diff --git a/server/ws/bunTmpdir.js b/server/ws/bunTmpdir.js
new file mode 100644
index 0000000..ac08f2e
--- /dev/null
+++ b/server/ws/bunTmpdir.js
@@ -0,0 +1,99 @@
+// opencode runs on the Bun runtime. Bun unpacks its embedded `libopentui.so`
+// into TMPDIR (default /tmp) and dlopens it at startup, so when /tmp is
+// mounted noexec the mmap(PROT_EXEC) fails and the TUI dies immediately
+// (opencode issues #26136 / #27580). ccserver works around it the same way as
+// the upstream launcher PR #26134: when the host TMPDIR is on a noexec mount,
+// point BUN_TMPDIR at ~/.cache/opencode/tmp so Bun's unpack succeeds.
+//
+// Pure helpers (parseMountOptions / mountHasNoexec) are exported for unit
+// testing; the wrappers read the live environment.
+
+import { readFileSync, mkdirSync, existsSync, realpathSync } from 'node:fs';
+import { homedir } from 'node:os';
+import { join, isAbsolute } from 'node:path';
+
+// Unescape the octal escapes /proc/self/mounts uses for mountpoint
+// characters that can't appear literally (space \040, tab \011, newline
+// \012, backslash \134) so the mountpoint compares equal to the real path.
+function unescapeMountPoint(mp) {
+  return mp.replace(/\\([0-7]{3})/g, (_, oct) => String.fromCharCode(parseInt(oct, 8)));
+}
+
+// Extract (mountpoint, options) pairs from /proc/self/mounts-style lines.
+// Fields are whitespace-separated: device mountpoint fstype options dump pass.
+export function parseMountOptions(lines) {
+  const out = [];
+  for (const line of lines) {
+    const trimmed = line.trim();
+    if (!trimmed) continue;
+    const parts = trimmed.split(/\s+/);
+    if (parts.length < 4) continue;
+    out.push({ mountpoint: unescapeMountPoint(parts[1]), options: parts[3] });
+  }
+  return out;
+}
+
+// Longest-prefix mount match for `path` (the kernel picks the most specific
+// mount), then check whether that mount's options contain `noexec`.
+export function mountHasNoexec(path, lines) {
+  let best = null;
+  for (const { mountpoint, options } of parseMountOptions(lines)) {
+    const prefix = mountpoint.endsWith('/') ? mountpoint : `${mountpoint}/`;
+    if (path === mountpoint || path.startsWith(prefix)) {
+      if (!best || mountpoint.length > best.mountpoint.length) {
+        best = { mountpoint, options };
+      }
+    }
+  }
+  if (!best) return false;
+  return best.options.split(',').includes('noexec');
+}
+
+// true only on Linux when the resolved TMPDIR (default /tmp) sits on a
+// noexec mount. Relative or nonexistent TMPDIRs report false -- that is the
+// historical behavior (no override), and a path Bun itself can't use is not
+// one to mirror into BUN_TMPDIR.
+export function isTmpNoexec() {
+  if (process.platform !== 'linux') return false;
+  const tmpdir = process.env.TMPDIR || '/tmp';
+  if (!isAbsolute(tmpdir) || !existsSync(tmpdir)) return false;
+  let real, lines;
+  try {
+    real = realpathSync(tmpdir);
+    lines = readFileSync('/proc/self/mounts', 'utf-8').split('\n');
+  } catch {
+    return false;
+  }
+  return mountHasNoexec(real, lines);
+}
+
+// The replacement Bun temp dir, matching upstream launcher PR #26134.
+function bunTmpDir() {
+  return join(homedir(), '.cache', 'opencode', 'tmp');
+}
+
+// Returns the BUN_TMPDIR value when the host TMPDIR is noexec (creating the
+// directory), or null otherwise -- the caller then launches exactly as
+// before. mkdir failure quietly falls back to the historical /tmp launch.
+export function bunTmpdirOverride() {
+  if (!isTmpNoexec()) return null;
+  const dir = bunTmpDir();
+  try {
+    mkdirSync(dir, { recursive: true });
+  } catch {
+    return null;
+  }
+  return dir;
+}
+
+// The env fragment for pty env assembly: { BUN_TMPDIR: dir } when an override
+// is needed, {} otherwise -- so callers just spread it. An explicit BUN_TMPDIR
+// from the environment is left alone: Bun uses it over TMPDIR, so the noexec
+// TMPDIR problem does not apply there, and overriding it could turn a working
+// user configuration into a dead launch (e.g. when HOME is not writable and
+// the fallback mkdir fails).
