diff --git a/src/tmux/command.js b/src/tmux/command.js
new file mode 100644
index 00000000..f4d50c54
--- /dev/null
+++ b/src/tmux/command.js
@@ -0,0 +1,27 @@
+const runtime = require('../core/runtime');
+
+function assertArgs(args) {
+  if (!Array.isArray(args)) {
+    throw new TypeError('tmux args must be an array');
+  }
+  for (const arg of args) {
+    if (typeof arg !== 'string') {
+      throw new TypeError('tmux args must contain only strings');
+    }
+  }
+}
+
+function runTmux(args, options = {}) {
+  assertArgs(args);
+  return runtime.run('tmux', args, options);
+}
+
+function isTmuxAvailable() {
+  const result = runTmux(['-V'], { stdio: 'pipe' });
+  return result.status === 0 && !result.error;
+}
+
+module.exports = {
+  isTmuxAvailable,
+  runTmux,
+};
diff --git a/src/tmux/session.js b/src/tmux/session.js
new file mode 100644
index 00000000..60c3d3de
--- /dev/null
+++ b/src/tmux/session.js
@@ -0,0 +1,83 @@
+const tmux = require('./command');
+
+function requireName(name) {
+  if (typeof name !== 'string' || name.trim() === '') {
+    throw new TypeError('tmux session name must be a non-empty string');
+  }
+  return name;
+}
+
+function addCwd(args, cwd) {
+  if (cwd !== undefined) {
+    if (typeof cwd !== 'string' || cwd.trim() === '') {
+      throw new TypeError('tmux cwd must be a non-empty string');
+    }
+    args.push('-c', cwd);
+  }
+}
+
+function sessionExists(name) {
+  const result = tmux.runTmux(['has-session', '-t', requireName(name)], {
+    stdio: 'pipe',
+  });
+  return result.status === 0;
+}
+
+function createSession(name, cwd) {
+  const args = ['new-session', '-d', '-s', requireName(name)];
+  addCwd(args, cwd);
+  return tmux.runTmux(args);
+}
+
+function attachSession(name) {
+  return tmux.runTmux(['attach-session', '-t', requireName(name)], {
+    stdio: 'inherit',
+  });
+}
+
+function newWindowOrPane(options = {}) {
+  const {
+    target,
+    cwd,
+    name,
+    pane = false,
+    split = 'vertical',
+  } = options;
+  const args = pane ? ['split-window'] : ['new-window'];
+
+  if (pane) {
+    if (split === 'horizontal') {
+      args.push('-h');
+    } else if (split === 'vertical') {
+      args.push('-v');
+    } else {
+      throw new TypeError('tmux split must be horizontal or vertical');
+    }
+  }
+  if (target !== undefined) {
+    args.push('-t', requireName(target));
+  }
+  if (!pane && name !== undefined) {
+    args.push('-n', requireName(name));
+  }
+  addCwd(args, cwd);
+  return tmux.runTmux(args);
+}
+
+function sendKeys(paneId, command) {
+  if (typeof paneId !== 'string' || paneId.trim() === '') {
+    throw new TypeError('tmux pane id must be a non-empty string');
+  }
+  if (typeof command !== 'string') {
+    throw new TypeError('tmux command must be a string');
+  }
+  return tmux.runTmux(['send-keys', '-t', paneId, command, 'C-m']);
+}
+
+module.exports = {
+  sessionExists,
+  createSession,
+  attachSession,
+  newWindowOrPane,
+  sendKeys,
+};
diff --git a/test/tmux-command.test.js b/test/tmux-command.test.js
new file mode 100644
index 00000000..a3890c0b
--- /dev/null
+++ b/test/tmux-command.test.js
@@ -0,0 +1,68 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+
+const runtime = require('../src/core/runtime');
+const tmuxCommand = require('../src/tmux/command');
+
+test('runTmux delegates to runtime command helper with argv array', () => {
+  const calls = [];
+  const originalRun = runtime.run;
+  runtime.run = (cmd, args, options) => {
+    calls.push({ cmd, args, options });
+    return { status: 0, stdout: 'ok\n', stderr: '' };
+  };
+
+  try {
+    const result = tmuxCommand.runTmux(['display-message', '-p', '#S'], {
+      cwd: '/tmp/project',
+    });
+
+    assert.equal(result.status, 0);
+    assert.deepEqual(calls, [
+      {
+        cmd: 'tmux',
+        args: ['display-message', '-p', '#S'],
+        options: { cwd: '/tmp/project' },
+      },
+    ]);
+  } finally {
+    runtime.run = originalRun;
+  }
+});
+
+test('runTmux rejects non-array args', () => {
+  assert.throws(() => tmuxCommand.runTmux('tmux -V'), /args must be an array/);
+});
+
