diff --git a/src/cockpit/sidebar.js b/src/cockpit/sidebar.js
new file mode 100644
index 00000000..b65ea70c
--- /dev/null
+++ b/src/cockpit/sidebar.js
@@ -0,0 +1,206 @@
+const path = require('node:path');
+
+const DEFAULT_WIDTH = 36;
+const MIN_WIDTH = 12;
+
+const STATUS_DOTS = new Map([
+  ['active', '*'],
+  ['running', '*'],
+  ['working', '*'],
+  ['thinking', 'o'],
+  ['idle', 'o'],
+  ['ready', 'o'],
+  ['done', '+'],
+  ['complete', '+'],
+  ['completed', '+'],
+  ['merged', '+'],
+  ['blocked', '!'],
+  ['error', '!'],
+  ['failed', '!'],
+  ['stalled', '!'],
+  ['dead', '!'],
+]);
+
+const ANSI = {
+  reset: '\x1b[0m',
+  dim: '\x1b[2m',
+  green: '\x1b[32m',
+  red: '\x1b[31m',
+  yellow: '\x1b[33m',
+  cyan: '\x1b[36m',
+  inverse: '\x1b[7m',
+};
+
+function text(value, fallback = '') {
+  if (typeof value === 'string') {
+    return value.trim() || fallback;
+  }
+  if (value === null || value === undefined) {
+    return fallback;
+  }
+  return String(value).trim() || fallback;
+}
+
+function sidebarWidth(options = {}) {
+  const width = Number(options.width);
+  if (!Number.isFinite(width)) {
+    return DEFAULT_WIDTH;
+  }
+  return Math.max(MIN_WIDTH, Math.floor(width));
+}
+
+function truncate(value, width) {
+  const raw = value === null || value === undefined ? '' : String(value);
+  if (width <= 0) {
+    return '';
+  }
+  if (raw.length <= width) {
+    return raw;
+  }
+  if (width <= 3) {
+    return raw.slice(0, width);
+  }
+  return `${raw.slice(0, width - 3)}...`;
+}
+
+function boundLine(value, width) {
+  return truncate(value, width);
+}
+
+function repoName(state = {}, options = {}) {
+  const explicit = text(options.repoName || state.repoName || state.projectName || state.repo);
+  if (explicit) {
+    return explicit;
+  }
+
+  const repoPath = text(state.repoPath);
+  if (!repoPath) {
+    return '-';
+  }
+  return path.basename(repoPath) || repoPath;
+}
+
+function agentLabel(agentName) {
+  const compact = text(agentName, 'agent').replace(/[^a-z0-9]/gi, '').toUpperCase();
+  return truncate(compact || 'AGENT', 3).padEnd(3, ' ');
+}
+
+function statusDot(session = {}) {
+  if (session.worktreeExists === false) {
+    return 'x';
+  }
+  const status = text(session.status, 'unknown').toLowerCase();
+  return STATUS_DOTS.get(status) || '.';
+}
+
+function lockCount(session = {}) {
+  if (Array.isArray(session.locks)) {
+    return session.locks.length;
+  }
+  const count = Number(session.lockCount);
+  return Number.isFinite(count) && count >= 0 ? count : 0;
+}
+
+function sessionId(session = {}) {
+  return text(session.id || session.sessionId || session.branch);
+}
+
+function isSelected(session, index, state = {}, options = {}) {
+  const selectedId = text(options.selectedId || options.selectedSessionId || state.selectedId || state.selectedSessionId);
+  if (selectedId && sessionId(session) === selectedId) {
+    return true;
+  }
+
+  const selectedBranch = text(options.selectedBranch || state.selectedBranch);
+  if (selectedBranch && text(session.branch) === selectedBranch) {
+    return true;
+  }
+
+  const selectedIndex = Number.isInteger(options.selectedIndex) ? options.selectedIndex : state.selectedIndex;
+  return Number.isInteger(selectedIndex) && selectedIndex === index;
+}
+
+function colorize(value, color, options = {}) {
+  if (options.noColor || options.color !== true) {
+    return value;
+  }
+  const code = ANSI[color];
+  return code ? `${code}${value}${ANSI.reset}` : value;
+}
+
+function statusColor(dot) {
+  if (dot === '*') {
+    return 'green';
+  }
+  if (dot === '!') {
+    return 'yellow';
+  }
+  if (dot === 'x') {
+    return 'red';
+  }
+  if (dot === '+') {
+    return 'cyan';
+  }
+  return 'dim';
+}
+
+function renderSessionRow(session, index, state, options) {
+  const width = sidebarWidth(options);
+  const selected = isSelected(session, index, state, options);
+  const marker = selected ? '>' : ' ';
+  const dot = statusDot(session);
+  const label = agentLabel(session.agentName);
+  const branch = text(session.branch, '(no branch)');
+  const task = text(session.task, '(no task)');
+  const missing = session.worktreeExists === false ? ' missing worktree' : '';
+
+  const firstPrefix = `${marker} ${dot} ${label} `;
+  const first = `${firstPrefix}${truncate(branch, width - firstPrefix.length)}`;
+  const taskPrefix = '    ';
+  const taskLine = `${taskPrefix}${truncate(task, width - taskPrefix.length)}`;
+  const meta = `    locks: ${lockCount(session)}${missing}`;
+
+  return [
