diff --git a/.ai/contexts/ipc-bridge.md b/.ai/contexts/ipc-bridge.md
index 4a987bfe..372de541 100644
--- a/.ai/contexts/ipc-bridge.md
+++ b/.ai/contexts/ipc-bridge.md
@@ -226,6 +226,16 @@ write is bounded by the try/catch on the write"). Callers get a boolean instead
 of an exception; the first-resize nudge uses it to skip its follow-up when the
 PTY is already gone.
 
+A kill is once-only. With `useConptyDll`, node-pty's `kill` ends in
+`ConptyClosePseudoConsole(hpc)`, and the native handle is only dropped when the
+shell's exit thread runs; a second kill before that finds the handle still
+registered and closes an already-freed pseudo console, a heap corruption
+(0xc0000374) that kills the whole main process with no JS error. So `killPty`
+closes a given pty once (a `WeakSet` of killed ptys), and `resizePty` /
+`writePty` return false on a killed pty. `test/pty-ops-conpty-kill.test.js`
+proves it against the real node-pty in a child process (Windows only): with the
+guard removed the child dies with 3221226356.
+
 Swallowed errors are not silent: `setPtyOpLogger(log)` in `main.js` routes them
 to `log.debug` as `[pty] <op> skipped session=<id> reason=<message>`. Debug level
 is deliberate — the file transport is at `info` in packaged builds, so a resize
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 03b1dd3a..d2c90046 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ What changes for you in each release of Switchboard. How to write an entry: [doc
 ## Unreleased
 
 ### Fixed
+- Stopping a terminal twice in quick succession, or resizing it while it is being stopped, no longer closes the Windows pseudo console twice, which could kill the whole app with no error. (#405)
 - A sandboxed session, or a sandboxed schedule, whose Additional Directories include a `.claude` or `.git` directory, or a path inside one, is now refused instead of binding it read-write over its read-only protection; add the project directory instead. A session started in a `.claude` or `.git` directory is refused too, except below `.claude/worktrees`, and Additional Directories naming your home directory or a parent of it are refused however the path is written. A relative `add-dirs` entry in a schedule is taken from the schedule's directory. (#385)
 - A session that has exited no longer keeps a busy dot in the sidebar, and the status bar's running count drops as soon as the session ends instead of waiting for the next refresh. (#375)
 - Quitting, closing the window or reloading while a file in the file panel has unsaved edits now asks first, in any session, kept-aside tabs included: Save writes them (a file that changed on disk is not overwritten), Discard drops them, Cancel stays. If Switchboard does not answer within a few seconds, it closes anyway. (#373)
diff --git a/pty-ops.js b/pty-ops.js
index d92fcb1a..530bee30 100644
--- a/pty-ops.js
+++ b/pty-ops.js
@@ -5,6 +5,9 @@ const os = require('os');
 
 let logger = null;
 
+// see .ai/contexts/ipc-bridge.md ("PTY operations race the exit")
+const killedPtys = new WeakSet();
+
 /** Install the sink used to report swallowed PTY errors. `null` disables it. */
 function setPtyOpLogger(next) {
   logger = next && typeof next.debug === 'function' ? next : null;
@@ -34,15 +37,23 @@ function withPty(session, label, fn, sessionId) {
   }
 }
 
+function isKilled(session) {
+  return !!(session && session.pty && killedPtys.has(session.pty));
+}
+
 function resizePty(session, cols, rows, sessionId) {
+  if (isKilled(session)) return false;
   return withPty(session, 'resize', (pty) => pty.resize(cols, rows), sessionId);
 }
 
 function killPty(session, sessionId) {
+  if (!session || !session.pty || killedPtys.has(session.pty)) return false;
+  killedPtys.add(session.pty);
   return withPty(session, 'kill', (pty) => pty.kill(), sessionId);
 }
 
 function writePty(session, data, sessionId) {
+  if (isKilled(session)) return false;
   return withPty(session, 'write', (pty) => pty.write(data), sessionId);
 }
 