+test('runTmux rejects non-string args', () => {
+  assert.throws(() => tmuxCommand.runTmux(['new-session', 7]), /only strings/);
+});
+
+test('isTmuxAvailable probes tmux version', () => {
+  const calls = [];
+  const originalRun = runtime.run;
+  runtime.run = (cmd, args, options) => {
+    calls.push({ cmd, args, options });
+    return { status: 0, stdout: 'tmux 3.4\n', stderr: '' };
+  };
+
+  try {
+    assert.equal(tmuxCommand.isTmuxAvailable(), true);
+    assert.deepEqual(calls, [
+      { cmd: 'tmux', args: ['-V'], options: { stdio: 'pipe' } },
+    ]);
+  } finally {
+    runtime.run = originalRun;
+  }
+});
+
+test('isTmuxAvailable returns false when probe fails', () => {
+  const originalRun = runtime.run;
+  runtime.run = () => ({ status: 127, error: new Error('missing tmux') });
+
+  try {
+    assert.equal(tmuxCommand.isTmuxAvailable(), false);
+  } finally {
+    runtime.run = originalRun;
+  }
+});
diff --git a/test/tmux-session.test.js b/test/tmux-session.test.js
new file mode 100644
index 00000000..74576084
--- /dev/null
+++ b/test/tmux-session.test.js
@@ -0,0 +1,111 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+
+const tmuxCommand = require('../src/tmux/command');
+const tmuxSession = require('../src/tmux/session');
+
+function withMockedTmux(callback) {
+  const calls = [];
+  const originalRunTmux = tmuxCommand.runTmux;
+  tmuxCommand.runTmux = (args, options) => {
+    calls.push({ args, options });
+    return { status: 0, stdout: '', stderr: '' };
+  };
+
+  try {
+    callback(calls);
+  } finally {
+    tmuxCommand.runTmux = originalRunTmux;
+  }
+}
+
+test('sessionExists checks target session without shell command strings', () => {
+  withMockedTmux((calls) => {
+    assert.equal(tmuxSession.sessionExists('gx-cockpit'), true);
+    assert.deepEqual(calls, [
+      {
+        args: ['has-session', '-t', 'gx-cockpit'],
+        options: { stdio: 'pipe' },
+      },
+    ]);
+  });
+});
+
+test('createSession builds detached session argv with cwd', () => {
+  withMockedTmux((calls) => {
+    tmuxSession.createSession('gx-cockpit', '/repo');
+    assert.deepEqual(calls, [
+      {
+        args: ['new-session', '-d', '-s', 'gx-cockpit', '-c', '/repo'],
+        options: undefined,
+      },
+    ]);
+  });
+});
+
+test('attachSession attaches with inherited stdio', () => {
+  withMockedTmux((calls) => {
+    tmuxSession.attachSession('gx-cockpit');
+    assert.deepEqual(calls, [
+      {
+        args: ['attach-session', '-t', 'gx-cockpit'],
+        options: { stdio: 'inherit' },
+      },
+    ]);
+  });
+});
+
+test('newWindowOrPane creates named window argv', () => {
+  withMockedTmux((calls) => {
+    tmuxSession.newWindowOrPane({
+      target: 'gx-cockpit',
+      name: 'status',
+      cwd: '/repo',
+    });
+    assert.deepEqual(calls, [
+      {
+        args: ['new-window', '-t', 'gx-cockpit', '-n', 'status', '-c', '/repo'],
+        options: undefined,
+      },
+    ]);
+  });
+});
+
+test('newWindowOrPane creates split pane argv', () => {
+  withMockedTmux((calls) => {
+    tmuxSession.newWindowOrPane({
+      pane: true,
+      split: 'horizontal',
+      target: 'gx-cockpit:0',
+      cwd: '/repo',
+    });
+    assert.deepEqual(calls, [
+      {
+        args: ['split-window', '-h', '-t', 'gx-cockpit:0', '-c', '/repo'],
+        options: undefined,
+      },
+    ]);
+  });
+});
+
+test('sendKeys targets pane and sends command as one argv value', () => {
+  withMockedTmux((calls) => {
+    tmuxSession.sendKeys('%1', 'gx status; echo still-literal');
+    assert.deepEqual(calls, [
+      {
+        args: ['send-keys', '-t', '%1', 'gx status; echo still-literal', 'C-m'],
+        options: undefined,
+      },
+    ]);
+  });
+});
+
+test('session helpers reject empty target values', () => {
+  assert.throws(() => tmuxSession.sessionExists(''), /non-empty string/);
+  assert.throws(() => tmuxSession.createSession('gx', ''), /cwd/);
+  assert.throws(
+    () => tmuxSession.newWindowOrPane({ pane: true, split: 'diagonal' }),
+    /horizontal or vertical/,
+  );
+  assert.throws(() => tmuxSession.sendKeys('', 'gx status'), /pane id/);
+});