+    selected ? colorize(boundLine(first, width), 'inverse', options) : boundLine(first, width),
+    boundLine(taskLine, width),
+    colorize(boundLine(meta, width), statusColor(dot), options),
+  ];
+}
+
+function renderSidebar(state = {}, options = {}) {
+  const width = sidebarWidth(options);
+  const title = text(options.title || state.title, 'gx cockpit').toLowerCase() === 'gitguardex'
+    ? 'gitguardex'
+    : text(options.title || state.title, 'gx cockpit');
+  const sessions = Array.isArray(state.sessions) ? state.sessions : [];
+  const lines = [
+    boundLine(title, width),
+    boundLine(`repo ${repoName(state, options)}`, width),
+    boundLine('-'.repeat(width), width),
+    boundLine('lanes', width),
+  ];
+
+  if (sessions.length === 0) {
+    lines.push(boundLine('  no active lanes', width));
+  } else {
+    sessions.forEach((session, index) => {
+      lines.push(...renderSessionRow(session, index, state, options));
+    });
+  }
+
+  lines.push(
+    boundLine('-'.repeat(width), width),
+    boundLine('[n] new agent', width),
+    boundLine('[t] terminal', width),
+    boundLine('[s] settings', width),
+  );
+
+  return `${lines.join('\n')}\n`;
+}
+
+module.exports = {
+  renderSidebar,
+  agentLabel,
+  statusDot,
+  truncate,
+};
diff --git a/test/cockpit-sidebar.test.js b/test/cockpit-sidebar.test.js
new file mode 100644
index 00000000..d0ef92dd
--- /dev/null
+++ b/test/cockpit-sidebar.test.js
@@ -0,0 +1,113 @@
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+
+const { renderSidebar } = require('../src/cockpit/sidebar');
+
+function lines(output) {
+  return output.trimEnd().split('\n');
+}
+
+test('renderSidebar renders an empty sidebar', () => {
+  const output = renderSidebar({
+    repoPath: '/work/gitguardex',
+    sessions: [],
+  }, { noColor: true });
+
+  assert.match(output, /gx cockpit/);
+  assert.match(output, /repo gitguardex/);
+  assert.match(output, /lanes/);
+  assert.match(output, /no active lanes/);
+  assert.match(output, /\[n\] new agent/);
+  assert.match(output, /\[t\] terminal/);
+  assert.match(output, /\[s\] settings/);
+});
+
+test('renderSidebar marks the selected session', () => {
+  const output = renderSidebar({
+    repoName: 'gitguardex',
+    selectedSessionId: 's2',
+    sessions: [
+      {
+        id: 's1',
+        agentName: 'codex',
+        branch: 'agent/codex/first',
+        task: 'first lane',
+        status: 'idle',
+        lockCount: 0,
+        worktreeExists: true,
+      },
+      {
+        id: 's2',
+        agentName: 'claude',
+        branch: 'agent/claude/second',
+        task: 'selected lane',
+        status: 'working',
+        lockCount: 2,
+        worktreeExists: true,
+      },
+    ],
+  }, { noColor: true });
+
+  assert.match(output, /^  o COD agent\/codex\/first$/m);
+  assert.match(output, /^> \* CLA agent\/claude\/second$/m);
+});
+
+test('renderSidebar marks a missing worktree', () => {
+  const output = renderSidebar({
+    repoName: 'gitguardex',
+    sessions: [
+      {
+        id: 'missing',
+        agentName: 'codex',
+        branch: 'agent/codex/missing',
+        task: 'repair missing lane',
+        status: 'stalled',
+        lockCount: 1,
+        worktreeExists: false,
+      },
+    ],
+  }, { noColor: true });
+
+  assert.match(output, /^  x COD agent\/codex\/missing$/m);
+  assert.match(output, /locks: 1 missing worktree/);
+});
+
+test('renderSidebar truncates long branch and task text', () => {
+  const output = renderSidebar({
+    repoName: 'gitguardex',
+    sessions: [
+      {
+        id: 'long',
+        agentName: 'codex',
+        branch: 'agent/codex/this-branch-name-is-too-long-for-the-sidebar',
+        task: 'this task description is also too long for the bounded dmux-style sidebar',
+        status: 'working',
+        lockCount: 0,
+        worktreeExists: true,
+      },
+    ],
+  }, { width: 30, noColor: true });
+
+  assert.ok(lines(output).every((line) => line.length <= 30));
+  assert.match(output, /agent\/codex\/this-br\.\.\./);
+  assert.match(output, /this task description i\.\.\./);
+});
+
+test('renderSidebar displays lock counts', () => {
+  const output = renderSidebar({
+    repoName: 'gitguardex',
+    sessions: [
+      {
+        id: 'locks',
+        agentName: 'codex',
+        branch: 'agent/codex/locks',
+        task: 'lock count lane',
+        status: 'working',
+        lockCount: 7,
+        worktreeExists: true,
+      },
+    ],
+  }, { noColor: true });
+
+  assert.match(output, /locks: 7/);
+});