diff --git a/test/pty-ops-conpty-kill.test.js b/test/pty-ops-conpty-kill.test.js
new file mode 100644
index 00000000..d91ce0eb
--- /dev/null
+++ b/test/pty-ops-conpty-kill.test.js
@@ -0,0 +1,63 @@
+// test/pty-ops-conpty-kill.test.js — the real node-pty + bundled conpty.dll, in a child process.
+// A second ClosePseudoConsole on the same handle corrupts the heap (0xc0000374) and takes the
+// process down; the child is the only place that can be observed. see .ai/contexts/ipc-bridge.md
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const path = require('node:path');
+const fs = require('node:fs');
+const os = require('node:os');
+const { spawnSync } = require('node:child_process');
+
+const HEAP_CORRUPTION = 3221226356; // 0xc0000374
+
+const CHILD = `
+const pty = require(process.env.NODE_PTY_DIR);
+const { killPty } = require(process.env.PTY_OPS);
+const term = pty.spawn(process.env.ComSpec || 'cmd.exe', [], { cols: 80, rows: 24, useConptyDll: true });
+term.onData(() => {});
+setTimeout(() => {
+  const session = { pty: term };
+  killPty(session, 's1');
+  killPty(session, 's1');
+  setTimeout(() => process.exit(0), 1500);
+}, 1000);
+`;
+
+// The packaged app finds conpty.dll beside the conpty.node it loads (scripts/after-pack.js
+// copies prebuilds/win32-<arch>/conpty/ there). A source-built node_modules has no such
+// folder in build/Release, so the child loads a temp copy that has only the prebuilds.
+function stageNodePty() {
+  const src = path.dirname(require.resolve('node-pty/package.json'));
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'sb-nodepty-'));
+  const dst = path.join(dir, 'node-pty');
+  const prebuild = path.join('prebuilds', `${process.platform}-${process.arch}`);
+  fs.cpSync(path.join(src, 'lib'), path.join(dst, 'lib'), { recursive: true });
+  fs.cpSync(path.join(src, prebuild), path.join(dst, prebuild), { recursive: true });
+  fs.copyFileSync(path.join(src, 'package.json'), path.join(dst, 'package.json'));
+  return { dir, nodePty: dst };
+}
+
+function runChild(extraEnv) {
+  const staged = stageNodePty();
+  try {
+    return spawnSync(process.execPath, ['-e', CHILD], {
+      cwd: path.join(__dirname, '..'),
+      env: { ...process.env, PTY_OPS: path.join(__dirname, '..', 'pty-ops.js'), NODE_PTY_DIR: staged.nodePty, ...extraEnv },
+      timeout: 30000,
+      encoding: 'utf8',
+    });
+  } finally {
+    // OpenConsole.exe may still hold the copy open for a moment; the temp dir is disposable.
+    try { fs.rmSync(staged.dir, { recursive: true, force: true, maxRetries: 10, retryDelay: 300 }); } catch {}
+  }
+}
+
+test('killing a conpty-dll pty twice back to back does not corrupt the heap',
+  { skip: process.platform !== 'win32' ? 'ConptyClosePseudoConsole exists only on Windows' : false },
+  () => {
+    const r = runChild({});
+    assert.notEqual(r.status, HEAP_CORRUPTION, 'child died of STATUS_HEAP_CORRUPTION');
+    assert.equal(r.status, 0, `child stderr: ${r.stderr}`);
+  });
diff --git a/test/pty-ops.test.js b/test/pty-ops.test.js
index 81b88957..fd9fc5bc 100644
--- a/test/pty-ops.test.js
+++ b/test/pty-ops.test.js
@@ -43,9 +43,9 @@ test('resizePty forwards cols/rows to a live pty and reports success', () => {
 
 test('killPty and writePty forward to a live pty and report success', () => {
   const pty = makeLivePty();
-  assert.equal(killPty({ pty }, 's1'), true);
   assert.equal(writePty({ pty }, 'hi', 's1'), true);
-  assert.deepEqual(pty.calls, [['kill'], ['write', 'hi']]);
+  assert.equal(killPty({ pty }, 's1'), true);
+  assert.deepEqual(pty.calls, [['write', 'hi'], ['kill']]);
 });
 
 // ── The race the crash came from ─────────────────────────────────────────────
@@ -140,3 +140,22 @@ test('a keystroke to a pty that exits mid-write does not escape handleTerminalIn
   // The composer still saw the keystroke — the guard must not skip the bookkeeping.
   assert.equal(session.composerState.pending, 5);
 });
+
+// ── A killed pty is closed once (double ClosePseudoConsole corrupts the heap) ──
+
+test('killPty kills a live pty once and ignores later kills', () => {
+  const pty = makeLivePty();
+  const session = { pty };
+  assert.equal(killPty(session, 's1'), true);
+  assert.equal(killPty(session, 's1'), false);
+  assert.deepEqual(pty.calls, [['kill']]);
+});
+
+test('resizePty and writePty do nothing once the pty was killed', () => {
+  const pty = makeLivePty();
+  const session = { pty };
+  killPty(session, 's1');
+  assert.equal(resizePty(session, 80, 24, 's1'), false);
+  assert.equal(writePty(session, 'x', 's1'), false);
+  assert.deepEqual(pty.calls, [['kill']]);
+});