diff --git a/CHANGELOG.md b/CHANGELOG.md
index 05d2c912be..0535dcf4ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@
 - Bedrock: Assistant text and reasoning are now preserved alongside tool calls in the same turn instead of being dropped. (#4457)
 - Bugfix: With checkpoints stored on S3, a resumed sample can now itself be resumed — previously a crash after resume lost the checkpoint chain and the next retry restarted the sample from scratch.
 - Bugfix: Crash recovery now reconstructs `sample.messages` from the first model role's conversation when a solver runs multiple agents concurrently, instead of returning whichever agent's model call happened to fire last. (#4414)
+- Web Browser: headful macOS scale-factor detection falls back to 1 instead of crashing when `pyobjc-framework-AppKit` is missing or no display is attached.
 - Bugfix: Errored samples that were scored (e.g. via `score_on_error`) now contribute their scores to metrics and `scored_samples`, matching what the sample log shows. (#4412)
 - Bugfix: Several raised errors (unexpected Anthropic input block, unsupported image data type, missing sample-buffer database) now interpolate the offending value into the message instead of showing the literal placeholder text.
 - Bugfix: Native compaction now truncates an oversized tool output before summarizing instead of crashing or silently summarizing an error when it would exceed the model's context window. (#3600)
diff --git a/src/inspect_tool_support/src/inspect_tool_support/_remote_tools/_web_browser/scale_factor.py b/src/inspect_tool_support/src/inspect_tool_support/_remote_tools/_web_browser/scale_factor.py
index 339c5647c4..a499a01ee5 100644
--- a/src/inspect_tool_support/src/inspect_tool_support/_remote_tools/_web_browser/scale_factor.py
+++ b/src/inspect_tool_support/src/inspect_tool_support/_remote_tools/_web_browser/scale_factor.py
@@ -1,6 +1,19 @@
+import logging
 import subprocess
 import sys
 
+logger = logging.getLogger(__name__)
+
+
+def warn_once(logger: logging.Logger, message: str) -> None:
+    """Emit ``message`` as a warning, but only the first time it is seen this process."""
+    if message not in _warned:
+        logger.warning(message)
+        _warned.add(message)
+
+
+_warned: set[str] = set()
+
 # Playwright launches Chromium with --force-device-scale-factor=1 by default, which
 # ensures consistent rendering and measurement behavior using CSS pixels instead of
 # native device pixels, regardless of the actual display's DPI. On HiDPI displays,
@@ -19,12 +32,29 @@
 # allows the code to accurately calculate node visibility.
 
 
-def get_screen_scale_factor() -> float:
-    if sys.platform == "darwin":
+def _darwin_scale_factor() -> float:
+    try:
         # `pip install pyobjc-framework-AppKit` is required to run headfully on macOS
         from AppKit import NSScreen  # type: ignore  # noqa: PLC0415
 
-        return NSScreen.mainScreen().backingScaleFactor()
+        screen = NSScreen.mainScreen()
+    except ModuleNotFoundError:
+        warn_once(
+            logger,
+            "pyobjc-framework-AppKit is not installed; falling back to a screen "
+            "scale factor of 1. Install it (`pip install pyobjc-framework-AppKit`) "
+            "to enable HiDPI scale-factor detection when running headfully on macOS.",
+        )
+        return 1.0
+    except Exception:
+        return 1.0
+    # mainScreen() is None when no display is attached (headless / over SSH)
+    return float(screen.backingScaleFactor()) if screen is not None else 1.0
+
+
+def get_screen_scale_factor() -> float:
+    if sys.platform == "darwin":
+        return _darwin_scale_factor()
     elif sys.platform == "win32":
         try:
             # Using GetDpiForSystem from Windows API
diff --git a/src/inspect_tool_support/src/inspect_tool_support/_remote_tools/_web_browser/test_scale_factor.py b/src/inspect_tool_support/src/inspect_tool_support/_remote_tools/_web_browser/test_scale_factor.py
new file mode 100644
index 0000000000..fbcb17a214
--- /dev/null
+++ b/src/inspect_tool_support/src/inspect_tool_support/_remote_tools/_web_browser/test_scale_factor.py
@@ -0,0 +1,37 @@
+import sys
+import types
+
+from scale_factor import get_screen_scale_factor
+
+
+def _install_fake_appkit(monkeypatch, screen: object) -> None:
+    """Put a fake ``AppKit`` on ``sys.modules`` whose ``mainScreen()`` returns ``screen``."""
+    module = types.ModuleType("AppKit")
+    module.NSScreen = types.SimpleNamespace(mainScreen=lambda: screen)  # type: ignore[attr-defined]
+    monkeypatch.setitem(sys.modules, "AppKit", module)
+
+
+def test_darwin_returns_backing_scale_factor(monkeypatch):
+    monkeypatch.setattr(sys, "platform", "darwin")
+    _install_fake_appkit(
+        monkeypatch, types.SimpleNamespace(backingScaleFactor=lambda: 2.0)
+    )
+    assert get_screen_scale_factor() == 2.0
+
+
+def test_darwin_headless_screen_falls_back_to_one(monkeypatch):
+    # NSScreen.mainScreen() is None when no display is attached (headless / over SSH);
+    # dereferencing it used to raise AttributeError instead of defaulting to 1.0.
+    monkeypatch.setattr(sys, "platform", "darwin")
+    _install_fake_appkit(monkeypatch, None)
+    assert get_screen_scale_factor() == 1.0
+
+
+def test_darwin_missing_appkit_falls_back_to_one(monkeypatch):
+    # pyobjc-framework-AppKit is an optional dependency; a missing import used to crash
+    # the probe instead of degrading like the Windows and Linux branches do.
+    monkeypatch.setattr(sys, "platform", "darwin")
+    monkeypatch.setitem(
+        sys.modules, "AppKit", None
+    )  # `from AppKit import ...` -> ImportError
+    assert get_screen_scale_factor() == 1.0