+  assert.ok(
+    /<th[^>]*data-priority="4"[^>]*data-sort-key="client_version"[^>]*>\s*Client\s*<\/th>/.test(thead) ||
+    /<th[^>]*data-sort-key="client_version"[^>]*data-priority="4"[^>]*>\s*Client\s*<\/th>/.test(thead),
+    'Client <th> with data-priority="4" and data-sort-key="client_version" not found in thead'
+  );
+});
+
+test('tbody row template contains a firmware <td> with data-value and class="mono"', () => {
+  const tbodyStart = src.indexOf('<tbody>');
+  assert.ok(tbodyStart > 0, '<tbody> not found in observers.js');
+  const after = src.slice(tbodyStart);
+  const trOpen = after.search(/`<tr\b/);
+  const rowEnd = after.indexOf('</tr>', trOpen);
+  const row = after.slice(trOpen, rowEnd);
+  assert.ok(
+    /<td[^>]*class="mono"[^>]*data-value="\$\{[^}]*o\.firmware[^}]*\}"|<td[^>]*data-value="\$\{[^}]*o\.firmware[^}]*\}"[^>]*class="mono"/.test(row),
+    'firmware <td> with data-value reading o.firmware and class="mono" not found in row template'
+  );
+});
+
+test('tbody row template contains a client_version <td> with data-value and class="mono"', () => {
+  const tbodyStart = src.indexOf('<tbody>');
+  const after = src.slice(tbodyStart);
+  const trOpen = after.search(/`<tr\b/);
+  const rowEnd = after.indexOf('</tr>', trOpen);
+  const row = after.slice(trOpen, rowEnd);
+  assert.ok(
+    /<td[^>]*class="mono"[^>]*data-value="\$\{[^}]*o\.client_version[^}]*\}"|<td[^>]*data-value="\$\{[^}]*o\.client_version[^}]*\}"[^>]*class="mono"/.test(row),
+    'client_version <td> with data-value reading o.client_version and class="mono" not found in row template'
+  );
+});
+
+test('firmware display truncates "Build:" suffix and preserves full string in title=', () => {
+  // Look for any helper or inline expression that strips Build:... when
+  // rendering, AND a title= attribute carrying the unmodified firmware string.
+  assert.ok(
+    /Build:/.test(src),
+    'firmware truncation marker "Build:" not referenced anywhere in observers.js'
+  );
+  assert.ok(
+    /title="\$\{[^}]*escapeHtml\([^)]*o\.firmware[^)]*\)[^}]*\}"/.test(src) ||
+    /title="\$\{[^}]*o\.firmware[^}]*\}"/.test(src),
+    'firmware cell does not carry a title= with full o.firmware string'
+  );
+});
+
+test('thead+tbody column counts stay balanced after adding new columns', () => {
+  const thead = extractBlock(src, /<thead><tr>/, /<\/tr><\/thead>/);
+  const thCount = (thead.match(/<th\b/g) || []).length;
+  const tbodyStart = src.indexOf('<tbody>');
+  const after = src.slice(tbodyStart);
+  const trOpen = after.search(/`<tr\b/);
+  const rowEnd = after.indexOf('</tr>', trOpen);
+  const row = after.slice(trOpen, rowEnd);
+  const tdCount = (row.match(/<td\b/g) || []).length;
+  assert.strictEqual(tdCount, thCount,
+    `Observer table column mismatch: ${thCount} <th> vs ${tdCount} <td>`);
+});
+
+console.log(`\n${passed} passed, ${failed} failed`);
+process.exit(failed === 0 ? 0 : 1);
diff --git a/test-observers-headings.js b/test-observers-headings.js
index 305b6af7..f980c002 100644
--- a/test-observers-headings.js
+++ b/test-observers-headings.js
@@ -58,7 +58,8 @@ test('expected headings present and ordered', () => {
   let m;
   while ((m = re.exec(thead)) !== null) labels.push(m[1].trim());
   const expected = ['Status', 'Name', 'Region', 'Last Status', 'Last Packet',
-                    'Packet Health', 'Total Packets', 'Packets/Hour', 'Clock Offset', 'Uptime'];
+                    'Packet Health', 'Total Packets', 'Packets/Hour', 'Clock Offset', 'Uptime',
+                    'Firmware', 'Client'];
   assert.deepStrictEqual(labels, expected,
     `Headings out of sync.\nGot:      ${JSON.stringify(labels)}\nExpected: ${JSON.stringify(expected)}`);
 });