diff --git a/src/cursor_flasher/detector.py b/src/cursor_flasher/detector.py
new file mode 100644
index 0000000..f262094
--- /dev/null
+++ b/src/cursor_flasher/detector.py
@@ -0,0 +1,131 @@
+"""Accessibility-based detection of Cursor's agent state.
+
+Detection strategy (based on a11y tree analysis):
+- Cursor is an Electron app; web content is exposed via AXWebArea.
+- In-app buttons render as AXStaticText with their label in the 'value' attr,
+  NOT as AXButton elements (those are native window controls only).
+- We collect both AXStaticText values and AXButton titles, then match against
+  known keywords for "agent working" and "approval needed" states.
+"""
+from dataclasses import dataclass
+import re
+
+from ApplicationServices import (
+    AXUIElementCreateApplication,
+    AXUIElementCopyAttributeNames,
+    AXUIElementCopyAttributeValue,
+)
+from Cocoa import NSWorkspace
+
+CURSOR_BUNDLE_ID = "com.todesktop.230313mzl4w4u92"
+
+AGENT_WORKING_EXACT = {"Stop", "Cancel generating"}
+AGENT_WORKING_PATTERNS = [re.compile(r"^Generating\b", re.IGNORECASE)]
+
+APPROVAL_EXACT = {"Accept", "Reject", "Accept All", "Deny"}
+APPROVAL_PATTERNS = [
+    re.compile(r"^Run\b", re.IGNORECASE),
+    re.compile(r"^Allow\b", re.IGNORECASE),
+]
+
+
+@dataclass
+class UISignals:
+    agent_working: bool = False
+    approval_needed: bool = False
+
+
+def _text_matches(text: str, exact_set: set[str], patterns: list[re.Pattern]) -> bool:
+    if text in exact_set:
+        return True
+    return any(p.search(text) for p in patterns)
+
+
+def parse_ui_signals(elements: list[dict]) -> UISignals:
+    """Parse flattened UI elements into detection signals."""
+    agent_working = False
+    approval_needed = False
+
+    for el in elements:
+        role = el.get("role", "")
+        label = ""
+        if role == "AXStaticText":
+            label = el.get("value", "")
+        elif role == "AXButton":
+            label = el.get("title", "")
+
+        if not label:
+            continue
+
+        if _text_matches(label, AGENT_WORKING_EXACT, AGENT_WORKING_PATTERNS):
+            agent_working = True
+        if _text_matches(label, APPROVAL_EXACT, APPROVAL_PATTERNS):
+            approval_needed = True
+
+    return UISignals(agent_working=agent_working, approval_needed=approval_needed)
+
+
+class CursorDetector:
+    """Polls Cursor's accessibility tree for agent state signals."""
+
+    def __init__(self):
+        self._pid: int | None = None
+
+    def poll(self) -> UISignals | None:
+        """Poll Cursor's a11y tree and return detected signals, or None if Cursor isn't running."""
+        pid = self._find_cursor_pid()
+        if pid is None:
+            self._pid = None
+            return None
+
+        self._pid = pid
+        app_element = AXUIElementCreateApplication(pid)
+        elements = self._collect_elements(app_element, max_depth=15)
+        return parse_ui_signals(elements)
+
+    def _find_cursor_pid(self) -> int | None:
+        workspace = NSWorkspace.sharedWorkspace()
+        for app in workspace.runningApplications():
+            bundle = app.bundleIdentifier() or ""
+            if bundle == CURSOR_BUNDLE_ID:
+                return app.processIdentifier()
+        return None
+
+    def _collect_elements(
+        self, element, max_depth: int = 15, depth: int = 0
+    ) -> list[dict]:
+        """Walk the a11y tree collecting button and static text elements."""
+        if depth > max_depth:
+            return []
+
+        results: list[dict] = []
+        err, attr_names = AXUIElementCopyAttributeNames(element, None)
+        if err or not attr_names:
+            return results
+
+        role = ""
+        title = ""
+        value = ""
+
+        for name in attr_names:
+            val_err, val = AXUIElementCopyAttributeValue(element, name, None)
+            if val_err:
+                continue
+            if name == "AXRole":
+                role = str(val)
+            elif name == "AXTitle":
+                title = str(val) if val else ""
+            elif name == "AXValue":
+                value = str(val) if val else ""
+
+        if role == "AXStaticText" and value:
+            results.append({"role": role, "value": value})
+        elif role == "AXButton" and title:
+            results.append({"role": role, "title": title})
+
+        err, children = AXUIElementCopyAttributeValue(element, "AXChildren", None)
+        if not err and children:
+            for child in children:
+                results.extend(self._collect_elements(child, max_depth, depth + 1))
+
+        return results
diff --git a/tests/test_detector.py b/tests/test_detector.py
new file mode 100644
index 0000000..8855ab9
--- /dev/null
+++ b/tests/test_detector.py
@@ -0,0 +1,72 @@
+import pytest
+from unittest.mock import patch, MagicMock
+from cursor_flasher.detector import (
+    CursorDetector,
+    parse_ui_signals,
+    UISignals,
+)
+
+
+class TestParseUISignals:
+    def test_no_elements_means_no_signals(self):
+        signals = parse_ui_signals([])
+        assert signals.agent_working is False
+        assert signals.approval_needed is False
+
+    def test_stop_text_means_agent_working(self):
+        elements = [{"role": "AXStaticText", "value": "Stop"}]
+        signals = parse_ui_signals(elements)
+        assert signals.agent_working is True
+
+    def test_cancel_generating_means_agent_working(self):
+        elements = [{"role": "AXStaticText", "value": "Cancel generating"}]
+        signals = parse_ui_signals(elements)
+        assert signals.agent_working is True
+
+    def test_accept_text_means_approval_needed(self):
+        elements = [{"role": "AXStaticText", "value": "Accept"}]
+        signals = parse_ui_signals(elements)
+        assert signals.approval_needed is True
+
+    def test_reject_text_means_approval_needed(self):
+        elements = [{"role": "AXStaticText", "value": "Reject"}]
+        signals = parse_ui_signals(elements)
+        assert signals.approval_needed is True
+
+    def test_run_this_time_means_approval_needed(self):
+        elements = [{"role": "AXStaticText", "value": "Run this time only (⏎)"}]
+        signals = parse_ui_signals(elements)
+        assert signals.approval_needed is True
+
+    def test_both_signals(self):
+        elements = [
+            {"role": "AXStaticText", "value": "Stop"},
+            {"role": "AXStaticText", "value": "Accept"},
+        ]
+        signals = parse_ui_signals(elements)
+        assert signals.agent_working is True
+        assert signals.approval_needed is True
+
+    def test_irrelevant_text_ignored(self):
+        elements = [{"role": "AXStaticText", "value": "Settings"}]
+        signals = parse_ui_signals(elements)
+        assert signals.agent_working is False
+        assert signals.approval_needed is False
+
+    def test_button_role_also_detected(self):
+        elements = [{"role": "AXButton", "title": "Accept"}]
+        signals = parse_ui_signals(elements)
+        assert signals.approval_needed is True
+
+    def test_partial_match_on_run_command(self):
+        elements = [{"role": "AXStaticText", "value": "Run command"}]
+        signals = parse_ui_signals(elements)
+        assert signals.approval_needed is True
+
+
+class TestCursorDetector:
+    def test_returns_none_when_cursor_not_running(self):
+        detector = CursorDetector()
+        with patch.object(detector, "_find_cursor_pid", return_value=None):
+            signals = detector.poll()
+            assert signals is None