diff --git a/autohands/build_util.py b/autohands/build_util.py
index 4c718d0b..e652758c 100644
--- a/autohands/build_util.py
+++ b/autohands/build_util.py
@@ -225,8 +225,19 @@ def execute_notebook(f, report=None, env=None):
         # stderr is always captured so a clean `sys.exit(0)` skip guard can be
         # told apart from a genuine cell failure (is_clean_skip_exit); stdout
         # keeps streaming live unless the report collector wants it.
+        # Run via run_notebook.py rather than `jupyter nbconvert --execute`:
+        # nbconvert starts the kernel in the notebook's own directory, but the
+        # workspaces document (and their auto-simulate guards require) execution
+        # from the repo root. nbconvert has no CLI flag for the kernel cwd, so
+        # the runner sets resources['metadata']['path'] via the Python API.
+        # Still a subprocess, so isolation/timeout/env are unchanged.
         subprocess.run(
-            ["jupyter", "nbconvert", "--to", "notebook", "--execute", "--output", f, f],
+            [
+                sys.executable,
+                str(Path(__file__).parent / "run_notebook.py"),
+                str(f),
+                str(Path.cwd()),
+            ],
             check=True,
             timeout=TIMEOUT_SECS,
             stdout=subprocess.PIPE if report is not None else None,
diff --git a/autohands/run_notebook.py b/autohands/run_notebook.py
new file mode 100644
index 00000000..96d1e09c
--- /dev/null
+++ b/autohands/run_notebook.py
@@ -0,0 +1,71 @@
+"""
+Execute one notebook with the kernel's working directory pinned to the
+workspace root.
+
+Why this exists
+---------------
+``jupyter nbconvert --execute`` starts the kernel in the **notebook's own
+directory**, not in the directory nbconvert was launched from. The workspaces
+document the opposite convention (autolens_workspace/AGENTS.md: "Scripts are
+run from the repository root so relative paths to ``dataset/`` and ``output/``
+resolve correctly"), and the auto-simulate guards rely on it::
+
+    if al.util.dataset.should_simulate(str(dataset_path)):
+        subprocess.run([sys.executable, "scripts/imaging/simulator.py"], check=True)
+
+That root-relative path resolves from a script run at the root and cannot
+resolve from a notebook run in ``notebooks/<topic>/`` — the subprocess exits 2
+("can't open file") and every auto-simulating notebook fails.
+
+nbconvert exposes **no CLI flag** for the kernel's working directory. The knob
+is ``resources['metadata']['path']``, which nbclient turns into the kernel's
+``cwd`` (see ``nbclient/client.py``, ``_async_start_new_kernel``), and that is
+reachable only from the Python API. Hence this module: ``build_util`` still
+runs it as a **subprocess**, so process isolation, the ``BUILD_SCRIPT_TIMEOUT``
+and the per-notebook environment are all unchanged — only the kernel's cwd
+moves.
+
+Failure contract
+----------------
+On a cell error the ``CellExecutionError`` is deliberately left **uncaught** so
+Python prints its own traceback to stderr. That output contains the
+``CellExecutionError`` marker and ends with the failing cell's terminal
+``<ename>: <evalue>`` line, which is exactly what
+``build_util.is_clean_skip_exit`` parses to tell an intentional
+``sys.exit(0)`` skip from a genuine failure. Do not wrap it in a handler that
+reformats the message, or that skip detection breaks.
+
+Usage::
+
+    python run_notebook.py <notebook-path> <workspace-root>
+"""
+
+import sys
+from pathlib import Path
+
+import nbformat
+from nbconvert.preprocessors import ExecutePreprocessor
+
+
+def run(notebook_path: str, root: str) -> None:
+    nb_path = Path(notebook_path)
+    nb = nbformat.read(nb_path, as_version=4)
+
+    ep = ExecutePreprocessor(timeout=None, kernel_name="python3")
+
+    try:
+        # The whole point: pin the kernel's cwd to the workspace root rather
+        # than letting it default to the notebook's own directory.
+        ep.preprocess(nb, {"metadata": {"path": str(root)}})
+    finally:
+        # Write back even on failure so a partially-executed notebook keeps its
+        # outputs, matching `nbconvert --output <f> <f>` (which writes in place).