+export function bunTmpdirEnv() {
+  if (process.env.BUN_TMPDIR) return {};
+  const dir = bunTmpdirOverride();
+  return dir ? { BUN_TMPDIR: dir } : {};
+}
diff --git a/server/ws/bunTmpdir.test.js b/server/ws/bunTmpdir.test.js
new file mode 100644
index 0000000..b9e18d2
--- /dev/null
+++ b/server/ws/bunTmpdir.test.js
@@ -0,0 +1,146 @@
+// Unit tests for the noexec-TMPDIR -> BUN_TMPDIR switch (see bunTmpdir.js).
+// The pure helpers are tested with synthetic /proc/self/mounts lines; the
+// env-reading wrappers only get cases that are deterministic everywhere (a
+// relative or nonexistent TMPDIR must report not-noexec without touching
+// /proc, and the override must match the real mount state of the test
+// TMPDIR).
+
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { mkdtempSync, rmSync, readFileSync, realpathSync } from 'node:fs';
+import { homedir, tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { parseMountOptions, mountHasNoexec, isTmpNoexec, bunTmpdirOverride, bunTmpdirEnv } from './bunTmpdir.js';
+
+test('parseMountOptions extracts mountpoint and options from real-style lines', () => {
+  const lines = [
+    'sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0',
+    'proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0',
+    'tmpfs /tmp tmpfs rw,nosuid,nodev,inode64 0 0',
+    '/dev/sda1 / rw,relatime 0 0',
+  ];
+  const mounts = parseMountOptions(lines);
+  assert.equal(mounts.length, 4);
+  assert.deepEqual(mounts[0], { mountpoint: '/sys', options: 'rw,nosuid,nodev,noexec,relatime' });
+  assert.deepEqual(mounts[2], { mountpoint: '/tmp', options: 'rw,nosuid,nodev,inode64' });
+});
+
+test('parseMountOptions unescapes octal escapes in mountpoints', () => {
+  const mounts = parseMountOptions(['/dev/sda1 /tmp/My\\040Folder ext4 rw,noexec 0 0']);
+  assert.equal(mounts[0].mountpoint, '/tmp/My Folder');
+  assert.equal(mounts[0].options, 'rw,noexec');
+});
+
+test('parseMountOptions skips blank and malformed lines', () => {
+  const mounts = parseMountOptions(['', '   ', 'garbage', 'overlay /mnt overlay rw 0 0']);
+  assert.equal(mounts.length, 1);
+  assert.deepEqual(mounts[0], { mountpoint: '/mnt', options: 'rw' });
+});
+
+test('mountHasNoexec detects noexec on the matched mount', () => {
+  const lines = [
+    '/dev/sda1 / rw,relatime 0 0',
+    'tmpfs /tmp tmpfs rw,nosuid,nodev,noexec,relatime,inode64 0 0',
+  ];
+  assert.equal(mountHasNoexec('/tmp', lines), true);
+  assert.equal(mountHasNoexec('/tmp/foo/bar', lines), true);
+  assert.equal(mountHasNoexec('/', lines), false);
+  assert.equal(mountHasNoexec('/usr', lines), false);
+});
+
+test('mountHasNoexec tolerates option order and surrounding options', () => {
+  for (const options of ['noexec,rw', 'rw,noexec,relatime', 'rw,relatime,noexec']) {
+    assert.equal(mountHasNoexec('/tmp', [`tmpfs /tmp tmpfs ${options} 0 0`]), true, `options: ${options}`);
+  }
+  assert.equal(mountHasNoexec('/tmp', ['tmpfs /tmp tmpfs rw,nosuid,nodev 0 0']), false);
+});
+
+test('mountHasNoexec uses the longest-prefix mount, not the first match', () => {
+  const lines = [
+    '/dev/sda1 / rw,relatime 0 0',
+    'tmpfs /tmp tmpfs rw,nosuid,nodev,noexec,relatime 0 0',
+    '/dev/sdb1 /tmp/foo ext4 rw,nosuid 0 0',
+  ];
+  // /tmp/foo is the longest match for paths under it and is not noexec.
+  assert.equal(mountHasNoexec('/tmp/foo', lines), false);
+  assert.equal(mountHasNoexec('/tmp/foo/x', lines), false);
+  // Everything else under /tmp falls back to the noexec /tmp mount.
+  assert.equal(mountHasNoexec('/tmp', lines), true);
+  assert.equal(mountHasNoexec('/tmp/bar', lines), true);
+  // A mountpoint with a trailing slash matches like any other.