+        nbformat.write(nb, nb_path)
+
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        print(f"usage: {Path(__file__).name} <notebook-path> <workspace-root>",
+              file=sys.stderr)
+        sys.exit(2)
+    run(sys.argv[1], sys.argv[2])
diff --git a/tests/test_run_notebook_cwd.py b/tests/test_run_notebook_cwd.py
new file mode 100644
index 00000000..36c6c89c
--- /dev/null
+++ b/tests/test_run_notebook_cwd.py
@@ -0,0 +1,97 @@
+"""Regression tests for the notebook kernel's working directory.
+
+`jupyter nbconvert --execute` starts the kernel in the **notebook's own
+directory**, but the workspaces document execution from the repo root and their
+auto-simulate guards shell out to root-relative simulator paths::
+
+    if al.util.dataset.should_simulate(str(dataset_path)):
+        subprocess.run([sys.executable, "scripts/imaging/simulator.py"], check=True)
+
+Under nbconvert that subprocess exits 2 ("can't open file") and every
+auto-simulating notebook fails. `autohands/run_notebook.py` fixes this by
+pinning the kernel cwd to the workspace root via
+`resources['metadata']['path']`.
+
+These tests lock in the two properties that matter: the kernel really runs at
+the given root (not the notebook's directory), and a root-relative subprocess
+therefore resolves. They are skipped when a Jupyter kernel is unavailable.
+"""
+
+import json
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+PROJECT_ROOT = Path(__file__).parent.parent
+RUNNER = PROJECT_ROOT / "autohands" / "run_notebook.py"
+
+jupyter = pytest.importorskip("nbformat")
+pytest.importorskip("nbconvert")
+
+
+def _write_nb(path: Path, source: str) -> None:
+    path.parent.mkdir(parents=True, exist_ok=True)
+    nb = {
+        "cells": [{
+            "cell_type": "code",
+            "execution_count": None,
+            "metadata": {},
+            "outputs": [],
+            "source": source,
+        }],
+        "metadata": {"kernelspec": {
+            "display_name": "Python 3", "language": "python", "name": "python3",
+        }},
+        "nbformat": 4,
+        "nbformat_minor": 5,
+    }
+    path.write_text(json.dumps(nb))
+
+
+def _outputs(path: Path) -> str:
+    nb = json.loads(path.read_text())
+    return "".join(
+        "".join(o.get("text", "")) for o in nb["cells"][0].get("outputs", [])
+    )
+
+
+def _run(nb_path: Path, root: Path):
+    return subprocess.run(
+        [sys.executable, str(RUNNER), str(nb_path), str(root)],
+        capture_output=True, text=True, timeout=300,
+    )
+
+
+def test_kernel_cwd_is_the_given_root_not_the_notebook_dir(tmp_path):
+    """The whole point of the runner: cwd is the root, not notebooks/sub/."""
+    nb = tmp_path / "notebooks" / "sub" / "cwd.ipynb"
+    _write_nb(nb, 'import os\nprint("CWD:", os.getcwd())')
+
+    result = _run(nb, tmp_path)
+
+    assert result.returncode == 0, result.stderr
+    out = _outputs(nb)
+    assert f"CWD: {tmp_path}" in out
+    # the failure mode this exists to prevent
+    assert str(nb.parent) not in out
+
+
+def test_root_relative_subprocess_resolves(tmp_path):
+    """The auto-simulate guard shape: a root-relative script path must run."""
+    (tmp_path / "scripts").mkdir(parents=True, exist_ok=True)
+    (tmp_path / "scripts" / "sim.py").write_text('print("simulated")')
+
+    nb = tmp_path / "notebooks" / "deep" / "guard.ipynb"
+    _write_nb(
+        nb,
+        "import subprocess, sys\n"
+        'r = subprocess.run([sys.executable, "scripts/sim.py"], check=True)\n'
+        'print("GUARD_OK")',
+    )
+
+    result = _run(nb, tmp_path)
+
+    assert result.returncode == 0, result.stderr
+    assert "GUARD_OK" in _outputs(nb)