+  assert.equal(mountHasNoexec('/tmp/foo', ['tmpfs /tmp/ tmpfs rw,noexec 0 0']), true);
+});
+
+test('mountHasNoexec returns false for unknown paths', () => {
+  assert.equal(mountHasNoexec('/nonexistent', ['/dev/sda1 / rw,relatime 0 0']), false);
+  assert.equal(mountHasNoexec('/tmpfoo', ['tmpfs /tmp tmpfs rw,noexec 0 0']), false);
+});
+
+test('isTmpNoexec is false for a relative TMPDIR', () => {
+  const before = process.env.TMPDIR;
+  process.env.TMPDIR = 'relative/tmp';
+  try {
+    assert.equal(isTmpNoexec(), false);
+  } finally {
+    if (before === undefined) delete process.env.TMPDIR;
+    else process.env.TMPDIR = before;
+  }
+});
+
+test('isTmpNoexec is false for a nonexistent TMPDIR', () => {
+  const before = process.env.TMPDIR;
+  process.env.TMPDIR = join(tmpdir(), 'ccserver-no-such-tmpdir');
+  try {
+    assert.equal(isTmpNoexec(), false);
+  } finally {
+    if (before === undefined) delete process.env.TMPDIR;
+    else process.env.TMPDIR = before;
+  }
+});
+
+test('bunTmpdirOverride matches the real mount state of the TMPDIR', () => {
+  const before = process.env.TMPDIR;
+  const dir = mkdtempSync(join(tmpdir(), 'ccserver-bun-tmpdir-test-'));
+  process.env.TMPDIR = dir;
+  try {
+    // The mkdtemp dir sits on whatever mount the host TMPDIR does, so the
+    // expected outcome is computed from the live mount table instead of
+    // assuming exec (which would fail exactly on the noexec hosts this
+    // feature targets). The dir is realpath-resolved like isTmpNoexec does:
+    // the mount table lists real paths only, so a symlinked host TMPDIR
+    // (e.g. /tmp -> /var/tmp) must not break the comparison.
+    let lines = [];
+    try {
+      lines = readFileSync('/proc/self/mounts', 'utf-8').split('\n');
+    } catch {
+      // Non-linux or unreadable: no mount info, so no override is expected.
+    }
+    const overrideDir = join(homedir(), '.cache', 'opencode', 'tmp');
+    const expected = mountHasNoexec(realpathSync(dir), lines) ? overrideDir : null;
+    assert.equal(bunTmpdirOverride(), expected);
+    assert.deepEqual(bunTmpdirEnv(), expected ? { BUN_TMPDIR: overrideDir } : {});
+  } finally {
+    if (before === undefined) delete process.env.TMPDIR;
+    else process.env.TMPDIR = before;
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+test('bunTmpdirEnv leaves an explicit BUN_TMPDIR alone', () => {
+  const beforeTmp = process.env.TMPDIR;
+  const beforeBun = process.env.BUN_TMPDIR;
+  const dir = mkdtempSync(join(tmpdir(), 'ccserver-bun-tmpdir-test-'));
+  process.env.TMPDIR = dir;
+  process.env.BUN_TMPDIR = join(dir, 'user-choice');
+  try {
+    // Bun prefers BUN_TMPDIR over TMPDIR, so a user-set value makes the
+    // noexec check moot -- whatever the mount state, it must be preserved.
+    assert.deepEqual(bunTmpdirEnv(), {});
+  } finally {
+    if (beforeTmp === undefined) delete process.env.TMPDIR;
+    else process.env.TMPDIR = beforeTmp;
+    if (beforeBun === undefined) delete process.env.BUN_TMPDIR;
+    else process.env.BUN_TMPDIR = beforeBun;
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
diff --git a/server/ws/sessionManager.js b/server/ws/sessionManager.js
index 82993aa..b315b9c 100644
--- a/server/ws/sessionManager.js
+++ b/server/ws/sessionManager.js
@@ -7,6 +7,7 @@ import { fileURLToPath } from 'node:url';
 import { buildSandboxSpawn, resolveApp, sandboxAvailable, loadSandboxConfig } from './sandbox.js';
 import { buildMcpConfigArgsAndEnv } from './mcpConfig.js';
 import { shouldInjectNotify, notifyEnabled, getNotifySockPath, notifyBrokerRunning } from './notify.js';
+import { bunTmpdirEnv } from './bunTmpdir.js';
 import {
   isValidApp,
   appResumeArgs,
@@ -263,6 +264,15 @@ export function createSession({ cwd, cols, rows, claudeSessionId, shell, sandbox
       // scrolls natively, and its own drag-selection + copy-on-select writes
       // to the browser clipboard via OSC 52 (handled client-side).
       ...mcpEnv,
+      // /tmp being mounted noexec makes Bun fail to unpack + dlopen its
+      // embedded libopentui.so, so opencode's TUI dies at startup (opencode
+      // #26136/#27580). Direct host launches switch BUN_TMPDIR to
+      // ~/.cache/opencode/tmp when the host TMPDIR is noexec. Sandboxed
+      // launches don't: the sandbox's /tmp is a fresh tmpfs that is always
+      // executable, and the host-side cache dir is not bound into bwrap (with
+      // a fresh HOME it would not even exist), so setting it there would
+      // break what it is meant to fix.
+      ...(shell || sessionApp !== 'opencode' || useSandbox ? {} : bunTmpdirEnv()),
     },
   });
   } catch (err) {